You Can Postpone Phosphorus, Potassium, And Zinc Fertilizer Applications When….

 

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

 

 

 

 

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. The sufficiency approach is often described as "fertilize the crop" versus the "fertilize the soil" crop removal approach. 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.

 

 

 

Recommended fertilizer rates vary with the soil test level and 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. While we are discussing nutrients in general, this discussion is really focused on phosphorus use since that is the nutrient that is most likely to be deficient after nitrogen.

 

At lower soil test values, the recommended rates of fertilizer increase. Increased yield levels are removing more nutrients per year, so soil depletion rates may be faster than in the past. Recently we changed our recommendation procedures to recommend some additional phosphorus before the critical level is reached. Based on yield maps and grid soil sampling, maintaining fields above the critical level should be the goal. If a field is at the critical level or above it, one or two years without P fertilization should produce similar yields to applying phosphorus annually.

 

Unlike with phosphorus, potassium does not build up quickly in the soil since its accumulation depends on soil physical properties and amount of drainage. Its depletion rate is slower than that of phosphorus since many Nebraska soils contain clay which releases potassium slowly.

 

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 noted 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. Experience has shown that some fine-textured soils in eastern Nebraska under high residue conditions in wet, cool springs will exhibit sulfur deficiency symptoms. Application of 15 lb sulfur may avoid these symptoms, but may not increase yields.

 

 

 

 

This 176-page Nebraska Extension book is a guide to using nutrients from all sources for the production of Nebraska's major agronomic crops. This 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. Download (4,723 KB) or view this Nebraska crop nutrient management guide on-line.

 

Crop removal values noted in this article are from the Nutrient Management for Agronomic Crops in Nebraska. Fertilizer costs are based on fall 2016 estimates for anhydrous ammonia ($415/ton), monammonium phosphate ($420/ton), potassium chloride ($310/ton), and ammonium sulfate ($370/ton). These change all the time, so be sure to get local prices to use in your calculations.

 

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 $52.12/acre (Table 3).

 

 

 

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 $52.12 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 -2004 (Figure 1) illustrate the probability of yield increase with P fertilization (40 lb P2O5/acre). Over all sites there was a 3 bu/ac increase with P applied, but when the soil tests were low, under 10 Bray 1P, following soybeans there was a significant yield increase of 14 bu/ac due to P application.

 

When corn followed corn, there was an 11 bu/ac difference in yield between fertilized and nonfertilized. All but two of these locations had average yields greater than 200 bu/ac. Of the 34 sites, one had a yield increase and one a yield decrease with K fertilization (43 lb K2O/acre), with soil K levels ranging from less than 100 ppm to over 600 ppm K.