x PRACTICAL PURPOSE 279 



termed the balance-sheet of plant growth ; and his 

 conclusions were adopted by the renowned German 

 chemist, Liebig, about 1840. Liebig took up the 

 complicated problems of soil constitution and fertility 

 with the practical purpose ' in mind of increasing its 

 productiveness. He traced clearly the relations between 

 the nutrition of plants and the composition of the 

 soil ; and he was the first to study carefully the mineral 

 constituents of plants, and to recognise the import- 

 ance of certain substances, especially potash and 

 phosphates. 



The principle of replacing artificially the substances 

 removed from the soil by crops was given a scientific 

 foundation by Liebig's work, and is now followed by 

 every progressive farmer. Knowing the nature of a 

 soil and the needs of a plant, suitable artificial fertilisers 

 can be applied to make up any deficiency in the main 

 constituents required for vigorous and profitable growth. 

 For example, a soil may be rich in humus, and in com- 

 pounds of nitrogen and . phosphorus, and yet be almost 

 barren land because of deficiency in another essential 

 constituent potassium. Dr. Cyril Hopkins tells an 

 impressive story of the result of applying potassic 

 fertilisers to such land in Illinois. A man who had been 

 farming soil of this kind came to see a demonstration 

 field of the Illinois Agricultural Experiment Station, 

 and brought with him his wife and children. 



As he stood looking first on the corn on the treated and 

 untreated land, and then at his wife and children, he broke down 

 and cried like a child. Later he explained to the superintendent 

 who was showing him the experiments that he had put the best 

 of his life into that kind of land. " The land looked rich," he 

 said, " as rich as any land I ever saw. I bought it and drained 

 it and built my house on a sandy knoll. The first crops were 

 fairly good, and we hoped for better crops, but instead they grew 



