Calcium is not a cure-all for avocado problems, but prudent, informed applications can improve the health and productivity of an avocado orchard.

Several forms of calcium are commonly used as soil amendments. Gypsum is often used to increase soil permeability, reduce crusting, loosen hard soils, develop soil structure, and remove exchangeable sodium from the soil. Calcium carbonate (calcite, limestone) is frequently applied to raise the pH of an acidic soil. For soil with a neutral pH, gypsum works better as a calcium source. 

Maintenance of calcium supply is essential for plant growth. In avocado orchards, calcium governs cell wall strength, plant growth, fruit development, fruit quality and storage. Calcium levels affect avocado fruit yield, size, and postharvest quality. Fruit ripening is also slowed by high internal levels of calcium and cold-induced disorders are decreased.

In a study of Californian avocado orchards, they have observed calcium amendments can reduce avocado root rot by creating a favourable environment for the growth of the tree. Gypsum can increase permeability, allowing water to drain freely through the soil, which allows more oxygen to reach the roots. Vigorous root growth is important in disease resistance.

The Californian study also found soils suppressive to avocado root rot in California and Australia were found to have high levels of organic matter, high levels of available calcium, available nitrogen, and high microbial activity. High levels of nutrients available for both plants and microorganisms in a soil may affect avocado root rot in two ways: by increasing plant growth and root regeneration, and by nourishing microorganisms antagonistic to root rot.

Gypsum works well when incorporated into a long-term programme as an available source of calcium and sulphur, which can be applied on its own or alongside other fertiliser applications. The overall rate of release from gypsum is controlled by the rate with which the calcium and sulphate are taken up from the soil solution. Speed of action can be hastened by cultivating gypsum in and by adding organic matter to the soil.

Talk to your advisor for further information and guidance on applications of gypsum in your orchard.

References

- California Avocado Society 1997 Yearbook 81: 69-78, The Effects of calcium on avocado growth and root health, Authors: Messenger, Menge, Amrhein, Faber

- The Biological Farmer, Gary Zimmer, 2017

- www.gypsum.co.nz