40 



SOIL CONDITIONS AND PLANT GROWTH 



factor now coming in. These relations are shown in the 

 curves of Fig. 5. 



A further illustration is afforded by experiments made by 

 the writer on the growth of tomatoes in pots of sand in which 

 supplies both of water and of nitrate were varied. The results 

 are shown in the curves of Fig. 6. The series of curves is 

 expressible by a surface, which is the proper way of represent- 

 ing the effect of two varying factors on plant growth. No 

 account was taken of temperature variations : to do this would 



Most Wacer.(.22 6%) 



Fig. 5.- 



MoTc Wattr.(j84%) 



Little Water . (U 3 %) 



— ^-Nitrogen added over and adove supply in Soil 



-Influence of water supply on the effectiveness of manures. (Von 

 Seelhorst and Tucker.) 



necessitate the construction of a series of surfaces each valid 

 for a particular temperature, or to adopt some mathematical 

 device equivalent to projection in a fourth dimension. 



In all these experiments it has been assumed that the ob- 

 served effect is caused by the direct action of the added sub- 

 stance on the plant, and the assumption is justifiable because 

 the plant is grown in sand which is very inert. When, how- 

 ever, a step nearer to natural conditions is taken, and the ex- 

 periment is carried out in soil it is no longer safe to assume 

 that the soil remains unaltered while the conditions are varied. 



Experiments on the growth of tomatoes in soil made 



