Chapter 5: Proper Application of Waste Products — Type A


Proper waste application involves knowledge of the waste application system, the soils and crops, and the required buffers that must be adhered to. This chapter will explain the required buffers and all other factors that must be considered when you are trying to determine when and how much to irrigate.

What Buffers Are Required When I Make Land Applications?

List the necessary buffers for waste application.

There are three different sets of buffers you should consider when land applying animal waste. The first are those that are required by law. Under the existing 15A NCAC 2H .0200 Nondischarge Rules, you can not land apply animal wastes within 25 feet of perennial streams. Perennial streams are defined as streams indicated by a solid blue line on a United States Geological Survey map. Effective October 1, 1995 for new or expanding swine farms, you may not apply waste of any form within 50 feet of residential property lines and rights-of-way. Furthermore, you may not apply waste within 50 feet of perennial streams. Again, these 50-foot buffers only affect swine farms constructed or expanded after October 1, 1995. You should also review your waste management plan or permit to see if any additional buffers are required.

A 100-foot buffer is required around wells for location of lagoons, other waste treatment facilities, and land application sites. The same processes that can contaminate a well from a lagoon can occur from a land application site if improperly managed or the well is poorly constructed. Even on a properly managed site it is best to maintain the buffer to reduce the potential for problems.

Describe why wind speed and direction should be considered when irrigating.

The last buffer is a "good neighbor" buffer. Maintain a minimum distance of 200 feet from any homes or other buildings frequented by people. Furthermore, do not make land applications on days with excessive wind. Drift on these days may irritate neighbors or pollute surface waters.


Irrigation Scheduling

Proper land application of lagoon liquid involves the use of water management strategies to best achieve a balance between:

A responsible system operator must understand how his/her wastewater should be managed, have knowledge of the capacity of his/her system to store and apply wastewater when appropriate, and be able to make prudent management decisions concerning when and how much wastewater to land apply. For a liquid waste management system utilizing irrigation, this decision-making process is called irrigation scheduling, and is generally based on the following flowchart (Figure 5-1).

Irrigation scheduling is the process of answering two basic questions:

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Figure 5-1. Nutrient management and irrigation scheduling decision-making flowchart.



Determining When to Irrigate

List the five factors that must be addressed before irrigating animal waste.

There are five basic questions that should be answered when deciding to irrigate:

  1. Do I have an actively growing crop (or will a crop be planted or actively start growing within 30 days)?

  2. Do I have a nitrogen deficit remaining for this crop cycle?

  3. Is the liquid level in my lagoon above the minimum storage depth?

  4. Are my land application fields dry enough to be irrigated?

  5. Do I have a waste analysis within 60 days of this irrigation?

If the answer to all five questions above is yes, then you should schedule an irrigation. The answer to Questions 1, 3, and 5 are straightforward and should be easy to determine. Question 2 requires a knowledge of how much nutrients you can apply and how much has already been applied. This will be addressed further in Chapter 6 (Record Maintenance). The next section will discuss how to determine soil-water content and how to determine irrigation rates.

Basic Soil-Water Relationships

Explain the importance of soil-water terms: saturation, field capacity, permanent wilting point, gravitational water, and plant-available water relative to irrigation scheduling.

Before attempting to measure or estimate soil-water content, you should understand some basic soil-water relationships. Understanding these relationships will help you determine if fields are dry enough to be irrigated and at what rates. Important terms you should be familiar with include: saturation, field capacity, permanent wilting point, gravitational water, and plant-available water. These relationships are discussed in great detail in Extension Service publication AG-452-1, Soil, Water and Crop Characteristics Important to Irrigation Scheduling.

To summarize briefly, soil is composed of three major parts: air, water, and solids. The portion occupied by air and water is referred to as the pore volume. At saturation, all soil pores are filled with water. Any water added to the soil under this condition will either run off or leach below the root zone. Saturated conditions are undesirable for good crop growth or wastewater irrigation. Field capacity refers to the point where the soil has had time to drain away excess water from the large pores by gravity, but still remains in a very moist condition. Permanent wilting point is where there is so little water left in the soil that plants cannot remove the water for their use.

Not all of the water added to soil is retained in the soil for use by plants. Wastewater should be applied to soil such that it remains in the root zone for uptake by the crop. Any wastewater not retained in the root zone will likely transport waste nutrients to wherever it goes, resulting in a pollution threat to either surface water, groundwater, or both.

To interpret soil-water measurements and apply them to irrigation scheduling, you must be able to distinguish between two categories of soil-water:

  1. Gravitational Water — the water in the soil that is free to drain or move by the force of gravity. Gravitational water is computed as the volume of water in the soil between saturation and field capacity. When gravitational water is present in the root zone, the soil is "too wet" to be irrigated with wastewater.

  2. Plant-Available Water (PAW) — the amount of water held in the soil that is available to plants. It is computed as the difference between the water content at field capacity (referred to as upper limit water content) and the permanent wilting point (often referred to as the lower limit water content). Irrigation should be scheduled to maintain the water content of the soil between these two extremes. If there is no PAW deficit (i.e., the soil-water content is above the upper limit), gravitational water is present and wastewater irrigation should be delayed unless you are operating under emergency conditions.

Soil texture greatly influences the portion of the soil pore volume that can be occupied by gravitational water or plant-available water; therefore, it is important to know your soil texture to determine how much water can be irrigated.

The amount of plant-available water (PAW) that exists in the soil at any given time is commonly expressed as the depth of water per unit depth of soil. Typical units are inches of PAW per foot of soil depth. Estimates of PAW for various soil textural classes are given in published soil survey reports. These estimates range from less than 0.2 inch of PAW per foot of soil for coarse sandy soils to nearly 2 inches of PAW per foot of soil for silty clay and clay soils (Table 5-1).

Table 5-1. Average Estimated Plant-Available Water for Various Soil Textural Classes
Textural Class Plant-Available Water Maximum Recommended Application Volume
  inches of water per foot of soil
Coarse sand and gravel 0.2 to 0.6 0.2
Sand 0.5 to 1.0 0.4
Loamy sand 0.8 to 1.5 0.6
Sandy loam 1.0 to 1.8 0.7
Loams 1.3 to 2.0 0.8
Silt loam 1.3 to 2.2 0.8
Silty clay loam 1.3 to 1.8 0.8
Clay loam 1.1 to 1.9 0.8
Sandy clay loam 1.1 to 1.8 0.8
Silty clay 1.2 to 1.9 0.8
Clay 1.2 to 1.9 0.8

At the start of irrigation, the water content in the soil should be lower than field capacity (upper limit). The difference between the existing water content and the field capacity water content is the amount that should be irrigated. The drier the soil, the more wastewater that can be safely applied per application, provided this amount does not exceed the required nitrogen application rate. Determining the water content of the soil tells you if the soil is dry enough to be irrigated and if so, how much wastewater can be applied. Maximum recommended wastewater irrigation amounts as influenced by soil texture are shown in Table 5-1.

Estimating Soil-Water Content

Describe techniques for determining the amount of water present in the soil and when to irrigate.

Determining whether or not the field is "dry" enough to be irrigated is not always obvious. There are three practical methods of determining if the field is dry enough to be irrigated. These include:

  1. a subjective method that involves "feeling" the soil,

  2. objective methods utilizing soil-moisture measuring devices, and

  3. an accounting approach (checkbook method) to estimate soil-water.

One of these three methods for determining soil-water content should be used to estimate the amount of water present in the soil at the start of irrigation.

Feel Method

The feel method involves estimating soil-water by feeling the soil. This method is easy to use, and many growers schedule irrigation in this way. This method is "subjective" since the results depend on the experience of the person doing the measurement. The value of this method depends on the experience of the operator. Some "feel" guidelines for estimating soil-water content by the feel method are given in Table 5-2.

Use of the feel method is demonstrated in the following example. Suppose your irrigation field is a sandy loam soil with a 15-inch root zone. You feel the soil and observe that it forms a weak ball which falls apart. Based on the guidelines given in Table 5-2, you can irrigate 0.3 to 0.4 inch of water per foot of root zone depth. For a 15-inch (1.25 feet) root zone depth, the permissible irrigation amount is:

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Table 5-2. "Feel" Guidelines for Estimating the Amount of Plant-Available Water to Be Replaced with Wastewater Irrigation as a Function of Soil Texture
Available Water Remaining in the Soil Sands
Loamy Sand
Sandy Loam Clay, Clay Loam, Sandy Clay Loam All Other Textures

Maximum Recommended Wastewater Irrigation
(per foot of effective root zone depth)

100% (i.e., field capacity) When ball is squeezed, no free water appears on soil but wet outline of ball is left in hand
Wastewater Irrigation None None None None
75% to 100% Sticks together only slightly Forms a ball that breaks easily Forms a ball; very pliable Easily ribbons between thumb and forefinger; feels slick
Wastewater Irrigation 0.1 to 0.2 inch 0.2 to 0.3 inch 0.2 to 0.4 inch 0.2 to 0.4 inch
50% to 75% Appears dry, will not form a ball Forms weak ball which falls apart Forms ball; slightly plastic; slightly slick Forms ball; forms ribbon
Wastewater Irrigation 0.2 to 0.3 inch 0.3 to 0.4 inch 0.3 to 0.5 inch 0.3 to 0.6 inch
25% to 50% Appears dry, will not form a ball Appears dry, will not form a ball Somewhat crumbly but holds under pressure Forms ball under pressure; somewhat pliable
Wastewater Irrigation 0.3 to 0.5 inch 0.3 to 0.6 inch 0.3 to 0.6 inch 0.3 to 0.7 inch
0 to 25% Dry, loose, single-grained, flows through fingers Dry, loose, flows through fingers Powdery, dry; easily breaks into powdery condition Hard, cracked; may have loose crumbs on soil surface
Wastewater Irrigation 0.3 to 0.5 inch 0.3 to 0.6 inch 0.3 to 0.7 inch 0.3 to 0.7 inch

Soil Moisture Measurement Devices

There are many different methods or devices for measuring soil water. These include the gravitational method, tensiometer, electrical resistance blocks, neutron probe, Phene cell, and time domain reflectometer. These methods differ in reliability, cost, and labor intensity.

Tensiometer and electrical resistance blocks are the most cost-effective and reliable devices for on-farm measurement of soil-water for irrigation in North Carolina. Tensiometers are best suited for sandy, sandy loam, and loamy soil textures while electrical resistance blocks work best in silty or clayey soils. Manufacturers of these devices provide calibration charts and recommended ranges for traditional "fresh" water irrigation. You should be aware that the calibration curves and recommendations supplied by the manufacturer for these devices were developed for general conditions and are not adequate for specific soil conditions and fields. You should also be aware that wastewater objectives and recommendations are different from fresh water recommendations. The differences will be discussed later in this chapter. For irrigating wastewater, you will get better results if all soil-water measuring devices you use are calibrated for the major soils you are irrigating.

Checkbook Method

The checkbook method is an accounting approach for estimating how much soil-water remains in the effective root zone based on water inputs and outputs (like a daily balance on a bank account based on deposits and withdrawals). Wastewater irrigation is scheduled when the soil-water content in the root zone drops below a threshold level. Some of the simpler checkbook methods keep track of rainfall, evapotranspiration, and irrigation amounts. More sophisticated methods require periodic measurements of the soil-water status and moisture use rates of the crop.

Checkbook methods require detailed daily record keeping which can become time consuming for the more complex methods. One of the advantages of the checkbook approach is that it can be programmed on a computer. Computer programs have been developed to handle the accounting and provide timely and precise scheduling recommendations.

To use the checkbook method, you must be able to estimate the rate that water stored in the soil is being used up. Water is removed from the root zone by direct evaporation from the soil and transpiration by plants. Collectively, this is referred to as evapotranspiration (ET). Evapotranspiration is affected by many factors and varies seasonally and from day to day. On a hot, rainless day, ET may approach 0.25 inch per day. Thus, during dry, summer periods, 1 inch of water could be removed from the soil every 4 days. On the other hand, more than a month may elapse before an inch is removed during prolonged cold-wet winter periods. For these reasons, irrigation amounts must vary seasonally in response to variation in evapotranspiration.

Average daily PET (PET is potential evapotranspiration — the amount of water that can be removed by a combination of the crops and evaporation) estimates for several locations are shown in Table 5-3. These values can be used in the checkbook method on rainless days. When rainfall is characterized by thunderstorms of short duration, the daily values given in Table 5-3 are also appropriate. On days that are mostly cloudy or with rain during much of the daylight hours, assume the daily ET for that day is zero. The local NRCS maintains average daily values for each county. Actual values can also be obtained from the National Weather Service and are available on the Internet for several locations.

Table 5-3. Average Evapotranspiration from Vegetation Growing in Moist Soils
  Average Potential Evapotranspiration (in./day)
Month Asheville Charlotte Raleigh Plymouth Wilmington
January 0.03 0.03 0.03 0.03 0.04
February 0.05 0.05 0.05 0.06 0.08
March 0.07 0.08 0.09 0.10 0.11
April 0.11 0.12 0.13 0.13 0.15
May 0.12 0.15 0.16 0.16 0.17
June 0.15 0.17 0.17 0.17 0.18
July 0.14 0.16 0.17 0.17 0.18
August 0.12 0.14 0.15 0.15 0.16
September 0.10 0.11 0.12 0.13 0.13
October 0.06 0.07 0.08 0.09 0.10
November 0.04 0.04 0.05 0.06 0.06
December 0.02 0.02 0.03 0.04 0.05
Values computed from long-term daily maximum and minimum temperature records using the Thornthwaite method, corrected for measured pan evaporation. Single day value may be higher than shown (up to 0.25 inch/day) but values shown represent long-term average daily values for each month.

To use the checkbook method, you must begin computations when the soil is at a known water content. Field capacity is the usual starting point and should be assumed to occur soon after a rainfall or irrigation of an amount large enough to wet the effective root zone. For many of the well-drained loamy soils found in eastern North Carolina (root zone textures consisting of loamy sand, sandy loam, loam, or sandy clay loam) field capacity can be assumed to occur 1 day after a rainfall or irrigation in excess of 1 inch.

Example calculations for the checkbook method are shown in Table 5-4. In this example, the soil is a sandy loam with an effective root zone of 12 inches. The allowable irrigation volume is the depleted plant-available water content given in Table 5-1 not to exceed the recommended maximum of 0.7 inch for a sandy loam soil. Irrigation would normally be scheduled when about 50 percent of the plant-available water had been depleted.

Table 5-4. Use of the Checkbook Method for Irrigation Scheduling (Raleigh Location)
Date PAW 1 in soil at start of day Daily PET Rainfall Irrigation Drainage PAW in soil at end of day Storage2 Comments
inches % of PAW inches inches % of PAW inches
5-01 - - - - - - - - - Don’t know initial conditions at this time
5-02 - - - -            
5-03 - - - 1.5   0.1 1.40 100 0.00 Assume FC reached at end of day after rain
5-04 1.40 100 0.16 - - - 1.24 89 0.16  
5-05 1.24 89 0.16 - - - 1.08 77 0.32  
5-06 1.08 77 0.16 - - - 0.92 66 0.48  
5-07 0.92 66 0.16 - - - 0.76 54 0.64 OK to irrigate
5-08 0.76 54 0.16 - 0.64 - 1.24 89 0.16 Irrigated 0.64 inch
5-09 1.24 89 0.16 - - - 1.08 77 0.32  
5-10 1.08 77 0.16 0.75 - 0.27 1.40 100 0.00 Rain exceeded storage so had 0.27 inch of drainage or seepage
5-11 1.40 100 0.16 - - - 1.24 89 0.16  
5-12 1.24 89 0.16 - - - 1.08 77 0.32  
5-13 1.08 77 0.16 - - - 0.92 66 0.48 Lagoon marker indicates need to irrigate
5-14 0.92 66 0.16 - 0.48 - 1.24 89 0.16 Irrigated storage at start of day (0.48 inch)
5-15 1.24 89 0.16 - - - 1.08 77 0.32  
5-16 1.08 77 0.16 0.22 - - 1.14 81 0.26  
5-17 1.14 81 0.16 - - - 0.98 70 0.42  
5-18 0.98 70 0.16 - - - 0.82 59 0.58 Would like to irrigate
5-19 0.82 58 0.16 - 0.50 - 1.16 83 0.24 Irrigated 0.5 inch
5-20 1.16 83 0.16 0.63 - 0.23 1.40 100 0.00 Rain exceeded storage so had 0.23 inch of drainage or seepage
5-21 1.40 100 0.16 - - - 1.24 89 0.16  
5-22 1.24 89 0.16 - - - 1.08 77 0.32  
5-23 1.08 77 0.16 - - - 0.92 66 0.48  
5-24 0.92 66 0.16 0.08 - - 0.84 60 0.56 Lagoon marker indicates need to irrigate
5-25 0.84 60 0.16 - 0.56 - 1.24 89 0.16 Irrigated 0.56 inch
5-26 1.24 89 0.16 - - - 1.08 77 0.32  
5-27 1.08 77 0.16 - - - 0.92 66 0.48  
5-28 0.92 66 0.16 1.32 - 0.68 1.40 100 0.00 Rain with 0.68 inch drainage
5-29 1.40 100 0.16 - - - 1.24 89 0.16  
5-30 1.24 89 0.16 - - - 1.08 77 0.32  
5-31 1.08 77 0.16 - - - 0.92 66 0.48 Lagoon marker indicates need to irrigate
1 Based on a maximum plant-available water content of the example soil of 1.4 inches.
Soil storage at end of day is amount of plant-available water depleted. It is computed as:

Soil Storage (end of day) = PAW (start of day) + PET - rainfall - irrigation + drainage

From Table 5-1, 50 percent depletion for a sandy loam soil ranges from 0.5 to 0.9 inch. Assume the maximum plant-available water content for this soil is 1.4 inches (sandy loam soil in Table 5-1) so that the target depletion value for this soil is 0.7 inch which corresponds to the maximum recommended for a sandy loam. The crop grown is coastal bermudagrass, and irrigation is recommended during the period from May to October. Since no irrigation was planned prior to May, it is not necessary to maintain a checkbook balance of the soil-water content prior to May. In this example, the checkbook balance begins May 1 and continues throughout the irrigation period.

Since no records were being maintained, the water content on May 1 is not known. The initial content on May 1 can be estimated using the feel method, Table 5-2, or a starting value can be assumed 1 day after a 1 inch or greater rainfall (i.e., field capacity value). Using the latter approach, 1.5 inches of rain occurred on May 3. Since the maximum plant-available water content is 1.4 inches, we can assume that the soil storage is "full" 1 day after the 1.5 inch rainfall. Therefore, we start the water balance on May 4 assuming the plant-available storage on that day is 1.4 inches. Each day thereafter, a balance is computed based on additions to the root zone (rainfall or irrigation) and removal from the root zone (ET, drainage, or seepage).

Soil-water content can be measured or computed reliably by several different methods. Success results from selecting the method that is appropriate for the soils and crops being irrigated and that you feel confident in using. To help select the method that is right for you, contact your Cooperative Extension Service center, Natural Resources Conservation Service, Division of Soil and Water Conservation, or Certified Technical Specialist.

Determining How Much to Irrigate

Irrigation should be scheduled and timed so that:

  1. no surface runoff occurs during irrigation,

  2. the root zone is not completely saturated at the conclusion of irrigation, and

  3. the irrigated water does not leach below the root zone.
List five factors that may influence the amount of wastewater that can be irrigated.

The amount of wastewater that can or should be applied during any single irrigation cycle is dictated by how much water the soil can "soak up." This varies from day to day and is influenced by:

  1. Rainfall — when and how much it last rained.

  2. Crop maturity — water uptake rate of the crop.

  3. Soil type — texture, structure, depth, and cover.

  4. Effective root depth — dependent on soil characteristics and crop type and age.

  5. Evapotranspiration — influenced by temperature, wind, and relative humidity.

Wastewater irrigation should replace the water that has evaporated from the soil or been removed by plants. This is referred to as the plant-available water deficit. Most water taken up by plants is removed in the upper half of the root zone. For the purpose of scheduling irrigation, this zone is referred to as the effective root depth. Soil conditions in North Carolina limit maximum rooting depths of most crops to 24 to 36 inches, so the effective root depth is typically only 12 to 18 inches. It is within this depth that we estimate or measure the plant-available water deficit to be replaced by irrigation. If the irrigation volume applied exceeds the PAW deficit, the excess either runs off or leaches below the root zone and could potentially contaminate groundwater.

If the soil was completely dry so that the crop was severely wilted, the plant-available water deficit would be the value shown in Table 5-1. In reality, the soil is rarely this dry, so the amount to be replaced will be less than the values shown in Table 5-1. At any given time, the plant-available water deficit could range from none to the maximum values shown in Table 5-1. Recommended wastewater irrigation volumes for a range of soil-water contents and soil textures are shown in Table 5-2.

Wastewater irrigation objectives and recommendations are different from fresh water recommendations. When irrigating with fresh water, the objective is to keep the soil moist for optimum growing conditions. This is desirable for all crops, but during dry periods, all of the water needed by the crop cannot be supplied from wastewater. The fresh water irrigation required in addition to rainfall to maintain moist conditions throughout a typical cropping season is 6 to 10 inches. Usually, the irrigation volume required to supply the nitrogen requirement is only 2 to 5 inches. Thus, not all PAW deficits can be replenished by wastewater irrigation without over application of N.

Evaporation during irrigation is also a consideration in determining how much to irrigate. The amount irrigated should not be less than the anticipated potential evapotranspiration (PET) for the day. These amounts are shown in Table 5-3. If the amount irrigated is less than the daily PET, most of it will evaporate soon after application with little infiltration into the soil. When this occurs, most of the applied nitrogen is lost to the atmosphere through the process of ammonia volatilization, which is also environmentally undesirable. In general, when the soil is dry during the summer, the irrigation amount should not be less than 0.25 inch of irrigation. When temperatures are cold and PET is very low, such as during the winter, smaller irrigation amounts even as low as 0.1 inch are acceptable. Since most irrigation occurs during the growing season when PET is near the daily maximum, minimum irrigation amounts will typically be 0.2 inch or greater.

Explain the relevance of irrigation precipitation rate to soil infiltration capacity.

Another factor affecting irrigation amount is the soil intake rate (infiltration rate). The soil intake rate is the rate that the soil can soak up the irrigated lagoon liquid. The soil intake rate decreases the longer water is applied. The intake capacity of most clayey or silty soils begins to be exceeded by the time 0.5 to 0.6 inch has been applied. Continuing to irrigate could result in surface ponding and possible runoff of the irrigated wastewater, which is a water quality violation that could result in penalties or fines. Soil intake rate is also dependent on the crop type, thickness of the stand, and slope of the land. Some typical ranges for various soil textures are shown in Table 5-5. Application rates in excess of these values could result in ponding and runoff.

Table 5-5. Approximate Water Infiltration Rates for Various Soil Textures and Slopes
0 to 3% 3% to 9% 9+%
inches per hour
Sands >1.00 >0.70 >0.50
Loamy sands 0.70 to 1.00 0.50 to 1.00 0.40 to 0.70
Sandy loams and fine sandy loams 0.50 to 1.00 0.40 to 0.70 0.30 to 0.50
Loams and silt loams 0.30 to 0.70 0.20 to 0.50 0.15 to 0.30
Sandy clay loams and silty clay loams 0.20 to 0.40 0.15 to 0.25 0.10 to 0.15
Clays, sandy clays, and silty clays 0.10 to 0.20 0.10 to 0.15 <0.10
Source: Sprinkler Irrigation Association Journal. For poor vegetative cover or surface soil conditions, actual rates may be as much as 50 percent less than shown.


Explain how to determine how much water to irrigate.

Sandy soils have high intake capacity and runoff is not much of a concern. But, sandy soils also have low plant-available water-holding capacity (PAW). For example, consider a deep sandy soil that has only 0.5 inch of plant-available water per foot of soil depth. For an effective root depth of 1.5 feet, the maximum plant-available water deficit is only 0.75 inch.

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For this example, if the application amount during any single irrigation cycle exceeds 0.75 inch, some of the applied wastewater will leach below the root zone and potentially pollute groundwater.

Explain how/why irrigation amounts need to be adjusted seasonally.

Taking all of the above factors into account, recommended wastewater irrigation amounts for a single irrigation cycle are in the range of 0.25 to 0.75 inch per foot of effective root zone depth. There may be an occasion when the appropriate irrigation amount falls outside this range, such as when irrigation must occur during cold periods when PET is low, when the soil has an unusually deep root zone providing a greater amount of plant-available water storage, or to satisfy emergency action guidelines. But, these situations are exceptions and should not occur on a regular basis.

Regardless of the calculated rate, you as the system operator should monitor each waste application to verify adequate infiltration of the waste into the soil. An irrigation cycle should be stopped if ponding and runoff start to occur.

Operational Considerations

A key component of the irrigation design is to select the proper combination of system components so that the system precipitation rate does not exceed the intake rate of the soil. Several terms may be used to express the rate at which water is being applied to a field during irrigation. Terms you should be familiar with include: discharge rate, precipitation rate, and application volume.

Discharge Rate

Define precipitation rate, discharge rate, and application volume.

Discharge rate is the volume of water exiting a sprinkler per unit of time, and is normally expressed in terms of gallons per minute (gpm). Discharge rate can also be referred to as sprinkler flow rate. Manufacturers publish discharge rates for their sprinklers as a function of operating pressure and orifice diameter of the nozzle. You should always have a copy of the manufacturer’s discharge specifications for the sprinklers on your system. Discharge characteristics for three typical sprinklers used for wastewater irrigation are given in Table 5-6. For example, a Rainbird Model 70 sprinkler operated at 55 psi with a 9/32-inch diameter nozzle has a discharge rate of 17.2 gpm. Discharge characteristics for typical big guns are shown in Table 5-7. For contrast, notice how much higher discharge rates are for gun sprinklers than for rotary impact sprinklers.

Explain how to obtain sprinkler discharge rates.


Table 5-6. Discharge Characteristics for Rotary Impact Sprinklers Used with Permanent Stationary Irrigation System
Nozzle Size Operating Pressure (PSI)
(inch) 50 55 60
  Flow Diameter Flow Diameter Flow Diameter
Nelson F70APV
1/4 12.8 128 13.6 131 14.0 134
9/32 16.0 134 16.8 137 17.6 140
Rain Bird 70 CWH
1/4 12.9 124 13.6 126 14.2 128
9/32 16.3 131 17.2 133 18.0 135
Senniger 7025 RD-1-DFF
1/4 13.0 127 13.6 131 14.2 138
9/32 16.3 133 17.1 137 17.8 142

Precipitation Rate

Precipitation rate is normally expressed as unit depth of water (inches) per unit of time, (usually an hour). The precipitation rate (inches per hour) depends upon discharge rate and coverage diameter. The precipitation rate is computed by first converting the discharge rate to a unit depth of water (inch) per unit of area (such as acre or square feet), then dividing by the wetted area of the sprinkler. Another important concept is total application volume (also expressed as application depth, inch), which is computed based on the amount of time the system operates at a given rate on a given field. Your target application volume represents the total volume (gallon/acre) needed to satisfy the PAN needs of the crop. This application volume is used for planning; rarely can you apply this much water during one irrigation cycle.

Wastewater analyses are often expressed in terms of pounds of plant-available nitrogen (lb PAN) per 1,000 gallons of wastewater. When irrigating, it is often preferable to express irrigation amounts as an equivalent depth of water per acre, for example, 1/2 inch per acre. Therefore, it is often necessary to convert between application volume expressed as gallons per acre and application volume expressed as an inch per acre. One inch of water spread over an acre, referred to as acre-inch, is equal to 27,154 gallons.


If you wish to apply 0.4 inch of irrigation water, how many gallons per acre should you apply?

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Your waste analysis indicates that you have 2.5 pounds PAN per 1,000 gallons. If you wish to apply 75 pounds of PAN, how many inches should you apply.

First, find the waste application rate (see Formula 4 in Chapter 3).


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Then convert the application volume to a depth:


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If your target application volume to achieve the required annual PAN is 3.0 inches, and you should apply no more than 0.5 inch at each irrigation, how many times must you irrigate?

To answer this, use the following formula:

Formula 7


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This calculation is used only to estimate the number of irrigation events needed for a given situation. Based on earlier comments, you will be making adjustments for application volumes based on soil and weather conditions. You may have application volumes ranging from 0.25 to 0.6 inch per cycle for the above example. HOWEVER, you must never exceed the irrigation volume for a field that is dictated by your waste management plan.

Note: Most irrigation systems do not completely cover a field with wastewater during operation. For example, a 30-acre pasture may only receive wastewater application onto 26 acres due to the layout of the field, buffers that must be observed, and operational parameters of the irrigation system. A wastewater application design and the appropriate records must reflect the area which receives wastewater; in this case 26 acres as opposed to the total field size.

Table 5-7. General Flow Rates and Coverage Diameter for Big Gun Sprinklers
Taper Bore Nozzle
  Gun Model
100T 150T 150T 200T 200T
Nozzle Diameter (inch)
Pressure 0.5 0.75 1.0 1.5 2.00
50 50 205 115 260 205 310
60 55 215 126 275 225 325 515 430 912 512
70 60 225 137 290 245 340 555 450 980 528
80 64 235 146 300 260 355 590 470 1047 548
90 68 245 155 310 275 365 625 485 1105 568
100 72 255 164 320 290 375 660 500 1167 592
110 76 265 172 330 305 385 695 515 1220 607
120 180 340 320 395 725 530 1277 622
Ring Type Nozzle
  Gun Model
100R 150R 150R 200R 200R
Nozzle Diameter (inch)
Pressure 0.71 0.86 0.97 1.56 2.00
50 74 220 100 245 130 265 350 370 640 435
60 81 235 110 260 143 280 385 390 695 455
70 88 245 120 270 155 290 415 405 755 475
80 94 255 128 280 165 300 445 420 805 490
90 99 265 135 290 175 310 475 435 855 505
100 105 270 143 300 185 320 500 445 900 520
110 110 275 150 310 195 330 525 455 945 535
120 - - 157 315 204 335 545 465 985 545


Explain the importance of sprinkler overlap.

To attain acceptable application uniformity, stationary sprinklers are typically arranged in a square pattern at a spacing of 50 to 65 percent of the wetted diameter. A typical layout for stationary sprinklers is shown in Figure 5-2. The orifice size, spacing, and operating pressure are selected from manufacturer’s literature to achieve the desired overlap and uniformity of coverage.

fig5-2.gif (7711 bytes)

Figure 5-2. Typical layout of a stationary sprinkler system. Sprinkler spacing is typically 50 to 65 percent of wetted diameter.


Determination of Precipitation Rates for Stationary Sprinklers

The precipitation rate for stationary sprinklers is computed from the formula:

Formula 8


ex5A-8.gif (2933 bytes)


Compute the precipitation rate for a stationary sprinkler irrigation system.

Procedure for computing precipitation rate:

  1. Determine the discharge rate (sprinkler flow rate) and wetted diameter from manufacturer’s literature.

    From Table 5-6, RainBird Model 70 with a 9/32-inch diameter nozzle operated at 55 psi:

    Sprinkler flow rate = 17.2 gpm

    Wetted Diameter = 133 feet

  2. Recommended sprinkler spacing is 50 to 65 percent of wetted diameter. Using a value of 60 percent:

    Design sprinkler spacing = 0.6 133 ft = 79.8 ft

    Sprinklers are normally spaced in equal multiples of 20 feet based on typical pipe length. Therefore, the design spacing would be 80 feet: for example, the sprinkler spacing along the lateral would be 80 feet and the lateral spacing would be 80 feet such that the effective area of a sprinkler would be 80 feet by 80 feet.

  1. Precipitation rate is then computed as:

ex5A-8.gif (2933 bytes)


Compute the application volume for a stationary sprinkler irrigation system.

The application volume is then computed as the precipitation rate multiplied by the operating time. In most cases, you will estimate the desired application volume based on soil conditions as described above. If this is the case, you then compute the time required to operate the system to achieve the desired application volume. For example, if the desired application volume is 0.6 inch, then the required operating time for the system would be:

Determine the operational time necessary to apply a desired application volum or nitrogen application amount.
  1. Time of operation is computed as:
Formula 9


ex5A-9.gif (2890 bytes)

So :

ex5A-10.gif (2322 bytes)


Determination of Precipitation Rates for Traveling Gun Sprinklers

The precipitation rate in inches per hour for a traveling gun sprinkler is generally not affected by travel speed. This situation occurs because at any given position within the wetted diameter, water is usually being applied for at least an hour or longer. The precipitation rate is affected by the angle of rotation of the gun sprinkler. For example, if the gun only makes a half circle (180 degrees of rotation), the precipitation rate is twice that of a gun making a full circle (360 degrees of rotation). The precipitation rate for a traveling gun sprinkler is computed by the formula:

Formula 10

Precipitation (application) rate for traveling gun; inches per hour,

ex5A-11.gif (3636 bytes)

where "w" is the angle of rotation expressed in degrees. If the gun sprinkler rotates a full circle, then w is 360 degrees, whereas if the rotation is only a half circle, w is 180 degrees. In practice, the angle of rotation should be in the range of 315 to 330 degrees.


Determine precipitation rate and effective coverage from manufacturer's literature for a traveling gun sprinkler.

What is the precipitation rate for a gun sprinkler if the operating pressure is 80 psi, the nozzle diameter is 1.0 inch, and the angle of rotation is 320 degrees?

From Table 5-7, for a gun sprinkler operated at 80 psi with a 1.0 inch diameter taper bore nozzle, the discharge rate is 260 gpm and the wetted diameter is 355 feet.

For a wetted diameter of 355 feet, the wetted radius is:

355 ft  2 = 177.5 ft

The precipitation rate is then computed as:

ex5A-12.gif (3527 bytes)


What would be the precipitation rate in the previous example if the angle of rotation was only a half circle?

ex5A-13.gif (3527 bytes)


Note: the precipitation rate for a traveling gun sprinkler increases rapidly as the angle of rotation decreases. Since the infiltration capacity of many soils is less than 0.5 inch per hour, it is important that the gun sprinkler rotate as much of a full circle as possible so that the precipitation rate does not exceed the infiltration capacity of the soil.

Determination of Application Volume (Depth) for a Traveling Gun Sprinkler

Compute the precipitation rate and application volume for a traveling gun sprinkler.

The volume of wastewater applied by a traveling gun depends on the flow rate, lane spacing, travel distance, and travel speed. The travel lane spacing should be approximately 70 to 80 percent of the sprinkler’s wetted diameter as shown in Figure 5-3. The application volume is computed by the formula:

Formula 11




What is the application volume for a gun sprinkler if the operating pressure is 80 psi, the taper bore nozzle diameter is 1.0 inch, and the travel speed is 3 feet per minute?

From Table 5-7, for a gun sprinkler operated at 80 psi with a 1.0 inch nozzle, the discharge rate is 260 gpm and the wetted diameter is 355 feet.

If the lane spacing is 75 percent of the wetted diameter, the lane spacing is

0.75 355 ft = 266 ft

The travel speed needs to be expressed in inches per minute. A travel speed of 3 feet per minute is equal to 36 inches per minute.

3 ft/min 12 in./ft = 36 in./min

The application volume is then computed to be:

ex5A-14.gif (3003 bytes)


fig5-3.gif (4729 bytes)

Figure 5-3. Typical layout of a traveling gun irrigation system. Lane spacing is typically 70 to 80 percent of wetted diameter.


Determination of Travel Speed for a Traveling Gun Sprinkler

In many cases, you may desire to determine the travel speed required to apply a desired waste application depth. The travel speed is computed by the formula:

Formula 12




Compute the required travel speed for a traveling gun sprinkler to apply the desired application volume.

What travel speed is necessary to apply 0.6 inch with a gun sprinkler if the operating pressure is 80 psi and the taper bore nozzle diameter is 1.0 inch?

From Table 5-7, for a gun sprinkler operated at 80 psi with a 1.0-inch nozzle, the discharge rate is 260 gpm and the wetted diameter is 355 feet.

If the lane spacing is 75 percent of the wetted diameter, the lane spacing is:

0.75 355 ft = 266 ft


ex5A-15.gif (2872 bytes)


System Calibration

Explain what effect changing nozzle diameter can have on discharge rate and wetted diameter.

Information presented in manufacturer’s charts are based on average operating conditions with relatively new equipment. Discharge rates and application rates change over time as equipment gets older and components wear. In particular, pump wear tends to reduce operating pressure and flow. With continued use, nozzle wear results in an increase in the nozzle opening, which will increase the discharge rate while decreasing the wetted diameter.

Explain the effects of changing pressure on droplet size, drift, precipitation rate, and wetted sprinkler diameter.

You should be aware that operating the system differently than directed in the design will alter the application rate, diameter of coverage, and subsequently the application uniformity. For example, operating the system with excessive pressure results in smaller droplets, greater potential for drift, and accelerated wear of the sprinkler nozzle. Clogging of nozzles can result in pressure increase. Plugged intakes or crystallization of mainlines will reduce operating pressure. Operating below design pressure greatly reduces the coverage diameter and application uniformity.

Explain why sprinkler systems should be field calibrated.

For the above reasons, equipment should be field calibrated on a regular basis to ensure proper application rates and uniformity. Field calibration at least once every three years is recommended. Calibration involves collecting and measuring flow at several locations in the application area. Many types of containers can be used to collect flow and determine the application rate. Rain gauges work best because they already have a graduated scale from which to read the application amount without having to perform additional calculations. However, pans, plastic buckets, jars, or anything with a uniform opening and cross-section can be used provided the liquid collected can be easily transferred to a scaled container for measuring.

Explain calibration procedures for stationary and traveling sprinklers.

For stationary sprinklers, collection containers should be located in a grid pattern throughout the application area at several distances from sprinklers. Collection gauges should be spaced 1/4 the effective sprinkler spacing apart. For traveling guns, sprinklers should be located along a transect perpendicular to the direction of pull. Set out collection containers no further than 25 feet apart along the transect on both sides of the gun cart. You should compute the average application rate for all collection containers and the application uniformity for the system. Calibration procedures are outlined in Appendix I for each type of system. You should obtain a uniformity coefficient of 0.5 or greater for stationary sprinkler systems, and the uniformity coefficient for a traveling gun system should exceed 0.6. If the uniformity coefficient computed for your system is less than these values, contact your irrigation dealer or technical specialist for assistance.


Pump and Haul Waste Management Systems

Wastes that have a higher solids content than that which can not easily be handled through an irrigation system may require land application through a pump and haul system. Typically, wastes or slurries with solids concentrations above 3 to 4 percent are difficult to handle as a liquid and cause excessive wear and clogging to standard irrigation pumps and equipment. Liquid applicators and tank spreaders are an alternative to irrigation for applying these materials.

List factors that must be addressed before applying animal waste with spreader or tanker equipment.

Many of the decisions on when and how much to irrigate wastewater are determined by the liquid nature of the waste and the potential for runoff. With slurry or solids application, these decisions surrounding liquid application are not as critical. Certainly, it is still your job as the system operator to ensure that the applied wastes will not run off the property, but the solid nature of the wastes greatly reduces the tendency of these materials to run off site. Therefore, the decision process for waste application is more related to the stage of crop growth and whether the crops need nutrient applications. Another important issue is the "trafficability" of the fields, or how easily your equipment can be operated to obtain uniform waste application without rutting the field or causing soil compaction. Once the decision has been made to perform waste application, you must be aware of your equipment’s waste application rate. This requires the calibration of the land application equipment.

A certain percentage of the nutrients in slurry and solid manures is tied up in the organic portion of the waste and is not immediately available for plant uptake. These nutrients will slowly become available to plants over the course of several years. To satisfy your waste management plan, you are only required to keep track of the nutrients that are available for the first crop. It is possible, but quite tedious, to develop a system to determine the "carry-over" nutrients from the organic portion of the manure. It is beyond the scope of this training to do this exercise, and as mentioned it is not required at this time. However, you may wish to consider this issue to help minimize the potential for over application of nutrients (especially nitrogen) that may be detrimental to your crops, soils, or groundwater.

Liquid Manure Spreaders

Describe calibration procedures for pump and haul waste management systems.

Liquid tank spreaders must be accurately calibrated to apply wastes at proper rates. Calibration is the combination of settings and travel speed needed to apply wastes at a desired rate and to ensure uniform application. To calibrate, you must know the spreader capacity, which is normally rated by the manufacturer in gallons.

Calibration method:

  1. Spread at least one full load of waste, preferably in a square or rectangular field pattern for ease of measuring, with normal overlaps.

  2. Measure the length and width of coverage, recognizing that the outer fringe areas of the coverage will receive much lighter applications than the overlapped areas.

  3. Multiply the length by the width and divide by 43,560 to determine the coverage area in acres.
Formula 13

Coverage area (area of rectangle in ft2) = length (ft) width (ft)


Formula 14

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  1. Divide the gallons of wastewater in the spreader by the acres covered to determine the application rate in gallons per acre.

Formula 15

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Repeat the procedure at different speeds and/or spreader settings until the desired application rate is achieved.


Your waste application method is a tractor-drawn tanker (honeywagon) with a 2,500-gallon capacity. You apply a load to a field and measure the application area as 22-feet wide by 280-feet long. What is the application rate in gallons per acre?

First, figure the coverage area:

ex5A-18.gif (2752 bytes)

Then figure the application rate:

equate19.gif (3699 bytes)


Review Questions

  1. What buffers must be observed during waste application?

  2. A proper pumping schedule seeks to balance what three things?

  3. What are the five questions you should ask yourself before deciding to irrigate wastewater?

  4. Explain the importance of estimating soil moisture prior to waste application.

  5. Explain the importance of plant-available water and soil-intake rate.

  6. Explain how you can measure or calibrate your actual waste application (in inches) from an irrigation system.

Conversion Factors and Important Formulas

In the previous three chapters, you have been introduced to a number of formulas used to help determine how to properly apply animal waste at agronomic rates. The use of these formulas requires that you know the following items about your operation:

  1. Type of operation

  2. Number of animals

  3. Pounds of plant-available nitrogen (PAN) per 1,000 gallons of liquid or per ton of waste

  4. Acreage of each field to receive waste applications, crop rotation for that field, and field number

  5. Records or estimate of crop yield for each field

  6. Type and specifications of waste application equipment, including:
  1. Estimate of infiltration rates on each field *

*Note: This number will vary with field conditions. See Chapter 5 for specifics on estimation of infiltration rates.

With the above information, you can make all of the calculations that will allow you to properly operate your waste application equipment so that agronomic rates are not exceeded. Important conversion factors and formulas that are used for proper waste application are summarized on the following pages. Most of these formulas are also included on the Record Forms IRR-2, SLUR-2, and SLD-2 (see Chapter 6) so that you can actually make your calculations on these forms as you work through them. The record forms do not include the equations for irrigation equipment application rate or travel speed.

Conversion Factors

1 acre-inch = 27,154 gallons

1 acre = 43,560 square feet

lane spacing for traveling gun = 70% to 80% of wetted diameter

lane spacing for stationary gun = 50% to 65% of wetted diameter

mg/l = pounds per 1,000 gallons

Important Formulas—Type A

  1. Volume of wastewater generated

equate1.gif (3267 bytes) 

  1. Waste application rate for priority nutrient (pounds per acre)

equate2.gif (3571 bytes) 

  1. Acres needed for waste application

equate3.gif (4048 bytes) 

  1. Application rate (gallons per acre)

equate4.gif (3511 bytes) 

  1. Application rate (tons per acre)

equate5.gif (3536 bytes) 

  1. Pound of plant-available nitrogen applied per acre

equate6.gif (3651 bytes) 

  1. Number of waste applications needed

equate7.gif (4446 bytes) 

  1. Precipitation (application) rate for stationary equipment, inches per hour

equate8.gif (3731 bytes) 

  1. Time of irrigation system operation (hours)

equate9.gif (3250 bytes) 

  1. Precipitation (application) rate for traveling gun; inches per hour, where "w" is the angle of rotation expressed in degrees

equate10.gif (3845 bytes) 

  1. Application volume (depth) for traveling gun; inches

equate11.gif (3674 bytes) 

  1. Travel speed for traveling gun; inches per minute

equate12.gif (3612 bytes) 

  1. Area of a rectangle

equate13-1.gif (2340 bytes)

Area of a circle

equate13-2.gif (2152 bytes) 

  1. Coverage area for application

equate14.gif (2827 bytes) 

  1. Application rate for spreader

equate15.gif (4051 bytes)


Example Problems

On the next few pages are example problems using these formulas. For more practice using these formulas, there are additional problems in Appendix E.

Problem 1

You maintain a farrow-to-finish operation of 2000 sows. You plan to have permanent bermudagrass hayland for the cropping system. The tables show that your soil type should have a realistic yield expectation of 6 tons per acre. Recent NCDA results for your waste analysis show that your lagoon has 2.8 pounds of plant-available nitrogen (PAN) per 1,000 gallons.

The irrigation equipment available to you is a traveling gun that puts out 200 gallons per minute with a wetted diameter of 300 feet. You have a sandy loam soil with a 2 percent slope. Based on your soils, the recommendation from the technical specialist, and from Tables 5-2 and 5-3, you determine no more than 0.5 inch of wastewater can be applied per application event.

Questions: (Use tables from this chapter for reference.)

Question 1:

How much wastewater does your facility produce per year (in gallons)?


From Table 3-1: Total gallons produced per sow per year = 10,481.

ex5A-20.gif (3178 bytes)

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Question 2:

How much nitrogen is needed per acre to maintain the bermudagrass yield of 6 tons per acre?


From Table 3-5: Bermudagrass needs 50 pounds N per ton of yield.

ex5A-22.gif (3547 bytes)

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Question 3:

How many gallons of wastewater per acre are needed to satisfy the nitrogen needs of the crop? How many inches?


You need 300 pounds N per acre. Each 1,000 gallons of wastewater supplies 2.8 pounds N.

ex5A-24.gif (3431 bytes)

ex5A-25.gif (2538 bytes)

1 acre-inch = 27,154 gallons; so:

ex5A-26.gif (2234 bytes)


Question 4:

How many total acres is needed to apply the wastewater generated at the agronomic rate determined in previous Questions 2 and 3?


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Question 5:

At what speed should you operate your traveler to obtain the recommended application rate?


ex5A-29.gif (3614 bytes)


Given: that flow is 200 gpm, wetted diameter is 300 feet, and wastewater application depth is 0.5 inch.

Lane spacing should be 70 to 80 percent of wetted diameter (use 75 percent); so

ex5A-30.gif (1899 bytes)


ex5A-31.gif (3355 bytes)

Note: If equipment does not have a variable setting, you should use the next higher setting. Example, if equipment settings are 20, 30, 40, or 50 inches per minute, and your calculations show 34.2 inches per minute, you should use the 40 inches per minute setting. Then you must recalculate the actual amount of water and nitrogen you have applied.

To calculate wastewater application, use:

ex5A-32.gif (3674 bytes)

Question 6:

Based on the number of inches of wastewater to be applied from Question 3 above, how many wastewater applications must be made to the field (at the recommended rate of 0.5 inch per application) to obtain the agronomic loading rate?


From Question 3 above, 3.94 inches of wastewater application is needed to meet the agronomic (nitrogen) needs of the crop.

From Question 5 above, 0.5 inch may be applied per application, so:

ex5A-33.gif (4481 bytes)


ex5A-34.gif (3828 bytes)


Problem 2

You land apply liquid waste with a tank spreader that holds 2,000 gallons. You typically operate such that the width of the waste application is 20 feet. You wish to perform a calibration run to determine waste application rate, so you select a speed of 2 MPH on your tractor. Recent waste analysis results show there are 3.0 pounds of nitrogen per 1,000 gallons of waste.

After applying the waste, you measure that you moved 300 feet to empty the tank.

Question 1:

What is your application rate in gallons per acre?


You applied 2,000 gallons on an area 20 feet by 300 feet; you must first figure the coverage area:

ex5A-35.gif (2948 bytes)

ex5A-36.gif (2764 bytes)

Now figure the application rate:

ex5A-37.gif (3410 bytes)


ex5A-38.gif (3220 bytes)

Question 2:

How many pounds of nitrogen per acre are being applied at this rate?


There are 3.0 pounds nitrogen per 1,000 gallons:

ex5A-39.gif (3644 bytes)


ex5A-40.gif (3106 bytes)

Question 3:

If you increase your application speed to 4 mph, how much nitrogen is being applied per acre?


If you go twice as fast (4 mph versus 2 mph), you will have covered 600 feet at the same waste discharge rate setting. Therefore, you have put 2,000 gallons on an area 20-feet wide by 600-feet long. Perform the same type calculations as above:

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ex5A-42.gif (2810 bytes)

Now figure the application rate:

ex5A-43.gif (3410 bytes)

ex5A-44.gif (2048 bytes)

How many pounds of nitrogen per acre are being applied at this rate?


There are 3.0 pounds nitrogen per 1,000 gallons:

ex5A-45.gif (3644 bytes)


ex5A-46.gif (3037 bytes)

Note: If you double the speed you half the application rate.


Question 4:

If you drive at 4 mph and apply 7,143 gallons per acre, how many inches of wastewater have you applied?


Use conversion of 27,154 gallons per acre-inch:

ex5A-47.gif (2624 bytes)


Problem 3

Question 1:

The target nitrogen rate for your crop is 150 pounds per acre per year. However, you wish to split this into 3 applications of 50 pounds N per acre. Recent waste analysis results show 2.0 pounds of N per 1,000 gallons. You use a stationary big gun with a taper bore nozzle that operates at 70 psi with a 1-inch nozzle. How long must you operate this gun to apply an application that supplies 50 pounds N?


  1. From Table 5-7: this gun puts out 245 gallons per minute (gpm) over a diameter of 340 feet.

  2. You wish to apply 50 pounds N per acre, and your waste has 2.0 pounds N per 1,000 gallons. You therefore must apply:

ex5A-48.gif (3431 bytes)


ex5A-49.gif (3240 bytes) 


  1. How many inches of wastewater must you apply to reach your target of 50 pounds PAN per acre?

ex5A-50.gif (2260 bytes) 


  1. Since gun spacing is 50 to 65 percent of diameter (use 65 percent), the spacing is:

ex5A-51.gif (1507 bytes)


ex5A-52.gif (3644 bytes)


ex5A-53.gif (2915 bytes) 


  1. How long must you operate to obtain your 50 pounds PAN waste application?

    Since a 50-pound PAN application requires 0.92 inch of wastewater, you must operate:

ex5A-54.gif (3252 bytes)


ex5A-55.gif (2088 bytes)

Problem 4


You land apply manure with a manure spreader because it is too dry and thick for irrigation or a tank spreader. Your waste analysis shows it contains 16 pounds PAN per ton of manure. Your cropping sequence is corn (grain) followed by winter wheat (grain). The average yield recorded over the past 5 years is 90 bushels of corn and 35 bushels of wheat. How many tons of manure per acre can you apply to maintain agronomic rates?


You must first find out the allowable nitrogen loading to the field, per acre.

From Table 3-5, you see that corn for grain uses 1.0 to 1.25 pounds of N per bushel (select 1.1), and wheat for grain uses 1.7 to 2.4 pounds of N per bushel (select 2.0).

The application rate is found by:

ex5A-56.gif (3569 bytes)

For corn:

ex5A-57.gif (3126 bytes)

For wheat:

ex5A-58.gif (3148 bytes)

With this cropping sequence, the total N needed is:

99 + 70 = 169 lb PAN/acre

Now you can determine how many tons per acre of manure to apply. Each ton of manure supplies 16 pounds of N, and you need 169 pounds of N per acre, so:

ex5A-59.gif (3524 bytes)


ex5A-60.gif (2778 bytes)


Note: This application rate is the total for BOTH crops and should NOT be applied with a single application. The applications must be made within 30 days of the crop being planted, and preferably split to maximize the use of nutrients in the material.


Problem 5

Question 1:

By your calculations, you have determined that you must apply 3.0 inches of wastewater to a field to meet the crop nitrogen requirements. You utilize a stationary big gun with a 1.56-inch ring nozzle operating at 80 psi. Determine the precipitation rate of wastewater from the gun, in inches per hour.


From Table 5-7, you get 445 gpm at a diameter of 420 feet

Gun spacing = 60% of diameter; or 0.6 x 420 feet = 252 feet

ex5A-60.gif (2778 bytes)


ex5A-62.gif (2955 bytes)

Question 2:

How long must your equipment be operated on this field to reach the crop nitrogen requirements?


You must apply 3.0 inches at a rate of 0.68 inch per hour. Use this formula:

ex5A-63.gif (3252 bytes)


ex5A-64.gif (2319 bytes)

Note: This is TOTAL run time. You would not apply all 3.0 inches in one irrigation event.

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Last Modified: 29-Aug-2003