SOIL EXHAUSTION AND ROTATION OF CROPS. 165 



I will mention some otlier experiments which may give us 

 light on this subject, made by Dr. Hellriegel, who has been 

 studying agricultural problems for some twenty years, having 

 been all this time employed in one of the Experiment Stations 

 kept up in Germany, partly by government and partly by 

 associations of individuals, for the purpose of making agri- 

 cultural investigations, by the help of chemistry and physi- 

 ology, and whatever aids can be brought to bear on these 

 questions. Dr. Hellriegel proposed to himself to ascertain 

 what quantities of the different materials which plants require 

 for their growth must be furnished to them in order to get a 

 crop. We have for some years known that phosphates and 

 sulphates of potash, lime, and magnesia, and nitrogen must 

 be given, but we need to know how much of each of these 

 various substances is necessary. In order to arrive at accu- 

 rate results. Dr. Hellriegel had to experiment under artificial 

 conditions. So he took for soil a perfectly pure sand, or one 

 as nearly free from everything that would furnish plant-food 

 as possible. In a large series of experiments, he mixed the 

 soil with a sufficient quantity of all the materials necessary 

 for the support of a crop, with in each case one single and 

 different exception. These excepted substances he added in 

 graduated quantities, putting one quantity in one box of soil 

 and a larger in another, and so on through a sub-series of 

 eight or nine boxes, in order to ascertain by the growth of the 

 plant, in which case he had hit the l^est proportion of these 

 ino-redients. His trials have been extended to the whole list 

 of the elements of the plant. In regard to water, for example, 

 he found that the growth was greatly influenced by the quan- 

 tity of this substance with which the crop was supplied. 

 There was a certain quantity of water in the soil necessary to 

 a maximum crop, other things being equal. In the sandy 

 soil which he experimented with, the largest yield of rye, 

 wheat, or oats was obtained when the soil held steadily ten 

 or fifteen per cent, of its weight of water. On increasing this 

 proportion, the straw in some cases was heavier, but the grain 

 was reduced in quantity. Thus the very fact that tlie amount 

 of rain fall is unequal in absolute quantity, and unequal in 



