Mar 21, 2023

Cover cropping for orchard soil health and root development

Determining how tree fruit orchardists can manipulate soil health and use planting practices to achieve optimum outcomes from root systems remains a priority for researchers. Cover cropping for orchard soil health is central to this work.

Astrid Volder, professor of plant sciences in the College of Agricultural & Environmental Sciences at the University of California-Davis, shared research updates in December at the Great Lakes Fruit, Vegetable & Farm Market EXPO in Grand Rapids, Michigan.

Defining soil health

“Soil health refers to the ability of soil to perform key ecosystem functions, including sustaining plant growth, minimizing erosion, regulating water flow, and buffering and filtering of potentially toxic materials,” Volder said.

She explained that soils need oxygen for respiration. Their ability to provide oxygen and nutrients to roots is crucial to tree fruit development.

“Soil is very dynamic,” she said. “Oxygen availability is strongly affected by how much water you have in your soils. The temperature within the soil is an impact, too, including seasonal fluctuations in temperature.”

Those factors influence microbial activity and nutrient ability. Within an orchard, soils vary widely, though temperature and rainfall are more predictable.

Root systems in orchards

Perennial root systems consist of large scaffolding structures as well as very fine, dynamic parts. “Roots change a lot in response to changes in your environment,” Volder said. “Fluctuations in moisture and nutrient availability will strongly affect where you will find roots that are going to be growing and where they’re being produced.”

Pruning practices also matter. “The big thing when you are establishing and thinking about the long-term health of your orchard is you want a nice, long structural root system,” she said. Improper planting, such as pushing roots together or staking trees too tightly, can damage structural root development.

“You should actually allow space for the trees to move to get good structural root development,” she said. Heading or pruning trees at planting is common in California, but it changes carbon flow to the roots. Data showed more roots formed when trees were allowed to move rather than being tightly staked.

“This is one of the reasons you should stake your trees very loosely,” Volder said. “You need to have very wide roots to actually anchor your tree very well. Deep roots are very important because those are going to be the ones that supply your tree with water.”

Research on root depth and management

Volder noted that soil bores often miss deep roots, making it difficult to measure their true reach. Tissue-cultured trees sometimes show roots pooling over each other, revealing planting issues.

“So far, we’ve found that if you prune the trees, you’re going to reduce the root to shoot ratio,” she said. “Whatever you do above ground is going to affect what happens below ground, and also later on with the interactions with the soil. The soil health is actually going to affect how much carbon interacts with the soil microbes.”

Pruning reduced root mass and distribution more than irrigation changes. “In California, that could be a major problem because we actually need the roots going very deep to access more water resources,” she said.

Linking soil health, pruning, and cover cropping

Researchers continue to explore how orchard practices like pruning, irrigation and cover cropping for orchard soil health shape tree growth. Roots remain more shallow if pruning is frequent, while irrigation reductions had less effect.

“Those structural roots have to be in place and have to be spread out and around your orchard and explore a lot of soil in order to provide space for fine, absorptive, first-order roots that do most of the hard lifting for water and nutrient uptake,” Volder said.

She also noted that root age matters. “Roots that are just born have very high uptake of nitrogen. Once you get to 10 days, that is much reduced. You need to have flashes of continuous new root growth in order to get the benefits from them.”

— Gary Pullano, FGN senior correspondent