LOSSES OF MOISTURE AND PLANT FOOD BY PERCOLATION. 7 



only twelve inches of the same soil. The depth and character of the 

 subsoil affect the soil volume occupied by the plants, the quantity of 

 water retained in the subsoil, and the quantity of water which can be 

 moved by soil forces, from areas below the roots, to within reach of the 

 roots. 



The rooting habits of the plant determine, to a certain extent, the 

 soil volume which the roots will occupy. There is a great difference in 

 plants in this respect. In arid climates, plants seem to send their roots 

 deeper than in humid sections. 



The quantity and distribution of the previous rainfall determines the 

 degree of saturation of the soil at the time of planting. The soil may, 

 or may not, be saturated at this time. In some sections, it is usual to 

 speak of having "a good season in the ground," meaning thereby that 

 the previous rains have placed the soil in a good condition of satura- 

 tion at the time of planting the crop. 



The previous treatment of the soil may have some effect upon the 

 depth of rooting of the plant and thus on the soil volume occupied. 

 Subsoiling may, with certain soils, be of advantage in this respect. The 

 previous treatment will also determine, to a certain extent, whether the 

 winter rains sink into the soil, or run off on its surface. ' It will also 

 determine whether the soil has been in good condition to absorb the 

 rains, and will affect the losses of water by evaporation. Shallow sur- 

 face cultivation, previous to planting the crop, may be needed for the 

 purpose of conserving soil moisture. Fall plowing may be needed to 

 open the soil to the winter rains. On the other hand, fall plowing of 

 other soils may be a disadvantage. Spring plowing may be all right on 

 some soils and in some seasons, rut, under other conditions, it may 

 cause loss of moisture through the drying out of the soil. 



It is not our purpose to discuss fully any of the factors above men- 

 tioned. 



The amount and distribution of the rainfall during the period of 

 growth of the crops affect not only the quantity of water which pene- 

 trates into the soil and the quantity which runs off, but also the length 

 of the period between rains governs the length of the time during which 

 the plant must rely upon the store of water in the soil. 



The loss of the rainwater which runs off from the soil surface de- 

 pends on the slope of the soil, the condition of its surface, and the 

 character of the soil. The average run-off, due to the average precipi- 

 tation, is considered to be approximately as follows :* 



'Wilson, Irrigation Engineering. 



