2 SOIL CONDITIONS AND PLANT GROWTH 



plant is burned it is reduced to a salty ash called alcaly by 

 apothecaries and philosophers. . . . Every sort of plant with- 

 out exception contains some kind of salt. Have you not seen 

 certain labourers when sowing a field with wheat for the second 

 year in succession, burn the unused wheat straw which had 

 been taken from the field ? In the ashes will be found the 

 salt that the straw took out of the soil ; if this is put back the 

 soil is improved. Being burnt on the ground it serves as 

 manure because it returns to the soil those substances that had 

 been taken away." But for every speculation that has been 

 confirmed will be found many that have not, and the begin- 

 nings of agricultural chemistry must be sought later, when men 

 had learnt the necessity for carrying on experiments. 



The Search for the " Principle " of Vegetation, 1630-1750. 



The earlier investigators sought for a " principle " of 

 vegetation to account for the phenomena of soil fertility and 

 plant growth. The great Lord Bacon (8) believed that water 

 formed the " principal nourishment " of plants, the purpose of 

 the soil being to keep them upright and protect them from 

 excessive cold or heat, but he also considered that each plant 

 drew a " particular juyce " from the soil for its sustenance, 

 thereby impoverishing the soil for that particular plant and 

 similar ones, but not necessarily for other plants. Van Hel- 

 mont regarded water as the sole nutrient for plants, and his son 

 thus records his famous Brussels experiment (131) : "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 1 69 pounds and 

 about three ounces. But the vessel had never received anything 

 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 sh^et 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 



