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Introduction

Soil testing is the best tool available to growers for determining the amount of nutrients needed for the coming crop year. Soil test results have the greatest value when the area sampled is uniform enough to be represented by one soil sample. The first decision, before actual soil sampling begins, is to determine how large an area can be represented by one soil sample. Any area in a field which is not representative of that field should either be avoided or sampled separately. Differences in soil type, salinity, slope, erosion, crop history, manure history and any other management differences are all reasons to divide fields into two or more separate soil samples.

Soil sampling equipment and contamination

Soil samples are generally collected with a soil probe or soil auger. This equipment allows the collection of a uniform slice of the soil profile to any depth. All equipment that touches the soil should be rust free, and not be galvanized or contain brass. When soil conditions cause soil to stick and plug the soil probe, a lubricant such as WD-40 is recommended. Research has shown this lubricant does not contaminate the soil sample.

Sample Depth

Separate soil samples from 0-6" and 6-24" depths are required for areas where the soil nitrate test is used (0-6 & 0-24" combination is also acceptable). In areas where the soil nitrate test is not yet used, a 0-6" or tillage layer sample is recommended. In established forages, lawns or vegetable gardens, a 0-6" sample only is recommended. The 0-6" sample and the 6-24" sample must be placed in separate sample bags. The phosphorus, potassium and most micronutrients are tested on the 0-6" topsoil sample. Laboratory analysis for nitrogen, sulfur and chloride are conducted on both the topsoil (0-6") and subsoil (6-24") sample depths.

When to soil Test:

Soil testing for P, K, Ca, Mg, Zn, Fe, Mn, Cu, B, %OM, pH and salts can be done at any time of year. Since these test levels do not fluctuate greatly, testing is needed only every 2-4 years. Many dealers and consultants test for these nutrients each year to develop better long term averages for fields.

Soil testing for nitrogen, sulfur, and chloride is generally conducted in the fall following harvest or early spring. Soil nitrogen levels must be tested every fall due to the large changes which may occur annually with a mobile nutrient like nitrogen. Mineralization or a slow release of nitrogen from plant residue and organic material continues until soil temperature drops to 50 degrees F. University research has shown that from August 15 - September 15, soil nitrate levels increase in the soil about 1/2 lb/acre per day (see figure).

Most dealers and consultants begin sampling in late August or early September and then make a small adjustment to the nitrogen guideline (reduce nitrogen guideline by 1/2 lb/day for samples taken before September 15).

Soil Sample quality is best on unworked fields. Sampling an undisturbed soil profile provides much better control of the sample depth. Differences in sample depth will greatly affect non mobile nutrients such as phosphorus. When sampling tilled fields, be sure to sample from a compacted area such as a wheel track for more consistant depth control. Soil sampling unworked fields also allows unproductive areas of fields to be visually identified while sampling. These areas can then be avoided or sampled separately to determine if soil fertility is causing the loss of productivity.

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