土壤田间持水量,即田间土壤所能保持的最大限度的水量,或称田间土壤最大毛管悬着水量。国内外农业科技人员,均将田间持水量到凋萎含水量之间的水分称为有效水,故田间持水量是有效水的上限。凋萎含水量是植物不能从土壤中取得水分而萎蔫时的土壤含水量,是有效水的下限。不同质地,不同结构土壤的田间持水量和土壤有效水含量是不同的。在干旱地区,对农作物来讲,适宜的土壤湿度小于此值时,水分对植物有效性就愈小,植物越难利用。如果土壤含水量超过田间持水量,水分就会下渗,导致地下水 Soil field water holding capacity, that is, the maximum amount of water that can be kept in the field soil, or the maximum capillary hanging water content of the soil in the field. Domestic and foreign agricultural scientists and technologists all call the moisture between field water content and wilting water content as effective water, so the field water capacity is the upper limit of effective water. Withering water content is the soil water content at the time of wilting when plants can not get water from soil and is the lower limit of effective water. Different texture, different structure of the soil field water capacity and soil effective water content is different. In arid regions, where the appropriate soil moisture is less than this value for crops, the less water available to the plant and the harder it is for the plant to utilize. If the soil moisture content exceeds the field capacity, water will infiltrate, resulting in groundwater