14 Richard H. Boerker 



4. The amount of evaporation from the soil. 



5. The amount of water withdrawn by other plants. 



6. The replacement of loss by capillary movement. 



7. The amount lost by seepage, percolation, etc. 



Of these factors, only four are important in the present investiga- 

 tions. These are the water-holding capacity of the soil, the 

 evaporation from the soil, the replacement of loss by capillary 

 movement, and the amount lost by seepage and percolation. The 

 water-holding capacity of a soil is determined by soil depth, soil 

 texture, and the amount of organic matter present. In soil tex- 

 ture two factors are important, namely, the size of the soil 

 particles, which affects the surface area of the particles and the 

 amount of pore space in the soil, and the density of arrangement 

 of these particles. It is largely for these reasons that loam will 

 hold more capillary water and will contain more air space than 

 sand or gravel. Evaporation from the soil naturally affects 

 greatly the amount of water available to the plant. This is 

 affected by climatic factors such as temperature, relative humidity, 

 and wind ; and by soil factors such as texture, color, depth and 

 the character of the surface. The replacement of the loss of soil 

 water by capillarity depends upon the rise of water from the 

 water table. This rise is conditioned by the degree of saturation 

 of the lower soil layers, the texture of the soil, the height to which 

 the water must be raised and the character of the intervening soil 

 layers. A fine-textured soil like loam or clay is much more favor- 

 able in this respect than a coarse-grained soil like sand or gravel, 

 principally on account of its great ability to obtain water from the 

 lower soil layers. The amount of water lost by seepage and per- 

 colation depends largely upon the texture of the soil. The coarser 

 the soil the greater is the amount of. water that percolates through 

 it and the less is the amount held by capillary forces. 



As far as it determines the amount of soil moisture available 

 to plants, soil texture is certainly the most important physical 

 property of the soil and it deserves a foremost consideration in all 

 problems that pertain to the germination of seeds. 



