Adjusting Fertilizer Recommendations in Times of High Input Cost

Amy L. Shober, Professor and Extension Specialist, Nutrient Management and Environmental Quality, ashober@udel.edu; Karen Gartley, UD Soil Testing Program Director, kgartley@udel.edu, and Jarrod O. Miller, Assistant Professor and Extension Specialist, Agronomy, jarrod@udel.edu

With fertilizer prices approaching record highs, you may wonder if you can cut back on fertilizer applications without sacrificing yield. The short answer is yes! If your soil test is within the “optimum” or “excessive” range, skip the fertilizer. Your soils have adequate nutrient concentrations to support crop growth and the probability that fertilization will result in profitable yield response is very low. We feel very confident in the recommendation to skip fertilizers when soil test nutrients are >50 FIV, in part because we know that our predecessors effectively “padded” our agronomic critical level just to be safe.

For soils with soil test nutrients in the “medium” (i.e., 25-50 FIV) category, there is a moderate probability of crop response to additional fertilizer. We suggest that you can reduce P and K fertilizer applications to match crop removal. Estimate crop removal rates of nutrients by multiplying your realistic yield goal by the appropriate crop nutrient removal rate (Table 1).

Applying fertilizer at a crop removal application rate will significantly reduce the amount of fertilizer that you need to buy. For example, the UD broadcast recommendation for full-season soybean with a realistic yield goal of 65 bu/A is 110 lb P₂O₅/A and 160 lb K₂O/A at a soil test level of 30 FIV (“medium” soil test P and K). In contrast, estimated crop removal of P₂O₅ and K₂O at 65 bu/A is 47 (round to 50) and 80 lb/A, respectively. With potash (0-0-60) prices at approximately $875/ton, applying potash to a 65 bu/A soybean crop at crop removal rates represents a cost savings of $57/A.

Table 1. Average P₂O₅ and K₂O removal rates in grain of common agronomic crops grown in Delaware (Binford, 2008).

For soils with soil test nutrients in the “low” (i.e., 0-25 FIV) categories, there is a high probability of crop response to additional fertilizer. We understand that you may feel uncomfortable applying crop removal rates. A 30% increase in fertilizer rate over crop removal for these “low” fertility soils will still save you money but better protect you from yield loss.

Here are some additional tips to get the most bang for your buck out of your fertilizer applications:

• Correct soil pH issues before fertilization. Keeping soils near the target pH for your crop ensures that nutrient availability is maximized across all nutrients. Applying lime at $30/ton is much more cost effective than applying high priced fertilizers, especially if those added nutrients sit in the soil in unavailable forms due to soil pH that is too low. Remember, only apply lime based on the results of an appropriate lime requirement test.
• Nutrients like P are easily fixed into forms that are not plant available when they come into contact with soils. Banding fertilizers for nutrients with a high fixation potential will greatly improve fertilizer use efficiency and will allow you to reduce rates over a broadcast application.
• Some nutrients, like K, are easily leached through Delaware’s sandy soils. Applying only what is needed for the current crop reduces the likelihood of excess nutrients that are highly leachable.
• Nitrogen application rates are not based on results of a soil test. Nitrogen management is tricky due to the potential for atmospheric losses and the biological nature of the N cycle. Use available tools (e.g., PSNT, chlorophyll meters, sensor-based applications) and make split applications to improve N use efficiency.
• Use an inoculant when planting leguminous crops like soybean to improve N fixation potential.
• Consider planting leguminous cover crops to increase N in the soil profile, but recognize that legumes will not fix N until flowering.

You might wonder why we feel comfortable reducing P and K rates so significantly during times of high input costs. This is because the University of Delaware nutrient recommendations for P and K were developed using a “build and maintain” approach (Figure 1). Essentially, following the UD recommendations for P and K results in fertilizer applications at rates that exceed crop need allowing you to “build” soil fertility when soil test concentrations fall below the agronomic critical level of 50 FIV (i.e., the point above which the probability plant response to fertilization is low). Once soil test concentrations are within the agronomic “optimum” range (i.e., 50-100 FIV), UD recommends fertilizing the soil with P and K at crop removal rates to “maintain” soil test concentrations within the “optimum” range. This “build and maintain” approach is most appropriate for building fertility on land that you own and in times of low input costs. However, this approach is not particularly appealing when farming rented land or in times when input costs are through the roof.

Figure 1. Under the “build and maintain” approach to soil fertility, fertilizer application rates are designed to increase (low or medium) or maintain (optimum) soil test concentrations. This results in fertilizer application rate recommendations that typically exceed crop need unless soils are in the “excessive” category.

A “sufficiency” approach to fertilization is much more appealing when input costs are high or when farming rented land (Figure 2). With the sufficiency approach, you fertilize the crop only when the soil test nutrient concentrations are below the agronomic critical level (i.e., <50 FIV). With sufficiency fertilization, you are feeding the crop only and not adding more fertilizer than needed to “build” your soil fertility. As such, fertilizer application rates are much lower than what is listed in our standard nutrient recommendations.

Figure 2. Under the “sufficiency” approach, fertilizer is applied only when soil test concentrations are below the agronomic critical level (i.e., 50 FIV for Delaware) and at rates designed to feed the crop and not the soil.

Our nutrient recommendations for agronomic crops were developed decades ago based on the results of regional field trials in soils ranging from deficient to adequate for a given nutrient necessary to obtain meaningful soil test correlation and calibration. We have no record of field calibration trials from our region to develop “sufficiency level” fertilizer rates for P and K. (Note: Recommendations for Ca, Mg, and micronutrients are based on a sufficiency approach.) As such, we recommend basing P and K fertilizer rates on crop removal rates when soils test is below the agronomic critical level. We feel that rates based on crop removal better reflect rates that would be recommended under a “sufficiency” approach and will allow you to feed the crop and not the soil.

Over the past several decades, we have made adjustments to the UD nutrient recommendations using best scientific judgment. This has allowed us to expand nutrient recommendations to additional crops and to adjust inputs based on higher yield potentials that are possible due to improvements in genetics. While not ideal, this method of maintaining our recommendations has been necessary as funds for conducting soil test calibration and correlation trials has been hard to come by. More recently, UD researchers are participating in a national effort to develop the Fertilizer Recommendation Support Tool (FRST). The FRST online decision support tool will help farmers and crop consultants make science-based nutrient management decisions using data from local/regional field trials.