Soil pH and plant health

Page last updated: Monday, 16 June 2014 - 2:10pm

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This web article has been written to help home gardeners solve nutrient deficiency symptoms in their garden plants. Despite the application of general fertilisers, plants can show symptoms of nutrient deficiency which is often related to the pH of the soil. This article explains the basics of soil pH, its effect on nutrient disorders and the general health of plants.

The pH of the soil is a measure of acidity or alkalinity. It basically indicates how ‘sour or sweet’ the soil is. The pH scale ranges from 0 to 14, with 7 being neutral. Values below 7 indicate an acid soil, and above 7, alkaline. Because the pH scale is logarithmic, a pH change by 1 unit means it is 10 times more acidic or alkaline.

Yellow rose leaves.
Iron deficient rose leaves.

Typically, Western Australian soils have a pH range between 4 and 8.5. In the metropolitan area, soils are more alkaline near the limestone-based coastal sands. Soils further inland, and most agricultural areas, are naturally acidic. In agricultural regions soils which were not very acid when cleared have become increasingly so through the continuous application of fertiliser.

In home gardens, where mineral fertilisers are frequently used, the soil pH may also be acidic.

Significance of soil pH nutrition

Soil pH determines the nutrient availability to plants. Some nutrients become ‘tied up’ in the soil at certain pH levels. For example, acid soils can lead to deficiencies of phosphorus, calcium, magnesium and molybdenum, as well as toxic levels of manganese and aluminium.

Zinc deficiency on garlic.
Zinc deficiency on garlic.

Alkaline soils may lead to deficiencies in iron, manganese, boron, copper and zinc. 

While most plants prefer neutral soil, some are suited to other pH levels. Examples of different preferences are:

Soil pH Ornamentals Fruit Vegetables
Acidic 4.5 to 6.0






Alkaline 7.0 to 8.0

Sweet Pea






A common pH-related condition in the metropolitan area is ‘lime-induced chlorosis’ which is an iron deficiency caused by high pH levels. It manifests as yellow-white leaves on plants growing in limestone-based coastal sands.

Cauliflower leaf whowing green veins and yellowed leaf tissue.
Magnesium deficiency in cauliflower.

This condition is rectified by applying iron sulphate, which will also drop the soil pH or an iron compost. Check if yellow leaves are due to iron deficiency by spraying the plant with an iron chelate. If the leaves green within a week apply an iron compost.

Iron compost

Moisten compost, coco peat or animal manure until just wet. To each 10 litres mix in one cup of iron sulphate. Dig holes 20cm deep around the root zone of the plant and compact the iron compost into the holes. Use three holes for a rose bush and up to eight holes for a big shrub. The roots will grow into the organic matter and the treatment lasts for several years. 

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