Eliminate Unnecessary Use of Phosphorus, Potassium and Sulfur Fertilizer

Eliminate Unnecessary Use of Phosphorus, Potassium and Sulfur Fertilizer

December 18, 2008

Potential Added Profit:


Richard B. Ferguson, Extension Soils Specialist

Phosphorus, potassium and sulfur are essential plant nutrients, and must be adequately supplied to crops if soil levels are limited to insure optimal yield. Adding fertilizer containing these nutrients, as well as nitrogen, iron, zinc, and occasionally boron, is sometimes necessary for optimal yield for crops in Nebraska. For phosphorus (P), potassium (K) and sulfur (S), soil testing is the best way to determine if the availability of these nutrients is low enough to require fertilization.

Table 1. Critical soil test levels for phosphorus
Crop Bray-1 P (ppm) Olsen P (ppm)
Corn 15 10
Wheat* 25 17
Soybean 12 8
Grain Sorghum 15 10
Alfalfa 25 17
*The critical level for wheat varies with the values of fertilizer & grain price, application method and soil pH.
Table 2. Critical soil test levels for potassium and sulfur
Crop Soil K (ppm) Soil SO4-S (ppm)
Corn 125 8
Wheat 125 -
Soybean 125 -
Grain Sorghum 125 8
Alfalfa 125 5

Current Critical Levels and Recommendations —
The Sufficiency Approach

Research has shown that the most profitable approach to using these fertilizers over the short term is the sufficiency approach, rather than a crop removal approach. This approach is often called "fertilize the crop" versus "fertilize the soil." When soil test levels for a specific nutrient exceed the critical level, it is not profitable to apply that nutrient for that crop year. Soil test critical levels (the test concentration above which no fertilizer is recommended) vary with the test procedure and crop. Response levels are summarized in Tables 1 and 2.

The Crop Removal Approach

Recommended fertilizer rates vary with the soil test level and the crop. The sufficiency approach will allow soil test levels to decline if they are relatively high and above the critical level. As soil test levels decline below the critical level, fertilizer is recommended. At lower soil test values, the recommended rates of fertilizer increase. Research has shown that those rates provide adequately for the crop and still increase soil test levels over time.

Recommendations for sulfur fertilization consider soil organic matter content (SOM), texture, irrigation water sulfur content, and the sulfur soil test concentration. Fine-textured soils or soils with soil organic matter greater than 1% will supply adequate sulfur to the crop and fertilization is not necessary. The critical levels above are for soils with soil organic matter less than or equal to 1% and less than 6 ppm SO4-S in the irrigation water. No response of wheat or soybean to sulfur fertilization has been documented in Nebraska.

Nutrient Management for Agronomic Crops in Nebraska, EC155

A guide to nutrient use from all sources for the production of Nebraska's major agronomic crops. Content in this 176-page, UNL Extension book is divided into two main areas: the basic principles of soil fertility for the primary, secondary and micronutrients and fertilizer recommendations for individual crops. View this guide on-line.

Another approach to fertilizer management is to replace the nutrients removed each year by harvest to prevent soil nutrient decline over time. This approach will usually insure maximum production, but at a cost.

Crop removal values are taken from EC01-155, Nutrient Management for Agronomic Crops in Nebraska. Fertilizer costs are based on fall 2008 estimates for anhydrous ammonia ($900/ton), monammonium phosphate ($1100/ton), potassium chloride ($700/ton), and ammonium sulfate ($500/ton).

The probability of yield increase is low if soil test levels for these nutrients are above the critical levels. Consequently, not applying these fertilizers when soil nutrient availability is adequate can save $118.56/acre.

Table 3. Fertilizer recommendations and cost at current fertilizer prices to replace the nutrients removed (P, K, S) in a 200 bu/acre corn crop.
Nutrient Removal
(lb/bu grain)
(lb/200 bu grain)
($/lb nutrient)
Fertilizer Cost
Phosphorus (P2O5)
Potassium (K2O) 0.28 56 0.22



Sulfur (SO4-S) 0.09 18 0.23 10.08
Total       118.56


The sufficiency approach for P, K and S management allows for significant savings in short-term fertilizer costs. With this approach, nutrient application is not recommended when the soil test level exceeds the critical level as the probability of yield response is low. With the crop nutrient removal approach, an additional $118.56 of nutrients per acre would be applied for situations of adequate nutrient availability and 200 bu/acre corn yield.

Results from field studies across Nebraska in 2002 and 2003 (Table 4) illustrate the probability of yield increase with P fertilization (41 lb P2O5/acre). All but two of these locations had average yields greater than 200 bu/acre. No yield increase was observed in 2002 with K fertilization (43 lb K2O/acre), with soil K levels ranging from less than 100 ppm to over 600 ppm K.

Table 4. Probability of yield increase with phosphorus fertilization (41 lb P2O5/acre), based on field studies conducted across Nebraska in 2002 and 2003.
Bray-1 P (ppm) Yield Increase
with P Fertilization
Bray-1 P (ppm) Yield Increase
with P Fertilization
Bray-1 P (ppm) Yield Increase
with P Fertilization
5 Yes 12 No 19 No
9 No 13 No 20 No
9 No 14 Yes 20 No
10 Yes 16 >Yes 36 No
11 Yes 17 No 49 No
11 No 17 Yes 74 No
12 No 17 No