Although organic nitrogen (N) fertilizers are commercially available, their high cost prohibits their use in many organic crop production systems. As a result, organic farmers rely primarily on manures, composts, and/or leguminous crops to meet the crop N requirements. In an organic soybean 鈥� winter wheat 鈥� corn rotation, livestock manure and/or compost are the common choice to maintain economic crop yields, particularly for corn; however, manure or compost application based on nitrogen needs of corn normally result in soil accumulation of phosphorus and other cations.
After winter wheat harvest in Ontario, the ground is frequently left bare from August until May. Leguminous crops planted after winter wheat and incorporated into the soil as a green manure before corn planting can be an attractive alternative especially when manure and compost are not available or an environmental concern. Although there are many benefits (in agronomy, economy , and environment) to using cover crops, they can be difficult to implement in crop rotations with long-season crops such as corn and soybean as there are not sufficient growing degree days left in the fall for successful establishment when the cover crop is planted after harvest. In some situations, cover crops may not provide enough land cover and sufficient N for the succeeding crop if there is insufficient legume cover crop growth and N fixation. The N contribution of a legume cover crop will certainly reduce fertilizer N inputs to the next crop in the rotation; however, the best cover crop management options to maximize N for a following crop and to maintain or enhance soil health remain largely unknown.
Frost under-seeding red clover in a winter wheat field could produce nitrogen and soil quality benefits for succeeding crops, and nitrogen credit values have been previously established for fall terminated red clover in corn-soy-wheat rotations in Ontario. However, the use of this practice typically declines with time in Ontario: it is difficult to maintain adequate red clover stands with conditions such as an insufficient freeze-thaw cycle, or a dry spring. Other cover crop options, such as cereal cover crop or radish, are available to Ontario growers, but they generally do not provide enough N or release N at the right time.
In a previous study, we found that winter-hardy leguminous crops seeded into wheat stubble (after wheat harvest) provided sufficient N to maintain corn grain yields in 3 consecutive years, with corn grain yield of 190 bu/ac in the leguminous cover crop plots (no chemical fertilizer) versus 92 bu/ac in the control (no chemical fertilizer and no cover crop). In this corn-soybean-wheat rotation the soybean yields were stable and good (60 bu/ac); however, wheat production posed challenges (yield declined to 50 bu/ac in the third year) due to an apparent N deficiency. In addition to planting leguminous cover crops after winter wheat harvest as an N source for corn, we further hypothesize that introducing a second leguminous cover crop phase in the corn year of the rotation (i.e. under-seeding legumes into standing corn at the 6-8 leaf stage) may provide a way to improve soil fertility/health and wheat yields by supplying additional C and N to the soil.
Accordingly, the overall goals of this study are (i) to develop a cropping system which will include two seasons of winter-hardy leguminous cover crops using the nitrogen fixed by leguminous cover crops as sole nitrogen source for corn and wheat in an organic soybean-winter wheat-corn rotation in southern Ontario and (ii) to test whether this cropping system could be agronomical viable and soil health (fertility) sustainable. The proposed rotation would provide growing crops in both the summer growing period as well as in the late fall and early spring periods (i.e. winter wheat would be the winter cover in one of the years and the winter-hardy legume cover crops would be the winter cover in the other two years). Farmland under the proposed system would have year-round cropping.
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