THE WATER-COST OF DRY MATTER 145 



mental period. Under irrigation, the method is quite 

 different, for the water is applied at relatively long inter- 

 vals, and when the available soil moisture has been largely 

 removed by the growing crop. The saturation of the soil 

 falls, therefore, from high to low, between successive 

 irrigations. 



All experiments on the subject, whether in pots or in 

 the field, show that, as a general rule, the more water 

 offered the plant, within practical limits, during the grow- 

 ing season, the larger the total yield of dry matter. The 

 increase in dry matter due to the increase in soil-saturation 

 falls upon every part of the plant roots, stems and leaves. 

 Von Seelhorst and Tucker, among the early experimenters 

 in this domain, showed, in a series of carefully conducted 

 tests, that the whole oat plant heads, straw and roots 

 increased as the water in the soil increased. In the pots con- 

 taining a low percentage of water, 591 grams of the whole 

 plant were obtained; in the pot with a medium percentage 

 of water, 725 grams, and in the pots with a high percent- 

 age of water, 922 grams. In most experiments, only the 

 parts of the plants harvested by the farmer are considered, 

 so that this experiment, is of special importance. The 

 increase in the total yield of dry matter does not, however, 

 continue indefinitely, as the soil-saturation increases. 

 Mayer, who was one of the first to study the effect of 

 varying quantities of water, found that for rye, wheat, 

 barley and oats, the yield increased with the increase in 

 soil-saturation up to a certain point, after which there 

 was a strong diminution in the yield of dry matter. Experi- 

 ments made elsewhere bear out this conclusion. As a 

 further general rule, then, increasing the soil moisture 

 increases the production of dry matter only within cer- 

 tain definite limits. If too much water is applied to the 

 j 



