The Composition and Use op Fertilizers. 207 



(c) Specific Functions in Plant Growth. — While magnesium 

 closely resembles calcium in many ways, it cannot replace in 

 plant nutrition. Magnesium accumulates in seeds, flowers, roots 

 and tubers to a much larger extent than it does in straw, steins, 

 leaves and wood, while the reverse is true of calcium. Hence 

 magnesium compounds are especially important in the formation 

 of seeds. In the absence of calcium compounds, magnesium 

 compounds may act as poisons in the plant. Magnesium appears 

 to be associated with nitrogen compounds in the formation of 

 proteids, and it also appears to have some effect upon the forma- 

 tion of the green coloring matter of plants (the chlorophyll). 



Sulphur. 



(a) Source. — Sulphur, uncombined with other elements, is 

 found near volcanoes. Combined with other elements, sulphur is 

 found in a great many minerals. Sulphur is also found in vegeta- 

 ble and animal products, combined with the elements carbon, 

 hydrogen and nitrogen. The properties of the element sulphur 

 are too well known to need any description. 



(b) Compounds.— When sulphur is combined with hydrogen 

 and oxygen in certain proportions it makes sulphuric acid, com- 

 monly called oil of vitriol. When the hydrogen of sulphuric acid 

 has its place taken by any metal, a sulphate is formed. For ex- 

 ample, when the metal potassium takes the place of the hydrogen 

 of the sulphuric acid, a salt is formed known as potassium sul- 

 phate, commonly called sulphate of potash; from sulphuric acid 

 in the metal calcium is formed the salt calcium sulphate, com- 

 monly known as sulphate of lime or gypsum. 



(c) Relation to Fertilizers. — Sulphur is known to be an essen- 

 tial constituent of plant food. So far as known, plants take it 

 up and use it in the form of sulphates. As a rule, there appear to 

 be in all soils amounts of sulphates sufficient to supply indefinitely 

 all the demands of crops. As the quantity of sulphur used by 

 plants is very small, soils do not readily become exhausted of this 

 element. Therefore, we do not need, in general, to add sulphur 

 compounds to the soil. Calcium sulphate (sulphate of lime) and 

 potassium sulphate (sulphate of potash) are often present in com- 

 mercial fertilizers, but they are generally used not on accounl of 

 the sulphur they contain. Some forms of sulphur compounds 

 render a soil barren, when present in any considerable quantity. 



(d) Specific Functions in Plant Growth. — Sulphur is required 



