University of Idaho nematologist Saad Hafez thinks Idaho's agricultural producers should consider including green-manure crops in their rotations.
The fall- or spring-planted crops-typically oilseed radishes or mustards-build and replenish soils when they're turned under after a few months' growth.
They also slash nematode populations by serving as nonhosts or poor hosts for the sugarbeet- and potato-damaging pests as well as by releasing biofumigating chemicals and activating natural enemies.
For the past eight years, Hafez
has studied crop rotations involving sugarbeets, potatoes, wheat, sweet corn, beans and oilseed-radish green manures.
Last fall, he
started a new study that includes onions and that's slated to continue for a dozen years.
Not only will Hafez
and other researchers measure the impacts of onions in these rotations, but they will examine the physical changes in soil produced by the green-manure crop.
"You get great benefits from green-manure crops, even if you don't have sugarbeet cyst nematodes or other nematodes," Hafez
For example, as organic matter builds, soil tilth and water-holding capacity increase and compaction and erosion decrease.
The organic matter slowly releases nitrogen into the soil, reducing the need for synthetic nitrogen without leaching excess fertilizer into > groundwater.
says more producers would use green-manure crops, but they haven't been able to fit them into their crop rotations.
"The stumbling block is the timing," he
"During the time that growers would be planting the green-manure crop, they are harvesting other crops or getting the ground ready for next year, so their labor and equipment are tied up."
Selecting the right green-manure crop is also critical: some will host diseases, weeds or even the very nematodes growers would like to control.
Others aren't as frost tolerant as they should be.
says new oilseed radishes not yet available in the marketplace should entice more Idaho potato and sugarbeet growers into giving green-manure crops a try within a few years: the varieties Defender and Comet cut populations of both sugarbeet cyst nematodes and potato-damaging Columbia root-knot nematodes by 95 and 99 percent, respectively, in his
"Their main advantage is that they reduce both of these nematodes," says Hafez
Previously, green-manure crops that discouraged one nematode encouraged the other.
In addition, Defender and Comet reach nematode-inhibiting growth stages in six to eight weeks-two weeks earlier than other varieties.
That will give growers more opportunities to squeeze them into their rotations after a fall-harvested crop or before a spring-planted crop.
The best older varieties of oilseed radish green-manures curbed populations of Columbia root-knot nematodes by 50 percent and sugarbeet cyst nematodes by 80-90 percent, Hafez
will evaluate the effectiveness of Defender and Comet in the field and hopes they will be available to Idaho growers by 2006.
In the 1998-2004 study, Hafez
and colleague Sundararaj Palanisamy determined that potatoes and beans suppress sugarbeet cyst nematodes more than wheat or sweetcorn do.
In the current and longer study, Hafez
will study not only the impacts of six crops but of fall-planted oilseed-radish green-manures included once, twice or three times over the course of 12 years.
"If one year is enough, we don't need two," he