The importance of soil fertility, especially in this new era of high soybean yields in the Midsouth, cannot be overstated.  Thus, a reminder about the soil sampling process is warranted even though it may be redundant to do so.

Rather than repeating the information that is abundant in current web postings and extension publications, I have highlighted what I believe are the more important points that are worth repeating.

• High-yielding crops such as irrigated corn and soybeans require large amounts of plant nutrients.  Thus, soils that produce these high yields will have lost considerable amounts of these nutrients during a crop year.  Intense monitoring to determine the level of nutrient removal is required in these production systems in order to ensure that necessary nutrients are replenished before the next crop cycle.


• Sampling for nutrient levels is best done when soil moisture level is suitable for tillage.


• Generally, sampling to a 6- to 8-inch depth is recommended since most soil test calibrations are set for this depth.


• When sampling row-crop fields, take samples between the rows.


• A representative sample should contain soil from 15 to 20 soil cores that are taken from a field or area that has similar soil characteristics and yield history.


• Use a soil map for each sampled field to ensure that composited samples best represent a consistent soil environment.


• Test each field at least once every 3 years.  In the case of fields that have a biennial rotation of crops, sample every 2 or 4 years following the same rotational crop at each sampling time.  This provides a consistent basis for comparing fields and detecting trends over time.


• As a general rule, sandy-textured soils should be tested more often than clay soils.


• To ensure sampling consistency and nutrient removal trends across years, sample at the same time of year and following the same crop in a rotational system.


• For spring-planted crops such as corn and soybeans, collect the samples soon after harvest.


• Soil tests should be used mainly to test for phosphorus (P), potassium (K), and pH.


• Amount of organic matter and the amount of nitrate in the soil are not reliable indicators of available nitrogen (N).  However, nitrate-N test results may be used to determine N loss following flooding or extended wet periods.


• Establish and follow a soil testing history for each field or area that is sampled on a regular basis.


• Soil test results–along with a history of the amounts of lime and fertilizer that are applied, the crops grown, and the yield history for each crop grown on a particular field–should be a part of the records that are kept for each field.


• The capacity of a soil to retain cations such as K, calcium, and magnesium is called the cation exchange capacity (CEC).  This property is helpful in identifying the soil texture class, but not for establishing soil fertility needs.  Basically, CEC goes from low to high as texture changes from sand to sandy loam to sandy clay loam to clay loam to clay.

I realize that many producers use a consultant or soil testing provider to do their soil sampling.  In that case, it is wise to periodically check to ensure that the above practices are being followed by these practitioners and that their protocol is established and consistent from year to year.  After all, expensive soil nutrients will be added according to the results from these tests, and both under- and over-fertilization are expensive.

For a summary of and links to soil sampling protocol, see the article on this website.

Composed by Larry G. Heatherly, July 2013, larryheatherly@bellsouth.net