Unexpected Effects of Tropical Cover Crops

Abstract

Danilo Almeida standing in a plot with five-month-old ruzigrass. Ruzigrass straw covering the soil where soybean is growing. In tropical regions, agricultural soils are often highly weathered and have low phosphorus (P) availability. Farmers use a variety of management techniques to maintain nutrients in the soil and continue to grow crops, including no-till farming. No-till practices have been successful in reducing erosion, and when combined with cover crops, may enhance P cycling. However, not all cover crop varieties will grow in these tropical conditions. Danilo Almeida started working with cover crops as a master's student, under the guidance of ASA member Ciro Rosolem. Almeida says that entering graduate school he wanted to develop research to improve the sustainability of agriculture, focusing mainly on grain crops that “demand large areas and have a great impact on the environment.” His dissertation work in the Department of Crop Science at São Paulo State University in Brazil focuses, in part, on the impact of ruzigrass (Urochla ruziziensis) as a cover crop on soil P availability. Almeida says grasses of the Urochloa (syn. Brachiaria) genus are the most cultivated species in Brazil, mainly in pastures. Because these grasses are adapted to the climate and soil conditions, they have potential to be used as cover crops. Almeida and colleagues share some of the results of this research, conducted in Brazil from 2012–2016, in an article recently published in Agronomy Journal (http://bit.ly/2Gw6Zf7). Plots were established within a larger study site, which has been in no-till farming since 1998. Almeida describes the site as “a traditional agricultural region, with soils representing several other important areas in Brazil, and also other tropical countries, where the soils are typically poor in P, rich in iron and aluminum, and with low pH.” In the experiment, soybean was grown as a cash crop. The experiment included two off-season treatments, fallow or ruzigrass, and three levels of P fertilizer applied to the soybean crop. The authors wanted to evaluate the long-term effect of ruzigrass use as a cover crop and test the hypothesis that ruzigrass would increase soil P availability and therefore increase soybean yield. The researchers did find differences in the soil properties between the ruzigrass and fallow treatments. In general, soil organic matter was higher under ruzigrass compared with fallow. Additionally, P resin was higher under ruzigrass compared with fallow although slight differences were observed from year to year. The effect of treatments on soybean crops was not as expected. The authors report an increase in soybean foliar P with increased P fertilization. However, foliar P was lower in soybean crops that followed a ruzigrass cover crop. And contrary to the hypothesized effect, soybean yields were roughly 10% lower when following ruzigrass as a cover crop. There was no evidence of ruzigrass having allelopathic effects. According to the authors, ruzigrass decreases soil P availability. The low P concentration in ruzigrass residues may reduce the mineralization rate, compromising the P cycling and decreasing P bioavailability to soybean. Since P plays a key role in the biological nitrogen fixation, soybean nitrogen acquisition was also impaired. According to the authors, these results demonstrate that P resin is not a good indicator of what is available to soy. Although ruzigrass did not have a positive impact on the soybean crop in this experiment, it is still a useful crop in Brazil, according to Almeida. “Ruzigrass still is … able to produce great yields even during the off-season,” he says. “It's easy to grow, very adapted to poor soils, and even able to reduce soil pathogens, such as nematodes, and control other weeds.” Knowing ruzigrass is unlikely to increase soil P availability, Almeida suggests that researchers should look into the timing, rate, source, and placement of P fertilizer to maximize cash crop production. He says it's also important to note that crop rotation with only two species, as usually observed among farmers and as tested in this work (soybean and ruzigrass), is not a real crop rotation and that it's necessary to diversify the species in the cropping system. For Almeida, one of the important impacts of this work is sharing results that do not confirm their hypothesis. “Most people archive the non-expected results,” he says, and focus instead on publishing results that confirm hypotheses. With this project, the researchers are “discussing and spreading the word of non-expected results, which is gaining attention, and stimulating new research.” Check out the Agronomy Journal article, “Soil Phosphorus Bioavailability and Soybean Grain Yield Impaired by Ruzigrass” at: http://bit.ly/2Gw6Zf7.