Keywords: Convection, diffusion, disease incidence, nutrient availability, pH, root-soil interface, water uptake.
The benefits and constraints of using high irrigation frequency in horticulture are discussed. High-frequency irrigation regimes enhance the time-averaged moisture content in the root region, and therefore increase the water availability to the plants. Yield improvements that follow increases of irrigation frequency are attributed to the improved availability of non-mobile nutrients, especially phosphorus and micronutrients. Frequent fertigation improves the uptake of nutrients through two main mechanisms: continuous replenishment of nutrients in the depletion zone in the vicinity of the root-soil interface, and enhanced transport of dissolved nutrients, because of the increased average water content in the medium. An increase in fertigation frequency enables the concentrations of immobile elements in the irrigation water to be reduced, so reducing potential environmental pollution. Frequent irrigation, in combination with NH4 nutrition, may be very effective for modifying the pH and, consequently, availability of nutrients in the rhizosphere. Thus, controlling irrigation frequency may be an important tool for improving the growth of sensitive plants under unfavourable soil-pH conditions. However, under a high irrigation frequency regime, it is recommended to modify the NH4/NO3 ratio, to diminish the risks of NH4 toxicity in sensitive crops.
The root system under frequent irrigation becomes shallower and has a greater density near the emitter. In addition, the root/shoot ratio decreases, mainly because of higher plant-P concentrations. Under saline conditions, frequent irrigation with saline water may be favourable to soilless-grown plants, but for soil-grown plants the high frequency may be disadvantageous because of the lower root/shoot ratio and the shallower root system. High irrigation frequency generally causes severe damage as a result of augmentation of the germination and the spread of soilborne pathogens. Therefore, it is necessary to reduce the watering level and/or irrigation frequency in order to decrease disease incidence.
Avner Silber, Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization, The Volcani Centre,
Bet Dagan 50250, Israel.
35 pages, 13 figures, 108 references.