SOIL MOISTURE 65 



precipitation and irrigation or to a limited extent from the gravitational 

 water that reaches the ground water level. The optimum water content 

 for the growth of plants is reached when its relative saturation is approxi- 

 mately 50 per cent. A certain percentage of the soil moisture is held in a 

 capillary adsorbed or colloidal form and is not frozen at the ordinary 

 freezing point of water. This portion of the water supply is of great 

 importance to the plant. The evidence indicates that a part of the water 

 of plant tissues is held in a similar manner and that this moisture is 

 significant in determining drought and frost resistant qualities of the 

 tissue in question. The percentage of the rainfall that percolates 

 beyond the range of the tree roots varies greatly with many factors, total 

 precipitation being one of the most important. Comparatively little 

 water that percolates beyond the range of the roots becomes available 

 for later use through capillary rise. The lateral movement of soil 

 moisture depends principally upon soil texture and the method by which 

 irrigation water is applied to the soil should be determined accordingly. 

 Root distribution is governed first of all by the growth characteristic 

 of the species or variety in question. To an important extent, however, 

 it is influenced by soil conditions, particularly soil moisture and soil 

 aeration. A deep, moderately wide-ranging root system is preferable to 

 one that is shallow, wide spreading or narrow. Though the great major- 

 ity of the roots of most orchard trees are in the upper foot or fifteen inches 

 of soil, there is little evidence that ordinary tillage results in an injurious 

 root pruning. Shallow soils, soils underlaid by hardpan or with a high 

 water table, should be avoided for fruit culture because of the restricted 

 root range that they necessitate and the consequent susceptibility to 

 drought injury of one kind or another. Depth of rooting can be con- 

 trolled to a considerable extent by cultural practices, such as tillage, the 

 use of cover crops or intercrops of different kinds, irrigation and drainage. 



