OF A G R I CULTURE. 35 



From the foregoing table we can form an approx- 

 mate estimate of the mineral ingredients needed by 

 different plants. 



If we examine the foregoing table, we see that the 

 mineral ingredients taken from the soil by different 

 plants are nearly all identical in kind, but vary con- 

 siderably in the proportions in which they enter into 

 the constitution of the ashes of different plants, or of 

 different parts of the same plant. That a soil may be 

 fertile for a particular crop, several chemical proper- 

 ties are essential. It must contain an excess of mineral 

 elements required by the crop, to allow the roots to 

 find an abundant supply in the limited spaces which 

 can be reached by the rootlets. This would, of course, 

 require in the whole mass of the soil much more of 

 each element than could be removed by a single crop. 

 The plant food must be in a proper chemical condition 

 to be taken up by the plant. For example : silica in 

 the form of sand cannot act as a fertilizer, because of 

 its insolubility ; but in such combinations as render it 

 soluble, it is one of the most important elements of 

 plant life. Wheat straw contains sixty- six per cent. 

 of silica or dissolved sand ; a corn- stalk, twenty- seven 

 percent. All the outer coating of wheat straw, corn- 

 stalks, oat straw, etc., is nothing but sand made soluble 

 by alkalies in the soil. 



The soil must be free from an injurious excess of any 

 of the mineral elements of fertility. Too much mag- 

 nesia, or even too much carbonate of lime, may be 

 injurious ; as for instance, the chalk lands in England, 

 wjiich are among the least productive in that country, 

 and also the salty spots in various portions of the 

 Southern States. The vegetable acids are useful, but 



