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2022 Midwest Vegetable Guide

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Field Production Information Midwest Veg Guide 2022 29 tensiometers. Soil tension or suction is a measurement usually expressed in centibars that describes how tightly water is held to the soil particles. The larger the value the drier the soil. Tensiometers directly read soil tension between 0 and 80 centibars and work best in sandy loam or lighter textured soils. Resistance blocks work in a wider range of soil textures, and some types, such as Watermark sensors, work as well in lighter textured soils, as do tensiometers. If the soil texture is known, use the Soil Water Deficit Estimates for Different Soil Textures and Selected Tensions table to estimate the inches of soil water deficit for a given tension reading; use the Soil Tension Values for Different Soil Textures For Use in Scheduling Trickle Irrigation table to estimate the point of 20 to 25 percent depletion. For example, let's say you have a sandy loam soil that has an AWHC of 1.5 inches per foot. A tomato crop would be irrigated before 50 percent (or about 0.7 inch) has been depleted in the upper foot of soil, or when a 6-inch tensiometer reads 45 centibars (Soil Water Deficit Estimates for Different Soil Textures and Selected Tensions). If we use the same soil for another example, a trickle-irrigated pepper crop would be irrigated when 20 to 25 percent (or 0.3 inch) has been depleted in the upper foot soil, or a 6-inch tensiometer reads 22 centibars (Soil Tension Values for Different Soil Textures For Use in Scheduling Trickle Irrigation). To obtain representative soil tension readings with any sensor, the sensors should be left installed throughout the irrigation season and preferably at two or more locations in the field. Two depths are generally desired at each location. These depths should be about one-third and two-thirds of the active root zone, or about 6 and 12 inches for a rooting depth of 18 inches. Estimating soil moisture by feel and appearance takes some practice. The Natural Resource Conversation Service (NRCS) provides instructions in Estimating Soil Moisture by Feel and Appearance, available at nrcs.usda.gov , or through your local NRCS office. A soil probe or shovel is used to collect samples from the desired depths. By observing the color and texture of the soil, squeezing it into a ball, pinching it between thumb and finger to form a ribbon, noting how well the ball holds together and how long a ribbon can be made, and comparing to photos or charts, it is possible to estimate soil water depletion and the percent of available water remaining. Soil Water Deficit Estimates for Different Soil Textures and Selected Tensions Soil Texture Soil Tension in Centibars (cbs) 10 30 50 70 100 200 1,500 1 Soil Water Deficit - Inches per Foot of Soil Coarse sands 0 0.1 0.2 0.3 0.4 0.6 0.7 Fine sands 0 0.3 0.4 0.6 0.7 0.9 1.1 Loamy sands 0 0.4 0.5 0.8 0.9 1.1 1.4 Sandy loam 0 0.5 0.7 0.9 1.0 1.3 1.7 Loam 0 0.2 0.5 0.8 1.0 1.6 2.4 1 1,500 cbs refers to the permanent wilting point and the soil deficit value is equal to the soil's total available water capacity. Soil Tension Values for Different Soil Textures For Use in Scheduling Trickle Irrigation Soil Texture 0% Depletion of Available Water Holding Capacity (Field Capacity) 1 20-25% Depletion of Available Water Holding Capacity 2 Soil Tension Values (in centibars) Sand, loamy sand 5-10 17-22 Sandy loam 10-20 22-27 Loam, silt loam 15-25 25-30 Clay loam, clay 20-40 35-45 1 At field capacity the soil contains 100 percent of AWHC; any excess water in the rootzone has drained away. 2 Start trickle irrigation for shallow-rooted crops at this point. Information adapted from Mid-Atlantic Commercial Vegetable Production Recommendations, New Jersey Ag Expt. Station, Rutgers; and Water Management in Drip- irrigated Vegetable Production by T.K. Hartz, UC-Davis, Calif., HortTechnology 6:165-67. Frost Control Irrigation can help protect vegetable crops from frost, although it is not a common practice in the Midwest. With the proper equipment, growers must begin sprinkling as soon as the temperature reaches 34°F. Place a calibrated thermometer at the lowest elevation in the field at plant level, facing skyward. Continue sprinkling plants until the air temperature is greater than 30°F and the ice has melted from the plants. To be effective, you need approximately 0.1 inch of water per hour, the sprinkling must be continuous, and the sprinklers should rotate at least once per minute. If conditions become windy and temperatures drop, it may be necessary to increase the amount of water to as much as 0.5 inch per hour. It is the process of the water freezing that gives off the heat to protect the crop. Therefore, liquid water must be present during the freezing period to protect the plants.

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