Straw mulch in tree fruit orchard rows may have potential to increase soil organic matter while conserving water and suppressing weeds, a soil scientist says.
Oregon State University researchers have experimented with straw mulch for several years, but last year, a trial to compare different ground-cover treatments and no cover was initiated by Dr. Xinhua (Frank) Yin, a soil scientist at OSU’s Mid-Columbia Agriculture Research and Extension Center in Hood River.
“Straw is very good for soil quality,” said Yin, noting that many of the Oregon soils in the Hood River and The Dalles growing region are low in organic matter but high in volcanic ash.
Yin’s research, which will continue for several more years, compares ground covers of straw and black or white synthetic fabric cloth with no ground cover to learn if ground covers influence fruit quality, storability, and yield of sweet cherries. He is also monitoring differences in soil fertility, soil quality, microbial biology, and plant nutrition. Additionally, Yin is comparing water efficiency between drip and microsprinkler irrigation systems and the effects of nitrogen use efficiency between broadcast application and fertigation methods.
In previous tests of the eight-feet-wide cloth covers in cherry orchards, Dr. Roberto Núñez-Elisea, horticulturist with Oregon State University at Hood River, observed warmer soil temperatures in the winter and cooler temperatures in the summer, larger trunk cross-sectional areas in trees, and higher yields in the plots with the covers.
In the first year of Yin’s study, equipment similar to a bale chopper was used to lay straw down the cherry orchard row five to six inches deep. The straw mulch costs $250 per acre for the material, not including application costs. Special hay chopping equipment was used to spread the straw down the row.
Yin hopes that the straw will last in the orchard for at least three years. Research will monitor how fast the straw is compacted by normal orchard tasks and equipment, and how fast it breaks down.
Though results are preliminary, Yin found a strong trend of higher yields in the trees with straw mulch and fabric covers, particularly with the white fabric cover. The average amount of fruit in the no-cover treatment was 43.9 pounds per tree compared with 55.3 pounds per tree in the white fabric cover. However, he said that the yield results were not statistically significant.
He found no difference in fruit quality—sugar content, firmness, and fruit size—between the ground covers or irrigation system, but saw a slight improvement in the percentage of marketable fruit in the trees with fabric covers. More cherry pitting was found in the microsprinkler treatments than under drip irrigation. No benefits were found with straw mulch or fabric covers in reducing fruit pitting relative to no cover.
“Differences in soil available nutrients, pH, or organic matter were primarily negligible between drip irrigation and micro sprinkler or among no cover, straw mulch, and black and white cover,” he stated.
However, in a postharvest leaf analysis, drip irrigation had slightly lower concentrations of nitrogen, phosphorus, potassium, calcium, and boron than the micro sprinkler.
He believes the uptake of these nutrients by tree roots may be slightly reduced due to the switch from microsprinkler to drip irrigation in the first year.
But unlike the irrigation systems, the three ground cover treatments and control had similar leaf nutrient concentrations except for nitrogen, he added. “The differences of nutrient concentrations in fruit were rarely significant between the two irrigation systems or among the four ground cover treatments.”
In the trial at the Mid-Columbia experiment station, leaf nitrogen content was 19 percent greater in the covered plots than in to the no-cover plots, a result consistent with previous studies. Scientists believe that because trees in the covered plots are larger than in the no-cover treatments, the total nitrogen uptake by roots is greatly enhanced.
Yin found that after applying fish meal or blood meal on top of the fabric covers the concentrations of soil available nutrients were similar to when the same fertilizers were applied below the covers. Leaf nutrient levels and leaf chlorophyll content were also similar between the two placement treatments. Above ground application my save costs.
“But we found out one thing,” Yin said. “If it was windy when the fertilizers were broadcast on the fabric covers, the wind would blow the fertilizer into small piles.”