Applying grass clippings to the tree row adds nitrogen at no cost.
The grass in the drive row of an orchard is a free resource that growers might be able to use to improve the sustainability of their operations, says Harold Huntley of Warren Morgan Orchards in Quincy, Washington.
In an effort to adopt more sustainable farming practices, Warren Morgan Orchards tries to evaluate practices objectively to determine if they are not just economically feasible, but economically justifiable, as well as environmentally sound, and socially equitable, Huntley explained during the Washington State Horticultural Association's annual meeting in Wenatchee. When looking at the environmental aspect, he considers if there's a better or more benign way to get the same result.
For example, to determine if composting is justifiable, Huntley looks at what the goals are and whether composting is the most environmentally sustainable way to achieve them.
On the positive side, compost adds organic matter to the soil, is a source of nutrients, increases microbial diversity, increases the cation exchange capacity of the soil, increases soil porosity, and reduces leaching of nitrogen.
On the negative side, hauling the material to the orchard increases the carbon footprint. It requires additional equipment, such as a spreader, and requires more passes through the orchard with the tractor, which increases the amount of diesel and number of man hours needed.
The cost of the material can be anywhere from fifty to hundreds of dollars per acre, depending on the material used and how much is applied.
Hauling and spreading of compost requires more diesel than commercial fertilizer because of its bulk and low nutrient content, Huntley said.
Compost is a more expensive source of nutrients than commercial fertilizer and is not totally benign, he concluded.
With this in mind, Warren Morgan Orchards decided to use composted mint, primarily as an organic-matter amendment rather than a source of nutrients, and supplement it with commercial fertilizer. Mint compost has a 2.5 percent nitrogen content and is produced locally, so it has a smaller carbon footprint than other types of compost hauled from Yakima or western Washington. Mint compost contains fewer salts or other contaminants than animal-based products, and there is less risk of food safety issues than with manure, Huntley said.
He also looked at whether mowing was sustainable. Warren Morgan Orchards was spending thousands of dollars applying compost in the fall, primarily to add organic matter, and in the spring was wasting all the organic matter in the drive row by mowing the grass. "It just didn't make sense economically, environmentally, or socially," Huntley said.
Since aesthetics are not among the goals of sustainability, it was difficult to justify mowing, Huntley said.
He had a mower modified so it discharges the grass clippings at the side, into the row. He likes the combination of compost and mulch from the grass clippings because the compost adds humus, which stores nutrients, and the decomposing organic matter feeds the soil microbes and aids in the release of nutrients.
Whereas orchard blocks would normally be mowed every 10 to 12 days, he has increased the interval to three or four weeks, saving several passes through the orchard. The mulch suppresses weeds, so fewer herbicide applications are needed.
In addition, the mulch helps conserve moisture, so less irrigation is needed, saving water and power. The ground cover in the drive row is healthier because it is not choked out by clippings.
The grass clippings add nitrogen to the tree row at no cost, Huntley noted. "You are not using any extra manpower or diesel to get it."
The mulch appears to promote biodiversity in the soil. He's seen an abundance of springtails, centipedes, earthworms, and molds, beneath the mulch which he didn't see with compost alone.
"There's something about keeping it moist with the mulch on top that was really good for biodiversity," he said.
Next year, Huntley plans to use compost tea to inoculate the mulch with microbes and increase the rate of decomposition. Though not sure if it will work, he said it costs very little to make the tea, so it's worth a try.
He could till the material in, but it would mean more tractor passes, more expense, and more carbon released into the atmosphere. "So we'll try the tea first," he said. "If it works, it is more sustainable than tilling."
In older blocks, he is renewing the ground cover. Blocks with a dandelion ground cover yield very little organic matter. He would like to use legumes to capture nitrogen from the atmosphere with the goal of being so efficient at recycling nitrogen in the orchard that they could stop hauling compost in from elsewhere.
"It may be a pipedream, but I like the concept. It's a worthy goal," he said. "Maybe it's time to take an objective look at ground covers and how we can manage them more sustainably."