PHOSPHORUS LOSSES

Approximately 95% of the total P in runoff from clean tilled row-crop fields is transported with the eroded sediment (about 2 lb total P per ton of sediment or 0.1%).  In close-seeded crops, such as alfalfa and small grain, erosion is minimal and about 95% of the total P is dissolved with the water.  In general, total P losses are not large.  For some 30-plus surface irrigated fields, the median P loss was 4.9 lbs/A for an irrigation season while the soluble P loss was 3.5 lbs P/A (Table 3).

Total P concentration in surface irrigation runoff correlates with sediment concentration, but not soil test P concentrations (Table 4).  Runoff dissolved P (DRP) and biologically available P (FeO-P) concentrations, on the other hand, correlate with soil test P concentrations, but not with runoff sediment concentration.
 
 
 
 
 

Table 3.  Surface Irrigation Seasonal Losses

 
range
median
Soil Erosion, tons/A 
0.2 - 63.0
3.5
Soluble P, lbs/A
0.0 - 2.1
0.15
Total P, lbs/A
0.2 - 116.5
4.9
Data from Berg and Carter (1980)
 
 

Table 4.  Linear Correlation Coefficients Between Average Runoff P Concentrations and Furrow Soil P Tests

 
Furrow Soil P Tests
  
Average Runoff P
NaHCO3-P
FeO-P
Water-P
Average Runoff Sediment
DRP
0.69*
0.74*
0.74*
0.04
FeO-P
0.68*
0.77*
0.59*
0.01
Total-P
0.08
0.15
0.12
0.72*
*Significant at 5% probability
Data from Westermann et. al. (2001)

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