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.

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. 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.

The Crop Removal Approach

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 or above the critical level, it is likely that P fertilization can be postponed for one or two years without losing yield potential.

Unlike with phosphorus, potassium does not build up quickly in the soil since its accumulation depends on soil physical properties and some potassium is leached when there is water drainage through the profile. 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; 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.

Few responses of wheat or soybean to sulfur fertilization have 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.

The probability of yield increases from applying phosphorus, potassium or sulfur are low if  soil tests are above critical levels. Postponing their application can save as much as $52.12/acre.

Table 1. Critical soil test levels for phosphorus

Crop	Bray-1 P (ppm)	Mehlich 3 (ppm)	Olsen P (ppm)
Corn	15	18	10
Winter Wheat*	25	29	17
Soybean	12	14	8
Grain Sorghum	15	18	10
Alfalfa	25	29	17
*The critical level for wheat varies with the values of fertilizer and 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
**Sulfur is not expected to increase yields when soil organic matter is greater than 1% and the soil texture is not sandy or sandy loam.

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)	Removal (lb/200 bu grain)	Cost ($/lb nutrient)	Fertilizer Cost ($/acre)
Phosphorus (P2O5)	0.4	80	0.40	$32.00
Potassium (K2O)	0.28	56	0.25	$14.00
Sulfur (SO4-S)	0.09	18	0.34	$6.12
Total				$ 52.12

Nutrient Management for Agronomic Crops in Nebraska, EC155

Nutrient Management for Agronomic Crops in Nebraska (EC155) provides a guide to using nutrients for the production of Nebraska's major agronomic crops. This book is divided into two topics: basic principles of soil fertility and fertilizer recommendations for individual crops.

Download (4,723 KB) or view this nutrient management for agronomic crops (EC155) on-line.

Crop removal values noted in this article are from EC155. 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). Fertilizer prices will change over time and vary by region.

The probability of yield increase is low if soil test levels for these nutrients are above the critical levels. Consequently, not applying these fertilizers, based on crop removal, when soil nutrient availability is adequate can save $52.12/acre (Table 3).

Find more articles in this series here.

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 $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 type of yield increases one might expect with P fertilization (blue bars, 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, and following soybeans, there was a significant yield increase of 14 bu/ac due to P application.

When corn followed corn (Bray P 2O/acre), with soil K levels ranging from less than 100 ppm to over 600 ppm K.

(This article was revised from a December 18, 2008 CropWatch article.)

Figure 1. Effect of previous crop (PC) and Bray #1P on corn yield (Nebraska NSFP 2002-2004).