ESSENTIALS OF PLANT 



33 



editions of plant growth are therefore more or less variable, 

 according as moisture or weather conditions are more or less 

 favorable to growth than are soil conditions. The rate of growth, 

 for example, may at one time be controlled by the phosphoric 

 acid supply of the soil, and at another by decrease in the rate of 

 supply of moisture (drought) or by an excess of water or by too 

 cool a temperature, or too little sunlight, etc. The conditions of 

 the soil and its supply of plant food are more or less affected by 

 soil moisture, weather, or other conditions. 



The varying effect of the season upon plots with different 

 fertilizer applications, may be seen by comparing the yield of 

 wheat in a wet year and in a dry year, with the average for 51 

 years, on the plots at Rothamsted, 1 England. 



There is nearly as much difference between the average crop 

 on the farm-yard manured plot, and the crop during the wet 

 season, as there is between the average yield with no addition, 

 and with the farm-yard manure. 



Limiting Conditions are Dependent Variables. The limiting 

 conditions are not independent of one another, but to a certain 

 extent influence each other so that variation in one may affect 

 several. 2 Increase in water in the soil, for example, decreases the 

 air content, and may decrease soil temperature. By transpiring 

 more water, the plant can take up more plant food. Increase in 

 phosphoric acid of the soil may also increase the activity of the 



1 Hall, an account of the Rothamsted Experiments, p. 54. 



' 2 Cameron, Proc. Am. Soc. Agron., 1910, p. 102. 



