Soil pH

Soil PH

The pH level of soil is a determination of the alkalinity or acidity of the soil. The soil pH scale ranges from 0 to 14, with 7 being neutral. Any reading that is greater than 7.0 is alkaline while any reading below 7.0 is acidic.

The Hunter Valley soil is by and large acidic and in the natural range of 4.5 to 7.

Why soil pH is important

Soil pH can significantly affect the nutrient uptake by plants in the soil-water system, as a result many plants growing in strongly acid or strongly alkaline soils may suffer nutrient deficiencies or toxicities, which in turn can negatively affect their productivity, appearance and in some cases kill the plant.

As a general rule, soil PH should be in the 5 to 8 area, depending on the plant and that plants requirements.

Plant nutrients are most readily available for plant uptake and toxicities are less likely to occur in soils with a pH near neutral (7.0). However, while most plants grow well in near neutral soils, some plant species require a particular pH level for optimum growth.

How to change soil pH

Heavy soils or soils with a high clay content require more alkalising or acidifying material to change their pH than sandy soils.

Raising soil pH

Liming materials. Liming corrects soil acidity. The three main types of liming materials are:

  • agricultural lime — calcium carbonate obtained by crushing limestone rock. It moves slowly in all but very sandy soils and is used mainly for treating strongly acid topsoils. The finest quality lime is preferred;
  • dolomite — about 60% calcium carbonate and 40% magnesium carbonate;
  • magnesite — 100% magnesium carbonate.

Other liming materials include burnt lime (calcium oxide), hydrated lime (calcium hydroxide) and waste materials such as kiln dusts, sewage residues and blast furnace slags.

Lime or a mixture of lime and gypsum can also be used to improve soil structure, but this is not recommended for alkaline soils (pH over 7).

To change the soil pH by one point, 5.0 to 6.0, apply at 120g/square metre for sandy soils and up to 380g/square metre for heavy clay soils.

The soil pH should be checked after 3 months and apply more lime or dolomite if necessary. Repeat this process until the correct PH is attained.

Lowering soil pH

Two materials normally used for lowering the soil pH are aluminum sulfate and sulfur. Aluminum sulfate will transform the soil pH instantly because the aluminum produces the acidity as soon as it dissolves in the soil. Sulfur, however, requires some time for the conversion to sulfuric acid with the aid of soil bacteria. The conversion rate of the sulfur is dependent on the fineness of the sulfur, the amount of soil moisture, soil temperature and the presence of the bacteria. If the conditions are not ideal, the conversion rate of sulpHur may be slow. For this reason, most people use the aluminum sulfate.

As a guide, to reduce the soil pH by one point from 8.0 to 7.0, elemental sulphur (99% sulfur) can be used at an application rate of 30g/square metre for sandy soils and up to 120g/square metre for clay soils. Check the soil pH after 3 months and apply more sulphur if necessary.

Pine needles, pine bark, rotting sawdust and leaf mulch all have an acidifying affect on the soil over time. Some fertilizers including sulphate of ammonia and urea can have an acidifying affect on the soil over time.


Soil Management for Orchards and Vineyards (1993), G O’Conner, J Strawhorn, K Orr, AGMEDIA – Department Of Agriculture, Victoria, ISBN – 0 7306 3018 8