RELATION OF PLANT GROWTH TO CONCENTRATION 7 1 



concentration over a wide range, in one case from 75 parts per 

 million to 750 parts per million, 1 and this effect seems to be 

 largely independent of the nature of the particular mixture of 

 solutes. But varying the relative proportions of the mineral 

 constituents has been shown by numerous experiments to produce 

 very marked changes in the growth of plants. Not only does a 

 control of the concentration and proportion of the mineral con- 

 stituents of a solution produce a more rapid, or a slower growth, 

 a greater or lesser total growth, but it produces differences in 

 the character of growth; as for instance, causing the tops to 

 grow relatively faster than the roots, or vice versa. However, 

 many effects of this type can be produced, and sometimes more 

 readily, by soluble organic substances, or mechanical agencies. 

 The mechanism of these effects is by no means clear, in many 

 cases. That other causes obtain than a sufficient supply of 

 mineral nutrients will be shown in the following chapters. Ex- 

 periments with wheat seedlings in water cultures, where the 

 weights of the green tops were taken as the measure of growth, 

 showed that the most favorable ratio was one of phosphoric 

 acid (PO 4 ) to three or four of potassium (K), about the ratio 

 which has been found to exist normally in the soil solution of 

 humid areas of the United States, namely, 6-8 parts per million 

 of phosphoric acid to 25-30 parts per million of potassium. 



All growing plants require for their growth and development 

 various organic compounds containing carbon, hydrogen, oxygen 

 and nitrogen. The higher crop plants with which agricultural 

 investigations appear to be more immediately concerned, seem 

 to have inherent power to produce these needed substances 

 within themselves. But it is becoming more and more evident 

 that the large problem of soil fertility, or the relation of the 

 soil to crop production, frequently if not generally involves the 

 growth and development of lower organisms including ferments 

 and bacteria. These may or may not in particular cases, favor 

 the growth of the desired higher plants. Many of these lower 

 organisms require certain organic compounds or thrive better 



1 Effect of the concentration of the nutrient solution upon wheat 

 cultures, by J. F. Breazeale, Science, n. s., 22, 146-149 (1905). 



