216 ON THE CHEMICAL CONSTITUENTS OF SOILS. 



There are certain plants which contain either no 

 potash, or mere traces of it. Such are the poppy, 

 \papaver somniferum,) which generates in its organ- 

 ism a vegetable alkaloid; Indian corn (^zea mays); 

 and helianthus tuherosus. For plants such as these 

 the potash in the soil is of no use, and farmers are 

 well aware that they can be cultivated without ro- 

 tation on the same soil, particularly when the herbs 

 and straw, or their ashes, are returned to the soil 

 after the reaping of the crop. 



One cause of the favorable action of the nitrates 

 of soda and potash must doubtless be, that through 

 their agency the akalies which are deficient in a soil 

 are furnished to it. Thus it has been found that in 

 soils deficient in potash, the nitrates of soda or pot- 

 ash have been very advantageous ; whilst those, on 

 the other hand, w^hich contain a sufficiency of alka- 

 lies, have experienced no beneficial effects through 

 their means. In the application of manures to soils 

 we should be guided by the general composition of 

 the ashes of plants, whilst the manure applied to a 

 particular plant ought to be selected wdth reference 

 to the substances which it demands for its nourish- 

 ment. In general, a manure should contain a large 

 quantity of alkaline salts, a considerable proportion 

 of phosphate of magnesia, and a smaller proportion 

 of phosphate of lime; azotized manure and ammonia- 

 cal salts cannot be too frequently employed. 



In the following part of this chapter I shall de- 

 scribe a number of analyses of soils executed by 

 Sprengel, together with observations on their sterili- 

 ty and fertility, as stated by that distinguished 

 agriculturist. It is unnecessary to describe the mo- 

 dus operandi used in the analyses of these soils, for 

 this kind of research will never be made by farmers, 

 who must apply to the professional chemist, if they 

 wish for information regarding the composition of 

 their soils. 



Under the term surface-soil, we mean that portion 

 of soil which is on the surface ; whilst by subsoil we 



