Sandy soils: is it best to water plants by drip irrigation?

Growing healthy plants in sandy soils in dry areas is challenging! This includes gardens in coastal areas as well as crop production on light textured soils, not to mention revegetation of desert areas. Hot temperatures and limited water supplies often make the job even harder. It is therefore vital to use precious water resources as efficiently as possible.

Surface (flood) irrigation is often not suitable for crops in sands, as it wastes water. There is poor water distribution due to rapid infiltration at the inlet end (head) resulting in overwatering, with the tail-end being under-watered.  There is also high surface evaporation from the open water surfaces in the borders or furrows (overall water efficiency is often < 50%).

Drip irrigation offers a better alternative. It applies water slowly via drops of water, onto or under the surface, and it wets only the root zone, not the entire soil volume. It is much more water efficient, often greater than 90% (combining distribution and application efficiencies).  Drip may improve crop quality and quantity while using less water per unit land than other irrigation systems.  It reduces evaporation losses, leaching, water runoff and erosion losses while maintaining ideal moisture in the plant root zone.  Fertilisers can also be supplied using this system. Drip can be applied above, on, or below the surface and on different terrains.  In recent years, improved dripper design, digital technology and wireless plant/soil sensors help to provide the exact amount required by the plants, reducing water wastage while maintaining ideal soil conditions. Solar pumps are also being used to reduce reliance on electricity or fuel power.  Combining drip with mulching and using water-wise plants offers the best water saving combination in sandy gardens.

Apart from gardens or crops, drip systems can be used to establish plant cover for land remediation and erosion control.  An example of this is found in the Middle East. Wind continually blows desert sand onto vital infrastructure such as highways.  Plants can be used to stabilise the sands and help prevent this, as illustrated in the picture below. So, how do you get plants to grow in searingly hot and dry conditions?   You apply drip irrigation along the edges of highways to establish and sustain plants and help stop the sand creep.

However, drip systems in sandy soils must be carefully designed and installed to achieve good results. The soil area wetted by the dripper (known as the water ‘bulb’) varies according to soil type (see diagram). In porous sands with low water retention, the bulb is long and narrow (often < 30 cm diameter). Drippers may therefore need to be spaced more closely in sands (for example every 20 to 30 cm) compared to clay soils, where the bulb diameter is much wider (often >75 cm).  Water is also often applied more often and in smaller quantities in sandy soils compared to clay soils, as this minimises leaching. With digital technology, applying exact amounts in varying locations has become much easier.

Attention must be also paid to potential salinity in the non-wetted areas when using drip. The salinity often remains low in the wetted bulb itself but can rapidly increase in the non-wetted area adjacent to the bulb, as the salts accumulate.

Once installed, drip irrigation requires very good management to sustain the practice.  Clogging of the drip lines must be avoided by proper water filtration and drip line maintenance

Want to learn more about drip and other forms of irrigation or water saving?  Have a look at our Irrigation-agricultural and  Irrigation-management courses or  try our Water-conservation-and-management course.