150 
On Modes of Comparing the 
are kept, was this year 19 tons of swedes per acre, while that of 
the mangold-wurzel was 26 ; and my own farm gave at the rate 
of 20 tons of swedes, and 32 of mangold-wurzel, but the season 
was peculiarly favourable to the latter crop. Assuming, then, on 
precisely similar land, 20 tons of swedes, and only 25 of mangold- 
wurzel, to be a fair average, it seems that, while one acre of 
swedes would only have given 640 lbs. of beef, an acre of mangold- 
wurzel would have given 1275, so that a crop of the latter is 
worth nearly double one of the former. I am quite aware that 
there are many circumstances under which the cultivation of 
swedes is preferable to mangold-wurzel. I would only apply the 
particular instance where such circumstances do not exist. 
In this case the facts are certain, and have long been made 
public, and yet how few of us are aware of the important conse- 
quences to which they lead, that, as in the last instance, by pre- 
ferring one crop to another, double the stock may be maintained 
by the land. It will doubtless require more manure, but it gets 
it just in proportion to the number of cattle that it feeds. What 
is lost to it in the flesh of the animal sent off must certainly be 
restored from extraneous sources : but it will be rare indeed where 
the necessary quantity of one sort or another cannot be obtained 
by the expenditure of a very small proportion of the increased 
returns from the doubled produce of the land.* 
* Of course this presumes all the manure from the cattle to be carefully 
made the most of. If it is, and the land kept in fine tilth, so as to derive 
the full benefit of those elements which for the most part, if not entirely, 
may be derived from air and rain, and which form the chief part of vegetable 
structure, as well as animal nutrition, the manure to be bought will be much 
less than is generally imagined, and in a great measure limited alone to the 
fixed ingredients of the soil, which, after all, form a small part of the grain, 
or the animal, which finds its way to market. Taking Liebig's supposi- 
tion that a J of a grain of ammonia may be contained in 1 lb. Hessian 
of rain-water, and allowing the rain in England to contain the same quan- 
tity, the average rain of a year should contain, upon a-.i English acre, 
161 "88 lbs., or nearly 162 lbs. of ammonia. According to Payen, 1000 lbs. 
of farm-yard manure contain 4 lbs. of nitrogen : thus a ton will contain 
8 '96 lbs. This nitrogen is in the shape of ammonia, which, according to 
Thomson's estimate, is composed, by weight, of hjdrogen 0*375 lbs., and of 
nitrogen 1 • 75. 
There is therefore to be added to the . . 8* 96 lbs. of nitrogen, 
I'92 „ hydrogen. 
Giving, as the total of ammonia contained in a-ljQ.gg ij^g 
ton of farm-yard manure, ... J 
The annual quantity of ammonia poured with the rain on an English 
acre was 161 '88 lbs., or a quantity equal to that which would have been 
given by 14" 87 tons of farm-yard dung (about two-thirds of a good dressing), 
the whole of which, if the surface of the soil is kept loose and light, is brought 
in contact with and appropriated either by the humic acid with which it 
combines, forming a soluble humate of ammonia; or, according to the dis- 
