Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Drip irrigation has become standard for plasticulture vegetable and strawberry production. However, when drip system leaks or plugs, crops can be negatively impacted.
Drip system leaks can be man-made or caused by insects or by wildlife. Drip irrigation companies have good quality controls; however, sometimes defects occur in manufacturing leading to drip tape with perforations, unwanted holes, or splits, especially along the seam.
Drip tape abrasion during laying can cause holes along the seam. Another common cause of leaks is from too high of water pressure. This will cause “blow outs” and is most common where pressure regulators are not used, and operators are manually adjusting line pressure. Another common cause of leaks is when stakes are being placed for tomatoes or other crops if they are driven through the tape. Lay tape so that it is to the side of where stakes are to be placed. Similarly, planting spikes, dibbers, or tools can pierce drip tape as can wire marking flags and marking stakes. Poor connections to headers can lead to leaks, as can driving over or stepping on connectors.
Animals seeking water during dry periods can damage drip tape. Soil insects can be a major cause of leaks on lighter weight (8 mil or less). Crickets, especially mole crickets will shred along the edges where the dip tape was folded. White grubs cause similar damage. Wireworms will chew a round hole in drip tape. Some weevil larvae can also chew through drip tape. Larger ants will chew around emitters. Rodents such as voles can chew through drip tape as can other animals such as rabbits, shrews, moles, and squirrels. Birds normally will not damage buried drip tape but will peck at emitters where drip tape is exposed at the ends of rows. Deer may damage drip tapes with hooves as can domestic livestock.
Damage to drip tape is likely from insect chewing.
Drip emitters can become plugged with fine particles, mineral deposits, or biofilms. When emitters become clogged, the plants nearest the clogs will receive less water and have more water stress and grow less or be stunted. This is most severe in higher density planted crops, such as peppers.
A common cause of plugged emitters is water containing high levels of dissolved iron. This can cause a proliferation of iron utilizing bacteria. These bacteria form heavy biofilms on the inside of the drip tube. They also oxidize the iron in the water (as part of their metabolism) and leave behind iron precipitates that can plug emitters. Chlorination of drip lines is needed to control iron bacteria.
Periodic treatment before clogging develops can keep the system functioning efficiently. The frequency of treatment depends on the quality of the water source. Generally, two or three treatments per season is adequate. Irrigation water containing high concentrations of iron (greater than 1 ppm) can also result in clogging problems due to types of bacteria that “feed” on dissolved (ferrous) iron. The bacteria secrete a slime called ochre that may combine with other solid particles in the trickle tubing and plug emitters. The precipitated (ferric) form of iron, known commonly as rust, can also physically clog emitters.
Treating water containing iron with chlorine will oxidize the dissolved iron, causing the element to precipitate so that it can be filtered and removed from the system. Chlorine treatment should take place upstream of filters to remove the precipitated iron and microorganisms from the system. Take care when adding chlorine to trickle irrigation systems, however, since concentration at or above 30 ppm can be toxic to growing plants.
Options for treating water with high iron include the following:
For iron treatment:
- Inject liquid sodium hypochlorite continuously at a rate of 1 ppm for each 1 ppm of iron in irrigation water. In most cases, 3 to 5 ppm is sufficient.
For bacteria treatment:
- Inject liquid sodium hypochlorite continuously at a rate of 5 to 10 ppm where the biological load is high or
- Inject 10 to 20 ppm during the last 30 minutes of each irrigation cycle or
- Inject 50 ppm during the last 30 minutes of irrigation cycles one to two times each month or
- Super chlorinate (inject at a rate of 200 to 500 ppm) once per month for the length of time required to fill the entire system with this solution and shut down the system. After 24 hours, open the laterals and flush the lines.
Irrigation or water treatment companies can also install treatment systems to remove iron from irrigation water. This requires a water test to determine the form of iron and the proper system for its removal.
Another common problem in some aquifers is well water with high levels of calcium and magnesium (“hard water”). In high water pH conditions, these can precipitate out as calcium or magnesium carbonates that will clog emitters. If you look inside the drip tubing, you will see a white or chalky film. In addition, if soluble phosphorus fertilizers are put into water with high levels of dissolved calcium or magnesium salts, they can precipitate out as calcium or magnesium phosphates, also plugging emitters. Acidification of water can reduce or eliminate this problem. Also, avoid running phosphorus through the drip if you have hard water.
Inadequate filtering is another possible cause of plugged emitters. While this is most common when using surface water from ponds, ditches, or streams. It can also occur in wells that have fine particles in the water.
The water sample on the left has excess iron that can plug drip systems. However, clear water may also have dissolved iron. iron that is not visible to the eye can be present as ferrous bicarbonate.