Like tree fruit orchards, large commercial nut orchards tend to grow a single crop species, in rows, with alleys between them. The traditional orchard practice of herbicide strips within rows, and bare ground or perhaps a sod alley — often disced or chemically killed during the growing season, remains the dominant practice. Things, however, are starting to change.
In the Midwest, where corn and soybean monocultures cover much of the agricultural acreage, the Woody Perennial Polyculture Research Site ( www.wppresearch.org ) at the University of Illinois at Urbana-Champaign, is a large-scale commercial agriculture research site studying a savannah-based agronomy.
Researchers are focusing on growing chestnuts and hazelnuts, with a eye on changing the commodity agriculture system currently in place. They’d like to replace it with one that supports soil health, better meets nutritional needs and fits seamlessly into the existing food system without requiring modifications in food manufacturing or consumer buying habits.
“We can change our agricultural system and have the new, big commodity crops be chestnut and hazelnuts,” Kevin Wolz, Ph.D student at the Woody Perennial Polyculture Research Site, said. “Chestnuts are corn on trees,” and can be used in all of the applications in which corn is used, while hazelnuts have properties similar to soybeans.
The goal is to find the sweet spot where commercial nut production co-exists with biological diversity, so the impacts of large-scale production are both economically and ecologically beneficial. Research at the WPP includes plots where each nut is grown alone, as well as grown with other fruit-producing plants interplanted within the orchard rows.
Perennial nut crops are more aligned with the native savannah ecosystem than corn and soybeans. They are conducive to being grown in a diverse polyculture system and require less inputs and less soil disturbance. In the native system, nut trees are a part of the canopy layer, and co-exist with shrubs, vines, brambles and grass. Such a system benefits the soil ecology, promotes habitat for beneficial insects and pollinators, and can improve erosion and water retention.
“Having a diversity brings you better ecological functioning,” Wolz explained. “A polyculture can also be through time,” an approach which is being used at WPP to design better nut orchards.
Chestnut trees at the WPP are grown in orchard rows, planted 30 feet apart, with 30 feet alleyways, allowing for large-scale, mechanical harvesting. Trees are mechanically planted, and a chemical pre-emergent herbicide is used during the first two or three seasons, to prevent weed competition during establishment. Currants, which are shade tolerant, are established within the rows at the same time. The currants provide a high-value harvest during the initial years of nut establishment, but won’t interfere with mechanical harvesting when the nuts are mature.
This system provides the farmer with a secondary crop, protecting against crop failure and producing income during the nut orchard’s establishment. While there may be minimal decreases in nut production, the secondary crop’s value more than augments any production loss.
The “complementary root structure” of the two crops, when planted together initially, allows them to perform different but vital roles in the soil ecology. Because the roots mature together, they do not compete for resources, each “getting nutrients off a different section of soil,” Wolz said.
In the WPP system, alleys are kept covered. Maintaining a mowed grass or cover crop alleyway decreases runoff and enhances soil organic matter. It can also provide a hay crop or livestock pasture, Wolz said.
Transitioning Commercial Orchards
Some orchard growers might argue that “managing an alley crop is extra work,” Rex Dufour, Western Regional Office Director, NCAT/ATTRA, said. Dufour works primarily with walnut and almond growers in California. “The reason they keep alleys clean is that these trees are shaken by specialized equipment, the nuts fall off, and they are swept up into the windrow,” to be picked up mechanically, he said. Any detritus from the ground needs to be minimized to keep the harvest clean.
But a growing number of producers are switching from discing the alleys. Allowing vegetation to grow, at least during the winter months, provides erosion control, decreases sediment runoff, and prevents the formation of a hard, compacted layer which water can not penetrate, Dufour said.
Dufour recently saw the positive benefits of a vegetative alley while taking soil samples. The alleys had been maintained in mowed grass for a number of years, with clippings left on the ground. A core soil sample showed moisture throughout the 14-inch sample depth, along with a healthy soil structure and fertility.
“You can design your alley crop to provide nitrogen with a legume, or biomass with grasses, or beneficial insect habitat with crimson clover,” Dufour said. “Putting in organic matter, that’s a long-term benefit for soils.”
In contrast, a sample taken from within the orchard row, where herbicide strips were maintained, unearthed a compacted soil layer four inches below the surface, which the probe could not penetrate. The soil was “dry as a bone,” and lacking in any organic matter.
This compaction creates an anaerobic root environment, conducive to plant pathogens, as soil oxygen is depleted. Mulching branches from pruning, and leaving them on the rows, is one practice that can improve soil structure and functioning within the crop row, as it increases organic matter, Dufour said.
Many nut-growing regions are faced with water concerns. Using micro-irrigation techniques is one way of conserving water use, but it isn’t enough. With increased periods of drought, or rain deluges causing flooding, soil health is of increased importance.
“In an active soil ecology, there are a lot of microbes that are helping the plant access nutrients in the soil, and water,” Dufour said. “A lot of growers aren’t aware of the role of organic matter in their soils.”
Dufour also advises growers to increase biodiversity. Almonds are bee-pollinated, and rental of commercial hives is costly. While smaller in number, native, wild bees are more efficient and effective pollinators on a bee-per-bee basis, and they are free.
“Set aside a little bit of land to put in a perennial hedgerow,” which serves as a native bee refuge, Dufour advised.
Growing a single species of nuts in a monoculture system is inefficient, and conventional nut production methods are not focused on enhancing soil ecology or biodiversity. Alternative practices which increase soil health, address common concerns such as nutrient and sediment runoff and water retention, and also provide economic benefits to nut growers, are being put into practice across the country.