SOIL EXHAUSTION AND ROTATION OF CROPS. 213 



nitrogen, and the wheat crop has got the repute, among some 

 writers, of wasting a great deal of nitrogen in its growth. 



On another plot of land, where Mr. Lawes raised barley, he ap- 

 plied 200 lbs. of ammonia-salts, which contained 40 lbs. of nitro- 

 gen, and raised 48 bushels to the acre. When he doubled his 

 dose, and put on 80 lbs of nitrogen, his grain was so heavy that 

 it lodged and failed to ripen, and the crop was spoiled. Without 

 the addition of any fertilizer, the soil gave him considerably less 

 than half that amount. 



I will mention some other experiments which may give us light 

 on this subject, made by Dr. Ilellriegel, 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 Ger- 

 many, partly by the government and partly by associations of 

 individuals, for the purpose of making agricultural investigations, 

 by the help of chemistry and physiology, and whatever aids can 

 be brought to bear on these questions. Dr. Ilellriegel 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 nitro- 

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

 these various substances is necessary. In order to arrive at 

 accurate results, Dr. Ilellriegel 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 pos- 

 sible. 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, within each case one single and different exception. 

 These excepted substances he added in graduated quantities, 

 putting one quantity in one box of S(^l 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 

 best proportion of these ingredients. His trials have been ex- 

 tended 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 quantity 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 



