Gordon Johnson, Extension Vegetable & Fruit Specialist;gcjohn@udel.edu
Each year we see fields were vegetable growth in drip irrigated plastic mulched beds is irregular. While root diseases, nematodes, or soil insect feeding can cause variable growth, the following other potential causes due to water quality problems, cultural practices, or irrigation system issues.
Plugged Emitters
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 seen most commonly in higher density planted crops such as peppers.
A common cause of plugged emitters is water containing high levels of dissolved iron. This often causes a proliferation of iron utilizing bacteria. These bacteria can 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.
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.
Improperly Designed or Maintained Drip Systems
Improperly designed drip systems can lead to over-watering or under-watering portions of the bed and cause variable crop growth. This most commonly occurs when systems are in too large of zones or have too small of supply lines where pressure and volume is too low or where length of run is too long. In these cases, the ends of the drip line will have much less water than the beginning of the run and will lead to a gradient of plant growth. Leaks in drip lines will also cause lower water delivery past the leak, leading to reduced plant growth.
Variable Depth of Planting and Transplant Handling
Many transplanted crops will show variability due to depth of planting. This is most common when the root ball is left partially exposed and dries out. If these plants survive they often will be stunted or will have reduced growth compared to plants around them. Planting too deep can also lead to variability in some plants. Rough handling or root ball disturbance can slow establishment of sensitive transplants leading to variability.
Variable Bed Formation
Variability in bed density and plastic laying can cause differences in plant growth. This is most common when plastic is laid in cloddy soils. This results in variable bed densities affecting root growth and water movement and variability in plastic contact with the soil surface leading to warm and cool spots thus slowing or speeding plant growth.