SO!L CONDITIONS AND PLANT GROWTH 



Hehnont considered he had found it in water, and thus records his 

 famous Brussels experiment (132). "I took an earthen vessel in 

 which I put 200 pounds of soil dried in an oven, then I moistened with 

 rain water and pressed hard into it a shoot of willow weighing 5 pounds. 

 After exactly five years the tree that had grown up weighed 169 

 pounds and about 3 ounces. But the vessel had never received any- 

 thing but rain water or distilled water to moisten the soil when this 

 was necessary, and it remained full of soil which was still tightly 

 packed, and, lest any dust from outside should get into the soil, it 

 was covered with a sheet of iron coated with tin but perforated with 

 many holes. I did not take the weight of the leaves that fell in the 

 autumn. In the end I dried the soil once more and got the same 200 

 pounds that I started with, less about two ounces. Therefore the 

 164 pounds of wood, bark, and root, arose from the water alone." 



The experiment is simple and convincing, and satisfied Boyle (50) 

 who repeated it with " squash, a kind of Indian pompion " and ob- 

 tained similar results. Boyle further distilled the plants and concluded, 

 quite justifiably from his premises, that the products obtained, " salt, 

 spirit, earth and even oil (though that be thought of all bodies the most 

 opposite to water) may be produced out of water ". Nevertheless the 

 conclusion is incorrect, because two factors had escaped Van Helmont's 

 notice the parts played by the air and by the missing two ounces 

 of soil. But the history of this experiment is thoroughly typical of 

 experiments in agricultural chemistry generally : in no other subject 

 is it so easy to overlook a vital factor and draw from good experi- 

 ments a conclusion that appears to be absolutely sound, but is in 

 reality entirely wrong. 



Some years later about 1650 Glauber (107) set up the hypothesis 

 that saltpetre is the " principle " of vegetation. Having obtained salt- 

 petre from the earth cleared out from cattle sheds, he argued that it 

 must have come from the urine or droppings of the animals, and must, 

 therefore, be contained in the animal's food, i.e., in plants. He also 

 found that additions of saltpetre to the soil produced enormous in- 

 creases in crop. He connected these two observations and supposed 

 that saltpetre is the essential principle of vegetation. The fertility of 

 the soil and the value of manures (he mentions dung, feathers, hair, 

 horn, bones, cloth cuttings) are entirely due to saltpetre. 



This view was generally accepted by later writers. Mayow (195) 

 studied the amounts of nitre in the soil at different times of the year, 

 and showed that it occurs in greatest quantity in spring when plants 

 are just beginning to grow, but is not to be found "in soil on which 



