Crop plants have needs that must be met in order for them to grow and reproduce. They need water, sunlight, CO2, above-ground space and some sort of matrix into which to extend their roots. Additionally, to meet the needs that are above and beyond what is provided by the seed, plants require nutrients in varying quantities. Nitrogen (N), phosphorus (P) and potassium (K) are considered primary macronutrients and are required in larger quantities than both secondary macronutrients (calcium, magnesium and sulfur) and micronutrients.
Nitrogen is needed to build proteins and can be provided to the plant by mineralization of soil organic matter and through the addition of manure or inorganic nutrients. Phosphorus is needed to produce both cell membranes and genetic materials (DNA, RNA) and potassium is needed to activate many of the enzymes that drive growth and development.
To estimate the concentration of plant-available nutrients and measure soil pH (which can affect availability) it is recommended that farmers collect from each field composite soil samples once every 3 to 4 growing years.
The end-goal of land that is put into crop production is to harvest (or remove) something that is then used or consumed by humans or livestock. Soybean, corn and wheat grain is removed during grain harvest. Each time grain is removed from a field some of the nutrients that had been taken up from the soil are removed as well.
As with other inputs, nutrients constitute a significant cost of production. Provided that there are adequate concentrations of plant-available P and K in a field, the strategy for many farmers is to apply only enough mineral fertilizer to replace what was is removed during harvest. This is called the 'maintenance' fertilizer requirement and is typically expressed on a per bushel basis – meaning that maintenance fertilizer rates are higher in more productive soils. The maintenance fertilizer rate required to produce 50 bushels of soybeans is double that of a 25 bushel per acre crop.
As new biotech traits are added or higher yielding or more disease-resistant varieties are developed, new corn hybrids and soybean and wheat varieties are added to the marketplace each year as others are removed. With the general increase in productivity (yields) over time there is a question as to whether the nutrient removal rates in grain, and therefore the maintenance fertilizer rates have also increased. There are several possibilities, that: 1) the maintenance rates that are currently recommended in the Illinois Agronomy Handbook (2009) still apply, but that the per-acre rate may be higher due to higher yields, 2) more modern varieties may be more efficient in some way and perhaps fewer nutrients are being removed than we would expect, or 3) there is some other as yet unexpected result.
This is why more research was needed to update the P and K grain removal rates and by extension the maintenance fertilizer rates. As part of a Nutrient Research and Education Council (NREC)- sponsored research project, during the 2014, 2015 and 2016 harvest University of Illinois researchers led by Dr. Emerson Nafziger have collected hundreds of grain samples from each of the state's nine crop reporting districts.
On behalf of this project, I have spent several sunny September and October days with scale house-workers collecting corn and soybean grain samples at grain elevators (Figure). These samples will be analyzed for P and K content and then combined with other results to update the P and K grain removal rates.
Results will be presented at the 2017 regional Illinois Crop Management Conferences in Mt. Vernon (Jan. 18), Springfield (Jan. 25), Champaign (Feb. 1) and Malta (Feb 15).