Part 1, Section 2: Soil Fertility Management
Soil Fertility Management
SOIL TESTING
Interpreting the results
Soil nutrient levels are given as parts per million (ppm) elemental P, K, and Mg. As a rule of thumb, to convert ppm to lb/A, multiply ppm × 2. The elemental results can be converted to oxide forms using the following conversions: P × 2.3 = P2O5, K × 1.2 = K2O, Mg × 1.6 = MgO
The results of the laboratory analysis are meaningless by themselves; they must be interpreted by relating the lab values to known crop responses under local conditions. The relationship between the soil test level and crop yield can be represented as a response curve such as the one illustrated in Figure 1.2-5. As soil test levels increase from very low levels, the yield will increase until it reaches a “yield plateau”—the point above which yield no longer increases with soil test level. The optimum soil test level lies just above that point. Eventually, if soil test levels continue to increase, yield reduction may occur in some instances. Once the response curve has been determined by field research in the area where the soil test will be used, the interpretation levels for the soil test (i.e., Below Optimum, Optimum, Above Optimum) can be established as shown in this figure. The interpretation, based on crop response research, is given as a bar chart that indicates whether the level for each nutrient is below optimum, optimum, or above optimum for the crop to be grown. The definition for each category is given below.
Below Optimum soil test level (Table 1.2-4) indicates that the nutrient is probably deficient and that the deficiency will likely limit crop growth. There is a high probability of a profitable return from correcting a low level. The greatest economic return per dollar invested in fertility is usually achieved through medium application rates to low-testing soils. However, the maximum profit per acre and the lowest cost per unit of crop produced is achieved as the nutrient application rate is increased to the recommended level. The recommendation for a low-testing soil is designed to gradually build up the nutrient level to optimum and to maintain it at that level.
| Soil test | Optimum Range | Comments |
|---|---|---|
| pH | 6.0-7.0 | A pH of 6.0-6.5 is considered adequate for most agronomic crops; however, 6.5-7.0 is recommended for alfalfa and barley. |
| Phosphorus (P) |
30-50 ppm | All agronomic crops. |
| Potassium (K) |
100-150 ppm 100-200 ppm |
Grain crops Forage crops |
| Magnesium (Mg) |
120-180 ppm 60-120 ppm |
Grass forage crops Other agronomic crops |
Optimum soil test level indicates that the nutrient is probably adequate and will likely not limit crop growth in a typical growing season. There is a low probability of a profitable return from increasing the soil test level above optimum. The recommendation for an optimum-testing soil is designed to offset crop removal in order to maintain the nutrient in the optimum range. If you are soil testing on an annual basis, no maintenance fertilizer is needed when the soil tests in the optimum range.
Above Optimum soil test level indicates that the nutrient is more than adequate and will not limit crop growth. There is a very low probability of a profitable return from applying a nutrient to a soil testing above optimum. Consequently, no fertilizer is recommended on a these soils. Too much of a plant nutrient may cause a nutrient imbalance in the soil and, as a result, in the plant. Additional broadcast applications of fertilizers or manures to soils that are very high not only result in unsatisfactory economic returns, but also they can adversely affect plant growth, animal health, and environmental quality.