200 
Causes of Fertility or Barrenness of Soils. 
in comparison to sand at 100 ; humus only at 49 : this power 
appears to bear a close relation to the weight of a soil. The 
power of becoming warmed by the sun's rays, which is another 
cause of the temperature of soils, appears to depend upon colour 
and dryness ; the darker the soil is, the greater its power of ab- 
sorbing heat. 
Thoroughly dry soils, whatever their colour or nature, though 
varying in temperature are never cold. Moisture being the 
principal cause of low temperature, we often have the remedy in 
our own power ; thorough drainage, by diverting the water tliat 
previously clogged up the pores of the soil, and was continually 
evaporating from the surface, into new channels by which it is 
carried directly to the ocean, not only warms and invigorates the 
particular case, but assists in improving the general climate of 
a district, by rendering it drier, and removes the seeds of those 
rheumatic diseases so prevalent and fatal in former times. 
V. — Suitableness of different Soils to different Crops. 
In these days of high farming, when the extraordinary powers 
of artificial stimulants enable us to grow almost any description 
of crop on every variety of soil, we are somewhat in danger of 
forgetting that certain soils are adapted to the giowth of pecu- 
liar descriptions of plants ; and that we shall act most wisely by 
so arranging our rotation as to repeat most fi-equently those crops 
which practical experience as well as chemical knowledge tell us 
are the natural produce of such soils. There is at the same 
time no doubt that, as knowledge increases and practice improves, 
less stress will be laid upon the importance of adhering to par- 
ticular rotations, and we shall be enabled to grow more frequently 
those crops which prove most remunerating. Just as laws were 
made for rogues, rotations are found necessary as a check to bad 
farming, and are evidently only adapted to an imperfect condition 
of agriculture. As our knowledge of the laws and principles of 
vegetable economy becomes perfected, we shall be able to make 
those applications of manures required by our crop, and which 
the soil did not possess in sufficient or available quantity. At 
present we have not attained this desirable point ; and though 
certain kinds of soil under judicious management may be made 
to bear the same crop very frequently, the rule holds good that 
certain crops and certain soils are peculiarly suitable for each 
other. The natural herbage of a stiff clay, porous sand, or dry 
chalk soil, will sufficiently prove this fact. All farmers know 
that the prevailing weed of strong land is the couch grass (tritlcum 
repens), the type of wheat, from which it may have originally 
sprung, for we have no wild variety of wheat as we have of barley 
and oats ; therefore we may consider that clay land is peculiarly 
