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What's the big deal with phosphorus?

Randy Pepin, Extension Educator
January 12, 2013

A lot of discussion revolves around nitrogen, whether from commercial fertilizer or manure, and its tendencies to volatize into the atmosphere or leach into ground water.  These are real nitrogen issues for our pocketbook and the environment.  Since phosphorus tends not to do either one of these, what is the big deal; any excess phosphorus will just sit there in the ground and not go anywhere, right?  We need phosphorus since it is a required nutrient for all plants and animals.

In our lakes, rivers and streams phosphorus is naturally a limiting nutrient.  When excess phosphorus enters these systems certain organisms flourish.  One easily observable organism that responds with rapid growth to excess phosphorus is green algae.  Most people have witnessed an algae bloom.  The water turns dark green rather the natural blue we expect.   When in excess, these algae can become very thick.  However, the greatest impact happens later when the algae and other aquatic plants growing in excess die, sink to the bottom of the water and begin to decay. This decaying process requires oxygen.  When the available oxygen is used to support this abnormally large decomposition process, other plants and animals, including most desirable species of fish, are unable to survive in the oxygen deprived water.  This impaired water can eventually become unfit for drinking, recreation and aesthetics.  The process of excess phosphorus entering a water system causing excess plant growth and eventually depriving the system of dissolved oxygen is called eutrophication.

What are the sources of the phosphorus entering our natural waters?  Industrial waste, municipal waste, impaired sewer systems, residential lawn runoff, and erosion from agriculture land and feedlots are all contributing factors.  This is the part of the discussion where everybody blames the other sectors rather than accepting responsibility for the phosphorus source over which they have control.  This is a typical human reaction because it requires us to spend money, change how we do things, or both.  Realistically, attaining significant reduction of eutrophication of our waters requires contribution by all the sectors mentioned above.

With higher costs of commercial fertilizer, the recognition of the economic value of manure is rapidly increasing.  One of the challenges with manure is that we cannot customize its analysis; corn needs more nitrogen in relation to phosphorus than typical manure provides.  So when most manure is applied to meet the nitrogen needs of corn following corn, the applied phosphorus is usually in excess of needs.  Frequent repetition of this practice in the same field will cause the soil phosphorus level to increase.

So back to our original question; if phosphorus is fairly stable in our soil, what’s the big deal?  Most soil phosphorus is bound to soil particles; however, manure contains some soluble phosphorus that can dissolve in water and leave the field with surface runoff water.  We have both water runoff and soil erosion in almost all cropping systems.  When this water runoff or soil erosion contains higher than normal levels of phosphorus, excess phosphorus can leave our fields, enter the environment and eventually into our waters.  Farmers perform many practices to minimize water runoff and soil erosion such as minimum tillage, contour tillage, strip tillage, crop rotation and buffer strips.  Other variables include degree and length of slope, soil type, crop cover, animal species, manure form, and timing, amount and method of manure application.  The proximity of the field to swamps, creeks, ditches, tile inlets, streams and lakes can all affect how much phosphorus enters our waters.

So what do we do?  This can be a complicated answer depending on the farm situation.  Knowing the analysis of our manure and soils is a start.  Having and following a manure management plan, implementing good conservation practices and understanding how phosphorus works in our environment are all important.  Grid sampling our fields to help us apply commercial fertilizer correctly is rapidly becoming an accepted practice.  If we can justify grid sampling for commercial fertilizer, why can’t we use it for making decisions on where to distribute our manure?  Livestock manure is a great asset; we must have the proper tools available to utilize it for greatest economic benefit and simultaneously protect our environment.

This winter there will be a series of workshops around the state demonstrating the concept of utilizing soil grid sampling to help in the distribution of manure.  The workshops will also illustrate the different nutrient availabilities in manure and how to utilize manure for maximum economic benefit.  Contact your local University of Minnesota Extension Educator for the time and location of the workshop nearest your farm.

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