CHAPTER XXII. 



MANURES. 



66S In order to understand the principles of manuring M C 

 must consider : 



(a,) A plant consists of two parts, an organic and a mineral part, 

 The mineral part is drawn from the soil alone, by the roots ; while of 

 the organic part one portion comes from the soil through the roots, and 

 another portion from the air through the leaves. 



(&,) The organic part of plants consists of four simple bodies, or 

 gases; carbon, hydrogen, oxygen, and nitrogen* In all the parts of 

 plants these four are associated also, with minute quantities of sulphur 

 and phosphorus. 



(C,) Of these four substances nitrogen appears to be drawn by the 

 plant almost exclusively from the soil ; the hydrogen and oxygen are 

 drawn partly from the soil and partly from the air chiefly in the form 

 of water. The carbon is derived only in small proportion from the 

 soil, being for the most part sucked in from the air by the leaves, in the 

 form of carbonic acid gas. Sulphur and phosphorus come from the soil 

 only. 



(&amp;lt;/,) The mineral part of the plant, which forms from half of one per 

 cent, to 15 to 20 per cent, of the whole weight of the dried plant, con* 

 sists of about 12 different substances. (See 93, p. 27.) Of these silica 

 exists cbicfly in the stems of grasses, grains, &c , and in smaller propor 

 tion only in the softer parts and juices of plants. Potash, soda, chlorine 

 and sulphuric acid are, for the must part, found in the sap ; litLC, mag 

 nesia, and oxide of iron in the solid parts of plantSi Phosphoric acid ia 

 necessary to, and is found in every part of a plant, but it collects in 

 larger proportion in thg grain or seeds as the season of ripening ap 

 proaches. {Sf.e Johnston s Experimental Agriculture, p. 7.) 



(c ) No plant therefore can flourish an\l come to maturity unless it is 

 able to procure in a proper form, awd absorb the abore substances, 



(569, We must consider also: 



