GREEN MANURES. 



GREEN MANURES. 



soon commences. It is requisite, however, for 

 this purpose, that moisture should be present, 

 and that the temperature of the soil should not 

 be less than about 45. If the atmosphere has 

 access to the vegetable matter, the putrefaction 

 proceeds with more rapidity, but its presence 

 is not essential. Putrefaction cannot, how- 

 ever, proceed if water is absent, and hence it 

 has been concluded that water is decomposed 

 during the process. The smell which proceeds 

 from the gases emitted varies according to the 

 vegetable substance which is putrefying. Thus, 

 as I have before remarked, those which con- 

 tain gluten emit ammonia; others, such as the 

 onion, evolve phosphuretted hydrogen. Almost 

 all emit carbonic acid gas and hydrogen gas, 

 which, combined with various vegetable mat- 

 ters, are commonly produced in very copious 

 volumes. When wood decomposes, a portion 

 of oxygen is absorbed from the atmosphere, 

 carbonic acid gas is emitted, and the whole 

 mass is gradually reduced to a dark vegetable 

 mould. This black substance is an excellent 

 fertilizer; plants grow in it with great luxu- 

 riance. The soils of some of the famed newly- 

 enclosed American lands owe all their fertility 

 to the abundance of this vegetable mould 

 which they contain. These are the American 

 soils from which we are told 20 successive 

 good crops of wheat have been obtained. 

 There are some lands in the Hundreds of Es- 

 sex, in Kent, and other places, whose luxuriant, 

 unfailing produce is hardly credible ; alternate 

 crops of wheat and beans have been obtained 

 from them from time immemorial. (Johnson 

 on Fertilizers, p. 168.) Vegetable mould, as ob- 

 tained from the trunks of oak trees, has been 

 examined by MM. Saussure and Einhoff; by 

 distilling it they obtained from 200 grains 

 (Rec. sur la Veg. p. 162) 



Cubic inches. 



- 124 



- 34 



Carburetted hydrogen - 

 Carbonic acid gas - - - - 



Grain.. 



Water containing acetate of ammonia - 53 



Empyreumatic oil ..... 10 



Charcoal --..-__ 51 



Ashes 8 



By the effects of cultivation, exposure to the 

 action of the atmosphere, and the roots of 

 plants, this mould becomes gradually exhaust- 

 ed in the soil, and the land is of course sensi- 

 bly impoverished. On this mould the alkalies 

 operate very powerfully, almost entirely dis- 

 solving it, and hence one great use of soda and 

 potash as fertilizers. 



It is also a continued source of carbonic 

 acid, which it emits slowly ; hence it might be 

 asserted, that in a good fertile soil there is an 

 atmosphere of carbonic acid, which is the most 

 nutritive food of the young plants raised in it ; 

 for when a plant is fully matured, and is fitted 

 to obtain most of its nourishment from the air, 

 the carbonic acid of the soil is no longer re- 

 quired. It is on that account that vegetable 

 mould is so fertile ; not by being itself assimi- 

 lated into the substance of the plant, but by 

 furnishing a slow but lasting supply of car- 

 bonic acid. 



With regard to the best time to turn under 

 clover, buckwheat, and other green crops, for 

 the purpose of enriching the ground, we have 

 584 



seen in the preceding observations that Davy 

 and others have decided in favour of the period 

 of full vigour, or when the plants may be in 

 blossom. It seems, however, that the results of 

 many well-conducted experiments and repeat- 

 ed observations lead to a different conclusion, 

 namely, that it is best to allow the green crop to 

 decay more or less before ploughing it in. In 

 the course of his agricultural survey of Massa- 

 chusetts, Mr. Colman found the opinion of some 

 most successful farmers to be in favour of allow- 

 ing the crop to mature and perish, before it was 

 subjected to the plough as a manure for the 

 soil. As the opinion of such men was at vari- 

 ance with the commonly received one, Mr. Col- 

 man addressed a letter to the well-known che- 

 mist, Dr. Dana, requesting his views on the 

 matter, as a question for chemical investiga- 

 tion. Dr. Dana's reply is contained in the 

 report of the commissioner. 



The essential element of fertility in a soil, he 

 says, has been called humus, geine, vegetable 

 extract, mould, as well as several other names, 

 all meaning a brownish-black, powdery mass, 

 the result of putrefactive decay, and the remains 

 of decomposed organic matter. This substance 

 combines with the alkaline, earthy, or metallic 

 bases of the plant or the soil, and constitutes 

 the means of growth or nutrition in the new 

 vegetable. Without it, there seems to be no 

 power in the earths of producing vegetation; 

 and if in too great excess, as it sometimes ap- 

 pears to be in very pure manures, it is destruc- 

 tive or unpropitious to all growth. In the 

 question now at issue, the inquiry, of course, 

 was, which furnishes to the soil the greatest 

 quantity of geine or humus, the green or the 

 dried plant. Dr. Dana decides in favour of 

 the latter. 



Fermentation appears to be the great agent 

 in the decomposition of organic matter; and 

 Dr. Dana's survey of the several kinds, such 

 as vinous, acetous, and destructive fermenta- 

 tion, seems to have a direct bearing on the for- 

 mation of the elements of fertility. The juices 

 only that contain sugar or starch, converti- 

 ble first into gum and then into sugar, by the 

 action of azotized vegetable principles, espe- 

 cially gluten, are capable of the vinous fermen- 

 tation. The conditions necessary to this fer- 

 mentation are moisture, air, and a temperature 

 not below 50, nor above 86. 



"If," says Dr. D.," we plough in green plants, 

 we put them in a temperature favourable to the 

 commencement of vinous fermentation; we 

 bury them full of sap, the requisite moisture 

 for vinous fermentation. The sugar and starch 

 of the plant, fermented by its gluten and albu- 

 men, are converted into gases and alcohol; the 

 former are lost in air, and the last washes 

 away or is changed to vinegar. All that re- 

 mains for the farmer is the altered gluten and 

 albumen, which soon putrefy and form geine. 

 All the starch and sugar of the plant are thus 

 lost." 



In his remarks on destructive fermentation, 

 Dr. Dana has the following observations : 



"Doubtless, all green plants ploughed in un- 

 dergo, to a greater or less extent, destructive 

 fermentation, which succeeds the vinous and 

 acid fermentations, perhaps caused by the ra- 



