SOIIv FERTlIvITY. 17 



Plant-food represents the third important factor in soil fer- 

 tility. Plants demand certain so-called elements, which are 

 really compounds. Fortunately most of these are present in 

 sufficient quantities in all soils. Only the nitrogen, phosphoric 

 acid and potash are likely to be lacking and to be needed in fer- 

 tilizers. Lime may sometimes be needed, but usually to correct 

 acidity of the soil, not to supply plant-food. 



Of these three needed elements, nitrogen is the most important 

 from a practical standpoint, because the most expensive. It 

 costs 15 to 18 cents a pound in the fertilizer market, while the 

 phosphoric acid and the potash may be had for 4^ or 5. 

 Furthermore, nitrogen is unstable and easily lost. If in a condi- 

 tion to be used by the plant, it may be readily dissolved in water 

 and washed away. It may likewise undergo changes which per- 

 mit it to escape into the air as a gas. This element may be 

 obtained, as are all the others, in stable-manure, but if this is 

 improperly handled a large proportion of the original supply 

 may be lost before reaching the field. Fully half the nitrogen 

 is contained in the liquid material. If this is allowed to 

 escape, one-half is lost at the outset. If the remainder is 

 thrown under the eaves and allowed to leach for six months, 

 half of what remained is probably gone, so that the proportion 

 which finally reaches the field is small indeed. Nitrogen may 

 also be obtained frorn nitrate of soda, which is the most quickly 

 available form. This is as readily dissolved as common salt, 

 hence should be used only when plants are ready to take it and^ 

 in frequent, light applications, not in large quantities at one time. 

 Nitrogen may also be obtained from sulphate of ammonia, dried 

 blood, tankage, hair or ground leather. In the latter forms it 

 is only slowly available for plant growth and may be of little 

 value to the crop to which it is applied. It, therefore, is not 

 enough to know that a given fertilizer contains the required per- 

 centage of nitrogen. The source from which that nitrogen 

 comes should also be known. For this reason the purchase of 

 chemicals and the home mixing of fertilizers possesses a marked 

 advantage in that the farmer may know not only the percentage 

 which the materials contain but the character of those materials. 



Phosphoric acid may be abtained from bone preparations or 

 from the phosphate rock so largely mined in the Southern States. 



