CHAPTER XVIII 



THEORIES CONCERNING SOIL FERTILITY 



ABOUT three hundred years ago Van Helmont, a Flemish alche- 

 mist, planted a five-pound willow tree in 200 pounds of dry soil. 

 He watered it with rain water for five years, and then found that 

 the tree had gained 164 pounds and that the soil had lost only 

 2 ounces, in weight. Therefore, he concluded, water is the source 

 of plant food. While it seemed to him that his evidence was strong 

 and positive, all know now that his conclusion was wrong, and that 

 the air, the water, and the soil are all essential sources of plant food. 



It will be noted that 2 ounces removed from the 200 pounds of 

 soil correspond to 1250 pounds from 2 million pounds of soil. 



Later, Bradley, in his "General Treatise of Husbandry and 

 Gardening," argued that water could be distilled or evaporated, 

 which was not the case with willow trees; and, hence, that water 

 is not the food of plants. He held that air must be the food of 

 plants. 



About two hundred years ago, Jethro Tull, the inventor of the 

 first seed drill, taught that neither water nor air could be the food 

 of plants because they were furnished alike to all plants; whereas, 

 two adjoining fields produced very different yields because one 

 was impoverished soil while the other had been enriched. Tull 

 wrote as follows: 



"It is agreed that all the following materials contribute in some manner to 

 the increase of plants, but it is disputed which of them is that very increase of 

 food: (i) Niter, (2) Water, (3) Air, (4) Fire, (5) Earth. . . . 



" Niter is useful to divide and prepare the food, and may be said to nourish 

 vegetables in much the same manner as my knife nourishes^me, by cutting and 

 dividing my meat; but when niter is applied to the root of a plant, it will kill 

 it as certainly as a knife misapplied will kill a man ; which proves that niter is, 

 in respect of nourishment, just as much the food of plants, as white arsenic is 

 the food of rats, and the same may be said of salts. 



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