Reducing Phosphorus Runoff

By James Hoorman, Hoorman Soil Health Services

Rain is again slowing down spring planting. April and May showers are saturating fields causing nutrient runoff and soil organic matter (SOM) losses.  While most scientists say phosphorous (P) is the main culprit, the harmful algae blooms (HAB) or cyanobacteria need a variety nutrients.  If rains continue into summer combined with warm weather and not much wind, HAB can multiply quite rapidly. Farmers have planted cover crops and applied a variety of best management practices to reduce HAB in Lake Erie, will it be enough?

Where is the P coming from, what is the source?  Human wastes account for roughly 16%, livestock manure 17%, and the biggest source is still from agriculture, from the soil.  Considering the large acreage (4.2 million acres in the Maumee River basin) it takes only a small amount of P loss to cause HAB in Lake Erie.  Farmers generally apply about 35-40# of P on corn and maintain about 95% of what is applied. HAB needs only 1/10 as much P as corn (1# P =500# of HAB), so now farmers are striving to be 99% efficient.  

Many factors affect P agricultural runoff, but weather is the biggest factor.  An analysis of rainfall records show that the amount and the intensity of annual rainfall events are increasing with time. Dr. Andrew Sharpley (retired), University of Arkansas, a leading P scientist says 80-90% of the P runoff occurs in 1-2 annual intense storm events. Rainfall intensity has increased so more P runoff occurs especially in events that have over 2 inches of rain. Small rains tend to soak in, but large intense rains wash SOM which floats, clay particles, and all attached nutrients into our ditches, streams, and rivers.

About 50-75% of soluble P is attached to SOM.  Unfortunately, SOM levels have decreased 50-70% over the last 100+ years due to soil tillage, so the soil has less SOM and less capacity to tie up P.  SOM slows down the water so that the P can be filtered and attached to the soil particles. In addition, about 80% of the P in surface water comes from only 20% of the land according to Sharpley. Roughly 30% of soluble P runs off in surface water while roughly 70% comes from subsurface drainage (tile lines). Conservation practices that protect the soil from soil erosion and P runoff may greatly decrease soluble P in surface water.

What can farmers do to decrease P runoff?  First, soil test and follow tri-state fertilizer recommendations. For corn and soybeans, 20-40 PPM Mehlich-III is an acceptable P soil test level with 30-50 PPM for wheat and alfalfa.   Where soil test levels are above 40 ppm Mehlich- III, do not apply additional P in a corn-soybean rotation. Fertilizing soils above these levels increases risk of P in surface runoff and tile drainage.

Second, try to avoid broadcasting fertilizer in the fall, try to inject, band, or put P fertilizer close to the seed (2 inches down, 2 inches to the side) which enhances the utilization of P fertilizer. Full width tillage has the potential to increase soil erosion and total phosphorus losses. Plant a cover crop immediately in the fall or after wheat harvest.  Live roots stabilize the soil, improve water infiltration, and live roots absorb soluble P.

Surface water flows from fields directed to tile via standpipes should be converted to blind inlets. As P risk loss potential increases, use edge of field treatments which control water movement or treat water as it is leaving the site. Drainage water management control structures, in ditch treatments such as two stage ditches with tallgrass prairie species (switchgrass) and other stream practices (buffers, field strip strips, grass waterways) can reduce nutrient loading.

While DAP (18%N-46%P-0) and MAP (11%N-52%P-0) are generally “cheaper” sources of P fertilizer, they are both highly soluble and easily leached. An older source of P fertilizer, triple super phosphate (48% P) is less soluble but contains more impurities (calcium, sulfur, and other micronutrients) which breaks down slower but allows soil microbes to use it more efficiently.

In summary, appropriate conservation practices should be implemented to minimize erosion and nutrient losses. Maintain 30% cover as crop residue/cover crop. Filter strips, grassed waterways and water diversion structures are appropriate tools. Live roots keep soil in place and tie up soluble nutrients. Strive to build soil health. Increase water infiltration by improving soil structure and reducing soil compaction which increases water retention, improves nutrient cycling, and reduced P and nutrient losses.  While everyone contributes to surface water P, farmers have to do everything they can to reduce P losses.  Hoping for good weather may not solve the problem this year. 

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2 comments

  1. you make the follwoing comment and I wonder if it is an error? “Roughly 30% of soluble P runs off in surface water while roughly 70% comes from subsurface drainage (tile lines).”

    Are these numbers mixed up?

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