A.
Components of soil testing
-
Sampling
-
Chemical analyses
-
Interpretation and recommendations
1.
Sampling
usually
the greatest source of error.A ten-acre field weighs 20,000,000
lbs. and may be represented by a one-lb soil sample.
a.
obtain soil boxes and information sheets.
b.
equipment -
1.
soil tube or auger
2.
Plastic bucket (clean). Galvinized buckets will contaminate
sample with Zn and copper.
c.
Taking the sample
1.
Divide fields into sampling areas which are as uniform as
possible in regard to soil type, topography, cropping history
and past fertilization.
2.
The sampling area may vary from 1-15 acres. Small areas that
are different for some reason and cannot be treated separately
should be avoided. If you can give the small spot special
attention, make it a separate sample.
3.
Depths of sampling
1.
Annual crops - plow depth, usually 6-8"
2.
Perennial - (alfalfa, pasture, turf) 4"
4.
Number of cores or subsamples per sample - Take 12-15 cores
at various spots in the sampling area. Place in the bucket
to make a composite sample representative of your field.
5.
Mix the composite sample well. Fill box to proper level. label
with name, address and field identification (be sure
to keep a record of how you identified the field, use
ASCS Map).
6.
Fill out the information
sheet. Problem
area sheets are available - sample with depth on good
and bad areas.
7.
Sampling
for precision farming systems.
2.
Chemical Analyses
Measures
a part of the total nutrients in the soil. The values are of
little use in themselves. The method must be calibrated against
nutrient rate experiments in the greenhouse and field.
a.
The North Carolina soil testing lab extracts soil samples
with Mehlich
3 extractant, which removes an estimate of plant available
nutrients.
b.
The concentration of plant nutrients in the extract is determined
c.
Different extractants are used at different labs.
3.
Interpretation
of analytical data
The
levels of nutrients in the extractant has to be calibrated with
crop response
| Yield
Quality |
P-
Index Value
|
Different
crops have different calibration curves because plants do not
have the same nutrient needs.
Lime
requirements - based on crop requirement (desired pH), present
pH, amount of acidity to be neutralized
Suggested
rate of N not based on soil test. Based on crop requirements
Plant
Analysis
-
A diagnostic tool to be used in combination with soil analysis.
-
Based on the concept that the amount of specific nutrient element
in the plant is directly related to thequantitv of that element
inthe soil.
-
Usually done only when a problem arises
-
Interpreting the results of plant tissue analysis is difficult.
Plant
sampling
1.
Usually done prior to or at the beginning of a reproductive
phase.
2.
Sample the correct plant part - on instruction sheet - usually
the
most recently matured leaves.
3.
Do not sample dead plants etc. - see instruction sheet
4.
Place sample in paper mailing envelope. (Not in plastic) so
it can
begin to air dry. Let succulent plants (cabbage, potatoes) air
dry a day before mailing.
5.
Take samples from an area with normal plant for comparison.
-
There is year to year and soil to soil variation in analysis.
crop varieties also differ
6.
Fill out the form
accurately.
7.
Take a soil sample at the same location and the same time and
send along with plant sample
8.
The fee for plant analysis is $5.00 per sample.
9.
Nutrient
sufficiency ranges in plant tissue.
Some
useful links
NCDA
Agronomic Services Soil Testing Section
Virtual
tour of the NCDA Soil Testing Lab