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ducted by the Potash & Phosphate Institute demonstrated, at a state and province scale, the varying phosphorus (P) needs across North America (Figure 1). Phosphorus levels also vary among and within fields, making more intensive soil sampling necessary in many situations. Figure 1. Percent of soil samples testing at or below “medium ” in P for states and provinces in North America (Fixen, 2002). Does a field area have enough P already or is more needed? The only way to tell is to take a good soil sample and have it tested by a reputable laboratory. The Extension Service in each province or state, as well as many laborato-ries, provide guidance on how to take and submit a soil sample and interpret lab results. At lower soil test levels, crops generally yield at a reduced percentage of what is possible when levels are higher. Figure 2 shows an example of how relative yield of wheat (percent of yield attainable when P is sufficient) increases with higher soil test levels. An important part of this figure is the variability in relative wheat yield response to soil P. While a relative yield of 100 % was first recorded at a soil test P of 15 parts per million (ppm), 90 % of the trial sites still responded to fertilizer P at this level. The influence of environment on crop response to soil