Soil and Scheduling
by Stuart Spaulding,
CLIA Certified Landscape Irrigation Auditor Training & Communications Manager at DIG Corp.
Any discussion of watering schedules for landscape drip irrigation systems should begin on site with an appraisal of the soil type. Drip irrigation systems, unlike sprinkler systems, apply water directly to the soil, and they deliver the water at a much slower rate, measured in gallons per hour rather than gallons per minute. This low application rate gives drip systems an advantage over sprinkler systems when it comes to scheduling, especially in heavier, denser soils that cannot absorb water very quickly.
Soils vary greatly from region to region in their texture, composition, and ability to hold water and moisture. Rich, deep loamy glacially deposited topsoil is quite uncommon in the US. More typical are compacted, layered soils, with a shallow upper layer of topsoil, but contrary to popular belief, even in rich loamy soils, and lighter soils with high sand content, the majority of nutrient absorption and moisture intake occurs in the top 2 feet of soil.
Plants and trees can only absorb nutrients in a water solution. If the root zone area is too wet or too dry, the nutrient intake at the root level will be inhibited. The goal is to maintain the optimum moisture level in this layer at all times.
Fortunately, with low-volume drip systems, that’s not too difficult to accomplish. With drip irrigation systems, this means relatively frequent irrigation cycles with a duration long enough to allow the water to percolate down to the lower level of the 2 foot layer.
The optimum duration can be estimated by using a soil probe and checking the moisture level in the soil at different depths after running the system for 30, 60, 90 minute periods. The ideal moisture level will be when the soil feels about as moist as a wrung out sponge.
A variety of factors determine how quickly the water in the root zone area is depleted and therefore how frequently the water should be applied. Some of these factors include plant species, plant density, and of course changes in weather. Dense clay based soils will hold moisture longer than sandy loamy soils, and so the latter will need to be irrigated more frequently.
More detailed information regarding plant water requirements, ET and irrigation scheduling can be obtained from the IA at www.irrigation.org.