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Reducing Water Quality Concerns in Phosphorous Application.

01 November 2011
Ohio State University

US - Phosphorous is back as a water quality concern, but it is not the same problem that Ohio farmers played an important role during 1980's in reducing phosphorus loading with the adoption of conservation tillage. The issue today is more complicated and while absolute answers are not available, implementation of best management practices will help keep phosphorous on fields where we want to keep it for crop production, writes Greg LaBarge and Harold Watters, Ohio State University Extension.

The issue of phosphorous in the 1960's and 1970's was related to the total phosphorous load going into lake systems across the state. While multiple sources of P were targeted, the majority of this loading from an agriculture standpoint was sediment bound phosphorous that eroded from fields via sheet and rill erosion.

A shift to no-till and conservation tillage crop production methods left a protective residue cover over soil, reducing erosion while lowering phosphorous levels in water. Reducing other sources of P from sewage treatment, detergents and multiple other sources were focused on as well. The health of the lake improved.

Today's phosphorous problem leading to harmful algae blooms finds it roots an increased level of dissolved reactive phosphorous (DRP) in Ohio's waters that have been observed since the mid 1990's. The levels of DRP have increased in spite of observations of steady total phosphorous levels equal to that of the mid 1980's entering Lake Erie. To mitigate the role that agriculture plays in the overall phosphorous levels, we will need to rely on implementation of BMP's in fertilizer and manure application.

In this fall fertilization season some practices that are thought to help keep the phosphorous were we want it in the plant root zone are:

  1. Soil testing. The key to an effective soil sampling program is being able to provide the laboratory a representative sample of the cropping area that a fertilizer recommendation is desired for. Characteristics of a representative sample are a single composite soil sample of 15 individual cores representing no more than 20 acres. The bulked sample should be thoroughly mixed with a subsample packaged and sent to the lab. Many laboratories participate in blind testing programs that test their procedures against known samples. When selecting a lab ask about results of these types of programs. Laboratories are very efficient and effective in processing soil samples sent to them for processing. More information on phosphorous soil testing can be found at:
  2. Phosphorous fertilizer recommendations from Ohio State University Extension follow a buildup/maintenance approach. The critical phosphorous soil test levels are 15 PPM (30 pounds per acre) for corn/soybeans and 25 PPM (50 pounds per acre) for wheat/alfalfa. The Tri State Fertilizer Recommendations publication can be found at:
  3. To answer the question "What do you mean do not put any P on, won't my soil test drop?" The answer to this question is a two part answer. First yes, soil test levels will drop, but if you are above the crop response range for the crop it really is not a problem crop production wise. If you are above 30 PPM there is no yield benefit and if you are way above this level there is an economic benefit to using this soil stored P. The second part of the answer is soil test do not drop 1 to 1 with crop removal. A 150 bushel corn crop removes (150 bushel * 0.37 Crop removal = 56 lbs). Phosphorous chemistry in the soil buffers the crop removal so that for each 15-20 lbs of P2O5 removal phosphorous levels in the soil are lowered 1 PPM. So our 150 bushel crop will lower the soil test at most 3 PPM.
  4. Application placement and timing should be an important consideration in making nutrient applications. Keeping nutrients on the land preserves the nutrients for their intended purpose of providing for plant growth. Most of the nutrients being added to water courses in Ohio are coming from 4-7 rainfall events during the year.
  • Avoid snow covered and frozen soils for broadcast fertilizer or manure applications. The potential for nutrient enriched runoff is very high with either snow melt or with heavy rainfall.
  • Avoid surface applications without incorporation. Minimally invasive tillage helps reduce nutrient runoff risk. Even with plant residues on the surface, while runoff volumes are reduced, nutrient concentration of the water are not reduced.
  • Plan fertilizers and manure applications prior to tillage activities.

Additional discussion on BMP's for reducing Phosphorous Loss can be found at:

Other practices can be beneficial in reaching the goal of lowering nutrients in particular phosphorous in Ohio's water courses. The industry has taken to discussing fertilizers application as the 4 R's or Four Fertilizer "Rights". These four "Rights" are not independent of each other but incorporated as a system where decisions in one area affect the "Rights" in another area. For more information on the fertilizer "Rights" see

  1. Right Source- Match fertilizer type to crops needs
  2. Right Time- Match nutrient availability to when the crop needs them.
  3. Right Place- Keep nutrients where crops can use them.
  4. Right Rate- Match amount of fertilizer to crops needs.

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