Assessing Nutrient Levels

Differences in pHnitrogenphosphoruspotassium, zinc, soluble salts, cation exchange capacity, and organic matter content are determined by chemical analyses. Soil sampling is the first step to help estimate field levels of specific chemicals. There are two primary objectives when sampling soils to determine nutrient levels. These are:

1. Sample the soil to obtain the average nutrient level and the average pH for a given field.

2. Sample the soil to map the variability of chemical properties over a specific field.

How a field is sampled depends on which of these is the primary objective.

The second objective is becoming more popular with site-specific farming. Even subdividing a field into three or four subfields can be profitable. Table 9.1 shows the soil test results of three samples collected from a single field. Each sample was obtained from 15 to 20 individual soil cores that were thoroughly mixed to make one sample. These three samples show a field average. If only two samples were taken, instead of three, the average nutrient level for phosphorus, potassium and zinc would be different. Without three samples, the low zinc level or the low phosphorus level in one area of the field may not be known. Therefore, how a field is sampled is important. The knowledge of the variability in a field is also important and is the basis for the concept of site-specific management.

Soil Test A B C Average
pH 6.8 7.4 7.2 ----
Phosphorus1 (Bray) 9 3 15 9
Potassium1 320 280 400 333
Zinc1 (DTPA) .25 1.75 2 1.33

1P, K, and Zn are reported as parts per million

Table 9.1.  An example of average nutrient levels and nutrient variability from three samples within the same field.