A large capital investment is made when a sub-surface drip system is bought. For the sake of looking after your investment, it is vitally important to do regular maintenance and to know which type of maintenance to do, to ensure that the system can be used for as long as possible. Irrigation expert from Agriplas, Chris Barnard, gives practical advice:

Clogging that occurs can be classified into three groups:

1.Physical clogging:

The types of clogging that occur are usually because of sand that gets stuck in the dripper or silt that settles in the dripper flow path. The installation and use of effective filtration is important to prevent this from happening. Regular flushing of the system and the lateral lines will also prevent it. Flush the system once every two weeks by opening five laterals at a time and then closing them, and then repeating the process. Another reason for physical clogging is root intrusion into the dripper outlet after the system is shut down.

The following treatment is suggested to combat root penetration:

Treflan treatment to stop root penetration of the dripper

• The active ingredient of this product is Trifluralin
• The dosage is 300 ml Treflan for every 5 000 dripper outlets
• The first treatment should start directly after the first irrigation
• Repeat the treatment twice per season thereafter
• The treatment must be done during normal irrigation times
• Ensure that the system is under pressure before dosage is started
• Ensure that all the chemicals are completely and correctly dosed
• After the Treflan treatment, clean water must be supplied to the system for 15 minutes
• Treflan must not be used on soil that is either too wet or too dry

2. Chemical clogging:

This type of clogging occurs where calcium or magnesium carbonates, calcium sulphates and heavy metals or fertiliser precipitates in the dripper flow path. Table 1 can be used in conjunction with a water analysis to predict the clogging hazard of the water being used to irrigate with.

Water classification for the prediction of clogging hazard

The precipitation of carbonates normally happens at a pH of more than 7,5. If the pH goes lower than 5, the carbonates should stay in solution. If iron and manganese in non-soluble form are precipitated, a pH of 2 – 3 of the irrigation water will be needed to get those elements in solution again.

The use of acids is advised for this purpose. The choice of acid must be done on recommendation of an advisor. The following types of acids can be used to lower the pH, and have a contribution to the nutritional status of the soil:

• Phosphoric acid – also as a source of phosphorous for the plant
• Nitric acid – also as nitrogen source
• Sulphuric acid – also as sulphur source
• Hydrochloric acid – no specific use for the plant

As a result of the cost implication and the danger of using acids, the following must be kept in mind:

• Check for flow deviations from the designed flow for the system
• If a deviation of more than 15% occurs, do a physical measurement of the drip flowrate at at least 10 drippers

• Determine if there is carbonate precipitate by physically checking the drippers
• Acids can be readily used on soils that have been brought to field capacity, limited changes in the soil pH
• Always add the acid to the water, and never water to the acid
• Use safety clothes

The correction of the water pH can be done with a titration test that is done in-situ of the irrigation water.

The following method is suggested:

Use one litre irrigation water, titrate the acid in millilitres in the water and mix well. Measure the water pH and repeat the process until the ideal pH is reached. The amount of acid needed is multiplied by 1 000, in order to determine the amount of acid per cubic meter of water. The answer is multiplied with the flow of the system (cubic meters per hour) to determine the total acid required (in millilitres). The acid must be injected into the system over a period of one hour. The system must then be left standing  overnight with the acid. The next morning the system must be flushed well, before irrigation is started again. If more treatments are planned, in-situ tests must be done regularly. By using this method it can be determined how much acid is needed.

3. Biological clogging

Biological clogging occurs where micro-organisms (bacteria et cetera) are growing in the system. The organisms form colonies against the wall of the pipe (biofilm) and reduce the oxygen levels which in turn changes the state of some of the elements from soluble to dissoluble, for example iron and manganese. Some of the older colonies break off and move into the dripper flow path where it can result in clogging.

The following factors can increase the development of the biofilm:

• High water temperature, especially during summer months
• If the velocity of the water is low, as is normally the case with dripper lines
• Availability of nutrients, especially where fertiliser is added into the water (fertigation)
• Coarseness of the pipe wall helps the micro-organisms to bind to the wall
• Water with a pH of more than 7

Various methods for the control of the biofilm are available:

• The use of ozone
• Hydrogen peroxide
• Ultra sound
• Different commercial mixtures
• HTH or calcium hypochlorite or normal chlorine

The use of HTH

HTH is not a chloride, but a chlorite. Chloride is found in table salt, PVC and in chemicals. Chlorite is the active form which is very dangerous, toxic and a very good disinfecting material. Different types of chlorine are available including sodium hypochlorite (by-product is sodium), chlorine gas as well as calcium hypochlorite (by-product is calcium). When calcium hypochlorite (HTH) is added to water, calcium hydroxide and hypochlorite acid develop, which is a disinfecting material that destroys the bio film. The optimal use of HTH must be pH 7,0 – pH 7,5. Various concentrations of chlorite can be added, depending on the extent of the problem. It must be kept in mind that HTH only contains 65% – 70% active chlorite so adjustments to calculations must be made accordingly.

Methods of chlorite administering

• Continually: 1 – 2 ppm
• Periodically: 15 – 20 ppm
• Last irrigation: 3 – 10 ppm
• Shock treatment: 150 ppm

It is important to know that wherever chlorine is added continually, a chlorite measurement must be done on the furthest outlets. The measurement can be done with a DPD 1 test (similar to those done on swimming pools).

Conclusion

Sub-surface drip irrigation works very well if it is designed, installed and managed properly.

Information courtesy an Agriplas advisory on sub-surface drip on lucerne compiled by Chris Barnard. For more information please contact Agriplas at +27-21-917-7177.