Using P Indexes to Compare Management Practices

The quiz for this lesson, which follows, is designed to give you practice using both the Nebraska and Iowa P Indexes.  Use both indexes to assess the risk of P delivery to surface waters for the various scenarios described in the quiz questions.  Each scenario will be a variation of the following 'base scenario':

The site is rainfed and has no P application. The soil is moderately eroded Monona silt loam with 5%-9% slope in Monona County on the southern Iowa drift plain.  Bray-P1 is 30 ppm.  The cropping system is row crops (straight rows-good) with tillage.  The site has an erosion rate (sheet and rill) of 10 tons/yr, with no gully erosion and a medium runoff rating.  The average distance to a concentrated water flow is 500 feet.  No erosion control practices are in place, and there is no subsurface drainage facility.  What are the risk values determined for this situation by both the Iowa P Index and the Nebraska P Index?   [Answer: The Iowa P Index gives a risk value of 5.31 for erosion, 0.27 for runoff, and 0.00 for drainage, for a total P index risk rating of 5.57, a high risk rating.  The Nebraska P Index gives a total risk rating of 12, a low risk rating.]