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Thursday, February 12, 2015

Subsoil Phosphorous Problems

Dissolved phosphorous in our surface water remains in the news.  One of the problems with dissolved phosphorous is that it becomes a problem at relatively low concentrations.  With nitrogen applied at agronomic rates, losses cut into yields.  With phosphorous, the losses are probably having an effect on yields.  Phosphorous losses also run counter to the basics we learned in beginning soils classes.  We were taught that phosphorous does not usually  leave the field except with soil particles, ie erosion. 

CSA news this month has an article that talks about phosphorous losses from subsoil.  The article goes on to say that there is good evidence that soil drainage is a culprit.  We can prevent or reduce phosphorous losses with tillage, but then we risk erosion losses.  We solve one problem and create another.  This study does not look at soil phosphorous test levels, but other information I have seen says that dissolved phosphorous in surface water increases as soil test phosphorous levels increase.  Nutrient stratification is also an issue.  We apply phosphorous over the top, even in no-till situations, but the surface applied phosphorous then concentrates at the surface and dissolves into otherwise fairly clean runoff.

In many ways the idea of over applying nutrients to store them for later is past.
  • We need to reduce soil phosphorous levels where concentrations are high. 
  • Tillage may be a good management tool where erosion is low risk.
  • Manure needs to be applied to fields where the nutrients are need.  We can't just use the same field over and over as a disposal site.
  • Applying nutrients on frozen or show covered ground can increase the risk of nutrient runoff, even though it won't happen every time.
  • Keep soil test levels below 40 ppm Meilich 3 P.   
The above article concludes that we probably don't know enough about how to manage phosphorous to prevent problems. 

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