4.3. Soil P availability
In calcareous soils, Olsen-P concentration is a good indicator for estimating soil P availability (Wang et al., 2010). In this study, Olsen-P was referred to as ‘available P’ to emphasize its availability. In the 2009 growing season with less rainfall, the balance of available P was positive, but negative in 2010 with more rainfall. This phenomenon was also observed in our previous study conducted on alfalfa (Jia et al., 2006). This can be explained on the basis that high crop biomass requires more soil available P in wet years than in dry years (Jia et al., 2006), which may decrease actual soil available P concentration in the wheat growing period.