AgWatch

The Science Of Soil Sampling Is Important For Healthy Fields

LAMAR, MO.
   Obtaining a quality soil sample is vital for receiving accurate nutrient recommendations for a field according to Jill Scheidt, agronomy specialist with University of Missouri Extension.
   “In a 20-acre field, there are about 40 million pounds of soil. Of those 40 million pounds, you send one pound to the lab for results. So make sure that one pound represents the entire field well,” said Scheidt.
   Soil samples need to be taken every three to four years. The average soil test assesses nitrogen, potassium, phosphorus, calcium, magnesium, organic matter, neutralizable acidity, cation exchange capacity and pH levels. Micro nutrients are not tested for unless the producer requests it at an additional charge.
   “Different soil types and soil needs are in the same pasture or field. Several samples bags need to be collected if the land is uneven,” said Scheidt.
   For example, if a pasture was once two pastures, separate samples should be taken on either side of the old fence line. If there is high animal traffic in a pasture, that area should be sampled separately. Hillsides and waterways should be sampled differently. If a pasture has been converted to a crop, or if a pond or tree line has been removed, separate samples are needed.
   Soil nutrient properties can fluctuate throughout the year depending on the season. The best time to take a sample for forages and spring-seeded crops is when the field is idle; usually after harvest in the fall or winter.
   For fields with winter wheat and fall-seeded crops, sampling during the idle time in the summer is best. Pre-plant or pre-side-dress nitrogen samples for corn should be taken in the spring as close to planned nitrogen application as possible.
   “It is best to wait at least three months after application of phosphorus fertilizer, lime or manure before taking a soil sample,” said Scheidt.
   Sample cores need to be at least six to eight inches deep. Too shallow of a sample can cause an overestimate of soil fertility levels.
   Every core should be the same depth and quantity to provide uniformity. A zigzag pattern of random soil sampling across the field works in most situations. If using a shovel instead of a soil probe, dig a hole and slice off one side.
   Collect 10-20 cores in a bucket, crumble and mix them well. Then remove sticks, rocks and grass and place about one pint of soil into a plastic bag or soil sample box. Always label the bag about where the sample was taken to identify it when the results are received.
   “The number of cores collected should reflect the variation of the land and land history,” said Scheidt. “More samples if the land is varied, less if it is more uniform. It is better to take too many samples as opposed to too little.”
   Grid soil sampling, which is sampling the field in 2.5 acres per sample, is economical when used in high yielding fields; especially when significant variations in soil tests are anticipated. It can also be useful in a field where the history is unknown. Grid soil sampling provides the most accurate results when paired with a variable rate spreader.
   Interpreting soil tests are the most difficult part of the process. The first section of the soil test report, given by University of Missouri laboratory, represents the current level of nutrients.
   Macro nutrients are expressed in pound per acre, and micro nutrients are expressed in particles per million (ppm) and rated on a scale of very low, low, medium, high, very high, to excessive.
   The lower section is the recommendations of the nutrients expressed in pounds per acre according to the desired yield goal and cropping option.
   Limestone tonnage recommendations can be calculated by dividing the Effective Neutralizing Material (ENM) by the guarantee of the limestone dealer. ENM guarantees usually range from 400-450; the greater the ENM, the finer the lime grade.
   Dolomitic lime may be suggested if the soil is low in magnesium, but there is little yield response, so it should only be applied when lime is suggested to raise the pH. ∆


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