FERTILIZERS 125 



should be considered as a living mass and not as dead, inert 

 matter. Indeed, the inanimate parts of the soil have their high- 

 est significance as the environment of the bacteria which they 

 contain, and in part nourish. 



To understand the effect and value of fertilizers, a knowledge 

 of the chemical and physical composition of soils, and of the re- 

 lation of their composition to plant growth is essential. These 

 things must be clearly understood, because fertilizers act upon 

 the plant indirectly through their influence upon the compo- 

 sition of the soil. 



At the beginning of the nineteenth century Sir Humphrey 

 Davy said that the substances which constitute the soil "are 

 certain compounds of the earths, silica, lime, alumina, magnesia 

 and of the oxides of iron and magnesium ; animal and vegetable 

 matters in a decomposing state, and saline, acid or alkaline com- 

 binations. ' ' 1 



He also fully understood that the soil furnished nourishment 

 for the plants, and that different plants flourish best in different 

 soils. While he described the soil elements, often with sur- 

 prising accuracy, and was the most expert chemist of his time, 

 he did not adequately appreciate the plant foods contained by 

 the soil, and his conception of the functions of the elements 

 which he described was often extremely vague. For example, 

 he held that the silica which plants contain imparts to them 

 their rigidity. He recognized in a general way, however, that 

 phosphoric acid, potash and lime enter into the composition of 

 plants, and he successfully combated many unscientific notions. 

 The derivation of soils from rocks was also known in his time. 



Mineral or artificial manures were first studied systematically 

 by Liebig, whose views found their way into the United States 

 before the middle of the century. The publication of his work 

 in 1840 marked a new era in agricultural chemistry. Before 

 his time it was very generally held that organic substances were 

 the chief food of plants. This has been called the humus 

 theory. It was rejected by Liebig, who went to the opposite 

 extreme and held that organic matter has no part in plant life. 

 Practical knowledge of the use of manures, wood ashes, slaugh- 

 terhouse refuse, gypsum, lime and plaster as fertilizers was 

 widely diffused and acted upon before the time of Liebig, but 

 1 Yearbook U. S. Dept. Agr., 1899, p. 203. 



