SOIL EXHAUSTION AND ROTATION OF CROPS. ^85 



same invisible tooth nibbles at every point of their surface, rough- 

 ening and corroding them until they are reduced to dust. Even 

 the sand-grains are ever cut smaller and finer until they dissolve 

 away from our sense of sight or feel, and the long imprisoned 

 potash and lime, the phosphates and the sulphates, are released. 



It is the "Tooth of Time" which thus levels mountains and 

 crushes boulders into soil, and it is the same tooth whose incisive 

 workings in the soil reduce the elements of the rocks to the impal- 

 pable state of food for the plant. Where circumstances remain 

 the same, these changes prepare the nutriment for plants at a 

 certain regular rate, and the natural strength of the soil is simply 

 the expression of this steady development of plant-food and the 

 corresponding production of vegetable matter. 



To turn now to what Mr. Lawes calls the "condition" of the 

 soil. Farmers are in the habit of saying, " This land is in poor 

 condition " — or, " This is good soil, but it is rather run down ; it 

 is in poor condition at present." Or, looking over the fields of a 

 neighbor, who has taken a little extra pains, "This is poor land, 

 but he has got it up into good condition." " Condition," then, is 

 artificial or accumulated strength ; a thing we cannot depend 

 upon, except as we can depend upon the continuance of the arti- 

 fice or temporary causes of which it is a result. "Condition" 

 refers to those elements of fertility which are capable of being 

 turned to account in the growth of crops within a limited time. 

 We may have a " condition," which is the result of natural causes, 

 as is illustrated by the manner in which Indian coim is grown in 

 some parts of South America, on land newly cleared from the 

 forest. "You know that in tropical latitudes, the year is usually 

 divided into two seasons — the wet and the dry. During the former, 

 abundant rains fall and vegetation "grows with wonderful luxuri- 

 ance. The other half of the year is comparatively dry, and plant- 

 life is inactive. At the close of the rainy season, the planters 

 chop down the timber, the brush, and everything that grows upon 

 the land where they propose to get a crop. When the fatten veg- 

 etation is sufficiently dry, they set it on fire, and everything burns 

 completely except the largest trees. When the fire has gone out, 

 toward the beginning of the next rainy season, they have a field 

 destitute of vegetation and coated with the ashes of the forest. 

 There, with the smallest preparation, they plant their corn in the 

 ashes, dropping it where they can, and get a magnificent crop. 

 The second year they put on corn again and get another large 



