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-P1Mehlich 3 or Olsen soil P tests. As STP increases, the potential for dissolvedbio-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?

Answer: Iron oxides is a common P adsorption reaction in soils of neutral to moderate acidicity.

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?

Answer: The potential of the soil to adsorb additional P decreases with increasing STP so that more of the applied P is in labile forms.

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.



Total P













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?

Answer: The advantage is that the 0-2? sample best reflects the risk of P loss. On-the-other-hand, the 0-8? sample is most appropriate for estimation of crop needs. Using the same sample for both purposes saves on sampling and analytical expenses. Many risk assessment tools are calibrated to attempt to account for P stratification effects. Other tools allow the user to specify sampling depth.