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Interpreting P and K soil test results (1/18/1999)
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by John E. Sawyer, associate professor and Antonio Mallarino, assistant professor, Department of Agronomy

Related Articles

Soil testing and available phosphorus
September 2000, p. 164-166

Integrated Crop Management is published by
the Department of Entomology, Iowa State University, Ames, Iowa.

Iowa State University


Soil testing is a key component for determining the need for phosphorus (P) and potassium (K) fertilization. Also, if fertilization is required, test results guide the rate of application recommended to optimize production. Through extensive field research, specific soil tests are calibrated against the expectation of response to applied P and K; that is, they provide both a relative index of the availability of P and K to the crop being grown and an indication of the magnitude of yield increase one might expect when nutrients are applied—thus providing the interpretation of test results.

In Iowa, P and K soil tests are reported in parts per million (ppm), with interpretative ranges from very low (very deficient level of available P or K in the soil), low, optimum, high, on to very high (much more than adequate level of available P or K in the soil). According to ISU field research, the optimum category is the most profitable to maintain over time due to several factors, including uncertainty in soil tests, greater penalty for under-fertilization, and the possibility of skipping fertilization one year (like this year with low crop prices). Crop removal P and K applications are suggested at optimum test levels. Example soil-test P (Bray P1 and Olsen P) and soil-test K (Ammonium Acetate K) interpretations for corn, along with suggested broadcast P2O5 and K2O application rates, are shown in Table 1. Note that the rate of fertilization decreases as soil tests increase, and eventually the suggestion is to not apply any P or K.

Is it really alright to not apply P or K when soils test high or very high, as these recommendations suggest? In general the answer is yes. Long-term research results, like those shown in Figure 1, below, indicate that the economic return to P or K application is low at high or very high test levels. Some specific soil or climatic conditions may result in small yield increase from nutrient application at these test levels (e.g., starter may be recommended for corn at high test levels), but for most situations the probability and magnitude of response is low.

In the short term, P and K can be withheld on soils testing slightly above optimum; however, realize that with crop harvest and resultant removal of nutrients soil tests will decline and increased fertilization will eventually be required. Soils testing very high have little probability of yield increase from nutrient application, and could have P and K withheld for several years before fertilization would be required. Soils should be tested regularly to monitor changes in test levels when fertilization is withheld.

For more information on soil testing, especially for interpretations and recommendations for P and K, please refer to ISU Extension Publication Pm-1688. This publication is available from county extension offices or from the Extension Distribution Center (515-294-5247). Recommendations in Pm-1688 may be viewed at

Table 1. Phosphorus and potassium recommendation for corn grain production.
Phosphorus (P) soil test (ppm)
Soil test category Very low Low Optimum High Very high
Bray P1:
Low subsoil P 0-8 9-15 16-20 21-30 31+
High subsoil P 0-5 6-10 11-15 16-20 21+
Olsen P:
Low subsoil P 0-5 6-10 11-14 15-20 21+
High subsoil P 0-3 4-7 8-11 12-15 16+
P2O5 to apply (lb/acre)
100 75 50 0 0
Optimum: removal at 140 bu/acre.
Potassium (K) soil test (ppm)
Soil test category Very low Low Optimum High Very high
Ammonium acetate extractable K:
Low subsoil K 0-60 61-90 91-130 131-170 171+
High subsoil K 0-40 41-80 81-120 121-160 161+
Fine textured 120 90 40 0 0
Sandy textured 100 70 40 0 0
Optimum: removal at 140 bu/acre.
All data from Pm-1688, General Guide for Crop Nutrient Recommendations in Iowa.

Figure 1. Net Returns for P and K

Last updated 7/12/1999 by John VanDyk.
This information subject to a usage policy.