OPTIMUM PROPORTION OF WATER 73 



periments. Water was determined in the Broadbalk soils 

 down to 3 ft. below the surface in January 1 869 ; the highest 

 mean percentage of water found was 26-71, the lowest 23-17. 

 Taking the weight of dry fine soil down to 3 ft. as n-6 million 

 Ib. per acre (see Table IV), the water contents to that depth 

 become respectively 18-68 and 15-47 inches per acre. In the 

 case of the Wisconsin soils, in a natural state of consolidation, 

 but saturated in the laboratory (Table IX), the upper three 

 feet of soil contained 15-98 inches when fully saturated, and 

 13-12 inches after draining for four days. Both the soils may 

 be generally described as clay loams. 



The quantities of water per acre just mentioned, supplemented 

 by the summer rainfall, would be fully equal to the demands 

 of the largest crop that could be grown (p. 55). The quantity 

 of water contained in a soil saturated by rain is not, however, 

 permanently held, but rapidly diminishes by percolation and 

 evaporation. The diminution of the quantity of water in the 

 soil below the point of saturation is indeed essential for healthy 

 plant growth. 



Optimum Proportion of Water. When a soil is fully sat- 

 urated with water air is of course entirely excluded ; this con- 

 dition is most injurious to the health of plants. Many plants 

 may indeed be grown with their roots immersed in water, if this 

 water is freely exposed to air ; but the water in a subsoil is 

 exposed to air only when the interstices of the soil are but 

 partly occupied with water. Experiments have been made by 

 Hellriegel, and by Wollny, in which agricultural plants were 

 grown in jars of soil in which certain proportions of water 

 were constantly maintained. It appeared that when the soil 

 contained 80 per cent, of the water required to saturate it, the 

 proportion was too high, and that when the water amounted 



