Answer: June-bearing strawberries set buds for the following year’s fruit in the fall. (Most perennial fruit crops set their fruit buds in the spring or early summer.) To get a good bud set, the plants must have adequate chilling and not be nutritionally stressed. Therefore, fertilizer applications are usually warranted in the late summer, giving the organic fertilizer material enough time to break down and provide nutrients for the plants during the crucial fall bud-set.
Timing is critical in supplying nitrogen to berry crops and the nitrogen release rates for organic fertilizers may not match the nitrogen needs of the crop. A study on organic fertilizers in California found great variability in the nitrogen availability of different sources of fertilizers. These included guano, feather meal, liquid fish emulsion, fish meal, pelleted chicken manure, compost, and a green manure crop. Initially, the soil nitrate nitrogen from the green manure crop and compost kept the level of nitrogen at adequate amounts (50 to 75 ppm) for three to four weeks and then declined to background soil levels below 10 ppm.
Supplemental fertilizing is therefore necessary to carry the crop through the season. Strawberry producers using the annual plasticulture system must rely on soluble organic fertilizers applied through drip irrigation lines. Farmers using these systems must face solubility and the capacity of these products to be filtered through fine mesh without plugging drip emitters. Products injected into the system may not emerge at the same concentration. In other systems, foliar or side-dress applications will be warranted.
While all perennial fruit crops will benefit from the fertility provided by pre-plant cover-cropping and green-manuring, strawberries are so prone to weed problems that pre-plant preparations to reduce weed pressure are practically mandatory in organic production. A thick cover crop of a grass/legume mix will help to smother out many weeds and will provide important long-term improvements in soil fertility and soil organic matter. In areas such as coastal California, long growing seasons and high land rents may make the extended use of cover crops uneconomical. However, many growers believe that the long-term benefits of cover crops and rotations to soil fertility and pest and disease suppression are worth the cost.
Compost can be used as a supplement or alternative. Spreading and incorporating the compost on the beds only, avoiding the furrows, will help concentrate fertility and microorganisms where they are most needed. Compost application rates vary from 10 tons/acre to 3 tons/acre. Supplemental fertigation is necessary to carry the plants through the production season: Research from Ohio has shown that vermicompost (compost made from earthworm waste) applications increased strawberry growth and yields significantly. These responses seemed not to be dose-dependent. Strawberries at one site grew fastest and yielded most in response to the 10 ton/hectare (4.05 ton/acre) vermicompost application rate, whereas strawberries responded positively and similarly to both the 5 ton/hectare (2.02 ton/acre) and 10 ton/hectare rates of application at another site. These responses could not have been mediated by the availability of macronutrients, since all plots were supplemented with inorganic fertilizers to equalize macronutrient inputs for all treatments. Based on other research in the laboratory, however, the responses could have been due to production of plant growth regulators by microorganisms during vermicomposting.
The foliar application of aerobically-prepared compost tea increased yields in a British Columbia study. Besides reducing incidences of Botrytis, the compost tea treatment increased yields in strawberries by 20 percent compared to the control and water sprays.
You can learn much more in the ATTRA publication Strawberries: Organic Production. It provides an overview of organic strawberry production methods. It also covers integrated pest management and weed control techniques that can reduce pesticide use in strawberry production. Included are discussions of weeds, pests, diseases, greenhouse production, plasticulture, fertility, economics, and marketing.
In addition, to learn more about organic fertilizers, vermicomposting, compost and cover crops, check out these ATTRA publications:
Alternative Soil Amendments
Worms for Bait or Waste Processing (Vermicomposting)
Vermicomposting: The Basics
Overview of Cover Crops and Green Manures
ATTRA Soil Resources
Cover Crop (340) in Organic Systems
Cover Crop Options in Hot and Humid Areas
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