12 AGRICULTURE FOR COMMON SCHOOLS 



foot deep to last 1,521 years, magnesia to last 3,300 years, 

 phosphoric acid to last 542 years, and nitrogen to last about 

 250 years, when properly farmed. This is doubtless true of 

 a fairly fertile soil. Although there are such large quantities 

 of these different substances in the soil naturally, the plant 

 very often suffers from a lack of them. This is because they 

 are bound up in the soil in such a way that the plant cannot 

 make use of them. In such cases it is necessary to apply them 

 to the soil in the shape of fertilizers, or steps must be taken 

 to make what is in the soil available. The plant foods most 

 often needed are nitrogen, phosphoric acid, potash, and lime. 

 We shall speak of these again. 



A great many persons believe that a chemist can analyze 

 a soil and tell just what that soil will grow and what kind of 

 fertilizers it needs. This is a mistaken notion. There are 

 two kinds of analysis. One is a chemical analysis. In such 

 an analysis the different kinds of plant food are determined 

 and the amount of each. There is usually found to be 

 plenty for all the needs of the crop. However, such an anal- 

 ysis cannot determine how much of these substances the 

 plant can use when it grows in the soil, so that certain ele- 

 ments must sometimes be supplied in the form of compost or 

 manures, even though the analysis shows an abundant supply 

 in the soil. The other kind of analysis is called mechanical. 

 In this analysis the soil grains are separated into groups of 

 various sizes and the amount of organic matter is deter- 

 mined. Such an analysis tells something about the ability of 

 the soil to hold water, and the ease with which it can be 

 drained. It also gives an idea of how easy a soil will be to 

 cultivate. If one knows something about the kind of land on 

 which different farm crops grow well, he can tell from this 



