Basics on Irrigating Vegetables

Basics on Irrigating Vegetables presented by Mike Orzolek

Mike Orzolek, Professor of Vegetable Crops in the Department of Horticulture at Pennsylvania State University, has done extensive research on stand establishment, plastic mulches, high tunnels, weed management, and tillage systems. On Saturday he presented on the topic of irrigation.

Orzolek argued that drip irrigation is the most uniform and efficient means of irrigating a crop. Overhead irrigation is not uniform and leaves the foliage wet. Drip tape irrigates the soil only, leaving the foliage dry, which prevents fungal growth and infection. It also allows the grower control over how much moisture is added to the soil, allowing for efficient irrigation.

To know how much moisture a vegetable crop needs it helps to think of evapotranspiration (ET) rates, which is the rate of loss of soil moisture due to evaporation from the soil surface and transpiration from the plants. Calculating this rate helps the grower understand how much moisture must be added to the soil in order to maintain an adequate supply for the crop. Next, the grower can calculate the gross irrigation requirement (GIR), which takes into account the ET rate plus irrigation inefficiency. Knowing how much moisture is lost due to ET plus how much moisture is misapplied by the irrigation system can help a grower get an estimate of the daily irrigation requirement for his or her crops.

Other ways to determine the irrigation schedule are to observe the plants and measure the soil moisture content. It is important to irrigate before plants show signs of wilt. After a plant wilts, it is already suffering the consequences of inadequate moisture. Measuring the soil moisture level can help a grower determine if it is time to irrigate. Tensiometers, resistance blocks, and neutron probes are tools that can measure the moisture content of the soil. Computer software is also available, which can map soil moisture levels and help predict irrigation schedules. It is always important to irrigate when the plants need water, even if rain is in the forecast. It is better to have over-irrigated plants than to have plants that are water-stressed.
(by Luke Freeman Senior at the University of Missouri, majoring in Sustainable Agriculture.)