Fieid-scale variation of soil phosphorus within small Alberta watersheds

Field-scale variation in soil-test phosphorus (STP) and the degree of soil phosphorus saturation (DPS) can affect predictions of phosphorus loss from agricultural soils. 14 Our objectives were to characterize STP and DPS and relate these to topographic characteristics within eight small watersheds in Alberta, Canada. An ungrazed grassland site (2 ha [5 ac]) and seven cultivated sites (22 to 248 ha [54 to 613 ac]) represented a range of tillage and manured conditions. Soils were sampled by landform position at three depths (1 point per I to 5 ha [2 to 12 ac]) in fall 2003. Levels of STP in 0 to 2.5 cm (0 to I in) were up to two times greater than in 0 to 15 cm (0 to 6 in) at non-manured sites. Within-field variation in STP and DPS was not well related to landform class or topographic index, although some variation was attributable to previous management. Current agronomic sampling strategies should give adequate representations of STP and DPS where management and soil properties are similar.