Organic Orchard Management:  Nutrition for Organic Orchards

As specified under the National Organic Program (NOP), “Organic producers must rely upon animal manures, compost (organic matter of animal and/or plant origin that has been decomposed by microorganisms), and cover crops to supply some, if not all, of the required nutrients for healthy crops.”  Under NOP regulations, many non-synthetic fertilizers like fish emulsion, poultry manure and blood meal are permitted, but in many cases supplemental fertilizers are allowable ONLY after documenting a deficiency.  Both soil and leaf samples can be used for documentation and growers will need to work with their accredited certifying agent to develop an acceptable nutrient program that prevents rather than corrects nutrient deficiencies.

When relying on organic fertility, growers should be aware of the differences between synthetic and organic fertilizers as well as organic soil amendments.  Following are several nutrient amendment options available to increase the nutrient health of organic orchards, starting with naturally derived soil amendments, namely compost and cover crops.  These soil amendments differ from both synthetic and organic fertilizers in the benefits they provide to soil health and the availability of the nutrients they contain.  While nutrients supplied to trees is limited, these soil amendments also increase soil organic matter, balance pH levels, increase microbial activity, improve soil structure and tilth, improve drainage in clayey soils, improve water-holding capacity in sandy or gravelly soils, and help to suppress some root diseases.  Organic fertilizers while often lower in nutrient content by volume then synthetic fertilizers contain additional naturally occurring nutrients including micronutrients.  They also are higher in available nutrients than composts, but do not provide the previously mentioned added benefits of organic residuals. 

Several additional regional resources exist for exploring enhancing soil fertility in organically managed orchards at the following links:
USU Fact Sheet Strategies for Managing Soil Fertility and Health in Organic Orchards (July 2018)
Washingon State University Organic Tree Fruit Soils & Nutrition
WSU Tree Fruit Soils & Nutrition

Compost

Compost can be applied pre-plant to increase soil organic matter, but also post-plant as a mulch to  increase soil water holding capacity and nutrient cycling.  This can be particularly helpful in sandy or coarse soils.  

Compost use in organic systems is regulated by the NOP to insure proper application and use of organic residuals to reduce risks to food safety.  NOP regulation §205.203 specifies that: “Compost must be produced through a process that combines plant and animal materials with an initial C:N ratio of between 25:1 and 40:1.  Producers using an in-vessel or static aerated pile system must maintain the composting materials at a temperature between 131°F and 170°F for at least 3 days.  Producers using a windrow system must maintain the composting materials at similar temperature for 15 days, during which time, the materials must be turned a minimum of five times.” Animal manures may also be used in organic orchards, but they must be incorporated into the soil at least 90 days prior to harvest.

Most commercial compost suppliers will provide a nutrient content analysis report to help calculate application rates for their composts as formulated.  Growers should also verify that the compost supplier is following all of the current NOP regulations, and that the accredited certifying agency will approve that compost for organic production.  Biosolids are not approved for use in organic production (7CFR§205.105).

In addition to following these guidelines, growers should obtain composts or feedstocks for compost from trusted sources to insure they do not apply an amendment that has been contaminated with residual herbicides such as aminopyralid, clopyralid, aminocyclopyralid or picloram as the presence of these substances can compromise organic certification.  Applying compost contaminated with these herbicides or any other substance prohibited by 7CFR§205.105 will void certification for 36 months after application.  If in doubt, growers can have the compost tested.  If the compost is free of prohibited substances a record of testing should be kept on file.  

In the case of residual herbicides which are expensive to test for, growers can perform a bioassay by planting a fast growing, herbicide sensitive annual like peas in soil amended with the compost in question and again in potting soil (control).  Plants grown in contaminated soil will emerge with cupped, curled or fringed leaves and appear stunted compared to the control.  This compost should not be used.  Residual herbicides can remain in soil and compost for years, especially in the Intermountain West’s dry cold climate.

For more information on using compost in orchards including benefits, testing and calculating application rates visit the WSU web page, Tree Fruit Compost Considerations.

Cover Crops

Cover crops in orchards can be used both as a pre-plant tool in orchards needing renovation from previous uses or planted alongside trees as a tool to encourage beneficial insects, improve site aesthetics and provide long-term inputs of nutrients and organic matter to the orchard system.  Pre-plant cover cropping can improve soil conditions, provide nutrients and organic matter. They can also be used alongside tilling to reduce weed competition before establishment.  Nitrogen-fixing legume cover crops are often seeded along with one or more species of annual grass. Nutrient availability will depend upon the growing conditions, species used, seeding rates, and prior soil nutrient status.  Typically, the cover crops are mowed/chopped and then incorporated into the soil prior to tree planting.  When using as a pre-plant tool, care should be given to terminate cover crops, typically at flowering, to avoid reseeding of the crop, which can become a weed.  

Organic growers can also maintain permanent covers in rows and/or alleys of bearing orchards to suppress weeds and protect soil.  In choosing which system and plant species to use growers must weigh the tradeoffs between competition and supply of nutrients, cover water use, the potential for increased pest habitat vs beneficial habitat and management of the cover (Rowley, et al).  See Chapter 8 for more details about orchard floor and weed management.

Fertilizers

Any material, including fertilizers, that is used in a certified organic system must be approved under NOP regulations by your certifier. Growers must also demonstrate deficiency before applying foliar fertilizers.  If deficiencies are evident based on visual symptoms or foliar test results, growers should consider other factors contributing to nutrient uptake including water and irrigation management, soil compaction and health.  

Many companies now make custom organic fertilizer blends.  These products tend to be more expensive than purchasing the materials in bulk and blending them on-site prior to application.  There are also numerous companies making liquid nutrients for foliar applications.  These can be useful for correcting deficiencies, making maintenance applications for return bloom (e.g., boron and zinc) and improving fruit quality (e.g., calcium).

Applying nutrients to leaves in a spray solution can provide the plant with nutrients such as calcium and zinc that are taken up poorly by the root system, as well as to help correct immediate nutrient deficiencies.  Under NOP regulations many of these products are only allowed if there is a documented nutrient deficiency.  Growers should contact their certifier to learn how to best document deficiencies, but soil and leaf analyses as well as visual symptoms will likely need to be documented.  

Organic sources of foliar nitrogen are derived mostly as a by-product of seafood processing, and come in the form of fish emulsions, fish powders, and fish oils.  Rates will depend upon the specific product.  Several companies make chelated foliar fertilizer products that are compliant with the NOP.  However, there are few replicated trials comparing different organically allowed foliar fertilizer products in orchards.  Solubor is a good source of foliar boron, and has proven to be an effective material for increasing leaf boron levels in orchards.

In most Intermountain West soil types, it is recommended that growers apply at least two “spring tonic” sprays that contain boron, zinc, iron and nitrogen in order to stimulate fruit set and flower bud initiation.  Also recommended are two to three applications of Epsom salt (for magnesium) at 15 lb/100 gallons of spray, starting at petal fall and continuing for several cover sprays.  

Additionally, repeated calcium sprays from the end of shoot growth to harvest have been shown to help improve fruit storage duration, particularly in Honey Crisp.  Calcium chloride is typically used as a calcium source, but other formulations may also be acceptable under NOP regulations.  The above foliar fertilizer recommendations are based upon trials in non-organic orchards, and it is not known whether recommendations for organic systems would be different.

Microbial Stimulants

Numerous microbial-based products are marketed with claims that they stimulate soil biological activity.  While these products may be acceptable under NOP regulations, there is little independent scientific confirmation of the manufacturers’ claims.  Well-managed organic orchards that include regular organic matter inputs (e.g., cover crops, manures, mulches, composts) typically already have relatively high soil organism biomass and activity, and additional microbial “stimulation” should not be necessary and is unlikely to be cost-effective. 

See Nutrition for additional information on micronutrient sprays, timing, and rates.