Cross-check Your Soil Test Reports to Determine Spring Soil Fertility Plans

By Charles White

Carefully managing soil fertility when fertilizer prices are high is important to maintain profitable crop production. Given the high fertilizer prices, the approach to fertilizer application rates this year may be different than your typical practices. Double checking the soil test reports from the most recent soil sampling of your fields can help you make final adjustments to your fertilizer application rates that may help you reduce fertilizer costs this year. If you don't have a soil test from within the last 3 years, or if you have not been following the fertilizer recommendations derived from your recent soil tests, you should strongly consider collecting new soil samples as soon as possible this spring.

The first soil fertility parameter to check should always be soil pH, because it controls many aspects of nutrient availability and crop health. Correcting soil pH with limestone is also relatively inexpensive when you consider liming will provide benefits over several years of crop production. The optimum range of soil pH for most agronomic crops is 6.0 to 6.5, but for barley and alfalfa it is 6.5 to 7.0. If your soil pH falls below these optimum ranges, then follow the limestone application recommendation on the soil test report. Limestone takes up to 6 months to fully react in the soil and neutralize acidity, so planning ahead is important, especially for sensitive crops like alfalfa and barley.

The next fertility parameter to check is potassium. Potassium levels in the soil can draw down faster than other nutrients because of potentially heavy withdrawals by forage crops or harvesting of straw and fodder. However, many fields that are regularly manured receive plentiful potassium inputs and may have above optimum potassium levels. The optimum range of Mehlich 3 extractable potassium is 100 to 150 ppm K for grain crops and 100 to 200 ppm K for forage crops. Note that some labs report potassium levels in units of lbs/acre or K₂O, in which case you need to convert the reported nutrient levels back to a ppm K basis.

After checking the potassium levels, check the Mehlich 3 extractable phosphorus level. The optimum range of phosphorus for agronomic crops is 30 to 50 ppm P. Similar to potassium, some labs may report phosphorus levels in units of lbs/acre or P₂O₅, which will require a conversion to units of ppm P to interpret where the P level is relative to the optimum range.

If your P or K levels are in the optimum or above optimum range, there is no need to apply that nutrient this year. This is a departure from our normal "build and maintain" philosophy of managing nutrients, where we recommend maintenance applications of P and K fertilizers to replace crop removals when soil test levels are in the optimum range. However, soils that have P or K levels in the optimum range have enough of that nutrient available to grow a crop without yield losses for at least one year and possibly multiple years. Therefore, while fertilizer prices are high, I recommend not fertilizing when soil test levels are in the optimum range. The recommendation made here to not fertilize when soil test levels are in the optimum range is likely different than what is stated on the fertilizer recommendations of your soil test report, which is why it is important to cross-check your plans.

If you have soil test levels for P or K that are below the optimum range, fertilizer applications this year are warranted because of the increased likelihood of yield declines due to nutrient deficiencies. Our normal recommendation philosophy of "build and maintain" calls for an addition of fertilizer designed to raise the soil nutrient level up to the optimum level (the build component), plus an addition to replace crop removal (the maintain component). Following the "build and maintain" approach when fertilizers are relatively inexpensive is a good investment in building up the store of soil nutrients to meet the needs of your crops, and raising nutrient levels to the optimum range puts you in a good position to forego nutrient applications in years when the fertilizer market experiences price shocks. However, it is not the right time to invest in soil nutrient building when fertilizer prices are as high as they are today. Instead, it will be more profitable in the short-term to apply just enough nutrients to meet the crop needs this year. In other words, don't invest in buildup this year, just focus on a minimal level of sufficiency. This year, for fields with soil test levels in the below optimum range, I suggest P and K fertilizer application rates be calculated to just meet crop removal rates. Crop removal rates 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).

¹65 percent moisture.
²For legume-grass mixtures, use the predominant species in the mixture.
³Dry hay equivalent, 10 percent moisture.
⁴Includes straw.

After cross-checking your soil test reports and revising your fertilizer application rates with the guidance above, consider application methods and nutrient sources that can help increase efficiency and reduce costs. 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.

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°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.

Cross-checking your soil test reports to evaluate soil pH and Mehlich 3 P and K levels will help you identify the fields that are priorities for lime and fertilizer applications this year. Revising the fertilizer recommendation approach to not apply fertilizers on optimum testing fields and only applying crop removal rates on below optimum fields will help to improve short-term profitability in times of high fertilizer prices.