- 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.
- For every bushel of corn harvested, we can expect 0..42 lb/A P2O5 and 0.27 lb/A K2O removal.
- For soybeans those numbers are .85 lb/A/bu P2O5 and 1.20 lb/A/bu K2O.
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 of 1 ppm of P and K, it takes 16 lbs/A P2O5 and 6 lb/A K2O respectively. For example, if we want to move our test level from 15 ppm to a level of 20 ppm for P2O5 it will require 16 times the 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 0.42 lb/A P2O5 and 0.27 lb/A K2O removal. For soybeans those numbers are .85 lb/A/bu P2O5 and 1.20 lb/A/bu K2O.
Table 1: Removal Rates P & K Corn
Table 2: R
In cases where P & K levels were just into the “sufficient” range., 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.
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.