Winter is often a desolate time for farm fields. Often times they lay bare, the crops harvested and sold, the weather too cold to plant again. But researchers in China have finished conducting a long-term study at the Chinese Academy of Agricultural Sciences in Hunan Province that suggests farmers should cover their winter soil with green manure. Green manure is another way of saying cover crops, which are planted when cash crops do not occupy the soil. The need for research on cover crops has emerged from the issue of fertilizer use, which has boosted yields but has slowed the efficiency at which these crops are able to take up the nutrients. This is not only costly for farmers who end up wasting fertilizer, but previous studies have shown that it’s also harmful to the air and soil, causing soil acidification and degraded soil structure.

milkvetch

Milkvetch, the highest performer of the study.
Courtesy of NRCS Montana

From 1983 to 2016, researchers explored how incorporating green manure into rice rotations effect yield sustainability, soil nutrients and how they’re balanced, and enzyme activity. Yield sustainability was calculated using the sustainable yield index (SYI), which represents actual yields over long periods of time. The higher your SYI, the more likely it is that the area will produce high yields over a long time period. The four cropping treatments used were rice-rice-fallow, rice-rice-milkvetch, rice-rice-rapeseed, and rice-rice-ryegrass. Each treatment was replicated three times.

What these researchers found makes a pretty good case for green manure use. They found that green manures significantly increased yields and SYI. Milkvetch performed exceptionally well – trumping the fallow treatment in terms of grain yield by 45% in early harvested rice and 46% in late harvested rice. Soil pH was also found to be lowest in the milkvetch treatment. The green manures also resulted in more soil contents of carbon, nitrogen, and phosphorous- all essential nutrients for plants.

So what’s milkvetch’s secret? Well, most researchers think that it performed so well because it’s a legume. This plant family has the unique superpower of being able to fix nitrogen in the soil. This is really important because nitrogen is a nutrient that plants need to grow, but most nitrogen occurs in the atmosphere where plants can’t access it. Legumes have nodules on their roots called rhizobia that are able to biologically fix this atmospheric nitrogen into the soil where plant roots can access it. Hopefully this research will encourage farmers to cover their winter soil with green manure instead of letting it go naked.

Article:

Qaswar, M.; Huang, J.; Ahmed, W.; Li, D.; Liu, S.; Ali, S.; Liu, K.; Xu, Y.; Zhang, L.; Liu, L.; Gao, J.; Zhang, H. Long-Term Green Manure Rotations Improve Soil Biochemical Properties, Yield Sustainability and Nutrient Balances in Acidic Paddy Soil under a Rice-Based Cropping System. Agronomy 2019, 9, 780. https://www.mdpi.com/2073-4395/9/12/780