4.3 Leaching Fraction

For risk assessment purposes, the efficiency of irrigation scheduling and management can be evaluated using the Leaching Fraction (LF) concept.

Leaching Fraction (LF) is a measure of the excess water that is applied during an irrigation event. It is the amount of water that runs out the bottom of the container divided by the total water applied to the container times 100 to convert to percentage.The Leaching Fraction diagram illustrates how this data is collected.

The process assesses how well the irrigator uses container size, substrate knowledge, plant size, and weather conditions in his/her process of applying irrigation water efficiently.

Leaching fraction (%) = Leachate from plant (gal) / applied irrigation volume (gal), x 100

The goal in irrigation application should be to maintain an adequate and uniform moisture level in the substrate. Leaching should be minimized if fertilizer is managed efficiently.

  • A leaching fraction of 15% or less is considered a best management practice, unless experience proves otherwise.
  • A leaching fraction test done on plant-occupied containers in the management unit (with normal irrigation practices), serves as a risk assessment test of irrigation scheduling efficiency.

It is important to understand how to apply this test. This test should not discussed with the grower / irrigation manager in detail in advance, as it may influence how they irrigate.

You should ask the grower when he/she is going to next water the different management units. Then, prepare the necessary lined containers for the test. Select the management unit area to be tested.

Put your containers in place - one plant-occupied container and one empty lined container next to each other. Place some in the middle and some toward the edges. Ask the irrigation manager to irrigate in his/her normal manner. Then remove your containers and collect the data.

Variation from middle of a bed to the edge or at different distances from sprinklers point out non-uniformity of application. This test is to evaluate how well the irrigation manager does; later a best management practice can be written if the leaching fraction is high or non-uniform. The irrigation manager can improve his management practices, if necessary.


Measuring Leaching Fraction

The procedure uses 5 to 10 plant occupied plants in each growing area (management unit) you are wanting to test:

  • Prepare double this number of empty, lined containers. Place a "spacer" block of wood in half of the lined containers, and then set a plant-occupied container in each empty, lined container.
  • Pair the rest of the empty, lined containers and set them near the plant-occupied lined containers in the growing area. When the crop is ready to be irrigated, ask the irrigation manager to operate the irrigation system for the normal length of time.
  • Collect all lined containers, allowing the leachate to fully drain from the plant-occupied containers. Take care not to tip the containers with plants as you remove them , as this will allow additional water to drain, affecting your LF measurement.
  • Measure the weight or volume of water in each container.
  • For each pair of containers (one with a plant and one without) divide the volume of leachate collected from the plant by the volume of applied irrigation water caught by the empty container next to it. If the water was weighed, divide the appropriate weights. The Leaching Fraction diagram illustrates this process (see above).
  • Use the formula above to calculate the average LF for the management unit .