Original site in English

Ask a Specialist - How can I improve fertility in my organic fruit trees?

The fertility needs of perennial fruit crops are very different from those of annual vegetable or row crops. Trees and shrubs must be managed for both long-term soil fertility AND yearly nutrient uptake. With the goal of balancing annual fruit production and long-term plant health, there is less focus on high nitrogen inputs in most cases. These plants demand a more complex approach and, as with all aspects of growing perennials, a lot of planning ahead.

For long term success, maintaining healthy orchard soil is closely related to ground cover management. Different species, as well as different growing systems, will have different needs, and there is no one right approach. In general, the smaller and younger your perennial plants are, the more they’ll be impacted by weed and grass competition. So, newly planted trees, a planting of dwarf apple trees, or berries, for example, will need to be protected from weeds more than a mature orchard of semi-dwarf or larger trees.

Organic mulches — including wood chips, hay or straw, leaves or compost — will simultaneously suppress weeds and build healthy soil. This can be combined with a strategy of using living mulches or cover crops under your perennial fruits. As living mulch plants die back each year, or ground cover species are mowed, the accumulated organic matter helps build soil, especially when paired with periodic applications of compost.

For annual nutrient applications, it’s important to know what your plants need. Soil and foliar sample tests can provide information, as can observation of growth and production. The University of Minnesota Extension has an excellent resource on soil tests, including videos, instructions, and other recommendations, which we’ve listed at the end of this article. You will also need to choose a lab. The MOSES Organic Resource Directory, also shown in the resource list below, has quite a few listed. For fruit production and quality, micronutrients such as calcium, zinc, and boron (among others) are just as important as NPK, so make sure you get a test that includes those.

It is equally important to learn to recognize the signs of deficiency in your particular crops. For example, calcium deficiency in apples results in a condition called bitter pit, and makes fruit unsuitable for storage. The annual vegetative growth of plants and trees, and the appearance and color of leaves are other useful indicators. Productivity declines that are NOT the result of frost damage might also be a sign of a nutrient deficiency, especially in berry crops. A full picture of your plants’ needs will come from both comparing what you see to the results of a soil test.

Once you’ve identified acute deficiencies or nutrient needs, foliar nutrient sprays are a much more efficient way to correct them than soil applications. As with soil-applied nutrients, there are holistic, multi-purpose spray options, as well as more specific and targeted ones. In his books The Apple Grower and The Holistic Orchard (both published by Chelsea Green), Michael Phillips lays out a spray regimen for overall plant health. Used by many organic orchards, this program includes things like liquid fish (available from Dramm or, in the Midwest, Schafer Fisheries), seaweed or kelp, molasses, and sometimes beneficial active microbes. These mixes are often applied in the spring and early summer, and sometimes post-harvest as well.

Then there are the targeted micronutrients. Depending on the needs of your specific plants, there are organic options for almost anything — calcium, zinc, boron, magnesium, manganese, etc. See, again, the MOSES Resource Directory for sources. For organic certification, you may be required to have a soil test that indicates a deficiency in a particular nutrient before that input can be approved. It is always best to check in with your certification agency before you make input decisions.

Rachel Henderson is a farmer on the MOSES Organic Specialist team. She grows a variety of fruits on the 60-acre farm she owns with her husband, Anton.

University of Minnesota Extension - Soil Testing

Issue: May 2022
By: Rachel Henderson