Calcium (Ca), number 20 in the Periodic Table, is the element that we think about when our soil is acidic (pH is low) and it needs some sweetening. Lime (calcium carbonate) is the usual material for this correction. Yet Ca is much more than a bag of lime for a soil with a sweet tooth. In the first place, Ca is not the only element that sweetens the soil and raises the pH. Magnesium (Mg), potassium (K), and sodium (Na) are also positive ions that also serve in a similar capacity. Soils can be high in K and Na, and give a high pH, and be deficient in Ca. Soil pH is technically the measure of the hydrogen ion concentration, which is really a reciprocal of the calcium, magnesium, potassium and sodium ions.
Why is Ca often called the king of the nutrients by the holistic eco-gardeners? It's the king because it is so prevalent in plant tissue, and so vital in many of the growth and development processes in plants. In short, it is involved in plant membrane permeability, cell wall structure, enzyme activities, and in interaction with photohormones. Like every element that a plant needs, Ca does not act strictly on its own. Ca must be in the correct amount and in the correct ratio with other elements for the production of nutrient-dense produce.
One important ratio is that of the nutrients of Ca and Mg. Strive for a ratio of 7:1 or slightly greater. Too much Ca will cause Mg, phosphorous and minor element deficiency. Too much Mg results in compacted soils, and phosphorous, potassium and nitrogen deficiencies. Another little fact; without boron, Ca uptake and utilization is inhibited. If your soil tests show that no boron is present, add borax to bring it up to the 4 pounds per acre level.
All "liming" materials are not alike. If you are adding lime, choose one that fits your garden soil needs. Don't count on all local suppliers of lime to understand the differences. Read the labels. Of several "liming" materials that are available, here are the percent averages of the various elements.
High calcium limestone 38 % Ca
Dolomite 22% Ca 14%Mg
Epsom salt – Magnesium sulfate 10% Mg 14% Sulfur
Calcium sulfate – Gypsum 22% Ca 18% Sulfur
Sul-Po-Mag 11% Mg 9% Sulfur 21% Potassium
On your Soil Test Report, the Estimated Base Exchange (%) is given, but rarely explained or used by gardeners. It refers to the soil's ability to hold plant nutrients. This is called the Cation Exchange Capacity, and it is the relative amounts of Ca, Mg, K, and Na, ions held in the soil. For those elements, I like to see numbers of 68, 12, 5, and 1 percent, respectively. Generally, when you get to these levels, your soil pH will be in the ideal area of 6.4. That is a slightly acidic soil, in which the microorganism are fully active and the plants are able to use the elements.
Fixing the Ca levels is just one step in the balancing of nutrients for your soil. I start by looking at the pounds per acre of Ca, and the ratio of Ca:Mg. Then, if needed, I decide what "liming" fertilizer is best for the situation. The process is often not a simple formula or cookie-cutter approach. I try to error on the side of adding too little rather than too much. The processes of Nature are complex and corrective, always striving to adjust the soil that leads to healthy plants. Of course that is done most effectively when the soil is free of toxic chemicals. Making the soil amendment adjustments slowly puts me in the camp of working with Nature, not acting like I am the one in control. In the long run, that strategy gives the best results.
If you are inclined to use the organic approach and are confused about what to do, how to interpret soil tests, etc, feel free to contact me. I'll do my best to help.