Soil Acidity and Proper Lime Use

Situation in North Carolina

Nature and Cause of Soil Acidity

Soil Testing and Target pHs

Lime Reactions in Soil

Benefits of Proper Lime Use

Determining the Lime Requirement

Calcitic Versus Dolomitic Limestone

Liming Product Standards for North Carolina

Lime Form

Application and Incorporation

Published by
North Carolina Cooperative Extension Service

Situation in North Carolina

Nearly all soils in North Carolina that produce grain and oil crops, tobacco, cotton, vegetables, fruits, some forest species, turf, many ornamentals, and forages need lime for optimum plant growth unless lime has been added recently. Soil test summaries compiled by the Agronomic Division, North Carolina Department of Agriculture, verify this need. Nearly 21,000 soybean and 13,000 cotton soil tests (July 1, 1989, through June 30, 1990) show that about 6 out of 10 fields will benefit from liming. Also, NCDA agronomists emphasize that a high percentage of the "problem samples" they receive have very low pH and therefore need lime.

Proper liming, in combination with other sound agronomic and pest control practices, will increase crop income in North Carolina. Using conservative estimates of yield increase from proper lime use, the return from cotton, soybeans, and peanuts (crops that are quite sensitive to low pH) could be increased by about $25 million. In addition, returns from tobacco, corn, commercial vegetables, forages (especially legumes), and turf could probably be increased another $10 million. Although response to lime is frequently rather subtle in contrast to a nitrogen application to corn, ignoring its regular use limits crop yields.

Figure 1. General relationship between soil pH and acidity.

Nature and Cause of Soil Acidity

Soil acidity is the term used to express the quantity of hydrogen (H) and aluminum (Al) in soils. On the other hand, soil pH is an indirect indicator of "soil acidity." Soil pH, which is the negative logarithm of the soil hydrogen concentration, is expressed on a scale from 1 to 14. Because the pH scale is logarithmic, soil with a pH of 6 is 10 times more acidic and soil with a pH of 5 is 100 times more acidic than soil with a pH of 7. Remember that the lower the pH number, the more acid the soil and therefore the greater the need for lime. This relationship is shown in Figure 1

North Carolina soils are highly weathered (leached) because of excessive rainfall and therefore are naturally acidic. This process has depleted the nutrient elements calcium (Ca) and magnesium (Mg) from naturally occurring minerals as well as those of previously applied agricultural limestone. Plants also remove calcium and magnesium. Decay of crop residue or the addition of animal waste or other organic matter increases soil acidity. Widespread use of fertilizer nitrogen also increases soil acidity.

Soil Testing and Target pHs

Because aluminum and hydrogen are the principal components of soil acidity in mineral soils (hydrogen is the principal component in organic soils) the North Carolina soil test report contains a measurement called the Ac value. This is the combination of aluminum and hydrogen in soils and is used to predict lime needs. Lime recommendations must take into account differences in acidity between soils and differences among various crops' tolerance to acidity. This explains why soils differ in the recommended or target pH. For most commonly grown crops, mineral (MIN) soils have a target pH of 6.0. For mineral organic (M-O) soils the target is a pH of 5.5, and for organic (ORG) soils it is 5.0. The reason for the difference is that soils high in organic matter generally contain less aluminum and are thus less toxic to plant roots at a relatively low pH.

Furthermore, crops differ in their ability to tolerate a low pH. Plants such as blueberries and azaleas are known to be especially tolerant, whereas others such as alfalfa, cotton, and tomatoes grow better at a higher pH. Because of the differences in crops and soils, the North Carolina soil test report recommends varying rates of lime to achieve the best pH for the particular soil class and crop combination under consideration.

Lime Reactions in Soil

The most commonly used lime for North Carolina agriculture is the dolomitic type (CaMgCO₃); calcitic lime (CaC0₃) is less frequently used. A liming material must have more than a high calcium content; it must also be capable of neutralizing acid (H). The chemical reaction of dolomitic lime with soils is as follows:

Aluminum hydroxide is insoluble; therefore the aluminum is effectively inactivated. Also, when hydrogen and hydroxide ions combine, water is formed and the hydrogen is therefore neutralized. Because lime dissolves very slowly, it must be ground finely before it pH can effectively neutralize soil acidity (Figure 2). Note that 40- to 50-mesh material raised the pH to a higher level than 8- to 20-mesh material did during an 18-month study.

Flgure 2. Lime screen slze and soil pH.

Benefits of Proper Lime Use

• More efficient use of fertilizer supplied phosphorus (P). Aluminum, particularly at a low pH, is chemically active and combines with fertilizer phosphorus, causing it to become insoluble. This tying up of fertilizer phosphorus means that less is available to the next crop. In some instances, fertilizer phosphorus has inadvertently served as a liming material, in that it has immobilized aluminum.

• Economical provision of essential magnesium if dolomitic limestone is used. Furthermore, the magnesium supplied in dolomitic limestone is released slowly over a period of three to four years and is therefore better protected from leaching than that supplied by fertilizer magnesium.

• Improved nodulation of legumes. The rhizobia in nodules on legume roots—those of soybeans, peanuts, alfalfa, and clover— synthesize greater amounts of nitrogen from the soil atmosphere for use by the legume where soil pH is not low. Such inoculation leads to an economical source of nitrogen and may supply the succeeding crop with substantial residual nitrogen. In addition, molybdenum (Mo), an essential element in a legume's nitrogen-fixing processes, is increasingly tied up as soil pH gradually declines below 5.5 and thus becomes unusable to the rhizobia bacteria. Therefore, a less-than-optimum molybdenum means nitrogen-deficient legumes.

• Reduced leaching of potassium. On the soil's exchange complex there are a limited number of sites that can hold nutrients such as potassium. When these sites are occupied by strongly attached aluminum (low pH), any potassium added in fertilizer is more susceptible to leaching. Proper liming will not completely prevent leaching of potassium but will tend to minimize it, particularly on soils with deep sandy surfaces.

• Improved performance of some herbicides. Triazines—atrazine and simazine—do not perform effectively below the optimum pH. Furthermore, there is increasing evidence that optimum pH also improves the performance of some nematicides.

Figure 3. Effect of soil pH on nutrient availability.

Determining the Lime Requirement

It is important to remember that soils in different parts of the United States have different optimum pHs. For example, most midwestern soils produce best crops at a pH of 6.5 to 7.0, but these values would cause micronutrient deficiencies in parts of North Carolina. Another problem is that laboratories use testing methods developed for their particular conditions. Many laboratories use a weighed soil sample and assume that the weight-to-volume ratio remains the same from one soil to another. The North Carolina laboratory uses a soil volume in its test because the soils of this state vary a great deal in weight-to-volume ratio.

According to the North Carolina Department of Agriculture's Agronomic Division, the amount of lime required depends on the pH desired for the intended crop, the present soil pH, the amount of acidity (Ac), and an adjustment for residual credit (RC)* from recent lime applications. Each sample is classified as mineral (MIN), mineral-organic (M-O), or organic (ORG) because the desired pH differs for each of these three groups. With computer assistance, NCDA agronomists offer lime suggestions calculated by the following equation:

Tons of lime per acre =

Ac x [(pH desired - present pH) /  (6.6 - present pH)] - RC

Example:

If soil pH = 5.0;

desired pH = 6.0; Ac = 1.2; RC = 0

then lime requirements are:

1.2 x [(6.0 – 5.0) / ( 6.6 – 5.0)]  - 0 = 0.76 ton/acre

*Residual credit is reduced by 8 percent per month from time of application to time of soil test for mineral soils and 16 percent per month for mineral-organic soils.

When the results of the calculation indicate that no lime is needed and the soil pH is 0.3 unit or less below the level desired, an applications of 0.3 ton per acre or 15 pounds per thousand square feet is recommended. When lime rates are calculated for a first and second crop, the highest of the two lime rates is suggested for the first crop and no lime is suggested for the second crop. Lime rates are reported in tenths of a ton; no lime application is recommended when calculations indicate less than 0.3 ton.

Calcitic Versus Dolomitic Limestone

North Carolina has few good natural lime sources. Calcitic marl liming materials (soft marine shell deposits) are available in the coastal plain, but there are no dolomitic lime deposits in the east. Dolomitic lime must be obtained from the Virginia or Tennessee mountains and is thus relatively expensive. Occasionally, by-product liming materials become available. If the neutralizing value is known and the lime is ground finely enough to react in the soil, these can be economical substitutes.

Liming materials containing calcium carbonate (CaCO₃) alone are called calcitic limes, and those with significant amounts of magnesium carbonate (MgCO₃) (6 percent magnesium or greater) are called dolomitic limes. Pure calcium carbonate is used as the standard for liming materials and is assigned a rating of 100 percent. This rating is also known as the "calcium carbonate equivalent." All other liming materials are rated in relationship to it. Dolomitic limes are slightly more efficient in neutralizing soil acidity and may have values slightly greater than 100.

Calcitic limes can be used on any soil high in magnesium. On the other hand, dolomitic limes should be used on soils low in magnesium. Many organic soils and some piedmont soils are naturally high in magnesium, whereas most sandy soils in the coastal plain are low. The soil test report will indicate which lime should be used. It is possible to use a magnesium fertilizer instead of dolomitic lime, but the costs of this source of magnesium are almost always considerably higher.

Liming Product Standards for North Carolina

Size standards and other criteria have been established for the sale of agricultural materials to ensure a quality product. They are as follows:

• Agricultural liming materials must be crushed so that 90 percent passes through a U.S. standard 20-mesh screen (with a tolerance of ± 5 percent).*

• For dolomitic limestone, 35 percent must pass through a U.S. standard 100-mesh screen; for calcitic limestone, 25 percent must pass through a U.S. standard 100-mesh screen (with a tolerance of ± 5 percent). *

• There is no minimum calcium carbonate equivalent requirement for limestone sold in North Carolina. However, the product must be labeled to show the amount necessary to equal that provided by a liming material having a 90 percent calcium carbonate equivalent. Lime recommendations in North Carolina are based on 90 percent calcium carbonate equivalency. For example, a product having a calcium carbonate equivalent of 80 percent would be labeled "2,250 pounds of this material equals 1 ton of standard agricultural liming material."

• Pelleted lime must slake down when it comes in contact with moisture.

Lime Form

Most agricultural lime is sold as a damp powder because dry lime is very dusty and difficult to handle. However, lime is occasionally excessively wet. Lime is sold by the pound; thus be aware that you may be purchasing a substantial amount of water and should adjust lime rates accordingly.

Lime is sometimes sold in pellet form. The pellets are formed from lime that has been finely ground; it is not large grains of solid limestone. The pelleted product is less dusty and easier to spread but is more expensive. Pelleted lime is slower to act than powdered lime. Soil reaction will be enhanced if the soil can be retilled thoroughly several days after the pellets have been mixed into the soil and have become soft. Pelleted lime is not an economical source for most field crops. Lime is also sometimes sold as a suspension, often called "liquid lime." It consists of fine lime particles mixed with water and a suspending clay. All the lime particles must be 100 mesh or finer. Up to 1,000 pounds of lime can be suspended in a ton of product. The main advantages are ease of handling and precise application. This material, although a fluid, does not react any faster than dry lime of the same particle size. Once it has been placed on the soil it is the same as dry lime. All of the lime in a suspension is fast acting, and a ton of product (1,000 pounds of fine lime particles plus clay and water) will raise the pH as fast as a ton of dry lime. However, the effectiveness of suspensions is short lived compared to regular agricultural limestone, and therefore the liming will probably have to be repeated every year. Also, suspensions are a considerably more expensive way to correct soil acidity.

Application and Incorporation

Lime moves little in the soil and neutralizes acidity only in the zone where it is applied. To be effective, therefore, it must be uniformly spread and thoroughly incorporated. The poorest and most common method of application is by spinner spreader. Double spinners are better than single spinners; however, all normally apply more lime immediately behind the spreader than to the sides. In practice, rates are adjusted by checking the spreader pattern, overlapping the pattern, and double spreading, making the second pass at right angles to the first. If done properly, this is an acceptable way to apply lime. In many cases, however, these precautions are not followed and lime is applied unevenly. The soil can suffer from both underliming and overliming. Reduced yields may result.

Special situations may occur in the coastal plain that lead to overliming. First, if excessive lime falls along a relatively narrow path at the center line of the spreader truck, the soil pH may increase somewhat above the desired level. Second, the delivered rate may be too high for sandy ridges that occur in certain fields. Third, there simply may have been too much lime applied uniformly across the field. These three circumstances may elevate the pH to the extent that within a year or two an "induced" manganese deficiency has been created, and the crop may exhibit a manganese deficiency.

Lime can be more evenly applied using full-width or boom spreaders. Full-width spreaders allow lime to fall to the ground by gravity. The rate is determined by the size of the openings in the box and by ground speed. Boom spreaders use drag chains, augers, or pneumatic pressure to move lime out the booms and drop it on the ground. If adjusted properly, both types of spreaders are vastly superior to the spinner type. The main limitations to their use are the high initial cost and more complex operation. Most growers will likely continue to spread lime using spinner spreaders, but if you choose that method you should be aware of the limitations and take every precaution to see that the lime is evenly spread.

The most commonly used lime incorporation tool is the disk. Its main limitation is that it incorporates lime only about half as deep as the disk blades penetrate. Even with repeated passes it will not incorporate lime well. Offset disks that throw the soil do better. The best incorporation implement is a heavy duty rotary tiller that mixes the soil as deep as the roots need to go. If the land is to be bottom plowed, do not bury the lime too deep. If plowing, the best approach is to apply half the lime, then disk and bottom plow, and then apply the other half and disk again; however, this process is costly and is not generally used. Certain other tillage practices, such as bedding or middle busting, will help with lime incorporation in the long run. Chisel plowing is very ineffective in lime incorporation. Although lime is applied on the surface to established pastures and lawns, it should be incorporated at establishment to reduce soil acidity.

A proper soil pH can increase your crop income. However, varying rates of lime are recommended depending on the best pH for the particular soil class and crop combination. To test your soil's pH, send a soil sample to Agronomic Division, North Carolina Department of Agriculture, Blue Ridge Road Center, Raleigh, NC 27611.