462 ANNUAL REPORT OF THp: Off. Doc. 



In the case of the sandy soil the same methods may be used, 

 though the results are directly the opposite of those in the case of 

 the clay. The addition of the lime, for example, has a tendency to 

 cement together and to solidify the stony particles, thus closing up 

 many of the pores and preventing the too rapid percolation of the 

 moisture to lower levels, as well as its rapid escape, while the intro- 

 duction of vegetable matter improves largely because the matter 

 itself has a greater absorbing and retaining power than the mineral 

 constituents of the original soil. 



The aim, therefore, in both of the cases referred to is to get into 

 and to hold in the soils of various kinds the water that falls in the 

 rains, and conserve it for the plant during its entire period of growth. 

 That is, all methods of improvement are based upon the principle 

 that water that is most useful to the plant is absorbed or held by 

 capillary attraction, and that its escape is due to the same cause 

 or capillary attraction. Where water exists in soils in the running 

 or free state, or in a form not useful to plants, where the water 

 level is too near the surface, the improvement then must consist 

 of operations which will lower its level in the soil so that an abund- 

 ance of room may be provided for the distribution of the roots of 

 the plants, and in order that the circulation of the air may not be 

 impeded, drainage is necessary here, and will be discussed later. 



Amount of Water Required by Plants. 



The amount of moisture that is consumed by a growing plant is 

 comparatively enormous, that is, the amount of water required for 

 growing a crop is very great in proportion to the dry matter con- 

 tained in it. On the average, in order to obtain one pound of dry 

 matter in the crop, over 300 pounds of water will be required; that 

 is, over 300 pounds of water must have been obtained by the roots 

 and carried up through the plant and sent into the atmosphere from 

 the leaf surface. Hence, some idea of the very great amount of 

 water that is necessary for the growth of a large crop is gained. For 

 example, a crop of hay from an acre of land, which contains a ton 

 (2,000 pounds) of dry matter, would not be regarded as a large crop, 

 yet io order to obtain it, there would be required over 300 tons (600, 

 000 pounds) of water, and since plants vary in their needs, the max- 

 imum requirements for a pound of dry matter may be as great or 

 greater than 600 pounds, or, in other words, large crops may draw 

 as high as 2,000,000 pounds of water from an acre. 



While the object in the conservation of soil moisture is to hold 

 it for use of the crop, it must not be understood that all crops are 

 equal in respect to their requirements. Crops differ widely in re- 

 spect to their use of soil moisture, depending upon the kind of crop, 

 the length of their period of growth and the object of their growtlh. 



