Plants require substantial amounts of nutrients, including calcium and sulphur for growth. A fertiliser programme
that maintains adequate levels of these minerals in the soil is necessary for continued cropping.
As a fertiliser, gypsum is a good source of sulphur and, being more soluble than lime, it is also a good source
of medium-term release calcium. It has the added advantage that it neutralises the by-products of organic decomposition.
Some New Zealand soils, particularly large parts of Otago, Canterbury and Gisborne, are low in sulphur and traditionally
superphosphate has been applied to amend this. The sulphur in gypsum is in a form that is readily available to plants.
Superphosphate applications raise the concentration of phosphorous in the soil whereas gypsum allows the application of
sulphur without input of phosphorous. Gypsum also alleviates problems of high aluminium and manganese in some clay
subsoils.
Calcium is a structural part of every plant cell wall and cell membrane. It also affects the activity of many plant
enzymes. Calcium is taken up passively in the transpiration flow and moves in the xylem sap to high transpiring
structures such as leaves and to a lesser extent fruit.
The calcium level in a fruit is usually much lower than that in the other organs (stems, leaves, roots etc.). As a
result, the fruit of many species (particularly apples, tomatoes, capsicum etc) tend to suffer from specific
calcium-deficit disorders such as bitter pit and blossom-end-rot. While the addition of luxury levels of calcium
fertiliser will not solve these problems it is important that soil-calcium deficiencies are promptly corrected for
these will only make matters worse.
A continual supply of calcium in the soil solution is required as calcium is not mobile within plant tissues, so is not
reallocated to the growing tips.
Deficiency symptoms include:
- Distortion of young leaves,
- Curled margins, and tips hooked back on young leaves
- Irregular or ragged shape of young leaves
- Brown spotting or scorching of young leaves
- Death of terminal buds
- Poorly developed root systems
- Fruit quality problems e.g. bitter pit in apples and blossom end rot in tomatoes.
Sulphur is necessary for proteins, vitamins and the synthesis of some plant hormones. It is essential for the production
of healthy, green leafy material. Without adequate sulphur, chlorophyll is not stable and plants suffer from chlorosis
(lack of chlorophyll). Like calcium, sulphur is not mobile within plant tissues, so a continuous supply is needed for
growing plants.
Deficiency symptoms:
- Young leaves are pale green or light yellow without spots (similar to nitrogen deficiency,
except nitrogen deficiency shows up primarily in older leaves as it is reallocated to new growth).
