Preplan for better soil test results

Soil testing is a very profitable practice to manage fertility input cost and promote environmental stewardship. The primary goal is to measure the soil’s ability to provide the soluble nutrient needed for crop production for two of our three macro nutrients (phosphorus and potassium) plus measure soil acidity which governs availability of micro nutrients and other soil functions. A secondary goal is to compare soil test results over time with crop response and fertilizer additions to identify trends in the fertility program as it is executed or what is becoming known as “adaptive management”. The key to accomplishing both these goals is to take a quality soil sample that represents the field area being sampled.

A soil sample sent in for analysis is around one pound. A 6.7 inch slice of soil with a surface area of one acre is approximately 2 million pounds known as an “acre furrow slice”.  As we include more acres in the sample you can see how small a volume is collected compared to the volume we want to represent. Some preplanning and establishing standard sampling criteria should be considered as good first steps to taking a quality soil test.

Preplanning involves determining the field area that will be collected for inclusion in the sample. There are a number of different methods on dividing up a landscape that can be effective. The overall goal is to have sample areas in the field that have similar crop yields, crop rotation histories, fertilizer application methods and sources of applied nutrient. Fields or field areas with a history of manure, banded fertilizer application or other unique characteristics require a different sampling strategy. Field areas represented by a single sample should be less than 25 acres.

The second focus area for a quality soil test is the sample collection.

First, a single soil sample is not a single core but a composite of numerous cores collected over the field area represented by the sample. Where broadcast applications have occurred a composite sample of 10-15 cores is suggested. Where a history of banded application exist in a field or manure application, then increase the number of cores to 20-25. The samples are bulked and then a subsample submitted to the lab.

The next critical collection item to define is the sample depth of the cores. Nutrients in the soil are naturally stratified with higher nutrient generally found on the surface due to residue breakdown and fertilizer placement then decreasing deeper in the soil profile due to plant removal. Each core taken should be taken to the same depth in the soil profile. Generally a 6 or 8 inch sample should be taken.

Finally, some other factors in sample collection should be included. Scrape the soil surface before taking a core so the sample core does not contain residue or live plant material.  If manure has been applied wait at least 6 months before sampling or if fertilizer is applied wait 2 months.  To compare sample trends, the soil samples should be done at approximately the same time of the year.

Controlling the sampling process is critical to think about implementing adaptive management principles. Adaptive management is a continuous loop of considering site factors, making a management decision, measuring outcome and then modifying the decisions to reach production goals. Adaptive management  from a soil fertility stand point involves  taking soil test, making fertilizer decisions, then measuring yield response followed by a follow-up soil test to provide a basis  to judge if the fertility program is providing nutrient in sufficient quantity to meet yield potential and maintaining soil test at the critical soil test level. An additional consideration in soil sampling to meet adaptive management is to use geo-referenced sampling to try and limit site sample variability from year to year.

The whole goal of soil sampling is to make a fertilizer recommendation for crop production. To provide the recommendation calibration studies are done with the soil test to measure crop response. For Ohio, the Tri-state fertilizer recommendations provide the calibration study history for recommendation development. For more information on the Tri-state Fertilizer recommendations or developing a soil sampling strategy several references are provided athttp://agcrops.osu.edu/specialists/fertility/fertility-fact-sheets-and-bulletins.

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