Effect of Forage Legumes in Short-Term Rotation on Phosphorus Fertility of Four Saskatchewan Soils
ABSTRACT Legumes are becoming increasingly important in cropping systems due to their beneficial effects on soil nutrient availability. Including legumes into a cropping system not only positively affect many soil physical properties and increases soil nitrogen (N) supply, but is also reported to have a positive impact on soil phosphorus (P) availability. Although a series of studies have examined the effect of grain legumes in rotation on increasing soil N and P fertility after several years, the effect of forage legumes like alfalfa (Medicago sativa L.) and red clover (Trifolium pratense L.) on soil P availability, the yield and P uptake of subsequently grown crops remain unknown in Western Canada. To address this gap, a four year field experiment was conducted in four soil zones of Saskatchewan: Dark Brown soil zone (Saskatoon), thin Black soil zone (Lanigan), Brown soil zone (Swift Current), and Gray soil zone (Melfort). The objectives of this study were (i) to evaluate the impact of including a two-year period of forage legumes alfalfa (Medicago sativa L.) and red clover (Trifolium pratense L.) along with annual crops on soil P forms, amounts and availability and uptake of P by the crops in rotation and (ii) to evaluate the effect of the different crop rotations over the four years on soil P dynamics and P balance. After two years of forage legume and annual crop rotations, it was observed that alfalfa and red clover removed more P from the soil in the hay harvest as compared to the amount of P removed in grain in the barley (Hordeum vulgare L.) - pea (Pisum sativum L.) and barley (Hordeum vulgare L.) - flax (Linum usitatissimum L.) rotations at all four sites (P < 0.10). However, the four crop rotations did not significantly affect the amount of soil available P and P supply rate at all four sites (P > 0.10) despite greater P uptake and removal by forage legumes relative to annual crops in 2010 and 2011 growing seasons. Also, similar amounts of labile and stable P were measured in the different sequentially extracted chemical fractions after two years of different crop rotations at all sites (P > 0.10). Grain yields of wheat (Triticum aestivum L.) and canola (Brassica napus L.) crops grown following two years of forage legume and annual crop rotations were positively affected by forage legume rotations at Saskatoon, Lanigan and Melfort (P < 0.10). Wheat P uptake was improved significantly by two years of red clover at Lanigan, Swift Current and Melfort (P < 0.10) but canola P uptake was not affected by different crop rotations at all locations (P > 0.10). The amount of soil available P and P supply rate measured after wheat and canola harvest were not affected by different crop rotations at all sites (P > 0.10) despite the greater P removal by forage legumes during the first two years of the four year crop rotation period at all sites and the enhanced P removal by wheat and canola crops following forage legume rotations at Lanigan and Melfort. This suggests that forage legumes are able to maintain soil P fertility in the face of greater P removal by crops in rotation, at least in the short-term. Four years of continuous cropping with the minimum amount of fertilizer P addition resulted in a significant reduction of soil P fertility over time at all locations (P < 0.10). The lack of a significant effect of rotation treatment on available P levels in the soil does not rule out that there is an effect but variability prevented its detection. Further research is needed to evaluate the effects of several cycles of this rotation over a number of years on soil P availability and the P uptake of crops in rotation.
DegreeMaster of Science (M.Sc.)
Copyright DateSeptember 2014