Soil Fertility Overview for Small Grain Production in Wisconsin: A one-stop shop

As with any other crop, fertilizer is an important aspect of management in small grain production. Nutrients are vital for crop development and yield outcomes, but they are also an input cost that should be optimized for profit; not yield alone. This document serves as a centralized resource containing soil fertility information from UW-Madison, Extension, and Nutrient and Pest Management (NPM) resources (see resources section below for more information). 

Soil pH Considerations for Small Grain Production

Soil pH is one of the most important soil fertility properties to consider as it impacts the nutrient availability to your crops. It also impacts soil microbial communities. Generally, the optimal soil pH for small grain production ranges from 5.6 to 6.6; with a specific target pH that optimizes crop performance for each individual crop (Table 1). 

Keep in mind, however, that soil pH should be managed based on the crop with the highest target pH in your rotation. Wheat, for example, has a target pH of 6.0; however, if soybean is in the crop rotation, the rotation’s target pH would be 6.3 (6.3 is the target pH of soybean). It generally makes more economic sense to manage pH, and therefore lime, in this way. For additional, generalized, information related to liming, see Chapter 5 in Extension Publication A2809).  

Phosphorus and Potassium Fertilizer Recommendations

Phosphorus (P) and potassium (K) fertilizer rate recommendations are determined based on several pieces of information including: 1. Crop P and K demand level; 2. Recent soil test P and K levels; and 3. Expected yield.  

1. Crop P and K demand levels (ranging from 1 to 4) are used as a method for grouping crops that have relatively similar P and K demand for optimal growth. Crops with a demand level of “1” need less P and K than those with a demand level of “4”. Small grains have a P and K demand level of “1”, indicating a relatively low P and K requirement compared to other crops. The only exception is wheat, where the demand level is “2”; suggesting that wheat requires slightly more P and K than other small grains due to higher nutrient removal rates.   
2. Soil test P and K interpretations are available for specific crops based on their relative P and K demand level (as mentioned above) in addition to the soil type of the field. Separate ranges are provided for “Loamy” soils and for “Sandy or Organic” soils (Table 2a and Table 2b below). Similar to managing soil pH, however, it is recommended to manage soil test P and K levels based on the highest P and K demanding crop in your rotation. Use the P and K results from your most recent soil test lab report and reference them against the ranges provided in the table below to determine the appropriate interpretation category. 

3. Phosphorus and potassium application rate recommendations for specific crops are based on current soil test levels (i.e. interpretation category) AND expected crop yield or yield goal. Generally, P and K application rate recommendations for soils testing in the “Optimum” category will match expected crop removal rates. For soils testing in the “low” or “very low” categories, recommended rates will exceed expected crop removal, with the goal of building soil test levels up to the optimum category over a period of 4 to 8 years. Conversely, for soils testing high or very high (for K), recommended rates are less than expected crop removal such that soil test levels will be “drawn down” to the optimum range over time.  

Lastly, it is important to account for the difference in nutrient removal (and therefore fertilizer rate recommendations) between when only the grain is being harvested compared to when the grain and the straw are being removed. See below for P and K rate recommendations for both harvest-scenarios for small grain crops in Wisconsin. 

Nitrogen Fertilizer Considerations

NITROGEN (N) fertilizer rate recommendations are determined in a slightly different way than for P and K recommendations are, and the process is also different between wheat and all other small grain crops. For wheat, N rate recommendations are made by using a maximum return to N (MRTN) approach, while other small grain-N recommendations are made based on soil organic matter content. 

Nitrogen for wheat

The MRTN rate guidelines for wheat were developed by conducting wheat yield response to N trials in on-farm and agricultural research station settings in major wheat producing areas of Wisconsin over many years. Analysis of the yield response to N data resulted in determining the average N application rates that resulted in maximum economic return on the N investment. As such, it is important to recognize that a combination of both agronomic (i.e. wheat-yield response to N functions) and economic (i.e. cost of N, price of wheat) factors are utilized in the MRTN approach; not just one or the other. The MRTN approach emphasizes maximizing profitability, not maximizing yield. 

Additionally, it is recommended that N is applied in the spring at “green-up”, and that fall N applications are avoided due to the risk of denitrification and leaching over the winter months. 

To determine the appropriate N application rate for wheat, we need to know the soil group (e.g. Loamy, Sandy, or Organic; Table 4.1 in A2809), previous crop, and the N:Wheat price ratio. By utilizing the ratio between the price of N fertilizer ($ / lb of actual N) and the value of wheat grain ($ / lb grain), rate selections are applicable to current / near-future market conditions and can be dynamic as those markets change over time.  

Example N:Wheat price ratio calculation, assuming $0.50 / lb N and $5.00 / bu of wheat: 
        N:Wheat price ratio = ($0.50 / lb N) ÷ ($5.00 / bu) = 0.10

Lower price ratios indicate market conditions where N fertilizer is relatively cheap and/or where wheat prices are relatively high. Conversely, higher price ratios reflect more expensive N fertilizer and/or lower wheat prices. For reference, a price ratio of 0.10 is “lower” than a price ratio of 0.15. Accounting for fluctuations in N cost and wheat price allows farmers to minimize overall financial risk. Figure 1 below provides an easy way to determine what the correct price ratio is for a range in N and wheat prices. 

Figure 1. N:Wheat Price Ratio Table