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Manure Phosphorus and Surface Water Protection II: Field and Management Factors

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Soil Test P

Soil test P (STP) is a measure to indicate the availability of solution and labile soil P to a crop. STP is determined for agronomic purposes using laboratory procedures, such as the Bray-P1, Mehlich 3 or Olsen soil P tests. As STP increases, the potential for dissolved, bio-available, and sediment bound P in runoff increases. Alternative 'environmental' soil P tests are sometimes suggested for use in assessing the potential for P loss in runoff.  The most common environmental soil P tests either measure water-dissolved P or water-dissolved P, plus P easily detached from sediment.  A common test for bio-available P determines the amount of P extracted from a soil sample after shaking it with a strip of filter paper impregnated with iron oxide.  The amount of P that reacts with the iron oxide bonded to the filter strip is then determined. The results of these agronomic and environmental tests for soil P are generally well related, and agronomic soil tests are commonly used in assessing the potential for P loss in runoff.
 

Discussion Question:
Why is an iron-oxide-impregnated filter strip a good test of soil P availability?


 

We need to consider stable soil P, in addition to soil test P, when assessing risk of P loss to surface waters.  In Lesson I, we learned  that stable soil P accounts for most soil P.  While total soil P, including the stable fraction, increases with increases in STP, total soil P can be substantial, even when STP is low.  In Figure 2, for example, we see that there is much total P, even when Bray-P1 is low, but that total P increases as Bray-P1 increases.

Figure 2.  Total soil P increases as soil test P increases, but there is much total soil P, even with a low STP.  (Graph by Charles Wortmann)

 

Discussion Question:
Considering Figure 2, why does the ratio of STP to total P increase as STP increases?



Can much P runoff loss occur when STP is low?  Sediment bound P losses due to erosion can be substantial, even at agronomically low STP levels. Because of this, the increase in P concentration is relatively less than the increase in STP.  Results from a runoff trial conducted over several years in eastern Nebraska indicated that Bray-P1 needs to increase by 200% and 500% to cause a 100% increase in the runoff concentration of dissolved P and total P, respectively.  On the other hand, a 100% increase of runoff volume or erosion loss resulted in approximately 100% increase in P fractions lost.  Total P loss can be substantial, even when STP is low.

Should soil samples be taken from the 0- to 2-inch depth (0-5 cm) only when testing soil for assessment of the potential for P loss? When testing soil to determine STP, the depth of sampling is important, as P concentration is generally greater in the two inches of surface soil than it is in deeper soil. Usually, there is little downward movement of P through the soil.  With surface application of P as fertilizer or manure and reduced or no-tillage systems, soil P becomes stratified with relatively high P concentrations in the surface two inches (5 cm) of soil (Table 2). This is also the soil that mixes with runoff water and contains the soil P that is most exposed to runoff risk.  In Table 2, we see that a sample collected for the 0- to 2-inch depth (0-5 cm) gives an STP result that is more than 2.5 times as high as the level for the 0- to 8-inch depth (0-20 cm).  Most states accept agronomic soil sampling, e.g., 0-8 inches (0-20 cm), for assessment of P runoff risk.  However, sampling of just the surface two inches (5 cm) may give a better indication of the potential for P loss.
 

                             Depth

Bray-P1
Total P
0-2'
380
1288
2-4'
154
754
4-8'
37
506
0-8'
143
747
Table 2.  Stratification of Bray-P1 in a soil where manure was regularly applied and incorporated with a disk.  Bray-P1 in the soil which is most exposed to runoff and erosion is much higher than indicated by a 0-8 inch (0-20 cm) sample.


Discussion Question:
What are advantages and disadvantages to using a 0-2' soil sample for assessing the potential for P runoff loss?



 

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