324 SCIENCE OF AGRICULTURE. Part II. 



with more oxygen ; none of them unite to azote ; and such of them as are capable of 

 attracting carbonic acid, are always saturated with it in those soils on which the practice 

 of fallowing is adopted." 



2175. Aeration and repose, or summer fallow. " The vague ancient opinion of the use 

 of nitre, and of nitrous salts in vegetation," Sir H. Davy says, " seems to have been one 

 of the principal speculative reasons for the defence of summer fallows. Nitrous salts are 

 produced during the exposure of soils containing vegetable and animal remains, and in 

 greatest abundance in hot weather ; but it is probably by the combination of the azote 

 from these remains with oxygen in the atmosphere that the acid is formed ; and at the 

 expense of an element which otherwise would have formed ammonia ; the compounds of 

 which are much more efficacious than the nitrous compounds in assisting vegetation." 

 It is proper to observe that this reason is more speculative than experimental, and seems 

 influenced, in some degree, by the opinion adopted by the author, that fallows are of little 

 use in husbandry. One obvious advantage of aeration in summer, or a summer fallow, 

 is, that the soil may thus be heated by the sun to a degree which it never could be if 

 partially covered with the foliage of even the widest drilled crops. For this purpose, if 

 the soil is laid up in large lumps, it is evident it will receive more heat by exposing a 

 greater surface to the atmosphere, and it will retain this heat for a period of unexpected 

 duration, from the circumstance of the lumps reflecting back the rays of heat radiated by 

 each other. A clayey soil, in this way, it is said (Farmer s Magazine, 1815), may be 

 heated to 120", which may in some degree alter its absorbent powers as to water, and 

 contribute materially to the destruction of vegetable fibre, insects, and their eggs. By 

 the aeration of lands in whiter, minute mechanical division is obtained by the freezing of 

 the water in the soil ; for, as water in the solid state occupies more space than when fluid, 

 the particles of earthy matters and of decomposing stones are thus rent asunder, and 

 crumble down in a fine mould. Rough stony soils will thus receive an accession to their 

 finer soil every winter. Soils which have been soured, sodden, or baked by the tread of 

 cattle, or by other means, in wet weather, are more speedily sweetened, as the expression 

 is, by exposure to the sun during the hottest weather of summer, than by exposure to the 

 frost of winter ; but in summer it is contended that the drying influence of the sun and 

 air exhausts the soil of its vegetable matter to such an extent as to counteract the good 

 effects of extreme heating by the sun. Those who maintain this doctrine contend that 

 the only use of a summer f.ilnr.v is to admit of freeing the soil of root-weeds. 



2176. Agricultural experience lias fully proved that fallows are the only means by which 

 stiff clays in moist climates can be effectually cleared of weeds. Supposing therefore 

 that no other advantage whatever was obtained, that no nutritive matter was imbibed 

 from the atmosphere, and the soil was neither chemically nor mechanically benefited by 

 aeration, this benefit alone, the effectual eradication of weeds, is sufficient to justify the 

 use of fallows on such soils. ' 



2177. Many of the objections to fallows have arisen in consequence of the parties not 

 previously agreeing as to what a summer fallow is. In England generally, or at least 

 formerly, a fallow was a portion of land left a year without culture or cropping, unless 

 being once or twice ploughed can be denominated the former, and an abundant growth 

 of coarse grasses and weeds can constitute the latter. The jacheres of the French are the 

 same thing. In Scotland, and in the best -cultivated districts, a summer fallow is a 

 portion of land begun to be cultivated after the crop is removed in autumn, and is fre- 

 quently, as need requires, ploughed, harrowed, and otherwise comminuted, and freed 

 from stones, weeds, inequalities, &c., till the autumnal seed-time of the following year : 

 it is thus for twelve months in a state of constant tillage and movement. The result .is, 

 that the land is thoroughly freed from roots of weeds ; from many seeds of weeds, which 

 are thus made to genninate, and are then destroyed ; and from many eggs of insects which 

 are thus hatched, but being without plants to nourish them in their larva state, speedily 

 die. The land is also thoroughly pulverised, and the top, bottom, and middle mixed 

 together ; stones are picked out, inequalities unfavourable to surface drainage removed 

 or lessened, and various other useful objects attained. Such a fallow can no more be 

 compared with what usually passes under that name, than the plough of Virgil (112.) 

 with that of Small. 



2178. That fallows of the common kind are much more universal than is necessary, 

 there can be little doubt ; but there can be as little doubt that fallows such as we have 

 described are much less frequent than they should be, and that wherever they are prac- 

 tised, the agriculturist's produce and profits will be found far superior to where they 

 are omitted : turnip soils are of course to be excepted, because the preparation for that 

 crop, on light soils, effects the same purpose in eight months, that the fallow does in 

 twelve. 



2179. The origin of fallows is commonly traced to the idea, that land naturally requires rest as well as 

 animals : but a want of hands first, and afterwards a want of manure, are much more likely causes. Men 

 must very early have observed, from what took place in the spots they cultivated as gardens, that pul- 



