MANURES. 



16: 



and frost, soon become powdery, and readily mix 

 with the soil. When phosphate of lime is pre- 

 sent in marls of this sort, their value is greatly 

 increased; but some of them contain a consider- 

 able amount of oxide of iron; and when this is 

 the case, they should be employed with caution, 

 otherwise bad results may arise from their use. 



5. Shell -marls consist of the remains of in- 

 fusorial animals, and of the shells of shell-fish, 

 mixed with sand, clay, and some organic matter, 

 the whole in a finely-divided state. These marls 

 are rich in carbonate of lime, and the organic 

 matter and phosphate of lime which they contain 

 add materially to their value. Shell-marls may 

 be beneficially applied to all soils deficient in 

 calcareous matter, likewise to sour peaty soils. 



6. Peaty marls. Marls are occasionally found 

 which contain a considerable amount of peat; 

 these form a valuable addition to soils deficient 

 in organic matter. As they are very wet when 

 first dug up, and retain moisture for a long time, 

 they ought not to be applied in a fresh state. On 

 this account they should either be made up into 

 a compost, or burned; but if the latter method 

 be adopted, the organic matter will be lost, and 

 the marl will merely be valuable for the lime and 

 other inorganic substances which it may contain. 



The quantity of marl which should be applied 

 to the acre is entirely dependent on the nature 

 of the soil, and the composition of the marl itself. 



Limestone Gravel occurs chiefly in Ireland ; it is 

 of the same appearance as common gravel, only 

 of a blue colour, and effervesces briskly with 

 hydrochloric acid. This gravel is sometimes very 

 fine, approaching to marl in its nature. It is used 

 with great advantage on bogs and strong clays; 

 it produces on the latter all the effects of a dress- 

 ing of marl, greatly increasing their friability. 



Calcareous Sands are much employed in some 

 parts of the country, not only for improving the 

 texture of the soil, but also for the sake of the 

 carbonate of lime, which some of them contain 

 to the extent of 60 per cent and more. 



In Devonshire and Cornwall, immense quan- 

 tities of shell-sand — a calcareous sand, consisting 

 chiefly of sand and the remains of shells, together 

 with a little organic matter — are carried many 

 miles inland, for the purpose of applying it to 

 the land. Deposits of shell-sand are likewise 

 found on the west coast of Scotland, and in the 

 Hebrides; also, on the north, south, and south- 

 west coasts of Ireland. 



Calcareous sands are of great benefit to all soils 

 deficient in lime; they are also advantageously 

 employed in improving heavy retentive soils, to 

 which they not only supply calcareous matter, 



but also improve their texture. Calcareous sands 

 are likewise applied with excellent results to 

 peaty soils, the vegetable acid of which they 

 neutralize. 



Some of these sands contain a considerable 

 amount of phosphoric acid and organic matter; 

 when such is the case, they are most valuable 

 fertilizers. 



Magnesia is found in the ashes of plants, and 

 is therefore absolutely essential to their growth. 

 In its caustic state it appears to be injurious to 

 vegetation; but one of its salts — sulphate of 

 magnesia, or Epsom salts, a compound of sul- 

 phuric acid with magnesia — has been used with 

 some success as a manure for Potatoes, Turnips, 

 and Onions. But since magnesium compounds 

 are found in tolerable abundance in most soils 

 of fair quality, and especially in garden soils 

 which have been largely dressed with stable 

 manure, comparatively little attention has been 

 given to their employment as fertilizers. It is 

 true, indeed, that the amount of magnesia taken 

 up by plants is large, although variable in 

 quantity. Thus, Raspberry canes take up 11 

 per cent, while the Pear-tree only requires 5 per 

 cent. The ashes of the fruit of the Fig show 

 9 per cent, while the ashes of the Cherry, Goose- 

 berry, and Green-gage Plum show only 5 per 

 cent. Again, the ashes of Datura flowers show 

 nearly 10 per cent of magnesia, while the 

 Dianthus is satisfied with 4 per cent. And in 

 the ashes of vegetables the Asparagus shows 13 

 per cent, and the Onion 5 per cent only. It is 

 possible that magnesia, when applied as a ma- 

 nure, may act directly by serving as food for the 

 plant, or it may be that it will act indirectly, 

 by uniting with the aluminous double silicates 

 which exist in the soil. 



Phosphate of magnesia is found in bones and 

 other organic manures. The use of these is, 

 doubtless, the cheapest and most advantageous 

 way of supplying magnesia, when we take into 

 consideration the other important and highly 

 fertilizing principles which such manure con- 

 tains. 



Ammonia, from its being the great source 

 from which plants derive their nitrogen, is one 

 of the most important components of manures. 

 To its presence, or formation, guano, the excre- 

 ments of animals, flesh, blood, &c 3 owe most of 

 their value as fertilizers. All manures contain- 

 ing ready-formed ammonia exercise a peculiar 

 stimulating action on vegetation, producing a 

 luxuriant and rich dark-green foliage, together 

 with a great increase in the total crop grown. 



Ammonia is supplied to plants by the decom- 



