Managing the orchard floor

These Regina on Gisela 6 rootstock cherry trees, grown under a bark mulch, are part of an Oregon State University research project studying the nutrient management of organic cherry trees.

Bark mulch and landscape cloth are two options for organic orchard floor management, and each provides different benefits, research at Oregon State ­University suggests.

Dr. Jennifer Moore-Kucera, with OSU’s horticulture department, is focusing on how the orchard floor can be managed in organic cherry orchards to meet the trees’ demand for nutrients while building organic matter and reducing potential nitrogen loss through leaching, denitrification, and runoff.

"By managing the soil properly, we can help insure long-term productivity of orchards and improve the overall health of the orchard system," she told growers at the Great Lakes Expo in Michigan last December.

Organic matter in the soil helps improve soil structure and porosity, enhances nutrient cycling, and increases the release of nutrients as it decomposes.

Release of nitrogen that can be taken up by the trees depends on the carbon-to-nitrogen ratio of the organic matter. Grass mowings, organic fertilizers, and poultry manure, for example, have a relatively low carbon-to-nitrogen ratio of about 12:1. When those materials are incorporated into the tree row, nitrogen is rapidly released. Release of nitrogen is slower with barnyard manure, which has a carbon-to-nitrogen ratio of 20:1, although some nitrogen is available to the tree. When the ratio is too high, as with grain straw (80:1) or sawdust (400:1), the microorganisms need too much of the nitrogen and tie it up in their biomass, so it is not released into the soil for uptake by the tree. This is known as immobilization of nitrogen, Moore-Kucera said.

Nutrient goals

Different materials or mixes of materials can help the grower achieve different nutrient goals. In her research, Moore-Kucera is exploring how soil and tree fertility respond, both short- and long-term, to a landscape cloth in the tree row versus mulch.

In a trial at OSU’s Lewis Brown Research Farm in Corvallis, Regina cherries were planted on Gisela 6 rootstocks in the spring of 2005. The soil, a silty loam, had 4.4 percent organic matter, and a pH level of 6.3. Precipitation ­averages 41 inches annually.

Bark mulch was applied as the mulch treatment in the summer of 2005 and again the following summer. Bark has a relatively high carbon-to-nitrogen ratio, so it doesn’t break down very quickly and tends to immobilize the nitrogen. However, trees don’t need much nitrogen at establishment, Moore-Kucera said. In the fall of 2006, a municipal leaf compost was added.

No fertilizer was applied to either treatment during the first two seasons, but in August 2007, Nutri-Rich 4-3-3 fertilizer was applied to the cloth treatment.

The bark mulch was not adequate to suppress weeds, so a cultivator was used to control weeds. The cloth treatment, on the other hand, was a very effective weed barrier. Just a few weeds came up at the edges of the cloth and around the tree hole, but they were easy to remove, she said.

Soil analysis

Soil samples were taken in 2006 and 2007 to compare the macro- and micronutrient levels in the two treatments.

They showed an increase in organic matter in the mulched blocks, which Moore-Kucera said made sense, since four to six inches of mulch had been added.

The organic matter in the soil under the landscape cloth treatment was maintained at first because the ground was not cultivated, and cultivation stimulates decomposition of organic matter. However, organic matter under the cloth treatment sharply declined in the fall of 2007, which she believed was because the trees were depleting their resources.

Although Moore-Kucera expected to see the highest nitrogen levels in the mulch treatment, the analyses showed the opposite. Soil nitrate concentrations were higher under the cloth. However, there was no visible difference in tree vigor between the two treatments, and the mulched trees appeared to be taking up nitrogen in high enough concentrations, she said. Another test, where the amount of ammonium is released under lab conditions, showed that the mulched trees had greater amounts of ammonium. She explained that the rate that ammonium is converted to nitrate and taken up by roots is very fast, so the soil nitrate test may not be an accurate representation of longer-term nitrogen status.

Moore-Kucera said soil enzymes are important in the mineralization process and the cycling of nitrogen. For example, the enzyme N-acetyl-beta-glucosaminidase (NAGase) is involved in nitrogen cycling. It breaks down chitin, a component of insect and fungal cell walls, and releases organic nitrogen. This nitrogen can be taken up by trees or more likely is converted by other enzymes into ammonium or nitrate for tree uptake, she said.

NAGase activity was greater under the mulch than the cloth. She was optimistic that this might be an early indicator of the long-term nitrogen mineralization potential. Currently, a test to measure this is not available commercially but holds promise because it is relatively inexpensive, easy, and fast.

The mulch treatment had higher phosphorus, manganese, and boron in the soil, but less copper. Moore-Kucera said soil copper is not well correlated with leaf copper, so she was not concerned about that.

Leaf analysis

Leaf samples taken in August 2007 showed that leaf nitrogen, phosphorus, potassium, and calcium levels were higher in the mulch treatment, but leaf manganese, boron, and zinc levels were higher in the cloth treatment. All nutrients except manganese and zinc were within the recommended ranges for cherry trees.

In 2006, the trunk cross-sectional area was greater under the cloth than the mulch, but by the following year, the mulched trees had caught up, and there was no difference. Trees with the cloth treatment had more fruiting spurs on average, although there was wide variation.

When samples were taken in October 2007, soil moisture levels were slightly higher (6.8 percent) under the mulch, and soil temperature was 2°F higher under the cloth.

Moore-Kucera said since the trees are only in their third leaf, there has been no fruit to evaluate yet. In the future, she will conduct an economic analysis. "If it doesn’t pay to do these systems, we won’t be recommending them," she said.

The best type of ground management depends on the orchard and the location, as well as the grower’s goals, she concluded. "You need to establish what your goals are. Are you just interested in building organic matter reserves, and are you willing to take a little reduction in tree vigor or yield?"