Impact of Cropping Sequence and Tillage System on Response to P Fertilization in Durum Wheat and Soybean

A study done to improve our ability to predict when P starter fertilizer is required by crops and identify management practices and fertilizer sources that may improve early season P availability to crops.


21 Mar 2006


One problem with P fertilization in prairie soils is that the applied P converts to relatively unavailable forms over time. Reducing the rate of this conversion could improve P use efficiency. Studies on highly calcareous soils in Australia indicate that ammonium polyphosphate slows the conversion and improves crop yield and P uptake. New coated phosphate compounds have been developed to slowly release the P into the solution at a rate matched to crop demand, potentially allowing the plant to utilize the P before it converts to unavailable forms. This would both reduce the risk of off-site P movement and improve the efficiency of use.

Crop sequence and tillage system can influence nutrient availability and response of crops to fertilizer applications, through effects on nutrient cycling, microbial population, soil temperature and root growth. Therefore, optimum P fertilizer management may change with management practices as well as with crop type.

Management may also influence trace element availability by release of trace elements from decaying crop residues and by impacts on soil microbiological dynamics. In studies recently completed at AAFC Brandon Research Centre, Cd concentration in flaxseed was approximately 25% higher when flax was grown after canola rather than wheat. Canola residue contains high concentration of Cd. In addition, canola is a non-mycorrhizal crop and flax grown after canola had lower mycorrhizal colonization than flax grown after wheat. Mycorrhizal associations can enhance P uptake by crops and reduce reliance on applications of P fertilizer. It has been suggested that mycorrhizal association may increase Zn uptake and reduce Cd uptake by crops. The increase in Cd concentration in flax growing after canola therefore may have been due to mobilization of Cd from high-Cd canola residues, reduced mycorrhizal activity in flax grown on canola rather than wheat, or other effects of the canola residue or organic acid secretions on Cd phytoavailability.

The study evaluates the effects of cropping sequence and tillage practices on yield and quality response of durum wheat and soybean to different forms and placements of P fertilizer, including two new slow release products. These two crops should differ in their P requirements and in their pattern of response to P fertilizer. Durum wheat is representative of cereal crops grown widely in the northern Great Plains region. It has been identified as an accumulator of Cd and efforts are being made to reduce its Cd concentration by breeding and by agronomic methods. Soybean is frequently used for human consumption and can also accumulate relatively high levels of Cd. Soybean is grown widely in the northern Great Plains, but is at the edge of its adaptive range in Manitoba. Therefore, early season P stress would be maximized for soybean in the Manitoba environment and a response to starter P may be more likely in Manitoba than in more southerly locations.

This study should improve our ability to predict when P starter fertilizer is required by crops and identify management practices and fertilizer sources that may improve early season P availability to crops. It should also provide evidence on the comparative efficiency of a range of P sources, as affected by preceding crop management. In addition, it should provide information on the impact of management on trace element accumulation, both in terms of potentially toxic elements such as Cd and potentially beneficial elements such as Zn, Fe and Cu.