Syncing the Amount of Water Available to Crops with Their Need for It

Vegetable growers work to ensure their crops have the water they need, but only the amount they need, from crop initiation through harvest. It’s a big, complex job requiring attention to water supply and demand, including soils and crops, and much preparation because the time to verify that drainage and irrigation capacities are adequate is before they are needed.

Indeed, when rainfall is excessive, the combined abilities of the soil and drainage system to accommodate the load is key, along with the crop’s ability to withstand flooding. Mapping fields soon after a “flooding rain” or lengthy irrigation cycle can identify where drainage improvements may be essential. Such improvements often result from combinations of “plumbing” (e.g., tiling), soil management and additives, and rethinking (lengthening and diversifying) crop rotations. If water is entering the high tunnel from outside it, grading the site may be necessary.

Irrigation is applied when there is clear evidence soil moisture is below crop need. Verifying the irrigation system is up to the task and knowing when to turn the system on and off are three critical tasks and decisions.

Whether overhead or drip, each irrigation system can deliver a specific amount of water in a fixed amount of time. However, using flow meters and periodically checking the condition of the system helps verify the flow rate to the crop is truly equal to the system’s calculated capacity. Leaks and blockages interfere with delivering the desired amount of water throughout the field or planting.

Knowing when and for how long to run the irrigation system requires clear and reliable reads on both the crop’s need for water and its availability from the soil. Years of farming experience and a strong understanding of soil conditions and irrigation systems are obvious assets in making those assessments. However, off-the-shelf technology (e.g., soil probes, tensiometers, gypsum blocks, flow meters) and decision aids (e.g., crop evapotranspiration calculators, irrigation charts in production guides) complement experience and reduce the irrigation learning curve. Irrigate to meet crop need and avoid under- or overshooting it significantly as often as possible.

Another point about crop water needs which affect the timing and/or amount of irrigation: they vary meaningfully with multiple crop-based, abiotic, and grower-based factors.

Two related ones may be critical at this point in the season many have experienced to date. First, alternating dry-wet periods have made it difficult for some to maintain soil moisture at levels consistently equal to crop need. For example, some have asked how much to credit rainfall under a variety of circumstances, including when plastic-covered raised beds are used. Moisture sensors under the plastic would be useful in making that determination. Second, soil conditions and irrigation practices can shape root systems which, in turn, influences the crop’s ability to take-up water. Overall, excess soil moisture, including because irrigation has been excessive, can result in shallow root systems. Shallow root systems limit plant access to water, making the crop more prone to displaying symptoms of low water stress although moisture levels deeper in the soil profile may be adequate. Wilting or other symptoms trigger irrigation, continuing the cycle. So, while it is never clear beforehand if the season will be dry, wet, or just right, the best practice is to irrigate according to crop need from the start and to rigorously cross-check assessments of crop need against the best-available information.

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