Soil health in Washington State

Q+A with Tianna DuPont. By Anna Mouton.

The United States is the second largest apple-producing country — total production in 2020 was 5.1 million tonnes. Seventy percent of these apples come from Washington State. Fresh Quarterly spoke to Tianna DuPont, tree fruit extension specialist with the Tree Fruit Research and Extension Center at Washington State University, to learn more about soil-health practices in the state.

Q What soil-health practices are growers using?

A Mulches are one of the soil-health practices that orchardists are using in Washington. Woodchip mulches in particular are becoming more and more common. That’s because we’ve got good research showing that it moderates both soil temperature and soil moisture. In six out of six studies published over the last ten years, the woodchip mulch increased tree growth compared to other soil management practices. So, it’s been one of our most consistent soil-health-related practices.

Another emerging practice is what we call mow-and-blow — mowing the alleyways and blowing those grass clippings into the tree row. That results in a really thin layer of mulch, which is advantageous for soil moisture retention as well as for carbon building. Out of three studies that have been done on mow-and-blow, two resulted in increased tree growth, so it’s still a newer practice with less data behind it.

Because mow-and-blow produces a very thin layer of mulch, it’s not very attractive to rodents, which is one of the big problems with mulches if they’re too thick. Some of the other materials which have been tested in the past resulted in pretty severe rodent issues. Voles will girdle the trees at the base, particularly with smaller apple trees on dwarfing rootstocks.

Q Where do growers source woodchip mulch?

A Many growers make their own woodchip mulch. For example, when they remove plantings in one orchard, they chip their old trees, and move that to a different site. There are also some folks who buy in woodchip mulch, but it’s quite expensive, so most of it comes from the orchards. But we do not have enough availability to cover all of the acres in Washington by any means.

Q Do growers make use of compost?

A Compost is certainly used. We don’t have great access to compost in Central Washington, and so there are restrictions on availability of compost as a carbon input. In some areas there’s more access to manure-based composts than in others. There’s one company that produces primarily manure-based compost, and then there’s poultry-litter compost that’s used as well. And one company in particular produces a non-manure-based compost, so it consists of all-plant material — yard waste, and waste from processing and from the field.

Q Are growers using cover crops?

A The standard practice in Washington is to plant a grass cover crop in the alleyway. Some folks that are focussing on mow-and-blow have chosen different grass species to grow more biomass. But oftentimes it’s the standard grass mixes that they’re using for mow-and-blow.

Some folks have experimented with growing alfalfa, and other cover crops, in the alleyway for mow-and-blow. There may be some concern here, because we do have issues with nematodes and leafhoppers with some of the forb [herbaceous perennials other than grasses] cover crops.

There are some research studies where they’ve grown a cover crop all the way into the tree row. However, in five out of eight of those studies there was reduced tree growth, due to competition from the cover crop right in the tree row. Interestingly, in a very long-term study — 16 years — they didn’t see reduced tree growth, because the root system had adapted and grown to a deeper layer.

In some organic pear and apple orchards, and in the old standard plantings on non-dwarfing rootstocks, having your cover crop all the way across the orchard can work quite well, and it helps to moderate vigour. However, most studies have been done on the dwarfing M.9-type rootstocks, and there we do see considerable competition when cover crops are grown in the tree row.

Q How do soil-health practices tie in with irrigation?

A We’re in Central Washington which is really quite dry, and an irrigated area. The average rainfall in Central Washington is 178–300 mm. So, soil moisture is really important, and that’s one of the reasons that these woodchip mulches and other practices are particularly interesting.

In conventional orchards, we generally keep the tree row bare for a 4–6 foot [1.2–1.8 m] strip, and then have grass in the work row. We have to irrigate the work row to have that grass strip. So, we generally use a combination of drip and microsprinklers, depending on the time of year, in order to maintain the trees and the grass. We try to conserve water as much as possible, and the combination allows you to manage the water to the trees more intensively while still keeping the larger surface area irrigated.

Q What can you tell our readers about your soil-health research?

A Dr Lee Kalcsits [tree fruit physiologist in the Department of Horticulture at Washington State University] and I are concluding a four-year study looking at soil-health indicators for Central Washington orchards. Orchardists in our area were really interested in what soil health is, and what it means for them, particularly how it relates to their bottom line — yields and pack-outs.

We measured 21 indicators of soil health in 101 orchards across Central Washington, and looked at biological, physical, and chemical properties of soil. We’re finding that soil-health indicators relating to root health and water regulation are particularly important in our orchards. That’s not surprising, because we have a history of replant disease and soil-borne pathogens being important in our orchards. We’re also in an irrigated climate where soil moisture is very important.

Another constraint that showed up in some of the orchards was compaction — having an area, usually 10–18 inches [25–45 cm] down, where the roots couldn’t penetrate. This is going to limit the roots’ ability to access that soil volume, as well as create an oversaturation effect sometimes.

Another indicator that looks to be important is available nitrogen — the nitrogen that’s mineralisable from the soil. In some of our soils there was quite a large pool of mineralisable nitrogen available. If folks are not measuring that slowly available nitrogen, they could end up overapplying nitrogen, which is obviously not only expensive, but can also cause a fruit-quality problem.

Q Is adoption of soil-health practices on the rise?

A Growers in Washington over the past ten years have had an increasing interest in soil health. Over the past five that I’ve been working in Washington, I’ve seen more and more acres adopting soil-health management practices, as well as growers working hard to learn more about their soil health. But there’s a lot of questions still to be answered, and also limitations in the availability and transferability of these practices.

Image supplied by Tianna DuPont.