Drip systems require special attention to fertilizing. Because the water flow is from each emitter downward and outward, fertilize that is not placed between the emitter and the plant roots may never reach the plant roots. The basic principle is to be sure fertilizer is placed within the wetted zone.
Some methods that work are:
- Placing controlled-release fertilizer pellets under each emitter (or between the emitter and the plant).
- Side-dressing with soluble fertilizer, as long as you side-dress between the emitter and the plant, and dig it into the wetted zone.
- Fertilizing through the drip system.
This last option — fertilizing through the drip system — is the method used by most commercial operations that rely on drip irrigation. To use this method, purchase a fertilizer injector for your drip head assembly. Two types of injectors are in common use:
- Some T and Y filters can be used as fertilizer injectors. The manufacturer probably sells fertilizer pellets made to fit the filter, or you can purchase pellets made for the purpose that fit inside your filter. To use, unscrew the filter as if you were cleaning it and drop in the pellet. As the system waters, fertilizer dissolves from the pellet to enter the irrigation water. Follow the pellet manufacturer’s instructions for frequency of use.
- The other type of injector, which is also assembled into the drip head, draws liquid fertilizer through a tube and injects it into the system at a pre-set (sometimes adjustable) concentration. With this type, you can add any liquid fertilizer to the bucket from which the fertilizer is drawn.
To avoid clogging the drip lines with solids precipitating out of solution, follow these rules:
- Don’t mix chemicals, including different fertilizers. Some combinations, such as calcium or phosphorus with sulfates, cause precipitates, which can plug up emitters.
- Be sure the fertilizer is completely dissolved (unless you are using a soluble tablet in a filter)
- Keep the fertilizer concentration dilute. When calculating the amount of fertilizer to use, make it dilute enough so that it will be injected for most of one irrigation cycle.
Use fertilizers that dissolve completely, without residue.
Fertilizing methods that do not work with drip irrigation are:
- Sprinkling dry fertilizer on the surface of the soil, even if you scratch or water it into the soil. Most of it will not get into the wetted zone.
- Using a liquid fertilizer with a hose or sprinkler. Again, most of it will not enter the wetted zone.
- Using an organic mulch with a high nutrient value, such as compost or manure. Because the drip system is under the mulch, it will not wash the nutrients from the mulch into the soil. Even if you place the emitters above the mulch, only a small area of mulch will be wetted by each emitter.