32 THE FARMERS' HANDBOOK. 



stated. These substances it is the function of the soil to supply, and where 

 the soil, by reason of its nature, or from having been exhausted, is unable to 

 meet the demand, we have to assist it by means of manure. 



The object, then, of manuring is to assist the soil in presenting to the 

 plant a portion of the food necessary for its growth ; and though the food- 

 stuff thus supplied is extremely minute in comparison with what the plant 

 derives from other sources, it is nevertheless of the greatest importance, and 

 its absence or insufficiency will affect the health of the plant quite as much 

 as will the absence of air or of sunlight or of water. 



Ingredients to be supplemented by Manure. 



Of these ash ingredients, the greater portion is found in all soils in quantities 

 more than sufficient for the requirements of any number of crops ; thus, iron, 

 magnesium, sodium, manganese, chlorine, sulphuric acid, silica, are present 

 in nearly all soils in greater quantities than the plant requires, so that there is 

 little danger of the soil becoming exhausted as far as they are concerned. 

 The ingredients, therefore, that we have to supplement by means of manure 

 are reduced to three or four, viz., lime, potash, phosphoric acid, and nitrogen. 



Lime is probably present in most soils in quantities sufficient for the pur- 

 poses of plant-food, and it is seldom necessary to supply it with that object. 

 Its principal functions when applied to land are, firstly, a mechanical one, in 

 modifying the physical character of the soil ; and, secondly, a chemical one 

 in rendering available certain insoluble plant-foods. (See pages 88 to 92.) 

 Lime acts, however, also directly as a plant-food, and the different classes 

 of artificial manures fall under one or other of the above headings, according 

 as they supply lime, potash, phosphoric acid, or nitrogen. 



The food thus supplied by the soil is taken up in solution by a peculiar 

 process by means of the roots. It is necessary, therefore, that such food 

 should be in a soluble form to be of any use to the plant, which is unable 

 to utilise insoluble material. By soluble we mean soluble in water or in 

 the weak acids present in the soil or secreted by the plant-roots. 



Insoluble and Soluble Plant-food. 



This is what is meant by the constituents being " available for plant- 

 food," and when the terms "latent" or "dormant" and "active" are 

 employed, they simply mean insoluble or soluble in the water imbibed by the 

 root. Those manures which are known as quick-acting manures are such as 

 contain their potash, phosphoric acid, and nitrogen in a soluble form at once 

 available for the plant ; those which are slow-acting contain these substances 

 in a more or less insoluble condition. 



In these latter cases a gradual decomposition takes place in the soil, due 

 to the action of chemical agencies within the soil and assisted by the action 

 of the roots of the plant, by which the insoluble compounds are converted 

 into soluble ones, and so become gradually available, the beneficial action 

 extending over a long period of time. * 



A good example of these two conditions is found in the state of the 

 phosphoric acid in bone-meal and in superphosphate respectively. Bone- 

 meal consists of bones ground to a powder, and the compound of phosphoric 

 -acid and lime which it contains is insoluble in water to any appreciable 

 extent. In course of time the agencies at work in the soil render it soluble. 

 Superphosphate is a compound manufactured by treating bones or mineral 

 phosphates with strong oil of vitriol, by which treatment a different compound 

 of phosphoric acid and lime is produced. This, being readily soluble, is at 

 once made use of by the plant. 



