Soybean rotational benefits

Within the US corn belt, rotation of corn (Zea mays) and soybean is preferred to continuous cropping (Wilhelma and Wortmann, 2004) because the rotation produces greater grain yield of both crops (West et al., 1996). Input costs are often less; in particular, less nitrogen fertilizer is needed for the corn-soybean rotation (Katupitiya et al., 1997) compared with continuous corn. A corn-soybean rotation also reduces deep leaching of nitrate nitrogen relative to continuous corn. Reduced stress from pests and diseases may also improve yields in rotations. This chapter concentrates on the nitrogen-saving benefits. In the first instance, it has already been indicated that in Brazil, 80% of the direct nitrogen for soybean comes from nitrogen fixation, with a global input of around 58%, so there are direct benefits for the grower of the soybean crop (Herridge et al., 2008; Salvagiotti et al., 2008).

The magnitude of the benefit has also been indicated to increase via the use of supernodulating and nitrogen-sparing soybean varieties (Herridge et al., 2001). A crop calculator for the nitrogen benefit of soybean has been developed for Canadian situations (Przednowek et al., 2004), which suggested that soybean is considerably inferior to the other legumes used in the study (pea, chickpea and faba bean). It is possible that because of the large amounts of nitrogen removed via soybean seed (150-200 kg N ha-1), soybean may frequently act as an nitrogen sink rather than a nitrogen source (Herridge et al., 2008; Salvagiotti et al., 2008) and any nitrogen benefit is due to nitrogen sparing rather than nitrogen fixation. However, Ennin and Clegg (2001) found evidence for soybean nitrogen fertilizer replacement values in Nebraska of up to 46 kg N ha-1 in rotation with maize (although only at high plant populations). This indicates a potential for substantial nitrogen savings in soybean rotation systems. However, most rotation articles indicate that nitrogen benefit only accounts for part of the rotational advantage. The size of the rotational benefit can also vary significantly (Bundy et al., 1993) depending on soil type and rotation used. However, in all circumstances the large removal of nitrogen in the soybean crop means that the benefit is generally small.

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