Of the three secondary nutrients needed at lower levels than NPK, calcium (Ca) is perhaps the most important. Calcium strengthens cell walls, helping to reduce bruising and disease in fruit, salad and vegetable crops. This means that a good supply of calcium produces food crops that are less prone to damage and have a longer shelf life. Crops short in calcium will have growth disorders such as corky skin.
Fruit and vegetables containing higher levels of calcium also have a higher nutritional value – for example, vitamin C and antioxidants in tomatoes. This means that eating fresh fruit with strong skins and a great, crisp bite will help provide us with the calcium we need for strong bones.
Magnesium (Mg) is also important for crop quality, but is also a key component of leaf chlorophyll and the enzymes that support plant growth. Low magnesium leads to reduced photosynthesis, which severely limits crop yields.
Grain fill in rice and dry matter content of potatoes can be significantly reduced if magnesium is undersupplied.
Sulfur (S) is an essential part of many amino acids and proteins. Without both S and Mg, crops suffer; growth slows and leaves turn pale or yellow. Sulfur is particularly important for ensuring the protein content of cereal crop grains.