LECTURE XVII. 



Of organic manures.— Vegetable and animal manures— Green manuring ; ploughing in of 

 spurry, the white lupin, the vetcii, buck-wheat, rape, rye, borage.— -Natural green manu- 

 ring. — Improvement of the soil by laying down to grass and by planting. — Use of sea- 

 weed. — Dry vegetable manures : dry straw, chaff, rape-dust, malt-dust, sawdust, cotton 

 seeds, dry leaves.— Decayed vegetable matter: use of peat, tanners' bark, and composts 

 of vegetable matter.— Charcoal powder, soot.— Relative value, theoretical, and practical, 

 of different vegetable manures. 



By organic manures are understood all those substances either of 

 vegetable or of animal origin, which, are applied to the land for the 

 purpose of increasing its fertility. It will be convenient to consider 

 these two classes of organic substances separately. 



The' parts of vegetables may be applied to the soil in three dif- 

 ferent forms — in the green, in the dry, and in the more or less natu- 

 rally decayed, fermented, or artificially decomposed state. 



§ 1. O/* green manuring^ or the application of vegetable matter in 

 the green state. 



By green manuring is meant the ploughing in of green crops in 

 their living state — or of green vegetables left or spread upon the 

 land for the purpose. 



1°. We have seen in the preceding lecture how important air and 

 water are to the decomposition of organic matter. Now green ve- 

 getable substances contain within themselves much water, undergo 

 decomposition more readily, therefore, than such as have been dried, 

 and are more immediately serviceable when mixed with the soil. 



2°. In the sap of plants also there generally exist certain compounds 

 containing nitrogen, which not only decompose very readily themselves, 

 but have the property of persuading or inducing the elements of the 

 other organic matters, with which they are in contact, to assume new 

 forms or to enter into new chemical combinations. Hence, the sap of 

 plants almost invariably undergoes more or less rapid decomposition 

 even when preserved from the contact of both air and water. When 

 this decomposition has once commenced in the sap it is gradually propa- 

 gated to the woody fibre and to the other substances of which the mass 

 of the stems and roots of plants is composed. Hence, recent vege- 

 table matter will undergo a comparatively rapid decomposition, ever 

 when buried to some depth beneath the soil — and the elements of which 

 it consists will form new compounds more or less useful to living plants, 

 in circumstances where dry and where many forms even of partially 

 decomposed vegetable matter would undergo no change whatever. 



3^. Further — when green vegetable matter is allowed to decay ir 

 the open air, it is gradually resolved more or less completely into car 

 bonic acid, which escapes into the air and is so far lost. But when 

 buried beneath the surface, this formation of carbonic acid proceeds 

 less rapidly, and other compounds — preparatory to the final resolution 

 into carbonic acid and water — are produced in greater quantity and 



