SOIL EXHAUSTION AND ROTATION IN CROPS. 



95 



of the plant. We cannot influence a field crop, except through 

 the roots. We do not manure the tops, or operate upon them 

 in any way. All out efforts to promote growth must be di- 

 rected to the root, and yet we do not know with precision 

 what is the extent and depth of the roots of the wheat plant, 

 for example, as compared with the roots of any other plant. 

 Wc simply know that some plants have more and longer roots 

 than others ; that clover, for instance, is deeper rooted than 

 wheat. 



Some important contributions to our knowledge of this 

 subject have been made quite recently, and I have placed 

 upon the blackboard some figures obtained by chemical analy- 

 sis of the residues of certain crops ; i. e., the stubble, and tlie 

 roots down to the depth often inches. At Proskau, in Prussia, 

 there is a Government Agricultural School, and Dr. Weiske, 

 one of the chemists connected with that school, a year ago last 

 summer, measured off certain plots of land, several yards in 

 dimensions, and carefully excavated the soil to the depth of 

 ten inches and with extreme pains dug out all the roots he 

 could get in that depth of soil. These he dried, weighed, and 

 analysed, and these figures show the average of his results, 

 calculated in pounds upon the surface of an acre. Unfortu- 

 nately, he did not state any thing about the quantity of the 

 crops ; but from the fact of their growing at Proskau, where 

 the soil has long been under cultivation, it is to be presumed 

 that these crops were good. 



COMPOSITION OF ROOTS AND STUBBLE — Ibs. per acre. 



The first column gives the amount of vegetable matter which 

 was contained in the roots and stubble. We are not informed 



