Calcium: the Neglected Soil Nutrient

Fertilizers typically come with an “NPK” rating. That stands for Nitrogen, phosphate and potassium. It would seem as if these are the only macronutrients needed for a productive soil, but nothing could be further from the truth. Calcium is, in fact, the bedrock of healthy organic soil.

Few mini farmers have heard of, much less understand, the importance of CEC — cation exchange capacity.

Ions are positively or negatively charged atoms or molecules. Anions (such as chlorine) have a negative charge whereas cations (such as sodium) have a positive charge.

Cation Exchange Capacity, while I won’t get into the chemistry of how it is measured, is a measure of the availability of minerals in the soil to plants. If a soil has low CEC, even high levels of other nutrients may not be readily available for the plants to use. So a high CEC allows for greater growth with lower nutrient requirements.

The primary determinants of CEC are the levels of humus and clay in the soil; as well as the age of the clay present. In some parts of the country, the clay is so old that its CEC is negligible whereas in others, it is quite high. Humus, as a colloid, has a high CEC and it is added in the form of well-aged compost.

Of course, high levels of clay alone, even clay exhibiting a high CEC, won’t do the trick because it can create anaerobic conditions in the soil; which are not conducive to the growth of the aerobic bacteria necessary for nutrient exchange in symbiosis with plant roots.

CEC is generally measured using four cations — calcium, magnesium, potassium and sodium. The ratio of calcium to magnesium in a soil has a large impact on its productivity; and you generally want to aim for a ratio anywhere from 3:1 to 7:1. If you are able to achieve this, you get optimum water-retention in your soil.

My soil is quite acidic. So much so that the water in my dug well has a pH of 4.8. This means that a large part of the CEC of my soil is naturally composed of hydrogen, H+.  When I apply lime to my soil, the calcium replaces the hydrogen. While hydrogen forms acids (such as hydrochloric or sulfuric acid), calcium forms bases, such as calcium carbonate or calcium hydroxide. Thus, adding calcium to the soil in the form of lime can increase CEC, create better water retention and establish better conditions for symbiotic bacteria.

Usually, because my soil is also deficient in magnesium, I apply dolomitic lime; which contains both magnesium and calcium in an appropriate ratio.

I just happen to have a chemistry lab that I can use to test all of this stuff; but some rules of thumb can generally work just fine.

In general, add 2″ of well-aged compost to each raised bed every year. Test your pH and if it is lower than 6.4; correct it by adding enough lime or dolomitic lime to establish that pH. Keep in mind that lime takes months to work; so it is best added in the fall for wintering over.