Should you be planting green manure?

Researchers see planting and terminating ‘green manure’ as an alternative to summerfallow


Researchers at Agriculture and Agri-Food Canada as well as the Saskatchewan Ministry of Agriculture have published two recent studies about the effects of green manure on farmland in the Prairies. In one study specifically, the researchers looked at using green manure as an alternative to summerfallowing.

Green manure “is a crop that is specifically planted in order to be terminated, it isn’t harvested for grain,” says Dr. Patrick Mooleki, a soil and nutrient management specialist with the Agriculture Knowledge Centre in Saskatchewan, and one of the studies’ authors.

Green manure is planted to add organic matter to the soil. “It can be terminated by spraying it, cultivating, or mowing it,” says Mooleki. It can be a legume or a cereal crop, but not one that you’ll harvest and sell. Growers planting a green manure crop know they won’t be selling crops for cash on those acres. For example, if lentils are grown as green manure, “it will be grown until it starts to flower, when it is in full flower, then you kill it. You terminate it halfway.”

Green manure can be a replacement for summerfallow. Summerfallow used to be a common practice to retain soil moisture. Farmers used to cultivate summerfallow acres to kill weeds. The cultivation also helped the organic matter break down and enhanced the nutrients in the soil. However, summerfallowing can be bad for the soil “because during summerfallow period, you are losing organic matter in the soil, as the rate of organic matter breakdown is increased with the tillage.” Those acres can become vulnerable to soil erosion.

Summerfallowing has decreased for the last two decades, but “we still want to have soil moisture recharge,” says Mooleki. Legume green manure crops add nitrogen as well as biomass to the soil. By terminating the crop midway, “when it hasn’t used as much moisture as a crop that goes to harvest,” you conserve soil moisture for the subsequent crop. Therefore, green manuring is an environmentally friendly way to conserve soil moisture and enrich the soil.

The study results

The study that Mooleki was involved with looked at how the kind of green manure used, and when it was seeded and terminated, as well as method of termination affected soil moisture, soil nutrients, and durum wheat yield and quality in the following growing season compared to other treatments such as summerfallow and stubble of harvested spring wheat, yellow peas and forage peas cut for silage. Yields for durum wheat after green manure were comparable to yields of durum wheat after summerfallow. However, Mooleki says, green manure is superior “because it doesn’t have the negative effects that you get with conventional summerfallow.” Grain yield of durum wheat on green manure was higher than that on stubble of grain crops or silage.

When should you try a green manure crop instead of summerfallow? “It depends on your cropping system,” says Mooleki. Organic farmers need to do this more frequently because they don’t have the option of using chemical fertilizer to add nutrients to the soil. Conventional farmers can use green manure if they are in an area where summerfallow is necessary once in a while, says Mooleki.

Since certain parts of the Prairies have moisture concerns, a green manure crop will help enrich the soil without stripping it of much-needed moisture and organic matter. A conventional farmer could plant a green manure crop as part of their rotation, says Mooleki, but it is up to the farmer to decide, as those acres will not produce any income that cycle. “You’re hoping that it is money well-spent come the following year when you harvest your subsequent crop.”

Something else to consider is what was planted on the field the year before and what will be planted after. “You have to consider your general crop rotation,” says Mooleki. For example, don’t want to plant a green manure crop of peas, for example, if you are planning to grow peas the following year.

The researchers found that it really didn’t matter when a green manure crop was seeded to see the positive effects on the soil and next year’s crop. If you are running late in the spring, you can seed the green manure crop later when you have time, says Mooleki. If you end up seeding your green manure crop late, you can even leave it to be killed (terminated) by frost; you’ll save time and money because you don’t have to go out and terminate it yourself.

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