528 SHELL SAND— LIME— MOULD. 



constitutes a portion, sometimes a most important portion, of 

 the food of plants. For garden purposes that kind of sand is 

 alone suitable which has been rounded in water. The angular 

 particles of " road-sand " form hard impermeable masses, and 

 should never be employed. 



Shell sand is not sand at all, but rounded particles of carbonate of 

 lime, fragments of sbells and corals long rolled in water. 



Lime occurs commonly in the form of its carbonate {chalk). 

 It is to some extent soluble in water, and forms an important 

 portion of the food of many plants, especially when combiued 

 with acids, as in the sulphate (gypsvm), or the phosphate {bone 

 earth). It readily dries, absorbs and detains heat, and has 

 great value in modifying the wet tenacious quality of clay. In 

 its caustic state it has the power of decomposing animal and 

 vegetable substances, the result of which is a compound partly 

 soluble in water, and peculiarly fit for the food of plants. It, 

 however, requires to be used with caution, in consequence of 

 the property it possesses of setting free ammonia, one of the 

 most indispensable constituents of the food of plants. 



Peat consists of the dead remains of roots, brancheSj and 

 leaves, mixed with various proportions of sand. It also com- 

 monly abounds in iron. "When reduced to a powdery con- 

 sistence by decay, it becomes mould or humus {black mould, 

 leaf-mould). It has the important property of restoring alkaline 

 matter to soU, of producing carbonic acid by slow combination 

 with oxygen, and of aiding greatly in preserving soil in an 

 open state, so as to allow water and air to pass freely through 

 it. In the state of mould it condenses gaseous matter by 

 virtue of its porosity. 



These four kinds of matter are all that concern the busiaess 

 of the cultivator. Each, in its pure state, has Httle or no power 

 of sustaining' healthy vegetation; but when mixed in various 

 proportions, they constitute the richest soils in the world. 

 Hence it is that gardeners constantly employ a mixture of 

 loam, peat, and sand, from which lime is only absent in 

 appearahce, abounding as it does in all good loam in the state 

 of chalk. Nature demands no other means of sustaining 



