- Harvested grain removes large quantities of phosphorus (P) and potassium (K) from the soil and they must be replaced with adequate rates of fertilizer in order to maintain soil test levels.
- The science of soil fertility teaches us that to build the soil test levels of 1 lb/A of P and K, it takes 9 lbs/A P2O5 and 4 lbs/A K2O respectively, 1 ppm takes 18 lbs/A P2O5 and 8 lbs/A K2O.
- Fertilizer needs must be assessed after every crop.
Corn and soybeans have relatively high total requirements for phosphorus (P) and potassium (K). A fairly large percentage of the P and K required by the plant is ultimately removed from the cropping system as harvested grain. In order to maintain soil test levels, the nutrients removed in grain must be replaced by fertilizer additions to the soil. Hence, these fertilizer recommendations are often referred to as ‘maintenance’ fertilizer. The science of soil fertility teaches us that to build the soil test levels by 1 lb/A of P and K, it takes 9 lbs/A P2O5 and 4 lb/A K2O respectively, or for 1 ppm we need 18 lbs/A P2O5 and 8 lbs/A of K2O. For example, if we want to move our test level from 30 lb/A to a level of 40 lb/A for P2O5 it will require 90 lbs of P2O5 investment to get back to that number. This can make soil test drawdown an expensive practice. Big yields in a given year mean a big draw from the soil resident fertility. Using University of Illinois nutrient removal numbers, which are close to surrounding states, for every bushel of corn harvested, we can expect 0.37 lb/A P2O5 and 0.23 lb/A K2O removal. For soybeans those numbers are .75 lb/A/bu P2O5 and 1.15 lb/A/bu K2O. The following tables indicate P2O5 and K2O removal rates for corn (Table 1) and soybeans (Table 2). P2O5 and K2O removal rates are also presented as affected by a two-year corn-soybean rotation (Table 3). Every state uses a unique set of removal numbers and it is recommended to use the ones from your state when calculating removal recommendations.
Table 1: Removal Rates P & K Corn
Table 2: Removal Rates P & K Soybeans
Table 3. P2O5 and K2O crop removal rates (in lbs/ A) in a two-year corn-soybean rotation.
In cases where P & K levels were just into the “sufficient” range, as determined by your individual state, high yields in one year followed by “typical” yields the next, could reduce soil test levels significantly. To avoid depleting P & K levels in the soil, yield data can be used to replace nutrients taken up by the previous crop. As a reference, in table 4, you can find the amounts of DAP and Potash needed to replace the amount of P and K taken off over a two-year period. As you can see, the DAP and Potash levels on this table are influenced by crop yields; the higher the yields, the greater the drawdown and therefore the greater the amounts to be replaced. If the actual corn yield in 2019 was 200 bu/A and the soybean yield on the same field is 70 bu/A in 2020, it takes 275 lb/A DAP and 210 lb/A Potash to replace the P and K removed over the last two years. If we have applied a two year spread we must evaluate after the first crop if there is still sufficient fertility for the second and if not may need to consider an additional application of fertilizer. Utilize the removal rates in the table along with grid soil sampling to determine if the rate of fertilizer applied is appropriate based on crop yield and soil test results. Putting on too little fertilizer can result in yield loss; applying too much reduces a grower’s return on investment (ROI) and can negatively impact the environment.
Table 4. Amount of DAP and Potash needed to replace the P and K removed by corn and soybean in a 2-year rotation
The focus of a successful fertilization plan should be to maximize yields while protecting the grower’s investment. For more information on crop removal rates and nutrient replacement needs based on your individual practices, contact your local FS crop specialist. Additional information on fall fertilizer strategies can be found in the article:
Strategies for Fall Fertilizer Application.