Subsurface drip irrigation (SDI) has been used on South African lucerne fields for more than 15 years. Commercial production of up to 25 tons of dry matter (DM) per hectare per year has been obtained, with water volumes of 25 to 30mm per week during peak periods. Apart from significant savings in water consumption, one of the great advantages of SDI technology is that the field can be irrigated directly after the lucerne has been cut and while the bales are still on the field. Growth can therefore be stimulated faster to achieve more cuttings per year.
Installation of drip lines
Drip lines are installed with a ripper at a depth of 20 to 30cm, with the lines approximately 1,2m apart. Generally a non-pressure compensated dripper supplying 2ℓ/h is used, with a 60cm spacing between drippers. Depending on the installation, the wall thickness of the dripper pipe can vary between 0,4 and 0,8mm. In soil with a high fraction of medium to coarse sand, drip lines have to be placed closer than 1,2m to each other to wet ample soil volume. It is vital to ensure that, for at least the first year of establishment, the lucerne is still irrigated with an overhead system. As the plants grow and the root system is established, overhead irrigation can be reduced and drip irrigation increased. It is possible to establish lucerne with a drip system alone, but it should be properly managed to ensure success.
Determining the soil potential is essential before considering drip irrigation. Deep, well-drained, high-potential alluvial soils of light to medium texture with good water retention ability and water distribution potential are ideal. In addition, water quality should be of an acceptable standard, especially with regard to iron (Fe), manganese (Mn), lime and electrical conductivity (EC).
Standard maintenance consists mainly of the regular flushing of the laterals and proper cleaning of the filters. For this purpose, the bases of the laterals are merged in a manifold and then rinsed using a rinsing tap.
Provision should also be made for the application of chemical agents to control Fe, Mn, lime, organic material and root penetration. It is crucial to make provision for the injection of chemical agents at the block valve. This junction point can then also be used to inject fertiliser. Just like any crop is treated against disease, it is imperative to act preventatively in respect of dripper clogging. System design is also very important in this regard. Since most of the irrigation components are underground, the system must be designed by an expert. Designers should take into account all safety factors, such as anti-vacuum valves, flow rate, lateral lengths and gradient for optimal functioning of the system. A water meter is useful, as it gives the first indication of possible defects in the system. -Izak Hofmeyr
Contact the Netafim irrigation and agronomy experts to learn more about how we can help you grow more with less. 021 987 0477, email@example.com, www.netafim.co.za