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Soil suitability for irrigation

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Summary

A wide range of soil types, ranging from sandy loam to clay loams, can be suitable for irrigation.

This page will help you understand:

the soil features which are desirable when implementing irrigation
unsuitable soil environments for irrigation.

Assessing soil suitability for irrigation

To determine soil suitability for irrigation, a soil survey should be undertaken by an experienced soil consultant, which will indicate potential soil constraints and drainage problems.

Most irrigated crops generally perform best in non-stratified soils (uniform texture and structure) with:

good depth (1.2 metres deep)
good organic matter content and nutrient supply
no physical barriers to root growth
optimal pH for nutrient availability (neither too acidic or alkaline).

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.

Listed below are the desirable and undesirable features of soils for intensive irrigated horticulture.

A good friable topsoil of sandy loam, loam or sandy clay loam, of 35 to 40 cm depth with an abundance of organic matter (2% organic carbon) in the first 20 cm.
Soil pH of 6-8 (both topsoil and subsoil).
A well-drained and aerated subsoil, preferably to 1 m 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.

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

Depth to groundwater

A soil drainage system may need to be installed to remove excess saline water from the rising underground water table.

Soil factors affecting rootzone depth of irrigated crops

This includes 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.

Soil variability, particularly Readily Available Water (RAW) variability

RAW describes the portion of soil water that a plant can easily extract without experiencing growth-inhibiting stress. Irrigation systems should be developed in a way which accounts for these variations within the landscape.

Previous land use

Agricultural land should have a chemical residue test to determine the presence of any residues (e.g. heavy metals, herbicides and salts), which may affect irrigated crop health.

Slopes

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.