Spatial distribution of soil phosphorus across selected New York dairy farm pastures and hay fields

Despite concerns that traditional soil sampling strategies are insufficient for assessing risk of surface water P contamination, there is no consensus that changes to these strategies, such as spatially explicit or discrete soil sampling within a field, represent enough of an improvement to justify the added cost. We conducted a study on four fields located on two dairy farms in Delaware County, New York, to characterize the spatial variability of P, Ca, Mg, Al, and Fe within two pastures and two hay fields at a 10-m scale and to interpret soil test P distribution relative to landscape and cultural practices. Pasture P distribution was characterized by various P "hot spots" that may be indicative of manure deposits by grazing animals. Hay fields contained large areas with elevated P relative to the rest of the field, with high-P areas occurring mostly near the gate and road where manure applications would be most accessible. Differences in soil properties associated with different soil map units also appear to partially explain P and Fe distributions in the mixed hay field. Results from a rainfall simulation study suggest that the use of a composite sampling strategy or an average P value for an entire field can potentially mask soil test P patterns such as those in the hay fields and result in an inaccurate estimation of P losses to surface waters. However, a composite sampling strategy may give a better estimation of P losses from pastures with "hot spots" because of the compositing effect whereby soluble P concentrations are affected by sorption processes that are controlled by low-P sediments.