Agriculture Reference
In-Depth Information
ing materials at least one full year before planting your trees. If the soil pH
in an established orchard is 5.0 or below, raise it gradually by adding liming
materials over 2 or 3 years to avoid damaging the trees and to allow the soil
pH to stabilize. Be sure to monitor the soil pH at least once yearly before
liming so as not to add too much liming material.
Lowering pH
While raising soil pH is relatively easy and inexpensive, lowering soil pH can
be both difficult and expensive, particularly on a large scale. In order to sig-
nificantly lower pH in an organic orchard soil, you usually need to add ele-
mental sulfur.
Lowering soil pH is more technically difficult than raising pH with liming
materials. Liming materials undergo simple chemical reactions with the soil
particles. Elemental sulfur, however, must be metabolized by soil bacteria
and converted into the acidic SO ion, which then reacts with basic (al-
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kaline) compounds in the soil. Although simple in theory, sulfur oxidation
depends on which types and how many soil microorganisms are present, the
soil type and moisture level, the carbon nutrient sources for the microbes,
soil temperature, and more.
Note also that sulfates, such as calcium sulfate and magnesium sulfate,
are salts formed by the reaction of SO ions and do not alter soil pH. This is
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because the SO has already reacted and is no longer acidic. In convention-
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al farming, ammonium sulfate is sometimes used to lower soil pH. In these
cases, it is the ammonium, not the sulfate, that reacts to acidify the soil. Am-
monium sulfate is sometimes recommended for acidifying soils, but it is not
allowed in organic production. Aluminum sulfate is sometimes recommen-
ded for acidifying soils, but it is not mentioned in the NOP. Although some
aluminum sulfate products are marketed as “organic,” nearly all aluminum
sulfate used commercially is manufactured synthetically. The naturally oc-
curring form is rare and typically found in burning coal mining waste dumps
and volcanoes. The improper use of aluminum sulfate can cause aluminum
toxicity in sensitive plants. It should not be used in an organic orchard.
