By the beginning of the irrigation season, the entire root zone is usually wetted by winter rainfall. Under low volume irrigation during the irrigation season only fifty percent or less of the root zone is wetted with each irrigation on most soil types. Soils with slow infiltration do not allow enough water to penetrate into the root zone to meet the plant’s water requirement. During an irrigation the water puddles while the soil beneath remains dry. Less than ten percent of the soil in the root zone may be wetted during an irrigation when water infiltration is a problem. Water storage in such a small volume of soil may amount to only two to three days of evapotranspiration. The tree may be under stress even though the amount of applied water exceeds the amount lost by evapotranspiration (ET). An infiltration problem is often associated with irrigation water low in salt and/or soils with inherently slow infiltration rates. Soil particles contain sites occupied by electrically charged ions such as calcium, sodium, and magnesium. In an optimum situation, a sufficiently high percentage of these sites are occupied by calcium which results in an aggregating or clumping effect among soil particles allowing water to penetrate. When the percentage of sites occupied by calcium is low and sodium predominates there is a repelling or dispersion of particles and water penetration is reduced. With increasing numbers of the exchange sites occupied by sodium ions the soil particles swell and repel each other creating a dispersion or loss of aggregation resulting in single particles. As this happens the porosity (or pore space) is reduced and the ability of water to enter is reduced. On the other hand as the exchange sites become more occupied by calcium the particles move closer together and aggregate or clump resulting in an increase in pore space. Therefore, soils that have a high percentage of the exchange sites occupied by sodium ions are dispersed and deflocculated and resist the entry of water while those with a high percentage of calcium ions are flocculated and favor water infiltration. With the use of low salt water over time, such as snow melt water, calcium may be removed from the soil particles exchange sites and these sites may then become occupied by another ion such as sodium.
Research addressing this problem of low infiltration was conducted in citrus under low volume irrigation by University of California researchers Peacock, Pehrson and Wildman. The soils type, at the experimental site of mature navel oranges, was a San Joaquin sandy loam characterized by a low infiltration rate. Canal water with a low salt content was used for irrigation. The trees were irrigated with a drip system every week day. Treatments began in June when soils typically begin to exhibit a reduced infiltration rate and were continued until mid-August but measurements continued until September. Simple devices for measuring the infiltration rate, called infiltrometers, were made from 12 inch PVC pipe and installed in the orchard. Chemical treatments and water were applied and rates of water infiltration were measured within these infiltrometers. Gypsum was applied weekly to the soil surface to maintain a slight excess continually on the soil surface and watered in resulting in gypsum application with each irrigation. Calcium nitrate and CAN-17 were each injected into the irrigation water. Calcium nitrate was introduced into the irrigation water at the rate of ten pounds per acre per irrigation. Calcium nitrate was applied daily, biweekly and in a single application. CAN -17 was applied daily, biweekly and in a single application. With these injections into the irrigation water, calcium was being introduced into the water at the rate of 3 milliequivalents per liter. Adding calcium continuously to irrigation water doubled infiltration rates over that of untreated low-salt water. It took 2-3 weeks before a treatment difference could be measured. However, the occasional additions of calcium nitrate or CAN-17 were not effective in maintaining infiltration rates. There were concerns that nitrogen application from these treatments could result in the nitrogen level in the tree being in excess of the tree’s nutritional requirements. Following this research equipment was made available on a commercial basis for regulated injection of materials into low volume irrigation systems.
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