SOIL CONDITIONS AND PLANT GROWTH 



When only little water is present the added -5 gram of nitrogen 

 is without effect, the supply in the soil being sufficient for the crop 

 needs : the water and not the nitrogen is the limiting factor. When 

 more water is added the plant can make more growth, and can there- 

 fore utilise more nitrogen : the added -5 gram now raises the crop by 

 10 grams. Again, however, the water supply sets a limit, and the 

 second -5 gram of nitrogen is without effect. When a liberal 

 supply of water is added the first -5 gram of nitrogen gives 20 

 grams of crop, double the previous increment ; but even this does not 

 represent the whole possibility, for the second *5 gram of nitrogen 

 gives a still further increase of 15-5 grams. 



The results of the phosphate series are somewhat different in detail, 

 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 



MostVfacer.(22-6%) 



I 



! 



GO 



65 



More Water.(l8-4%) 



Little Water (1437 ) 



i added over and. a'bove supply in Soil 



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



and Tucker.) 

 1 See footnote 3 on previous page. 



