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Impact of Long-term Phosphate Application and Level of Fertilizer Cd on Crops and Soils

Assess the rate of P application from various P sources having different inherent cadmium levels, on Cd uptake and Cd concentration of harvested grain from crops in western Canada.

IPNI-2002-CAN-MB16

Overview Related Article
19 Apr 2002

Justification

    2006 - 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.
    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. 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. 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.
    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. Durum wheat 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.
    This study should provide information on the impact of management on trace element accumulation, as well as 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.

    Note that this study was initiated in 2004, growing the preceding crops, with the first year of P fertilizer assessment conducted in 2005.

    Previous Years - Phosphate fertilizer contains varying amounts of Cd as a contaminant and there is concern that this Cd will accumulate in soils and enter the food chain. Monoammonium phosphate fertilizer has been shown to increase Cd concentration in crops in the year of application, regardless of Cd concentration. There are few longer-term studies evaluating the impact of level of Cd in phosphate fertilizers on Cd accumulation in soils and crops. A study conducted at a single location in Alberta showed long-term effects of Cd level with high rates of MAP, but the effect varied with lime application. Further information is needed to determine if the Cd added in phosphate fertilizers is available for crop uptake at a similar level in different soils, or if some soils convert the Cd to a form that is less available to plants. This will have important implications for both short- and long-term management of MAP and for regulation of Cd concentration in phosphate fertilizers.

    A number of soil testing methods are used for assessment of available soil Cd, with varying success. The standard in Europe is 1 M NH4NO3, which seems to be of little value on prairie soils. Study is needed to determine the effectiveness of soil testing methods on a range of soils and whether the tests predict the availability of both native Cd and Cd added in phosphate fertilizers.

    This research will help producers to maintain or expand our markets for durum wheat and flax, by reducing the Cd concentration of these crops. This is particularly important considering the increased concern of our customers for health and food quality.