Calcium is used as part of the cell wall structure of plant cells, including the middle lamella layer that holds cells together. Calcium is also important for cell membrane integrity and in the translocation and retention of other nutrients, and Ca is also used by plants as a messenger for response to environmental changes^.1,3,4

There is a delicate balance between the amounts of Ca, Mg, and K within the plant. Too much of any one of these nutrients can suppress the uptake of the other two. The uptake of Ca by the roots can also be affected by levels of NH4 in the soil.^4,5 The uptake of Ca is a passive process, meaning that the rate of uptake is dependent on the amount of Ca in the soil and that the plant does not expend energy to extract Ca from the soil. It is absorbed by the part of the root just behind the root tip. So, the presence of healthy, actively growing roots is important for the absorption of Ca, and uptake can be affected by root diseases and other factors that affect root function and growth.⁴

Calcium is not highly mobile within the plant, and the levels in specific tissues are affected by transpiration and the movement of water within the plant. Moisture levels in the soil and humidity levels around the canopy affect the translocation of water and the movement of Ca.^4,5 A deficiency of Ca can result in the failure of terminal bud and root tip development.

Chlorosis and necrosis of leaf margins can develop on new leaves, resulting in contorted growth, as is seen with tip burn of lettuce and cole crops. Chlorotic and necrotic spots can also develop on leaves deficient in Ca. Blossom end rot of tomato and pepper, blackheart of celery, and cavity spot of carrot are all the result of localized Ca deficiencies within the plant (Figure 2). Symptom development is often associated with an inability of the plant to translocate Ca to the tissue where it is needed rather than a lack of available Ca in the soil or within the plant as a whole.^4,5

Maintaining even soil moisture levels and avoiding alternating periods of overly dry and overly wet soils can help minimize the development of blossom end rot and other Ca deficiency related symptoms. Using agricultural lime to help correct overly acidic soils can help increase the availability of Ca to the plant, and the foliar application of Ca fertilizers (CaCl or CaNO3) to young, growing tissues can help alleviate localized Ca deficiencies in some situations.⁵

Magnesium is used as part of the chlorophyll molecule needed for photosynthesis. It is also used as an enzyme cofactor and activator, making certain enzymes work better. Mg is relatively mobile in plants, easily translocated from older to younger tissues if necessary, and deficiency symptoms usually develop on older plant tissues. The uptake of Mg can be inhibited by excessive levels of K, Ca, and NH4.^1,4,5

Deficiencies of Mg manifest as interveinal chlorosis and necrosis on older leaves. Severe deficiencies can result in the stunting of plants.^4,5 On sweet corn, Mg deficiency symptoms may be seen early in the season in cold, wet soil conditions. On potatos, deficiencies can cause a chlorosis of leaves beginning on the leaf tips and margins of older leaves, progressing to the internal portions of the leaf. On celery, chlorosis usually starts at the tips of older leaves and progresses to the margins and interveinal tissues. On tomatoes (especially greenhouse grown) interveinal chlorosis symptoms develop on the older leaves, with a mild purpling of affected leaves. Foliar applications of Mg-containing fertilizers can help manage deficiency symptoms. Soil and tissue nutrient analysis can also help detect excess levels of K, Ca, and N.⁵

Sulfur is a constituent (building block) of several amino acids including cystine, cysteine, glutathione, and methionine. Sulfur is also a cofactor for some enzymes.^1,4 Sulfur is usually absorbed by the plant as the sulfate ion (SO₄²-), which is a component of many fertilizers.⁵

Sulfur deficiency symptoms are most frequent on plants grown in sandy, low organic matter soils. Legumes (bean and peas) are sensitive to S deficiencies while sweet corn is relatively insensitive. Plants that are deficient in S typically develop a light green color, similar to that seen with N deficiency. S is not very mobile within the plants, and symptoms usually appear first on younger tissues, which distinguishes it from N deficiency. Excessive levels of S can result in toxicity symptoms of interveinal chlorosis and scorching of leaf margins that progresses inward. Exposure to the air pollutant sulfur dioxide (SO2) can result in symptoms that resemble frost damage or herbicide injury.⁵