Quite clearly, the results of farming depend on adequate and timely sup-plies of water. The critical factor is not rainfall but soil moisture throughout the life history of the crop involved. As everyone with house plants or a garden knows, the appropriate conditions depend on the plant species, the stage of growth, the properties of the soil, and air temperature and humidity. Water in the soil feeds and cools growing vegetation as it passes into the plant through the root systems. The flow of fluid transports nutrients into the stems and leaves. When the air is hot and dry, the tomatoes of the leaves open and a great deal of water vapor transpires into the air. In the process, the plant is cooled. In a sense, this is the vegetative equivalent to human perspiration. Because of uncertainties about the quantity and scheduling of watering needed to achieve optimum crop yields, irrigation practices have been less than maximally efficient. Vegetation can survive and even thrive in moderately dry soil, but when there is inadequate water, leaves begin to wilt. A heavy rainfall or deep irrigation will improve the health of the plants, but will not entirely undo the harm caused by the desiccation. This means, of course, that when irrigation is available, a grower should not wait for signs of distress; that is, the plant should not be used as a measure of water insufficiency. The crucial factor, soil moisture, should be monitored directly. A procedure widely recommended to the home gardener is poking a metal rod or digging a few inches into the soil to see if it is wet. Water should be applied when the soil is dry. Surprisingly, in arid regions, where irrigation is a necessary practice, ,a great many plants are killed by over-watering. This happens when an amateur farmer, seeing dry surfaces, incorrectly assumes that the dryness extends into the root zone.
Commercial farmers sometimes use soil augers to gags the state of the soil. Another approach is to install electronic moisture sensors whose out-puts are recorded continuously. Some authorities believe these procedures are inadequate because they do not measure the wetness at enough points through a deep layer of earth. An alternative approach to the monitoring of soil moisture is to maintain a water budget of the cultivated field. This involves measuring the water added by precipitation and irrigation and balancing it against the water leaving the field by evaporation and transpiration. This requires computation of the last two processes, which are often lumped together and called transpiration.
