MANUEING IN THEORY AND PEACTICE. 



257 



The formation of nitrates will take place most 

 abundantly in surface mould, as the proportion of 

 nitrogenous organic matter (the remains of A'ege table 

 and animal life) is most considerable, and the access 

 of air most free. 



The production will be greatly favoured by rain 

 and by tillage, and will be far more extensive in 

 summer than in winter time ; also much more ener- 

 getic in hot climates than in cold. The quantity of 

 carbonic acid formed will be found greater accord- 

 ing to the richness of the soil in organic nitrogen, 

 and is much increased when farmyard manure has 

 been applied. It is, like the nitrates, produced in 

 largest quantities during the summer months ; the 

 amount may be further increased by the application 

 of chalk or lime to the soil. 



Absorptive and Retentive Powers of 

 Soils. — One of the properties of humus, and also of 

 all animal manures, is to attract and retain moisture 

 from the atmosphere. These compounds also retain 

 ammonia with great tenacity — a property of immense 

 utility in connection with the supply of nitrogen to 

 plants. 



The extent to which farmyard dung increases the 

 water-holding powers of a soil may be shown by one 

 of the Rothamsted experiments. 



During a very wet winter samples of soil were 

 taken from the Wheat-field previously referred to ; 

 the percentage of water found in the soil manured 

 each year with farmyard manure, and in a soil 

 treated with artificial manures, was as follows : — 



Farmyard Manure. i Artificial Manure. 



. I 



1st 3 inches . . 39-67 i 1st 3 inches . . 26-53 



2nd „ . . . 35-62 ; 2nd „ . . . 22 93 



3rd „ . . . 28-85 3rd „ . . . 2062 



4th „ . . . 23-95 j 4th „ . . 2470 



It is seen that in the first nine inches of mould the 

 absorbing power of the soil was much increased by 

 the continued use of farmyard manure, but that its 

 effect ceased at this point. 



The absorptive and retentive character of a soil 

 is found to be in direct ratio not only to the quantity 

 of organic matters present, but also to the fineness 

 of its state of subdivision. 



It therefore becomes essential, in a practical point 

 of view, to secure a proper degree of both if the soil 

 is to withstand drought. In dry climates the 

 power of holding water renders a soil more valuable 

 to the horticulturist, whereas in localities where rains 

 are frequent, a simple determination of this property 

 will serve to indicate to the practical gardener on 

 ■which of his soils it is most important to him, in 

 65 



reference to surface water, that the operation of 

 draining should be most effectually performed. 



The more water a soil contains within its pores, 

 the more it has to part with by subsequent evapora- 

 tion; and, therefore, the colder it is likely to be. 

 Hence the greater necessity for draining, applying 

 ashes, lime, old mortar or road-scrapings to clayey 

 than to sandy soils. The more sand a soil contains in 

 proportion to clay and humus, the less moisture it is 

 capable of absorbing, and the more readily it parts 

 with what it may take up, which may be clearly 

 shown by some results of Professor Schiibler, to 

 whom we owe much important information regard- 

 ing soils. 



Table Shoaving the Absorptive and Retentive 

 Powers of Different Soils. 







Of 100 of Water 





Water 



absorbed there 



Description of Soil. 



absorbed by 



evaporated in 



100 of Soil. 



four hours at 







66° Fahr. 



Sand .... 



25 



88-4 



Light Clay 



40 



52-0 



Stiff CJay .... 



50 



45-7 



Heavy Clay 



61 



34-9 



Pure Clay 



70 



31-3 



Eich Garden Soil . 



96 



24-5 



Humus .... 



190 



20-5 



" During dry weather plants require a soil which is 

 both absorptive and retentive ; and that soil which is 

 capable of seizing atmospheric moisture, and holding- 

 it when the atmosphere is heated, is one of the best- 

 constituted soils. But stiff and heavy clays, which 

 take up the greatest quantity of water when it is 

 poured upon them, are not the soils which absorb 

 most moisture from the atmosphere in dry weather ; 

 they cake and present only a small surface to the 

 air ; and the vegetation on them is generally burnt 

 up almost as readily as on sands. The soils that are 

 most efficient in supplying the plant with water by 

 atmospheric absorption are those in which there is a 

 due mixture of sand, finely- divided clay, and car- 

 bonate of lime, with some animal or vegetable 

 matter, and which are so loose and light as to be 

 freely permeable to the atmosphere."* 



Loss of Plant-food by Drainage.— Most 

 cultivated soils contain a large amormt of mineral 

 food of plants, as well as organic nitrogen — that is to 

 say, nitrogen in combination with carbon, the resi- 

 due of previous vegetation. 



A certain amount of this organic nitrogen is every 

 year converted into nitric acid, which combines with 

 the lime in the soil and forms a most important and 

 * " Soil of the Farm," by Scott and Morton. 



