The objectives of this study were to
independently evaluate the P sorption capacity equations for Manitoba soils and to
determine the DPS threshold limit for Manitoba soils. Also, the influence of manure
application on the prediction P sorption capacity was evaluated.
To achieve these objectives, 40 independent surface soil samples (0-15 cm of 20
manured and 20 non-manured soils) representing the major soils of Manitoba were
collected. The chemical and physical properties of the soils were determined by routine
laboratory analysis. The P sorption capacities of the soils were determined using both
the single and multi-point sorption methods. Portion of the soils were further incubated
with different rates of P to provide different degree of P saturation. Manure application generally reduced the total capacity of the soils to sorb P. Manure
application affected the predictability of P sorption capacity of the soils because of the
influence of manure application on soil properties that directly influence P sorption such
as Mehlich-3 extractable Ca and Mg. It was found that the influence of soil texture on P sorption was
more pronounced in non-manured soils than in manured soils.
The pH range of the soils is another important factor that influenced the prediction of P
sorption capacity of the soils. The predictive equation worked better for soils with neutral
to high soil pH than for soils in the acid range pH probably because of the neutral to
high alkaline pH of the soils used to formulate the original equations. From our results, the DPS threshold of Manitoba soils is 20, values of DPS greater than this will lead to unacceptable loss of P from the soil to adjacent environments.