May 6, 2022
Assess summer cover crop benefits before organic vegetable in the fall
Dean Peterson, Organic Grower correspondent

Weed control and building soil nutrients are major challenges for organic growers, but a new study at Iowa State University (ISU) found the right choice of a summer cover crop can help address these issues before a fall vegetable.

The summer cover crop must be something that grows fast.

“We have an eight-week window in Iowa,” said Ajay Nair, associate professor in ISU’s Department of Horticulture and lead researcher in the study.

Sunn hemp, mung bean and cowpea were the legume cover crops in the study. The grasses included brown top millet, teff grass and sorghum sudangrass.

Two non-legume broadleaves were also part of the study. They were golden flax, which is not well studied as a cover crop in vegetable production, and buckwheat, which is more common.

ISU researchers evaluated the cover crops on weed suppression, and their effects on soil nutrient levels and vegetable crop yield and quality.

The summer cover crops were seeded in mid-June and terminated in mid-August with a flail mower. Cabbage seedlings were transplanted into the terminated cover crops and harvested in mid and late October.

Several cover crops suppressed weeds well.

“Buckwheat and sorghum sudangrass did really well, and there are some definite reasons why,” Nair said.

Buckwheat germinates quickly.

“You’ll see buckwheat seedlings as early as three to five days after seeding,” Nair said. “Sorghum sudangrass grows very quickly, produces a lot of biomass and is drought tolerant.”

In general, the more biomass produced the greater the weed suppression.

Teff grass suppressed weeds well and can also be used as a cover crop between plastic beds. It doesn’t need much incorporation, can be planted very shallow and can be broadcast seeded between the rows, said Moriah Bilenky, an ISU researcher in the study and now a postdoctoral researcher at Pennsylvania State University. “Teff grass between plastic beds is an option for small growers,” Bilenky said.

Sunn hemp suppressed weeds well, but not as well as most of the grasses. “It’s a legume but can also provide a lot of biomass,” Bilenky said. Sunn hemp establishes quickly and grows upright like sorghum sudangrass, which aids in weed suppression.

The legumes cowpea and mung bean produced less biomass than the grasses and allowed more weed growth. “It’s very difficult for cowpea and mung bean to compete with fast growing weeds,” Nair said.

However, soil tests taken in August at cover crop termination showed the legumes had contributed soil nitrogen – primarily soil nitrate. This is a big advantage when growing fall vegetables. The legumes in the study clearly produced a higher cabbage yield than the grasses although cabbage quality was acceptable under both systems.

This advantage to legumes relates to their nitrogen-fixing ability and low carbon to nitrogen ratio (C:N). The biomass of crops with a low C:N disintegrates faster, which returns its nutrients to the soil quicker. The biomass disintegrates faster because the soil microbes find it easier to look for nitrogen during decomposition.

Legumes like cowpea and mung bean haveaC:N of 10 to 15. AC:N of 10,for example, means there are 10 units of carbon for each unit of nitrogen.

The C:Ns of grasses are higher. Sorghum sudangrass can have a C:N of near 60. Sorghum sudangrass also has a much larger amount of biomass. This can mean more nitrogen immobilization where nitrogen is tied up during biomass decomposition.

However, in soil tests taken in November after cabbage harvest, the total nitrogen from grasses and legumes evened out – and evened out at soil nitrogen levels that were higher than in August.

There are reasons for the increase. By November, the decomposition of the grass biomass was catching up with the legumes and, overall, both the grass and legume biomasses had more time to decompose.

“We do see nitrogen levels go up, and we want to see them go up,” Nair said. This is part of the soil building benefit of adding biomass.

Mixes of grasses and legumes in a cover crop can be a way to balance the C:N.

“Grasses have a high C:N so add a legume,” Nair said. The rate of biomass decomposition also depends on conditions like soil moisture, soil microbial activity, and prevailing weather conditions.

Termination must always be discussed with cover crops.

“Teff is a great cover crop,” Nair said. “You can flail mow it and till, but it is clumpy – it has clumps of shoots – and we had to go over the plot with tillage twice.”

The second tillage was needed to limit clumps of teff from creating pockets under the plastic mulch and preventing good seed-soil contact.

Allelopathy is an issue with sorghum sudangrass. Growers should wait seven to 10 days after termination to plant or there could be some negative effects on direct-seeded, fall vegetables. Sorghum sudangrass must also be tilled well or its stalks can poke through the plastic.

Seed availability and price are always factors when selecting a cover crop. If a lot of compost has been added to a field and the amount of available nitrogen is not an issue, then sorghum sudangrass that adds lots of biomass may be a good fit.

“If my primary goal was weed suppression, I’d stick with sorghum sudangrass or buckwheat,” Nair said. If the goals are suppressing weeds and adding biomass, then sorghum sudangrass or teff grass are good options.

“Growers need to prioritize their goals,” Nair said.

ISU researchers didn’t plant an early spring vegetable before the summer cover crops, but that’s something to think about.

“After harvesting the spring vegetable by mid-June, can we squeeze in a summer cover crop before we start a fall vegetable?” Bilenky asked. “If soil conditions aren’t conducive for a spring vegetable or you’re trying to build biomass before starting to grow vegetables, then a summer cover crop alone is a better option.”

Photo at top: Ajay Nair, associate professor in ISU’s Department of Horticulture and lead researcher in the study. Photo by Iowa State University.

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