Soil phosphorus spatial variability due to landform, tillage, and input management: A case study of small watersheds in southwestern Manitoba


Wilson, H.F., Satchithanantham, S., Moulin, A.P., Glenn, A.J. (2016). Soil phosphorus spatial variability due to landform, tillage, and input management: A case study of small watersheds in southwestern Manitoba. Geoderma, [online] 280 14-21.

Plain language summary

Understanding where phosphorus (P) accumulates in annually cropped fields on the Prairies is important for developing fertilization strategies for more efficiently using P fertilizer or manure and reducing accumulation that leads to losses with runoff. This research shows that, in Manitoba, P tends to build up at the surface of soils in areas of fields that are more likely to be wet or have more water flow through them. This pattern is most easily predicted in no tillage systems or low P input systems like organic production. These management systems also exhibited lower soil P than the other fields studied. For all types of management systems studied, an accumulation of P in the soils of low lying areas was observed. The potential to use variable rates of P application to reduce accumulation in low lying areas is shown, but the need to develop more management system specific recommendations is also evident.


Understanding spatial patterns of soil phosphorus is critical for efficient nutrient management and for the design of soil sampling in agricultural watersheds. This study describes patterns of soil test phosphorus (Olsen-P) concentration and variability for undulating/dissected agricultural landscapes in southwestern Manitoba under a range of management systems (no-input organic, organic with manure application, no-till, fertilized and tilled). Sampling occurred at 504 locations located in 16 small watersheds draining eight different fields. At a regional scale, most variance in Olsen-P for the 0-15 cm depth was associated with differences among fields (0.344 [log (mg kg-1)]2, 47%). However, significant variance (0.263 [log (mg kg-1)]2, 36%) was associated with differences among sampling plots within each field. Olsen-P for the 15-60 cm depth was lower and variance was more strongly associated with differences in field. Among-field differences in Olsen-P for 0-15 cm were strongly associated with management. Elevated Olsen-P was observed for fields that historically received application of fertilizer or manure in combination with tillage and lower Olsen-P was observed for low input organic and no-till fields. For six of eight fields sampled, a high proportion of variance in Olsen-P at the 0-15 cm depth was predicted by the Topographic Wetness Index (TWI) (r2 = 0.46-0.96, p < 0.01), with better predictive ability observed in low input and no-till fields. Despite differences in this relationship associated with management history, a significant positive correlation with TWI was observed among all sampling plots (r2 = 0.25, p < 0.0001, n = 56). The tendency toward P accumulation in low lying areas highlights the potential to reduce input rates in these locations for a variety of farm management systems practiced on the glacially formed landscapes of the Canadian prairies, but the need to develop management system specific recommendations for targeted application are also illustrated in this case study.

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