The first time crops and orchards are planted in virgin soil, they grow beautifully. But the plants themselves change the biology of the soil, setting up subsequent plantings for severe stress.
It is a scenario that has plagued farmers since the beginning of modern agriculture. Soil problems have been addressed over the millennia by a variety of strategies — moving to new farmland, rotating crops, leaving land fallow, and heating soil under a plastic tarp in the sun. The most effective treatment by far has been soil fumigation.
However, for the past 20 years, with increasing regulations on soil fumigants and decreasing availability of those that are most effective, UC agricultural scientists have been working on hundreds of research projects to find new alternatives. Below are some promising alternatives now being tested.
• Superheated steam
Injecting superheated steam into the soil may be one way to replace chemical fumigants in agriculture. Steve Fennimore, UC Cooperative Extension specialist, said steam technology was already under study in California more than a century ago, but it was almost forgotten when highly effective chemical fumigation became the standard practice.
Working with a company in Kingsburg, Calif., Fennimore imported a machine that is used in Italy to prepare soil for greenhouse basil plantings. The machine, manufactured by Ferrari Constructione, has a 100-by-74-inch platform fitted with 99 10-inch spikes that inject steam into the ground.
“It’s surprising how fast it heats the soil,” Fennimore said. “Within two minutes, it will take 60-degree soil in the surface 8 inches and heat it to 200 degrees. It’s like a microwave.”
However, there are drawbacks associated with the steam machine. Treating the field is expensive. The machine must crawl 8 feet at a time up and down the field, each time pushing the spikes into the ground and leaving them positioned there for six minutes while the steam does its work. With one steam machine, it takes about 30 hours per acre.
“We tested a small machine that was easy to ship and most accessible,” Fennimore said. “It is not the final design by any means.”
Fennimore calculated that operating the steam machine, labor and fuel cost $3,848 per acre. Currently, chemical fumigation in California costs $2,700 to $3,000 per acre. Applying steam to raised beds rather than the entire field could cut steam cleaning expenses to about $3,000 per acre.
Another concern is air quality. The Italian machine runs on diesel fuel. In some parts of California, the operation of diesel engines is regulated to reduce air pollution.
“We’re concerned about the air quality issue and people point out all the time that we’re burning lots of carbon,” Fennimore said. “That’s true, but you can only solve so many problems at once. We’re trying to develop a practical system and we’re interested in switching to propane.”
Fennimore is encouraged by the success Italians have had with the steam system.
“The Italians have not been able to use (the fumigant) methyl bromide for a while now,” Fennimore said. “Yet, they have a number of the same industries that we have and they’re able to keep going.”
Using steam to treat land before replanting orchards and vineyards is another issue. The 10-inch depth of the steam injectors wouldn’t be sufficient for trees whose root zones reach 4 to 6 feet into the ground. As a gas, methyl bromide and other fumigants readily move through tiny gaps in soil and with proper application can treat deep down under the soil surface.
For orchard and vineyard replant purposes, Mike McKenry, UC Riverside nematologist, is studying steam application, not on the whole field, but in holes that give steam access to the full root zones of future trees. In his research, McKenry is using a steam generator powered by propane with an injector on a long hose.
“We will pull the steam generator on a tractor, there’s an auger on the front of it, we will dig down 4 feet and we will treat with steam prior to the planting of the tree,” McKenry said. “We believe it will provide a brief period of relief from soil pests and diseases.”
• Starve and switch
McKenry is also studying a fumigation alternative he named “starve and switch.”
While orchards grow, sometimes for decades, the chemicals excreted by the tree roots alter the flora and fauna in the soil. The resulting soil environment will severely hinder the growth of new trees when the orchard is replaced, a process McKenry calls “the rejection component.” The rejection component, Mc-Kenry said, inhibits the tree growth for at least a year after replanting the tree, after which, if no other problems are present, the trees may grow as if they had been planted in fumigated soil. But that means the farmer must wait longer to get a return from the investment.
Sanitizing the soil with a fumigant works remarkably well in correcting this problem. It takes care of nematodes, plant diseases, weed seeds, the rejection component and it even causes nitrogen in the soil to be made available, giving the tree a good start. But with fumigants on the way out, McKenry developed starve and switch.
“We kill the old roots with an appropriate systemic herbicide such as Roundup and fallow the field for a year so the microbes in the soil starve,” McKenry said. “Then we switch to a rootstock with tolerance to the rejection component.”
In a replicated trial of almond trees planted in spring 2009 — where some sections were treated before planting with the fumigant Telone, other sections subjected to starve and switch, and other sections planted with no special treatment — the trees’ canopies tell the story. One year after they were planted, the Telone trees are dense and tall, the starve and switch trees are nearly as tall, but with much less foliage, and the untreated trees decidedly smaller and less dense.
• New rootstocks
Replanting walnut orchards without fumigation has been notoriously difficult for farmers since there are no nematode-resistant rootstocks. But there is hope on the horizon. One tree from China — Juglans cathayensis No. 21 — has walnut breeders excited. It has natural resistance to root knot and root lesion nematodes. For the first time, a rootstock may be available in walnuts to try starve and switch, the process in which trees are killed with Garlon herbicide, the field left fallow for a year and then a tree on resistant rootstock is replanted.
McKenry has already tried grafting an English walnut to the rootstock and it takes the graft. However, at the graft union, there is a lack of compatibility, resulting in what is likely a weak connection that could break when trees are shaken for harvest.
“This rootstock will have to go back to Davis and be crossed with English walnut to bring improved graft affinity to the graft union,” McKenry said.