A California Avocado Growers trial report (1) on rootstock response to soil salinity outlines avocados in general are a salt sensitive species where chloride and sodium build up may cause detrimental effects on growth and yield. The report noted avocados are especially susceptible to leaf injury caused by the accumulation of sodium and chloride in the leaves. Also, high levels of chloride in the soil directly impair avocado root growth.

Trials quoted in the report outline avocado yields decrease in linear proportion to the levels of chloride that are contained in the leaf tissues. Salinity and chloride strongly interact resulting in decreasing avocado yields.

Salinity issues
The Californian trial report outlines sodium accumulation in the surface soil, which may be due to saline irrigation water or fertiliser salts, leads to dispersion of the soil aggregates and eventually leading to loss of soil structure. This results in a decrease in macropore space for root growth, decreased soil aeration, and increased problems with waterlogging. As drainage issues become more evident, it becomes increasingly difficult to leach the salt from the upper soil profile. The salinity issue can spiral out of control, leading to development of sodium and chloride ion toxicities that cause leaf burn, as well as diminished root growth and function, and can increase the incidence of Phytophthora root rot.

The report identifies the problem can be greatest in poorly drained soils that have a high bulk density or poor soil structure. This may also occur in otherwise well drained, but shallow soils that overlay a hardpan or rock layer that prevents the water from draining.

Gypsum for salinity issues
An important aspect of salinity management in heavy soils is the use of Gypsum, (chemical composition is calcium sulfate), which provides two ways to alleviate problems caused by salinity in these types of soils. The first effect is the displacement of sodium as calcium exchanges with sodium on the cation exchange complex in the soil. This causes the soil clay particles to flocculate (aggregate) into clumps and promotes soil aggregation and drainage. The sulfate ions then form ion pairs with sodium, which leaches out of the upper soil profile as sodium sulfate.

New Zealand vineyard managers often use gypsum every 2-3 years for soil salinity issues found in soils in the Blenheim and Hawkes Bay regions. For avocados grown in Northland’s sandy soils, the use of gypsum may assist with removing sodium build up from the upper soil profile.

An Australian report on avocado nutrition (2 ) outlines Gypsum is often used as the most effective product to increase soil calcium in avocado orchard situations, but it does have the potential to displace potassium and magnesium from the soil. If applying gypsum on a regular basis, monitor soil magnesium and potassium levels and correct with follow up applications if necessary.

The Australian report advises that gypsum applications are made in early spring to ensure a readily available supply of soluble calcium in the soil for the first six to eight weeks after fruitset. And to refrain from applying fertilisers, such as gypsum and potassium fertiliser at the same time as they are antagonistic.

For a number of reasons, gypsum applications are highly recommended for routine use in avocado production soils and are particularly important in soils with high clay content where promoting soil drainage and oxygen diffusion are essential for root health.

Talk to your local fertiliser advisor for more information on using gypsum in your orchard. Gypsum, it all starts with healthy soil.

References

1. Meeting the challenge of salinity: Evaluation of selected rootstocks for
response to salinity 

2. Avocado Plant Nutrition Review