Dr. Kim Polizotto, chief agronomist with PCS, recently published an article in which he discussed reasons why the soil nutrient build-up philosophy needs to be revisited. Polizotto listed possible reasons for the increased number of soils testing low in phosphorus and potassium including: less university research oriented toward optimizing crop yield; a trend toward the public sector promoting lowering crop inputs; the controversial lowering of recommended optimum soil test nutrient levels; and overall nutrient removal increases through increased crop yields.

I agree with Polizotto. Somewhere we got away from optimum economic production philosophy. The results have been less residual soil nutrients and an increase in crop nutrient deficiencies.

I continue to see widespread potassium deficiencies throughout the growing season in corn, soybeans and alfalfa. I am starting to see more phosphorus deficiencies, especially in early season corn.

Many crop producers have gotten away from build and maintenance programs, where they apply enough phosphorus and potassium to supply what is removed by the harvested crop plus enough to build the soil up to optimum levels. Many have gone toward maintenance (crop removal) programs. Maintenance programs can succeed as long as all other crop production factors that influence plant nutrient availability are optimum. Weather, crop diseases, insect damage, soil properties, time of fertilizer application and crop grown influence fertilizer use efficiency. These factors are unaccounted for in maintenance programs.

Because many factors affect soil phosphorus and potassium availability, it makes little sense to base fertilizer recommendations on removal (maintenance) alone. By doing so, you will be increasing the potential for crop nutrient deficiencies and their associated yield reductions.

A maintenance program also does not account for nutrients tied-up by the soil (fixation) or nutrients contained in plant roots and non-harvested crop residue.

There are practices such as band fertilizer placement, optimum fertilizer application timing, variety selection, planting dates, irrigation, liming, etc. that increase crop phosphorus and potassium fertilizer efficiency. By using these programs, you increase the chance for maintenance program success. However, using these practices in lieu of building soil nutrient levels to optimum, is not recommended.

The percent of farmland being cash rented has increased while the length of cash rent contracts has decreased. This has led to lower incentives for the farmer to increase long-term soil nutrient levels. Landlord/farmer contracts need to become more creative in terms of nutrient management.

Another issue affecting phosphorus and potassium use is higher recent nitrogen prices. Many producers have a fixed fertilizer budget and will reduce phosphorus and potassium application while maintaining the same nitrogen rates. This is a short-term fix that often leads to long-term soil quality problems.

Many producers choose to apply phosphorus and potassium fertilizers during the corn year of a corn/soybean rotation. Soybeans are high phosphorus and potassium users. I believe that the combination of soil fixation and luxury consumption (crop removing more nutrients than needed) restricts the efficiency of 24-month fertilization intervals, unless soil test levels for both phosphorus and potassium are in the optimum to high range.

Luxury consumption is especially relevant in forage systems. Most forages should be fertilized with phosphorus and potassium at least twice per season and never on a 24-month interval schedule.

Soils have a wide range of phosphorus and potassium fixation properties. For example, total potassium in the plow layer of Missouri soils ranges from 10,000 to 50,000 pounds per acre. However, only 0.1 to 2 percent of this potassium is available to a growing crop. The rest is held in relatively unavailable mineral forms.

An example of the range in crop nutrient removal (potential luxury consumption) was reported in a recent Agronomy Journal article written by J.R. Heckman, et al. They found corn grain phosphorus concentrations ranged from 0.22 to 0.54 percent depending upon hybrid. For 150-bushel corn, those ranges are 18.5 to 45.4 pounds of phosphorus removed per acre. For potassium, the ranges were 0.31 to 0.62 percent or 26.0 to 52.1 pounds of potassium per acre removed by a 150-bushel-per-acre corn crop. They also found that as yield increased, the amount of phosphorus and potassium per bushel increased. This inconsistency in nutrient removal weakens the maintenance-only philosophy of crop nutrient management.

I still recommend the build/maintenance management system for phosphorus and potassium. In this system, soil sampling on a regular basis (every 3 to 4 years) is required to check the soil phosphorus and potassium bank accounts. By doing so, you will eliminate these two nutrients as potential yield-reducing factors in your crop production system.