Soils in dry climates are typically alkaline, meaning that they contain a high percentage of lime (calcium carbonate). The main problem associated with alkaline soils is that certain nutrients essential for plant growth, particularly iron and zinc, become unavailable to the plants.

A wide range of garden plants grow quite satisfactorily in mildly alkaline soils, with just a small minority of acid-loving plants being unsuitable; camellias, rhododenrons and azaleas being the most widely known examples. Plant growth and development is generally affected though in strongly alkaline conditions, as the plants fail to take up nutrients.

Acidity and alkalinity are measured according to a scale known as pH. Ranging from 1 - 14, a pH of 7 is neutral, figures lower than 7 indicate acidity, while those higher than 7 indicate alkalinity. The vast majority of plants grow most comfortably within a pH range of 6.5 -- 7.5, or between mildly acidic to mildly alkaline. In dry climates, not only do the soils tend to be alkaline, but the water as well. Although a pH round about 8.0 is very common, there is no shortage of plants to choose from. As the pH rises beyond that however, measures need to be taken to reduce the soils alkalinity, and under conditions where the pH reaches 8.5, virtually no plants are able to grow properly.

It is best therefore to undertake a soil test in order to ascertain the soil's pH. Home kits are good enough to get a general impression, but in problematical circumstances, a laboratory test is more appropriate. In mild conditions, consistently adding well-rotted compost to the soil is often sufficient to lower the pH to the point that the garden plants as a whole will benefit. With higher alkalinity though, where the pH reaches say 8.2, it may be necessary to add a sulfur-based amendment. As the recommended rate at which the sulfur is applied differs according to the existing soil pH, it follows that this must never be done without reference to the findings of the soil test.

An alkaline level registering 8.5 on the pH scale means that the soil has become a sodic soil, referring to the high percentage of sodium present. In such conditions, the soil's crumbly structure breaks down, depleting it of oxygen. A sodic soil is effectively a destroyed one, and drastic steps have to be taken, usually involving the use of gypsum (calcium sulfate) and flooding, to leach the soil of excessive salts. This work should only be attempted in consultation with, and preferably by a soil expert.

Growing plants in alkaline soils is principally a case of correctly managing the soil on a long-term basis, and avoiding the few species that require acidic conditions. Probably the most critical factor in regulating alkalinity is to reduce the soil's salinity. Routine gardening tasks and practices should always be carried out with this in mind. Compost, as long as it is well rotted, should be preferred to chemical fertilizers, and irrigation practices should avoid regimes based on small quantities applied frequently, (which contributes to the salt build-up) and opt instead for deep, occasional waterings, which among other benefits, periodically leach out the excessive salts.