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Fluid Journal : Fluid Journal 1996-1998
Winter 1996 Pre-Bloom Main 1st Fruit Set Harvest 5 4 3 2 1 Figure 1. N uptake of tomato plants from pre-bloom through harvest, Hartz, University of California. N uptake - lbs/A/day N uptake - lbs/A/day Thinning Cupping Harvest 5 4 3 2 1 Figure 2. N uptake of lettuce plants from thinning through harvest, Hartz, University of California. N application - lbs/A/day 30 25 20 15 10 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Weeks Figure 3. Pepper fertigation template over 14-week period, Hartz, University of California. high-flow tape or tubing. Particular caution should be take to see heavy soils stay moist. Irrigation frequency. The use of two easy rules will help decide how often to irrigate: 1) deplete no more than 20 to 25 percent of available moisture in the most active root zone, and 2) limit individual applications to less than 0.5 inches. Time vs volume. The only way to be sure you are applying the correct volume of water is by monitoring an accurate flow meter. Thinking in terms of duration or hours of run can lead to serious problems. Faulty equipment can skew the delivery rate of water considerably. Over-irrigation. Although some degree of saturation is inevitable, it is important to minimize this stress to get the full advan- tages of drip irrigation. A tensiometer reading less than 10 cb generally indicates that saturation or near-saturation conditions exist. Such monitoring will also help minimize leaching of nitrate below the crop root zone. Determining N requirements Uptake pattern. Vegetable crops differ widely in their nitrogen needs and in the pattern of uptake over the growing season. Fruiting crops such as tomatoes, peppers and melons require little N until flowering begins, then increase their N uptake, reaching a peak during fruit set and early fruit bulking period. Non-fruiting crops such as broccoli, celery and lettuce show slow N uptake through the first half of the season, with N need accelerating until just before harvest. Figures 1, 2 and 3 show typical uptake patterns for tomatoes, lettuce and peppers. Soil N. Most soil N is tied up in complex forms in organic matter and is unavailable to the plant. The rate at which these complex forms are broken down into plant-available forms is constantly changing, controlled by a series of interactions of crop residues, soil microbes, soil moisture, and temperature. When crop residues are incorporated into the soil, they can contain substantial N. After incorporation, this residue is degraded over time by soil microbes, releasing inorganic nitrogen available for uptake by the next crop. This release of available nitrogen, called mineralization, occurs rapidly from the breakdown of fresh, high- nitrogen crop residues and more slowly from the older, more stable organic matter in the soil. The result is that succeeding crops can draw a substantial portion of the nitrogen they need, perhaps 50 percent or more, from this pool of soil N. Ammonium and nitrate are the common forms of plant-available nitrogen. Most vegetable crops will use both forms, although most rapid growth is usually favored more by nitrate than by ammonium. More extensive monitoring of soil nitrogen status is justified, since drip irrigation provides the ability to add nitrogen on demand. One form of measuring is the use of soil solution access tubes (SSAT). Clearly, using SSATs to determine N deficiency would not be appropriate. However they do provide a simple and inexpensive method for determining when soil N is clearly sufficient. Other methods of measurement are extraction of soil N in aluminum sulfate solution and a "quick test" procedure. Application frequency. In sprinkler or furrow irrigated cultures, nitrogen applica- tions are generally large and infrequent. Drip irrigation offers nearly limitless
Fluid Journal 1993-1995
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