THE NUTRITION OF THE ROOT 55 



The so-called raw phosphates and bone-meal contain this 

 nutritive substance in a very insoluble form, and must there- 

 fore be mixed with the soil a considerable time before sowing. 

 They are only to be recommended for fields known to be suffi- 

 ciently damp, and for autumn sowing, where the chief produc- 

 tivity takes place in the spring. If bone-meal is to act more 

 rapidly, it should be left to ferment in heaps before use. The 

 quickest effect is obtained from superphosphates. If it is 

 desirable to add nitrogen as well as phosphoric acid to the 

 soil, a mixture of ammonium sulphate and superphosphates, 

 so-called ammoniacal superphosphate, may be used. The third 

 chief nutritive substance, which contains potassium as well as 

 nitrogen and phosphoric acid in readily soluble form, is Peru 

 guano. These last two substances, on account of their being 

 immediately available by the plants, are used as top-dressing 

 for weak crops, and by gardeners for wholesale growing of 

 herbaceous plants, especially foliage plants. From the first of 

 these two preparations we must, however, distinguish the raw 

 ammonia superphosphate, which must only be employed in 

 fields which are well aerated and have a considerable moisture, 

 for this preparation contains a considerable amount of rhodan- 

 ammonium, which acts injuriously on plants. 



A preparation which has more recently been largely used 

 is the Thomas-slag, which is derived from ironworks, and 

 which must be ground as fine as possible. It contains about 

 40 to 60 per cent, of phosphate of lime in different com- 

 binations (tricalcium phosphate, &c.), besides silica, clay, iron 

 oxides, and traces of magnesium, chlorine, and sulphur. The 

 lime, besides occurring in combination with phospliorus, is 

 present in considerable quantities combined with silica, and 

 also in the form of quicklime. This manure is also most 

 effective in a damp soil, and when added at an early period. 

 In a dry soil it is only rendered soluble after a considerable 

 time. 



(c.) Organic Manures. 



In spite of the evident beneficial results of inorganic 

 manures, and in spite of the fact that they are indispensable 



