Sand dunes and highly calcareous soils occupy vast areas in arid zones. The soils are characterized by low available nutrient content and low to medium water-holding capacity of the upper soil surface. These features result in low vegetation density under arid climate conditions. The introduction of trickle irrigation coupled with liquid fertilizers turned desert sand dunes and highly calcareous soils into productive agricultural soils for high cash crops. The intensity of daily nutrient supply dictates the farmer's consideration of the form of nitrogen supplied to the crop and to the variable sensitivity of plants to the different forms of N supplied by various fertilizers through the irrigation system. The combination of high root temperature and high ammonium concentration is of particular importance. Plants that have relatively low carbohydrate content in their roots might suffer from ammonia toxicity if a high daily supply of ammonium is available. High concentrations of urea and very high calcium carbonate content in the soil are also dangerous to ammonium- sensitive plants like tomatoes. Planning the irrigation system and nutrient supply to the crops according to their physiological stage of development, and consideration of the soil and climate characteristics, can give high yields and high quality crops with minimum pollution, but salt accumulation on the soil surface should be prevented, either by sprinkle irrigation, or by plastic mulch during the growing season.
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