Topic 11


Soil Testing

Soil Test - A Chemical method for estimating the nutrient-supplying power of a soil before the crop is planted.

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

Collecting samples slide set

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

Purdue turf fertilizer calculator

Fertilizer calculations for greenhouse crops

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