December 18, 2008

Potential Added Profit:
$118.56/acre

 

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.

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.

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.

Summary

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.