326 SCIENCE OF AGRICULTURE. Part II. 



will, in winter, be considerably higher than that of a similarly constituted soil exposed 

 to the full influence of the weather. The early flowering of plants, in woods and hedges, 

 is a proof of this : but as such soils cannot be so easily heated in summer, and are cooled 

 like others after the sinking in of rains, or the melting of snows, the effect of the reflec- 

 tion as to the whole year is nearly neutralised, and the average temperature of the year of 

 such soils and situations will probably be found not greater than that of open lands. 



2153. Shading the ground, whether by umbrageous trees, spreading plants, or cover- 

 ing it with tiles, slates, moss, litter, &c. has a tendency to exclude atmospherical heat and 

 retain moisture. Shading dry loose soils, by covering them with litter, or slates, or 

 tiles, laid round the roots of plants, is found very beneficial. 



SwBSECT. 7. Rotation of Crops. 



2154. Growing different crops in succession is a practice which every cultivator knows 

 to be highly advantageous, though its beneficial influence has not yet been fully accounted 

 for by chemists. The most general theory is, that though all plants will live on the same 

 food, as the chemical constituents of their roots and leaves are nearly the same, yet that 

 many species require particular substances to bring their seeds or fruits to perfection, as 

 the analysis of these seeds or fruits often afford substances different from those which 

 constitute the body of the plant. A sort of rotation may be said to take place in 

 nature, for perennial herbaceous plants have a tendency to extend their circumference, 

 and rot and decay at their centre, where others of a different kind spring up and succeed 

 them. This is more especially the case with travelling roots, as in mint, strawberry, 

 creeping crowfoot, &c. 



2155. The rationale of rotation, is thus given by Sir H. Davy. " It is a great advan- 

 tage in the convertible system of cultivation, that the whole of the manure is employed ; 

 and that those parts of it which are not fitted for one crop, remain as nourishment for 

 another. Thus, if the turnip is the first in the order of succession, this crop, manured 

 with recent dung, immediately finds sufficient soluble matter for its nourishment ; and 

 the heat produced in fermentation assists the germination of the seed and the growth of 

 the plant. If, after turnips, barley with grass-seeds is sown, then the land, having been 

 little exhausted by the turnip crop, affords the soluble parts of the decomposing manure 

 to the grain. The grasses, rye-grass, and clover remain, which derive a small part only 

 of their organised matter from the soil,' and probably consume the gypsum in the manure 

 which would be useless to other crops : these plants, likewise, by their large systems of 

 leaves, absorb a considerable quantity of nourishment from the atmosphere ; and when 

 ploughed in at the end of two years, the decay of their roots and leaves affords manure 

 for the wheat crop ; and at this period of the course, the woody fibre of the farm-yard 

 manure, which contains the phosphate of lime and the other difficultly soluble parts, is 

 broken down : and as soon as the most exhausting crop is taken, recent manure is 

 again applied. Peas and beans, in all instances, seem well adapted to prepare ground 

 for wheat ; and in some rich lands they are raised in alternate crops for years together. 

 Peas and beans contain a small quantity of a matter analogous to albumen ; but it seems 

 that the azote, which forms a constituent part of this matter, is derived from the atmo- 

 sphere. The dry bean-leaf, when burnt, yields a smell approaching to that of de- 

 composing animal matter ; and in its decay in the soil, may furnish principles capable of 

 becoming a part of the gluten in wheat. Though the general composition of plants is 

 very analogous, yet the specific difference in the products of many of them, prove that 

 they must derive different materials from the soil ; and though the vegetables having the 

 smallest system of leaves will proportionably most exhaust the soil of common nutritive 

 matter, yet particular vegetables, when their produce is carried off, will require peculiar 

 principles to be supplied to the land in which they g^ow. Strawberries and potatoes at 

 first produce luxuriantly in virgin mould, recently turned up from pasture ; but in a 

 few years they degenerate, and require a fresh soil. Lands, in a course of years, often 

 cease to afford good cultivated grasses ; they become (as it is popularly said ) tired of 

 them ; and one of the probable reasons for this is, the exhaustion of the gypsum contained 

 in the soil." 



2156. The powers of vegetables to exhaust the soil of the principles necessary to their 

 growth, is remarkably exemplified in certain funguses. Mushrooms are said never to 

 rise in two successive seasons on the same spot ; and the production of the phenomena 

 called fairy rings has been ascribed by Dr. Wollaston to the power of the peculiar fungus 

 which forms it, to exhaust the soil of the nutriment necessary for the growth of the 

 species. The consequence is, that the ring annually extends ; for no seeds will grow 

 where their parents grew before them, and the interior part of the circle has been ex- 

 hausted by preceding crops ; but where the fungus has died, nourishment is supplied for 

 grass, which usually rises within the circle, coarse, and of a dark green color. 



2157. A rotation is unnecessary, according to Grisenthivaite ; and, in a strict chemical 

 sense, what he asserts .cannot be denied, His theory is a refinement on the common 



