Rejuvenating worn-out soil naturally

WSU mustard green manure researcher Andy McGuire in field of mustard.Top of page, Chopping and disking mature mustard.

Twenty years ago, Dale Gies began searching for a green manure to improve the depleted soil on his 650-acre farm in Moses Lake, Wash., where he grows potatoes, wheat, hybrid vegetable seeds and grain corn.

He started adding mustard green manures to his wheat-potato rotation about 15 years ago. He usually rotates his potato crops every two years, sometimes every three, which normally would deplete the soil further. Instead, his soil is improving.

“While growing potatoes in close rotation, we’ve basically doubled the organic matter in the soil,” Gies says. The green manures have had another benefit as well: Gies hasn’t fumigated for fungal diseases on his potatoes for years.

In 1999, Gies contacted Andy McGuire, agricultural systems educator at Washing-ton State University Extension, about his experience. McGuire says, “We started soil testing that fall.”

They tested a number of varieties of mustard that had been selected in Italy, Australia and other countries specifically for use as green manures. The tests confirmed that they improve soil quality and reduce wind erosion, as well as control many soilborne fungal pathogens and nematodes in potatoes, McGuire says.

About 1,800 acres of mustard green manures were planted in Washington State in 1999, McGuire says. In 2007, that number rose to about 23,000 acres in the Columbia Basin alone.

Benefits to the soil

Green manures increase the amount of organic matter in the soil and improve the structure and quality of the soil fairly rapidly, Gies says.

“The more often farmers grow them, the better,” adds McGuire. “For years and years we’ve been saying that longer rotations are better for potatoes. It seems kind of counterintuitive, but the more often you grow green manures, the better off you’ll be. It’s better to rotate every two years than every four.”

In addition to resisting wind erosion, better-quality soil doesn’t stick to potatoes the way poor soil does, so they’re easier to dig and faster to harvest, McGuire says. Plus, you aren’t hauling away as much soil.

It also increases water infiltration, water distribution and the water-holding capacity of the soil, all of which allow water to be used more efficiently.

Gies uses some water on mustard, he says, but he uses less than before on his other crops. “It probably averages out. I can put more water on at one time, less often, because better penetration reduces runoff. Our water-holding capacity has increased by about a third. That’s huge in sandy soil.”

Another benefit of green manures is that some of the nutrients in them, especially nitrogen, are released into the soil after the residue is incorporated.

“You’ll get some of the nitrogen back,” McGuire says. “About 50 percent the first year.” Gies expects 80 percent of what they put into mustard to be returned.

Effects on soilborne diseases

The manures can be effective on diseases such as potato early dying disease, a complex that includes Verticillium wilt, Verticillium dahliae and root lesion nematodes, McGuire says. They can knock out about 80 percent of Columbia root-knot nematodes.

Drilling mustard seeds in stubble.

Gies says the manures allow farmers to use kinder and gentler pesticides. “I consider the manures as IPM. We can reduce our levels of chemicals. We can reduce our dependence on them, and we can reduce some very expensive ones.”

No one knows exactly how they work. “We’re thinking there’s more than one thing going on. Biofumigation is possible,” McGuire says.

Biofumigation is thought to work partly because the roots, leaves and stems in mustards and other brassicas produce chemicals called glucosinolates, McGuire says. When the green manure is incorporated into the soil, the glucosinolates break down into chemicals that are similar to the active chemical in the fungicide metam sodium.

Irrigating young mustard plants planted after seed corn.

Another possibility is that the green manures stimulate beneficial nematodes in the soil, which prey on the ones that damage potatoes.

“Other chemicals might also have an effect,” he says. McGuire is teaming with additional scientists to work on a second stage of mustard green manure research. One of the answers they’re hoping to find is exactly how they affect soilborne diseases.

Economic effects

It’s easy to put a number on the costs of using green manures, McGuire says. Farmers spend more for seed, fertilizer, machinery, labor, irrigation and herbicide. In Washington State, that added up to about $133 per acre in 2006.

Another cost is that where growing seasons are short, it may not be possible to plant a second crop the same year. Where it is possible, the choices for a second one are limited.

Some economic benefits can be measured. Farmers spend less on nitrogen fertilizer and pesticides. If they replace metam sodium with mustard green manures, they could potentially save $41 per acre, McGuire says.

There are other economic benefits that result from improved soil, such as growing more, higher-quality potatoes more efficiently, McGuire says. Unfortunately, though, it isn’t possible to measure them.

Planting

“We’ve done a lot of variety testing on seeds,” McGuire says. “For the most part, commercial dealers are selling the best of what we’ve found. Do your homework. Were the mustards developed for green manures? Less expensive ones may not be as effective. They may be lower in glucosinolates. I would definitely recommend having a large percentage of oriental mustard in whatever I planted, whether a blend or a single variety.”

Gies is producing his own seed and selling it through dealers. “We have five different blends of mustard. The blend variety to use depends on the situation,” he says.

Before adding fertilizer, test the soil to determine how much nitrogen is available. Depending on the planting date and season, between 100 and 140 pounds of nitrogen per acre over the season gives optimum growth, McGuire says.

Mustard tolerates saline soils as well as barley does. It’s sensitive to broadleaf herbicides, and may even be affected by herbicides used on previous crops.

Drill 8 to 10 pounds of seed per acre. If you’re seeding by air or broadcasting, use 10 to 15 pounds per acre.

“If you’re planting under a center pivot, you don’t have to plant very deeply at all,” McGuire says. An eighth to a quarter of an inch is enough. When you’re flying it on, the seeds need to be pushed into the soil afterward.

Irrigate immediately after planting and maintain adequate soil moisture throughout the growing season.

Timing

The optimum planting time depends on the area’s local soil and air temperature conditions, McGuire says. When the manures are used as fumigants, they need to be incorporated into the soil while they’re still green, but before the danger of a killing frost, which occurs at temperatures below the low 20s Fahrenheit.

“The chemicals are volatile. They get released rapidly and they disappear fairly rapidly. We want them to be released when they’re in the soil.”

The best way to determine when to plant is to decide when you want to incorporate them, then work back from there, he says. Soil temperatures have to be at least 60 degrees Fahrenheit for eight to 10 weeks for the manures to grow enough plant material. That takes about 75 days, or 60 days minimum.

In areas with long frost-free periods, they can be grown either before or after another crop.

Spring-planted green manure crops should be followed by a short-season crop like sweet corn or dry beans, McGuire says. Wait about two weeks after you’ve incorporated the manure before planting the next crop. Manures that haven’t decomposed enough can stimulate damping off disease and attract unwanted insects, he says.

Fall-planted mustard greens can follow an early-maturing crop like peas, barley or wheat. It may be necessary to use herbicides on the weeds.

In the Columbia Basin, the best time to plant is usually close to August 10, McGuire says. In this area, July plantings do not compete as well with summer weeds. The mustard is in full bloom by the end of September and reaches its maximum biomass in late October or early November.

This is the time to disk the plants and incorporate them into the soil. If soil erosion is a problem in your area, leave enough residue to keep it in place. Cross-pollination with seed crops of brassicas is usually not a problem because the mustard is blooming at the wrong time.

Conclusion

While the manures are effective on many soilborne diseases, most farmers are using them to improve their soils, McGuire says.

“I don’t think a lot of farmers would use green manures just for pest control. They’re using the manures to improve the soil and finding it reduces pests. Most farmers still fumigate. Many are using both metam sodium and green manures.”

The author is a freelance writer based in Altadena, Calif.