How to manage weeds and nitrogen in organic orchards
April 19, 2017
Fixed-arm rolling cultivators — like this Wonder Weeder — are the most common method of weed management in Northwest organic orchards. But other implements may be needed if trees in the row are closer than 4 feet apart. (TJ Mullinax/Good Fruit Grower)
Managing weeds and maintaining nitrogen in organic apple production is an expensive and labor-intensive task.
In short, it isn’t easy. Organic growers everywhere struggle to adequately control weeds while maintaining nitrogen levels and the high quality of their soils.
Given that Washington is the largest U.S. producer of organic apples, harvesting more than 90 percent of U.S. fresh production (according to 2014 U.S. Department of Agriculture data), Washington growers might have some insight into how best to do both.
Here’s a quick summary of the practices commonly employed by organic apple growers in Central Washington to control weeds and maintain nitrogen levels in soils.
Organic growers in other areas should keep in mind that they may have fewer options.
For example, chicken “compost” or manure may be less available where little poultry production occurs or where competition is stiff from higher value organic crops.
Another factor is that some older orchards with rocky soils, solid-set sprinklers or large, widely spaced trees may be less amenable for some of the tillage practices used by many Washington tree crop growers.
Nitrogen (N) and weed fertilization programs for most organic tree crop growers in Washington are often interconnected in that tillage for weed control incorporates the compost and fertilizer.
Many growers use the most cost-effective N strategy, currently consisting of “composted” chicken manure, applied in fall and/or spring, plus supplemental feather meal or other higher concentration N fertilizer, generally applied in spring.
The manure is composted enough to kill pathogens for food safety concerns, but not enough to immobilize all the N in organic matter.
Most finished compost has about 1.0 to 1.5 percent total N and the N is slowly mineralized in the soil over many years, so it is not considered a fertilizer unless used for many consecutive years.
But the chicken compost coming from in-state and out-of-state farms often has 3 to 4 percent total N, and plant-available nitrogen will be about 50 percent in the first year, compared to about 10 percent for finished compost.
Typical application rates are 2 to 3 tons per orchard acre, concentrated in a 10- to 20-inch band on either side of the tree row and usually not in the grass alley.
A 2-ton application of this chicken manure at 3.25 percent N and 25 percent moisture content may contain nearly 100 pounds of total N.
In the first year after application, it will likely provide about 50 pounds of plant-available nitrogen per ton applied, assuming it is incorporated the day it is applied.
Growers should obtain recent nutrient analyses of compost and manure products and should also know the percent moisture.
About a quarter of the total N in the chicken manure product may be present as ammonium, which is subject to rapid loss to the air (as ammonia gas) if not incorporated into the soil with tillage, or through irrigation.
Once in the soil, the ammonium quickly converts to nitrate, which is taken up by tree roots or can also be lost to leaching or runoff if over-irrigation occurs.
This available N is ready for tree use shortly after application, while the slower release N will be available during the rest of the year and in subsequent years.
With an organic N source, getting the timing right can be challenging. Trees assimilate N from the soil when they are actively growing (spring and summer) and not during the dormant season.
There is generally heavy demand in spring to supply N for the new vegetative growth, and in late summer or early fall to build tree N reserves for the next spring.
Past research has shown rapid uptake of available N a few weeks before apple harvest without affecting fruit quality, compared with little tree uptake later in the fall when the trees are senescing and heading into dormancy.
Fall applications can result in substantial loss to leaching over the winter as soil temperatures are still generally high enough for conversion of some N to the nitrate form, which is not held by the soil.
The most common method of weed management in organic orchards in the region is a Wonder Weeder, which is a Lilliston-type rolling cultivator.
This implement is fixed so it has no hydraulic arm retraction to move it back and forth into the tree row, as a Weed Badger or similar implement does.
But it does have a steel spring shear bar that cuts off weeds between the trunks at ground level and then bends back when it contacts the trunk to avoid damage.
This can work well in older trees with wider in-row spacing, but is not as well-suited to trees closer than 4 feet apart in the row.
Care must be taken with young trees as they can be damaged by the shear bar. Use in cherries is limited due to concerns about bark wounds that could lead to bacterial canker infection. The shear bar must be replaced periodically.
In the center of the tree row, a Weed Badger or other articulating cultivator is more effective for large weeds and perennial weeds. In-row cultivation has the added benefit of disrupting rodent tunnels.
Where poly tubing is used (either drip or inverted microsprinklers), the tubing must be suspended above the ground to prevent damaging it with cultivation. Suspending the tubing is easiest in trellised high-density orchards.
In-row tillage implements operate at about a 3- to 4-inch depth to kill weed crowns, but this also cuts some tree roots.
For this reason, a few growers use wood chip mulch (applied with a commercial mulch spreader) as an alternative to tillage, which provides a healthy environment for tree root growth and supplies nutrients as it slowly breaks down.
The wood chips, which need to be reapplied every one to three years to provide ongoing weed control, are effective against annual weeds and are generally not conducive to vole tunneling. Wood chips do not control most perennial weeds.
Some growers instead use landscape fabric, which provides more durable weed control but creates attractive vole habitat and cuts the soil off from any carbon inputs.
To address these concerns, the edges potentially could be covered; also the fabric can be opened during the winter to expose the voles to predation and to allow for compost or other amendments to be applied.
This is a costly process, but the fabric can provide more control of certain perennial weeds, such as quackgrass, than other methods. For both methods, cultivation adjacent to the mulch may be used to incorporate the compost and fertilizer.
There are several other weed control tactics that might be combined for more effective weed control.
Growers have renewed interest in the “mow and blow” method, in which the mower moves clippings to the tree row, suppressing weed seed growth and adding residue that breaks down to feed microbes and add nutrients.
If only a thin layer is periodically applied, the clippings will not provide cover for voles but also may not control weeds as effectively.
Propane flamers desiccate leaf cells of primarily broadleaf weeds, but flamers have worker safety concerns and high fuel costs. Organic herbicides that are currently available can help, but they are generally expensive and not reliably effective, especially on grasses. •
Do you know what’s in your compost?
Growers should be aware that manure and compost also add unwanted nutrients and harmful salts and nutrients.
Some growers are finding that phosphorus (P) becomes excessive — 3 tons of broiler litter with 25 percent moisture and containing 150 pounds of total nitrogen (N) per ton will also contain about 120 pounds of phosphorous pentoxide (P2O5, or over 50 pounds of actual P), and P not taken up by plants tends to accumulate in the soil.
Very high P levels in the soil can interfere with availability and uptake of other nutrients, especially iron and zinc, which could lead to plant deficiencies. Similarly, 3 wet tons of broiler litter also contains about 150 pounds of potassium oxide (K2O), or 125 pounds of actual K. Excess K can build up in the soil over time and could reduce calcium uptake to negatively impact fruit quality.
There are now more specialty fertilizers from which to choose that only apply the targeted nutrients, and some organic growers are using these and reducing or eliminating compost applications. But compost applications can be made every two to three years to maintain the “carbon” cycle and provide a food source to keep soils biologically active and diverse.
– by Chuck Ingels, a farm adviser for University of California Cooperative Extension in Sacramento County, California. He compiled this information with help from David Granatstein of Washington State University for a workshop on organic fertilization and weed management.