P & K Fertility Decision with Tight Margins (2024)

Looking closely at fertilizer input needs is one way to deal with tighter profit margins. The biggest concern with reducing fertilizer is, of course, loss of yield. The best strategy to reduce fertilizer input while avoiding yield loss is to know the ability of soil to supply nutrients through soil testing. It is impossible to predict the impact of changes to a fertility program without measuring soil-available phosphorus (P) and potassium (K). Once the fertility supply in the soil is known, the Tri-State Fertility Recommendations can be used to determine if fertility is low and to what degree.

Soil pH is a key driver in determining nutrient availability. When soil pH drops below 6.5, phosphorus availability rapidly decreases. Below 6.0, the plant availability of many other nutrients such as potassium, sulfur, and magnesium also decreases (Fig. 1). In Ohio, there is a lot of acreage with low pH soils, especially in the eastern part of the state where acidic subsoil dominates. If soil tests reveal a low soil pH, applying lime this fall can be an inexpensive way to increase nutrient availability without applying fertilizer. Adjusting pH to the desired range of 6.2-6.8 is also beneficial for soil bacteria that mediate many soil processes. An example is the nitrogen-fixing relationship between rhizobium and legumes such as soybean.

Figure 1. Relative availability of elements essential to plant growth at different pH levels for mineral soils.

P & K Fertility Decision with Tight Margins (1)
Source: Ohio Agronomy Guide, 15th Edition

A few reminders about lime application:

  • Base the application rate on the buffer pH value on a soil test.
  • Use Table 3-6 in the Ohio Agronomy Guide to determine application rate.
  • Know the effective neutralizing power (ENP) of the lime source.
  • Adjust for tillage depth (ex. 3” tillage depth multiply rate by 0.38).
  • More liming details can be found in https://ohioline.osu.edu/factsheet/AGF-505-07

If soil pH is in the desired range, utilize the soil test reported P & K values to determine which fields have the lowest probability of yield reductions without applied fertilizer. Remember each crop has a critical soil test level where, below that level, the probability of a yield loss increases. When soil test values are above the critical value, we are in the maintenance range where we apply a crop removal rate to keep fertility levels above that critical level. Maintenance fertilizer application can occur in future years when profit margins improve. Table 1 shows the critical values as the lowest number in the range while the maintenance limit is the highest value. For example, if a field going to soybeans is at or below 20 ppm phosphorus, there is a higher probability that not applying phosphorus will result in yield loss. If a field going to soybeans is at or above 40 ppm the probability of a yield increase is very low.

Table 1. Optimal Level or Maintenance Range

Crop

Phosphorus (Mehlich-3 P)

Potassium (Mehlich-3 K)

Sandy Soils (CEC <5 meq/100g)

Loam and clay soils (CEC >5 meq/100g)

Corn, Soybean

20-40 ppm

100-130 ppm

120-170 ppm

Wheat, Alfalfa

30-50 ppm

100-130 ppm

120-170 ppm

Source: Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat and Alfalfa

A more detailed look at fertilizer response trials provides the frequency and magnitude of yield responses at various soil test nutrient levels for P and K.

Data from 439 field phosphorus trials showed that when soil P levels were below 20 ppm, about 34-67% of trials showed a positive yield response with a substantial yield bump of 7-13% (median relative yield of 87-93%; Table 2). If a field is solidly in the maintenance range of 20-40 ppm, yield loss due to no application of fertilizer for a year should be minimal to non-existent. About 86-88% of field trials showed no yield gain when soil P levels were above 20 ppm P (Rakkar and LaBarge, 2024). When P is not applied, how much a soil test P level drops from crop removal varies by soil type. P trials in Ohio have shown a drop of 0.8–2.1 ppm on an annual basis in a corn-soybean system (LaBarge and Cochran, 2023).

Table 2: Soil Phosphorus and Crop Response to Phosphorus Fertilizer in Ohio

P & K Fertility Decision with Tight Margins (2)
Source: Rakkar and LaBarge, 2024.

Soil test potassium impacts on yield show a similar trend to P. As soil tests decrease, the frequency and the magnitude of yield increase. Table 3 shows potassium response from 458 fields in Ohio. The data showed no yield response to K fertilizer in 95-98% of trials when soil test K was above 130 ppm, indicating a low risk of yield penalty. When soil test K values are between 100-130 there is a higher number of responsive trials, but relative yield increases were about 4%. It is when soil K levels were below 100 ppm that about 40-50% of trials showed a positive yield response with a substantial yield bump of 11-14% (or a median relative yield of 86%).

Table 3: Potassium Uptake and Ohio Crop Response
P & K Fertility Decision with Tight Margins (3)
Source: Rakkar and LaBarge, 2024.

Most have come to realize that manure is a great source of nutrients. It can be difficult to get ahold of, but it may be worth asking a livestock neighbor to see if it is a less expensive option. Municipal biosolids also offer a good nutrient source and are fairly easy to transport. The challenge here is paperwork but the municipality may work with farmers and consultants to complete the paperwork.

When considering nitrogen application next spring, remember that the university recommendations are based on the Maximum Return to Nitrogen model which considers nitrogen and corn prices. This is the best way to maximize economic return to your nitrogen application. You can find it at: https://www.cornnratecalc.org/.

Drought Considerations: Very dry soils can affect soil test results. It can be difficult to keep sampling depth consistent due to pushing a probe into the ground and soil falling out of the probe. Dry soils also impact K and pH values. Keep this in mind if large differences are seen between samples taken within 3-4 years of each other. To learn more about the impacts of drought on soil test results, please read: https://extension.entm.purdue.edu/newsletters/pestandcrop/article/keep-in-mind-soil-test-k-and-ph-are-affected-by-low-soil-moisture/

Resources

Rakkar, M., LaBarge, G. Potassium Uptake and Ohio Crop Response. Retrieved from: https://ohioline.osu.edu/factsheet/anr-0147

Rakkar, M., LaBarge, G. Soil Phosphorus and Crop Response to Phosphorus Fertilizer in Ohio. Retrieved from: https://ohioline.osu.edu/factsheet/anr-0146

P & K Fertility Decision with Tight Margins (2024)
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