Answer: Without rain or overhead irrigation, some bulky organic fertilizers—e.g., compost—will not readily break down and move into the soil. And because the fruit crops are usually perennials, cover crops cannot be relied upon for nutrition after the initial planting. This emphasizes the need for pre-plant consideration of nutritional needs of the plants, beginning with a soil test. For instance, adjusting soil pH with lime or sulfur should occur before the tunnel's cover goes on. Likewise, if soil organic matter needs improvement, cover crops or applications of manures or composts should happen before the high tunnel is covered. Allow time for breakdown and mechanical or natural incorporation of these into the soil.
Post-planting nutritional needs are going to be met with fertigation (running liquid fertilizers in the irrigation system), by precision hand-application of compost or pelletized organic fertilizers, or by hand-application of liquid organic fertilizers via watering buckets or hoses with fertilizer attachments. Regarding fertigation, liquid organic fertilizers, like fish emulsion or compost tea, have a reputation for clogging or gumming up the drip emitters (manufacturers of some of these products are now touting some formulations as "fertigation friendly" in order to address this problem). In such cases, proper filtration is the first line of defense. If clogging still occurs, there is an organically acceptable cleaner, Cleardrip-O.
Fertigation in Organic Vegetable Production Systems is an excellent primer on organic fertigation, as relevant to perennial fruits as it is to vegetables. It discusses the necessary equipment, fertigation products, how to calculate rates, and more.
If you're going to apply dry fertilizer materials, they will have to be placed where the soil is wet from the irrigation; otherwise, the nutrients from such materials will not move into the root zone to be available to plant roots.
Lastly, it's advisable to remove the plastic cover every winter in order to insure that the trees meet their chill requirement but also to allow rain to rinse out excess salts from the fertilizers. This is also a time when manure or compost could be applied with some confidence that it will be "washed" into the soil by rain and melting snow. Compost, especially compost made from plant matter and not animal manure, contains less salt and so should be considered a superior organic fertilizer material for most high tunnel applications.
Want to learn more? The ATTRA publication High Tunnel Tree Fruit and Grape Production for Eastern Growers. This publication identifies fruits that hold the most potential for profitable high tunnel culture. It also identifies several limitations and potential pitfalls growers must recognize if such a venture is to be profitable.
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