Chapter IX. 



THE RELATION OF PLANT GROWTH TO CONCENTRATION. 



That the concentration of the mineral constituents in the soil 

 solution under normal conditions is competent for plant support, 

 is shown by numerous experiments. Birner and Lucanus 1 in an 

 experiment that has long since become classic, found that they 

 could raise wheat to maturity in a well-water, the concentration 

 of which was approximately 18 parts per million with respect 

 to potassium, and 2 parts per million with respect to phosphoric 

 acid, while the corresponding concentrations of the soil solution 

 are normally about 25-30 parts per million of potassium and 

 6-8 parts per million of phosphoric acid. Nevertheless Birner 

 and Lucanus report that the wheat grown in the well-water 

 throve even better than that grown at the same time in a rich 

 garden mold. Since then many investigators in numerous trials 

 have obtained similar results. Recently wheat, corn, and some 

 of the common grasses have been grown to a satisfactory matur- 

 ity in tap water with a concentration of about 7 parts per million 

 of potassium and 0.5 parts per million of phosphoric acid. And 

 repeatedly wheat plants, grasses, cowpeas, vetches, potatoes and 

 other plants have grown in a satisfactory way in solutions made 

 by shaking up a soil in distilled water and separating from the 

 solid particles by means of filters of unglazed porcelain. 



There can be no doubt, therefore, that the soil solution is 

 normally of a concentration amply sufficient to support ordinary 

 crop plants, and is maintained at a sufficient concentration, so 

 far as mineral plant nutrients are concerned. Undoubtedly, 

 however, variations in the concentration of the soil solution can, 

 and often do, take place, and the results of laboratory experiment 

 indicate that they probably produce effects on plants. 



It has been shown in water-culture experiments with wheat, 

 that if a given ratio of mineral nutrients be maintained, relatively 

 small effect is produced on the growing plants by varying the 



1 Wasserculturversuche mit Haf er, von Dr. Birner und Dr. Lucanus, 

 Landw. Vers.-Sta., 8, 128-177 (1866). 



