By Charles White and John Spargo, Pennsylvania State University Extension Specialists
First and foremost, get a soil test for all your fields, including rented and recently acquired fields. Soil tests that have been collected within the last 3 years will provide information that is current enough to work with, so long as you have been following the fertilizer recommendations provided for crops grown since the time of soil testing.
If you have under- or over-fertilized compared to the recommendations since the last sampling, it may be worth collecting new soil samples to get the most accurate measurement of existing nutrient needs. Soil sample as soon as possible this fall so you can get results before the soil freezes over the winter.
After obtaining soil test levels of phosphorus (P) and potassium (K), consider what category of interpretive levels each field is in. For P, the optimum range for grain and forage crops is considered to be between 30 and 50 ppm Mehlich 3 P. For K, the optimum range for grain crops is 100 to 150 ppm Mehlich 3 K and for forage crops is 100 to 200 ppm Mehlich 3 K. Fertilization decisions for each field should be determined in relation to whether the field is below, within, or above the optimum range.
In normal years with reasonable fertilizer prices, Penn State typically recommends that fields with soil test levels in the optimum range be fertilized at approximately crop removal rates to maintain soil nutrient levels in the optimum range for the future. Soils that are in the optimum range of a nutrient to begin with, however, already have enough nutrient supplying capacity to grow crops without any deficiency for at least one, if not several, years.
Therefore, when fertilizer prices are very high, it is not necessary to pre-replace the nutrients that will be utilized if soils are in the optimum range. Rather, just rely on the existing soil nutrients for now, and replace the removed nutrients later when fertilizer prices are lower. Soil P reserves could hold out for several years, while soil K reserves tend to be depleted faster, especially where forage crops are harvested.
If fertilizer prices don’t retreat within a year, and your nutrient levels were at the bottom of the optimum range to begin with, consider getting a new soil test next year to be sure you haven’t fallen below optimal, and adjust your fertilization strategy according to the new results.
If you have soil test levels for pH, P, or K that are below the optimum range, fertilizer and lime purchases this year could be warranted because of the increased likelihood of yield declines due to nutrient deficiencies. If your budget is limited, consider addressing any soil pH concerns first. Crops often show a positive economic response to pH corrections and doing so can enhance the availability of other nutrients. For instance, existing P in the soil and newly added P fertilizer will be more available for crop use when soil pH is optimal.
Next consider the P and K fertilization strategy. In normal years, Penn State recommends a ‘build and maintain’ approach to fertilizing when a soil test is below optimum. The maintain component of the recommendation is designed to feed the crop what it will remove each year, while the build component is additional nutrients designed to raise the soil test level into the optimum range.
It isn’t strictly necessary, however, to build soil test levels into the optimum range, and may be prohibitively costly in years when fertilizer prices are high. Instead, this year, consider spoon-feeding the crop just what it will need to maintain itself, and save the nutrient building for another year when prices are lower.
Crop removal levels for P2O5 and K2O can be calculated based on the expected crop yield, multiplied by the removal rate for each unit of crop yield from the table below.
Table 1. Typical crop nutrient removal for phosphorus and potassium (Reprinted from Table 1.2-6, Penn State Agronomy Guide, 2021-2022).
|Pounds/Per Unit of Yield P2O5
|Pounds/Per Unit of Yield K2O
|Removal for Given Yield P2O5
|Removal for Given Yield K2O
|Corn silage (T)1
|Cool-season grass (T)2,3
|Small grain silage (T)1
1 – 65 percent moisture.
2 – For legume-grass mixtures, use the predominant species in the mixture.
3 – Dry hay equivalent, 10 percent moisture.
4 – Includes straw.
Once you’ve decided on a crop removal rate of P and K, consider an application strategy that will help maximize efficiency. For P fertilizers, the application should be banded rather than broadcasted, to reduce fixation by soil minerals, and be placed several inches deep into the soil so that crop roots can access the nutrients. Applying MAP (11-52-0) or ammonium polyphosphate (10-34-0) fertilizer with granular or liquid starter fertilizer units on a planter is a perfect strategy to increase efficiency of P fertilizer this year.
When using DAP (18-46-0) fertilizer, the band should be placed further from the seed (> 3 inches), or be applied several weeks before planting, to reduce ammonia toxicity on emerging seedling roots. Potassium, which easily diffuses through the soil, can be broadcast on the soil surface. Because muriate of potash fertilizer has a high salt index and can injure young seedlings, the application rate in a starter band should be limited. For field corn, the rate of N plus K2O in the starter should be limited to 70 lbs/ac if the band is placed about 2 inches from the seed.
Nitrogen (N) management practices should also be carefully considered this year. Because crop yields respond with diminishing returns to increasing rates of N, it’s important to consider what the most economical rate of N fertilizer is. This will be the rate of N where the crop yield gained from an additional unit of N fertilizer applied is equal in value to the cost of the N fertilizer.
When fertilizing at rates above this threshold you start to lose money because the additional N fertilizer costs more than what you gain in crop yield. If you have experimented with different N rates on your farm in the past, you may have a good idea of what rate of N to use to reach economically optimum yields.
Using yield response curves we have collected through recent on-farm research, we have calculated that recent corn prices for 2022 harvest ($5.85/bu) and N fertilizer prices ($0.90/lb N) are resulting in economically optimum N fertilizer rates that are 15-20 lbs N/ac lower than what they would have been two years ago, when corn was valued at $4.00/bu and N fertilizer cost $0.30/lb.
Furthermore, decreasing N fertilizer rates this amount relative to the economically optimum rates from two years ago only resulted in 1 bu/ac yield declines on the yield response curve.
Nitrogen use efficiency can be significantly improved by using the right rate, the right source, and applying it at the right time in the right place. The correct rate should be determined using a realistic approximation of yield potential, with appropriate adjustments for the current economic conditions outlined above. The best source of N fertilizer will be determined by where it is placed.
Urea N is prone to volatilization losses when left on the soil surface unless it is treated with a urease inhibitor or washed into the soil with a timely rainfall or irrigation. Split applications of N reduce the risk of N losses (leaching, volatilization, and denitrification) early in the season by improving the synchrony between crop N demand and N fertilizer supply.
For applications of UAN solution (30-0-0), which is 50% urea, use a urease inhibitor when broadcast spraying or apply the liquid in a band to enhance infiltration into the soil and reduce the surface area where volatilization can occur.
Manure is a valuable source of N, P, and K and is abundantly available in some parts of Pennsylvania. Efforts should be made to maximize the efficiency of the nutrients contained in manure in order to reduce the amount of supplemental fertilizer required to meet crop needs. To increase the efficiency of manure N, considering injecting liquid manure to reduce volatilization.
Alternatively, volatilization can be reduced by applying liquid or solid manures when air temperatures are cold (<40 degrees F). Late-winter and early-spring (e.g., mid-March) application timing provides a good balance between realizing cold temperatures at the time of application and mitigating nutrient losses that can occur during the winter. To reduce the risk for nutrient runoff this time of year, avoid spreading manure on fields with steep slopes, high erosion rates, poorly drained soils, or that are close to water sources.
Manure applications should also be prioritized on fields that require N, P, and K to maximize utilization of all three nutrients. If your soil tests are already optimum or above optimum in P and K, consider exporting some of your manure to neighboring farms that need those nutrients.
The strategies discussed above should help you to make more informed management decisions about the rate, source, timing, and application methods of fertilizers and manure to maintain crop yields while reducing costs on fertilizers this year. Many of the strategies to enhance fertilizer and manure nutrient use efficiency are best practices that should be implemented no matter the cost of fertilizer, the current high fertilizer prices just bring their importance to the forefront.
Other strategies we discussed, such as foregoing maintenance and soil building applications of P and K fertilizers are more designed to help you get through short-term price spikes. When fertilizer prices come down in the future, it will make sense to return to the traditional approach of “build and maintain” recommendations, replenishing the reserves of soil nutrients used up during this period of high fertilizer prices or building nutrients up into the optimum range.
Doing so will allow your soils to provide a buffer that gives you some additional flexibility in navigating short-term fertilizer price spikes.