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Understanding Calcium and pH

Gardeners tend to worry first about the NPK levels of their soil, neglecting the soil’s calcium. Without adequate calcium, all the fertilizer in the world will do you little good. Calcium benefits soil and plants by improving the soil’s physical structure, raising the pH of the soil and directly contributing to the plants’ nutritional needs.

Soil Structure: Free calcium ions have a double-positive charge, so they help to stick negatively charged clay soil particles together. This is called flocculation. Clumped particles—instead of dispersed particles—increase soil porosity, which means more oxygen in the root zone and better water penetration (think about the relative air space available in a bucket of tennis balls vs. a bucket of marbles).

Effects on pH: When lime is applied to acidic soil, the hydrogen ions react with the calcium carbonate to produce water and carbon dioxide. This reduces the level of free hydrogen ions in the soil and raises the pH. pH levels that are significantly below neutral will limit the availability of many important plant nutrients and inhibit plant growth.

Plant Nutrition: Calcium helps plants build strong cell walls, which means bigger healthier plants and better resistance to disease. It is important to the metabolism of nitrate and other plant nutrients. Also, it helps to regulate leaf stomata, so plants need adequate calcium to cope well with heat stress. Fruiting plants deficient in calcium may suffer blossom end rot or other quality problems.

How much lime should you apply? Don’t even try to answer this question without a professional soil test! The quantity will depend not only on the current pH and calcium levels of your soil, but also on your soil’s cation exchange capacity (CEC) and perhaps on what crops you plan to grow (most garden vegetables are pretty happy with a pH around 6.5, but if you’re growing larger quantities of one thing then it’s important to know that potatoes like a much lower pH than asparagus). Even on very acidic soils, most experts recommend that applications not exceed 3 tons per acre per year (about 140#/1000 sq ft). Lime recommendations on soil-test reports typically assume you are applying straight calcium carbonate. The effect on pH of other liming agents is reported as a “lime equivalent” relative to calcium carbonate, and quantities should be adjusted accordingly.

When and how should you apply lime? Calcium moves very slowly in the soil: if applied to the surface, it will move downward a rate of just 1" per year. The liming materials permitted for use in organic production may take 6 months to a year to react fully, longer if the soil is dry. For best results, liming should coincide with a tillage event to incorporate it as deeply into the root zone as is practicable. Applying in the fall gives the lime more time to react before plant growth, and probably moister soil conditions. The good news is a lime application is for the long haul: you typically need to apply lime only once every 5–10 years.

A soil test will help identify an appropriate calcium source for your location:

  • 8187 Aragonite AKA oyster shell meal. Powdered. Liming equivalent 94%.
  • 8214 Gypsum 23% calcium, 17% sulfur. Granulated. The sulfur buffers against any change in pH. Provides calcium when pH is already at desired level.
  • 8217 Hi-Cal Limestone Granulated calcium carbonate. Liming equivalent 100%.
  • 8220 Dolomitic Lime Calcium carbonate with 8.5% magnesium carbonate. Powdered. For use on soils with a magnesium deficiency. Liming equivalent 116%.
  • 8248 Wollastonite Calcium oxide with silicon dioxide. Liming equivalent 86%.