COMMERCIAL FERTILIZERS 61 



taining mostly sulphate of potash are separated out. There 

 are two grades : the high grade sulphate of potash contains 

 51 to 53 per cent, of potash, and the low grade about 26 

 per cent. 



Besides the nitrogen, phosphoric acid and potash fertilizers, 

 there are other materials which are frequently applied to the 

 land, sometimes to add an element of plant food to the soil, 

 but more often for the effect which they will have upon the 

 soil in improving its texture. Such fertilizers are called indi- 

 rect fertilizers. Lime is often put on land because the soil is 

 lacking in lime, but more often because the land is "sour," 

 and the lime will sweeten it. Lime also makes compact soils 

 more crumbly, hence more easily worked. Leached ashes 

 and coal ashes add nothing in the way of plant food, but the 

 leached ashes will furnish a good deal of lime. Common salt 

 is sometimes applied to land, especially sandy soil. It helps 

 the soil to hold moisture and also aids somewhat in making 

 lime and potash available. Large quantities will injure the 

 plants. Land plaster or gypsum is also often applied for the 

 lime which it contains. Where it can easily be obtained marl 

 is used for supplying lime. It is especially good for sandy 

 lands, because there is often a good deal of clay mixed with 

 the marl which tends to make the sand more compact. Fine 

 ground limestone is one of the best indirect fertilizers. It will 

 sweeten sour land as quickly as lime and will not burn out the 

 organic matter as lime does. Ground limestone should be 

 applied at the rate of two tons or more per acre. Usually 

 autumn is the best time to apply it. 



The best way to find out what elements of plant food are 

 needed in the soil is by actual experiment on land. A small 

 area which represents the whole field fairly well is laid out 



