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).

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.

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)

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

Answer: Events due to snow melt, spring tillage may be preferred to fall tillage.