50 STUDIES IN THE VEGETATION OF THE STATE 



Prom the foregoing tables it is easy to determine the 

 relative positions of loose soils used in a scale of limits. If 

 placed in order of the highest to the lowest, the order is 

 saline, humus, loam, clay, loess, sand. 



Sand lowers the limit of available water. All the results 

 from sand averaged 0. 3 per cent, it being air dry when the 

 plant was near the death point. The sand used did not con- 

 tain sufficient silt to hold much hygroscopic water, for it con- 

 tained less than one per cent of silt. The presence of sand 

 in the soil has a direct effect in lowering the limit as was 

 shown by plants in a few pots which had sand mixed with 

 either loam or clay. It must not be supposed, however, 

 that because a plant assisted by evaporation and gravity 

 might take water out of sand to less than one per cent when 

 it leaves eight per cent in loam under the same conditions, 

 that sand is a better soil in which to grow plants in a dry 

 climate. Sand being so porous evaporates water very read- 

 ily, the rate of evaporation decreasing with a decrease in 

 the size of the grains. For this reason it evaporates water 

 more rapidly than other soils. It also allows water to pass 

 rapidly through it by gravitation to the stratum beneath, 

 since the forces of capillarity and surface tension are not 

 sufficient to retain the water as in other finer soils. The 

 soil which absorbs water most quickly loses it by evapor- 

 ation and gravitation most rapidly. It also loses the nu- 

 trient soil solutions rapidly by leaching. 



The finer soils like the clays hold water the most tena- 

 ciously. This quality tends to raise the limit of water by 

 increasing the hygroscopic water. In such soils the forces 

 of capillarity and surface tension as they are designated by 

 soil physicists may be arrayed against the forces of solution 

 operating in the root hairs of the plants, and tend to negate 

 the latter. If it be maintained that the great difference be- 

 tween the limit of the fine clayey soils and the coarse sandy 

 soils is due solely to the relative increase of these two forces 

 in the former, the limit of non-available water in saline, 

 humus, loam and loess should have been lower than that of 

 clay soil and higher than that of sand, for these soils contained 



