SOIL CONDITIONS AND PLANT GROWTH 



but not in principle. The first dose of P. 2 O 5 in the dry soil gives an 

 increased crop, and so does the second, the first not having been large 

 enough ; in the wetter soil, however, the increase is much larger. 

 There is a still further increase in the wettest soil, but less than before, 

 some other limiting factor now coming in. 



These relations are shown in the curves of Fig. 4, the ordinary 





65 



MostWater.(22'6%) 



More Water.(ia-4%>) 



Little Water.(l43/ ) 



^Nitrogen added over and above supply in Soil 



FIG. 4. Influence of Water Supply on the Effectiveness of Manures. (Von Seelhorst 



and Tucker.) 



series expressing the operation of a limiting factor ; they would more 

 properly be expressed by a surface. From the practical point of view 

 the important result is that a given increase in the food supply may 

 produce no increased growth, small increase, or a larger increase, ac- 

 cording to the extent of the water supply. 



Phosphorus. Phosphates are by far the most efficient phosphorus 

 foods known for plants. The relationship between phosphorus supply 

 and growth has been measured by E. A. Mitscherlich (p. 24) in a series 

 of experiments on oats grown in sand with each of the three calcium 

 phosphates. For equal weights of the three salts the relative efficiencies 

 corresponded with the basicity ; for equal weights of P 2 O 5 , however, 

 the values were 2'66 : 2^31 : i'6$. This was in sand cultures ; in soils 

 different efficiencies were found : thus for the mono-phosphate the 

 values were : 



Sand. Soil i. Soil 2. Soil 3. 

 2'66 i'8o i'74 2'4O 



The effect of a phosphate on the crop is twofold. In the early 

 stages of growth it promotes root formation in a remarkable way. So 

 long ago as 1 847 Lawes (160) wrote : " Whether or not superphosphate 

 of lime owes much of its effect to its chemical actions in the soil, it is 



