SOIL CONDITIONS AFFECTING PLANT GROWTH 43 



soil, which in turn, reacts on the plant growth. If mathe- 

 matical treatment is to be attempted it must be on statistical 

 lines. 



The Effect on the Habit of Growth or Other Plant Char- 

 acteristics. 



Hitherto, we have dealt only with the actual weight of the 

 plant. Observations in the plant culture house or in the 

 field, however, show that two plants may have the same weight 

 and yet differ considerably in appearance, in proportion of 

 root or of seed to leaf and stem, in degree of maturation and 

 in other respects. These differences are often of considerable 

 technical importance, profoundly affecting the value of the 

 crop, and they are of great interest as indicators of soil condi- 

 tions. It cannot, of course, be assumed that a certain appear- 

 ance or character in the plant is always and necessarily 

 produced by the same soil conditions, but the appearances are 

 often symptomatic and serve to narrow the problem. The 

 detailed discussion of these differences is the province of the 

 modern science of Ecology, but the general results are of great 

 importance to the soil student.^ They will be dealt with 

 under the headings of the separate factors. 



Effect of Water Supply. 



Water. — The relationship between the amount of growth 

 and the supply of water is shown by Hellriegel's experiments 

 (130^, Table VII.) with barley grown under favourable con- 

 ditions in sand cultures. 



The yield rises as the water increases up to a certain point, 

 and then falls off because the excess of water reduces the air 

 supply for the roots. 



The grain suffers sooner than the leaves and stems. When 

 a series of plants is grown in this way, with varying water 

 supply, certain important qualitative differences are revealed. 



^ For an example of a survey carried out on these lines, see T. H. Kearney ; 

 L. J. Briggs, H. L. Shantz, J. W. McLane, and R. L. Piemeisel, Indicator 

 Significance of Vegetation in Tooele Valley, Utah, youm. Ag. Research, 1914, 

 I, 365-417- 



