BOIL EXHAUSTION AND ROTATION OF CROPS. 209 



Dr. Rinns. The land was what they call "clover-sick." 



Prof. Joiinson. "Clover-sick," and finally clover-dead ! 



Mr. Lawesmade another interesting trial on a piece of originally 

 similar ground, which had, however, been used as a kitchen-gar- 

 den probably for two or throe centuries. It was sown to clover 

 early in 1854, and from this one sowing the plant grew well, with- 

 out further manure, for six years and yielded'in that time fourteen 

 cuttings, at the rate of twenty-six tons of hay per acre for the six 

 years, or four and one-third tons yearly. 



In discussing the causes of clover sickness, Mr. Lawes sug- 

 gested that the assumption that clover requires a portion of food 

 to be supplied by the soil in the form of certain organic com- 

 pounds — vegetable matters or humus, such as are contained in 

 garden earth and come from the yard manure, would perhaps ex- 

 plain why the crop failed on ordinary soil, but should succeed in a 

 garden which had been heavily manured perhaps for centuries. 

 Mr. Lawes did not assert that this was the reason, only that it 

 might be. 



But I think we have facts enough to justify us in concluding 

 that that is not the reason. When a student in Germany, I saw 

 an experiment by Dr. Wolff of the Academy at Hohenheim, which 

 he was in the habit of making for the benefit of his classes. He 

 took a quantity of rather poor soil, and calcined it in a clay muffle 

 — a kind of oven which is heated by fire burning all around it, so 

 that its sides are brought to a bright redness. This operation 

 completely burned out all the organic matter of whatever kind 

 that was originally in the soil. To that soil he added the various 

 components of the ashes of plants which are given in the table 

 (see page 119), viz.: lime, magnesia, potash, soda, phosphoric 

 acid, etc., in proper proportions, together with a certain quantity 

 of saltpetre — nitrate of potash — and in that soil he raised the 

 most beautiful clover. You can grow anything to perfection in 

 that way. You do not meed a particle of organic matter in the 

 soil for the growth of any plant. Many plants have been grown 

 in simple water in which the mineral elements of the plant, in- 

 cluding nitrates, were dissolved or suspended. 



The suggestion that the result, in the case to which Mr. Lawes 



refers, was due to the absence of vegetable matter, must therefore 



be regarded as destitute of foundation. I believe that if he 



had spaded his land as deep as the roots of clover go, and had 



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