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Journal of Environmental Quality Abstract -

Soil Test Phosphorus and Clay Content Effects on Runoff Water Quality


This article in JEQ

  1. Vol. 29 No. 5, p. 1582-1586
    Received: Sept 1, 2000

    * Corresponding author(s): fred_cox@ncsu.edu
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  1. F. R. Cox * and
  2. S. E. Hendricks
  1. Soil Sci. Dep., North Carolina State Univ., Raleigh, NC 27695-7619.



Dissolved phosphorus (P) in runoff increases with an increase in soil test P, but slopes found for this relationship vary by an order of magnitude. A difference in clay content contributes to this variation. Experiments were conducted on Ultisols of 5 and 32% clay by incorporating poultry (Gallus gallus domesticus) litter and triple superphosphate (TSP) to create a wide range in Mehlich 3-extractable phosphorus (M3P). Mehlich 3 P increased similarly for the two sources with rate of P applied, but the increase was greater as clay content decreased. Total P in runoff was measured and it was related primarily to the quantity of sediment. Dissolved P (mg L−1) was measured with simulated rainfall and it increased with an increase in M3P (mg dm−3 in the surface 15–20 cm of soil). The slope of dissolved P versus M3P was 0.0040 for the 5% clay soil and 0.0014 for the 32% clay soil. With these relationships, M3P would need to be 250 mg dm−3 at 5% clay and 714 mg dm−3 at 32% clay to result in 1.0 mg L−1 dissolved P in runoff. Dissolved P in samples of runoff collected from natural rainfall with automatic samplers was substantially less than that with simulated rainfall. Thus, there was a marked difference due to method of measurement as well as the difference noted due to day content. Both factors should be considered if attempting to predict the level of dissolved P in runoff from soil test data.

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