36 



SOIL CONDITIONS AND PLANT GROWTH 



but not in principle. The first dose of P.jO^ 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 



— ^Mtrogen 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 PjO^, however, 

 the values were 2-66 : 2'3i : i'65. 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 2'40 



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 1847 Lawes (160) wrote : " Whether or not superphosphate 

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



