Soil suitability for irrigation

A wide range of soil types, ranging from sandy loam to clay loams, can be suitable for irrigation. In most cases, heavy clay soils should be avoided, as most plants do not tolerate sustained periods of waterlogging. There will be exceptions such as grapevines or other crops that are dormant in winter and can tolerate waterlogged soil during dormancy.

Most irrigated crops generally perform best in: non-stratified soils with good depth (1.2 metres deep), good organic matter content and nutrient supply, with no physical barriers to root growth, and which are neither too acidic or alkaline. Listed below are the desirable and undesirable features of soils for intensive irrigated horticulture.

Desirable features

• A good friable topsoil of sandy loam, loam or sandy clay loam, of 35 to 40cm depth with an abundance of organic matter (2% organic carbon) in the first 20cm
• A topsoil pH of 6-8
• A well drained and aerated subsoil, preferably to 1m depth, with good water holding capacity. Usually a loam, clay loam or non-sodic clay.
• The presence of limestone in the subsoil can be desirable, as these soils have the ability to drain well but also store water within their structure.
• A subsoil pH range between 6–8.

Undesirable features

• Soils that have a high clay content are prone to water-logging (shown as mottling) and remain cold and wet in spring. This can inhibit root growth, which begins as temperatures rise above 12°C.
• Shallow soils, which restrict root growth and subsequent crop development.
• Very deep or fertile soils can promote undesirable tree or vine vigour, which may be difficult to manage and affect fruit quality.
• When pH is below 5, key nutrients such as phosphorus, calcium and zinc become deficient, with increasing Aluminium toxicity.
• Limestone in the topsoil is undesirable as it causes lime induced chlorosis and limits root growth.
• High salinity or sodium in a soil.
• Excessive grit, gravel and stone in soils as it reduces water-holding capacity.

To determine soil suitability for irrigation, a soil survey should be undertaken by an experienced soil consultant. A comprehensive soil survey will also indicate potential drainage problems. In some irrigated areas a soil drainage system may need to be installed to remove excess saline water from the rising underground water table.

Rootzone depth potential of irrigated crops is affected by a range of soil parameters, including soil physical condition, hard rock or hardpan, soluble salts (including boron), alkalinity, acidity and sodicity. Some crops are more sensitive than others to at least some of these factors.

Soils are highly variable and as a result the amount of water stored in the soil that is easily available to the plant, known as the Readily Available Water (RAW), will also be variable. It is important that the irrigation system is developed accordingly to account for these variations.

Soils that have previously been used over long periods of time for agriculture should have a chemical residue test to determine the presence of any residues, which may affect irrigated crop health.

Irrigated crops are usually much easier to manage on flat land. However, gentle slopes often help in draining water and can greatly assist in air drainage to reduce the risk of frost.

Find out more:

• Assessing Agricultural Land
• Water Holding Capacity factsheet I map
• Deep Drainage Potential factsheet I map
• Rootzone Depth Potential sensitive crops (e.g. citrus, avocado) factsheet
• Rootzone Depth Potential intermediate sensitivity crops (e.g. stone fruits, almond, pome fruits) factsheet
• Rootzone Depth Potential hardy crops (e.g. grapes, olives) factsheet
• Rootzone Depth Potential root crops (e.g. potatoes, carrots, onions) factsheet
• Rootzone Depth Potential above ground annual crops (e.g. brassicas) factsheet
• A guide for irrigators on soil conservation
• Irrigation Glove Box Guide 2016
• Water - DEW