162 THE PRINCIPLES OF 



off the second growth plough it up for wheat, and you find 

 your ground well stocked with clover roots. Now, whether 

 recruited from the air ocean, above ground, or from the sub- 

 soil, or from both it matters not at present. You have plenty 

 of nitrogen stored up in the first few inches of the soil ; the 

 consequence is a heavy wheat crop. That is the way in 

 which farmyard dung acts and re-acts, echoes and re-echoes, 

 during the whole of the rotation. 



There are some disadvantages from the use of general 

 manures, the chief being that they store up in the soil 

 certain materials to an unnecessary degree. You add every- 

 thing, and perhaps do not require everything ; there is there- 

 fore an accumulation, it may be, of lime or potash, in the 

 soil beyond the requirements of the plants. 



Special manures may likewise be objected to, and they 

 also have their strong points. A special manure may contain 

 only one important constituent of plant food, as for example, 

 nitrate of soda. The soda is comparatively useless, because 

 there is no substance so universally distributed in nature, 

 and there is always enough of it in a soil for crop require- 

 ments. Nitrate of soda, therefore, boasts one active con- 

 stituent only, namely, nitrogen in the form of nitric acid. 

 Superphosphate may be considered as an example of a special 

 manure with more than one constituent, for it contains 

 phosphoric acid, lime, a very appreciable amount of sulphur, 

 besides other constituents. But it is wanting in nitrogen 

 and in alkalis, and therefore it is a special manure, but a 

 special manure containing more than one important plant 

 constituent. Lime may be regarded as a special manure, 

 although like all natural products containing impurities 

 some of which are of manureal value. Common salt is a 

 special manure, the chief constituent being chlorine. Sul- 

 phate of potash, sulphate of magnesia, sulphate of ammonia, 

 silicate of soda, chloride of ammonium, gypsum, are all 



