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Manure Phosphorus and Surface Water Protection II: Field and Management Factors

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Cover Crops, Tillage and Other Site Management Practices

Crop production practices which reduce water and soil movement from the field also reduce the risk of runoff P loss.  This makes sense, since P loss is dependent on transport factors, such as runoff and erosion.  Transport factors are further addressed in the next lesson.

In an Oklahoma field where the erosion rate was 3 tons per acre per year (6.6 mg/ha/yr), the use of cover crops resulted in much less soil loss and a 70-85 percent reduction in lost P fractions (Fig. 10).

Figure 10.  In a study in Oklahoma, inclusion of cover crops in the cropping system greatly reduced erosion loss and total P loss (t/ha = 0.44 t/ac; kg/ha = 0.89 lb/ac).  (From Sharpley and Sheffield, Livestock and Poultry Environmental Stewardship Curriculum)

 Phosphorus loss was primarily as sediment P, with relatively little loss of dissolved P.  That the reduction was greater for total P than for dissolved P reflects the close association total P loss has with soil loss.  Reduced tillage is expected to reduce total P loss, but not necessarily dissolved P loss, especially if reduced tillage results in high STP at the soil surface.

Deep plowing, in cases of excessively high P concentration at the soil surface, may be a sound practice to protect against excessive runoff P loss, if the plowing can be accomplished without significantly increasing soil erosion.  The best implement for this is the mini-moldboard plow, now known as a conservation-tillage plow.  A normal plow will increase the risk of runoff P.  Occasional tillage of no-till fields that receive high levels of manure P may be a partial solution to over-application of manure P.  Research is currently being conducted in Nebraska to address this issue.


Discussion Question:
How can plowing be done to minimize risk of erosion?



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