Understanding the potassium calcium magnesium relationship

Managing mineral nutrition for hydroponically grown crops is essential for maximizing crop productivity and quality. Deficiency symptoms can occur when there are insufficient nutrient concentrations in solution. However, it is also possible to observe deficiency symptoms even when there is a sufficient concentration in the nutrient solution due to nutrient antagonisms. This article will focus on the relationship between potassium (K), calcium (Ca) and magnesium (Mg).

Understanding K, Ca and Mg

The relationship between K, Ca and Mg is an important one for hydroponic operations. To understand how to manage the relationship between these nutrients it is important to review a little bit of their chemistry. When fertilizer salts are dissolved into solution, the salts dissociate, and nutrients are available in their charged ionic form. All three of these nutrients — K, Ca and Mg — are all positively charged ions, or cations. Furthermore, in their ionic state they have a similar charge or valance. K is missing one electron and is a monovalent cation (K+). Ca and Mg are missing two electrons and are divalent cations (Ca2+ and Mg2+). In addition to their ionic charge and valance, Ca and Mg nutrients are all taken mass flow when water is taken up by plants. Due to the similar chemical properties of these elements, as well as some being taken up by the same processes, these nutrients can compete with one another.

Developing antagonisms

A nutrient antagonism is when an excessive concentration of one nutrient inhibits the uptake of another. Since K, Ca and Mg have similar properties, and are taken up in a similar fashion, too much of one nutrient can inhibit the uptake of another nutrient. For example, if the concentration of Ca gets too high, it can impede the uptake of Mg. Or if the K concentration gets too high, Ca uptake can be inhibited. The ideal ratio of K:Ca:Mg to each other in hydroponic nutrient solutions to avoid uptake varies a bit, but is usually three to five parts K and Ca to one part Mg (3-5K:3-5Ca:1Mg). However, this tends to vary with plants. For example, lettuce and leafy greens do well when Ca and K are balanced with each other. Fruiting crops do well with a greater proportion of K.

Antagonism-induced nutrient deficiencies can cause a variety of hydroponic food crop disorders, ultimately reducing productivity and marketability in crops. For example, a K deficiency in tomatoes can results in yellow shoulders, where fruits do not fully ripen. A Ca deficiency in tomatoes and peppers can result in blossom end rot and produces fruits with necrotic portions of fruit where the calyx or petals of the flower were attached, whereas Ca deficiencies in lettuce and other leafy greens can cause marginal necrosis or “tip burn” on leaves. Finally, Mg deficiencies cause chlorosis on leaves and, while commonly observed on tomato plants, it is most problematic for basil.

Creating and maintaining a balance

To keep K, Ca and Mg in balance in nutrient solutions, the water, fertilizer and pH adjusters used to make up and maintain nutrient solutions should all be evaluated. By starting out with the right proportions of K:Ca:Mg, and maintaining them throughout production, antagonisms and the deficiencies they induce can be avoided.

The most proactive approach to managing nutrient antagonisms after mixing fertilizers is to perform regular nutrient solution analyses."