Over the next few months, the University of Minnesota Extension will discuss potential practices dairy farmers could consider to reduce their farm’s excess phosphorus balance, which is importing more phosphorus into a farm system than is exported. 

A number of years of net phosphorus imports into a farm system usually results in excess phosphorus levels in the soil. The problem is that excess phosphorus entering surface water contributes toward eutrophication, or rapid growth of plant life. This excess plant life eventually dies and sinks to the bottom, and the decaying material causes an oxygen shortage for desirable fish and plant life. We typically observe eutrophication as algae blooms. 

Agriculture is certainly not the only contributor of excess phosphorus into the environment, but we are not immune from it either.

The nitrogen-to-phosphorus ratio in manure is much narrower than the nitrogen-to-phosphorus ratio needs of corn. In many situations, manure has been applied to supply the nitrogen needs of corn, and consequently more phosphorus is applied than is necessary for crop production. Repeating this practice regularly over a period of years frequently escalates the soil phosphorus level. 

Unlike nitrogen, which can be lost through volatilization, denitrification or leaching, phosphorus is quite stable once mineralized into the soil and only will leave the field mostly through soil erosion or plant uptake. Any erosion off of fields with higher than necessary phosphorus can potentially result in more phosphorus entering our lakes, rivers and streams than would with normal levels of soil phosphorus. 

Implementation of conservation methods such as buffer strips, contour farming, minimum tillage and other practices are excellent in helping to reduce potential phosphorus runoff. However, there is a certain amount of soil erosion in all systems, even full vegetative cover, so keeping the soil phosphorus level from escalating is a constructive strategy under all cropping or grassland systems.

One spring operation that brings phosphorus into a farm system is corn starter fertilizer containing phosphorus. Adequate phosphorus is necessary for normal growth of any plant and corn is no exception. In the present economic culture of striving for every last bushel or pound, we typically do not want to "miss anything". So many farmers automatically spend money on corn starter fertilizer containing phosphorus and consider it "insurance.”

Current University of Minnesota suggestions for soils testing from 21 to 25 ppm Bray 1-P is to add 10 to 15 pounds per acre of banded phosphate. When soil phosphorus is higher than 25 ppm Bray 1-P, no phosphorus is generally suggested. Both corn grain and corn silage research on soils testing higher than 30 ppm Bray 1-P in phosphorus with a history of manure application is limited and yield response from adding fertilizer with phosphorus has been inconsistent. Occasionally in the early vegetative growth stages of corn, we observe visual differences between plants that receive starter fertilizer with phosphorus and those that do not. Early vegetative growth has not always proven to provide extra yield in the fall.

So when might we consider not using starter fertilizer containing phosphorus? This is potentially easier managed if the field has been grid soil sampled. The high nutrient variability observed with grid soil sample maps could explain some of the yield inconsistency experienced when adding or withholding starter fertilizer containing phosphorus on fields that have only a composite soil sample. 

If there is considerable variability in soil test phosphorus across the field, applying starter fertilizer containing phosphorus in the field areas below 25 ppm Bray 1-P would be recommended and would likely improve consistency of early plant growth and yield in those areas. We could then consider withholding starter fertilizer containing phosphorus in those areas in the field over 25 ppm Bray 1-P. This use of precision agriculture can save us some input costs, maximize yields, reduce our phosphorus imports and help protect our environment.

So is it a good idea to withhold starter fertilizer containing phosphorus on high phosphorus testing soil? It certainly should be considered; many livestock farmers have done it and they think it has been a good decision.