638 TRANSACTIONS OF SECTION M. 
50 inches per annum, not too much segregated into seasons and some at least 
falling in the winter. 
If the rainfall is excessive or the drainage inadequate to carry it off, the 
formation of peat is induced, resulting in such uncultivated areas as the 
bogs of Ireland and the moors of Eastern England, Holland, and Germany. 
Given suitable rainfall and temperature the texture of the soil becomes a 
factor of importance; if too coarse and sandy, so little of the rainfall is retained 
that we get all the effects of drought secondarily produced. In itself the 
open texture of a coarse sandy soil is favourable to plant development; under 
irrigation, or where the situation is such as to result in permanent water a 
short distance below the surface, fine crops will be produced on sandy soils that 
would remain almost barren if they only depended upon the rainfall for their 
water. In Western Europe large areas of heaths and waste land owe their 
character to the coarse and open texture of the soil. At the opposite extreme 
we find clays so heavy that their cultivation is unprofitable; such soils, how- 
ever, will carry grass and are rarely left unoccupied. For example, in the 
South-East of England there are a few commons, i.e., land which has never 
been regarded as worth enclosing and bringing into particular ownership, 
situated on heavy clay land; most of such land is pasture, often of the poorest, 
or, if at any elevation, has been covered with forest from time immemorial. 
One last factor in the soil is of the utmost importance to fertility and that 
is the presence of lime—of calcium carbonate, to be more accurate—in quantities 
sufficient to maintain the soil in a neutral condition. Old as is the knowledge 
that lime is of value to the soil, we are only now beginning to realise, as 
investigation into the minute organisms of the soil proceeds, how fundamental 
is the presence of lime to fertility. A survey of the farming of England or 
Western Europe will show that all the naturally rich soils are either definitely 
calcareous or contain sufficient calcium carbonate to maintain them in a neutral 
condition even after many centuries of cultivation. Examples are not lacking 
where the supply of calcium carbonate by human agency has been the factor 
in bringing and keeping land in cultivation. I have discussed one such case 
on the Rothamsted estate and several others have come under my notice. 
The amelioration of non-calcareous soils by treatment with chalk or marl from 
some adjacent source has been a traditional usage in England and the North 
of France: Pliny reports it as prevailing in Gaul and Britain in his day, and 
the farmer of to-day often owes the value of his land to his unknown prede- 
cessors who continuously chalked or marled the land. Upon the presence of 
carbonate of lime depends the type of biological reaction that will go on in 
the soil, the beneficial bacterial processes that prepare the food for plants 
only take place in a medium with a neutral reaction. The Rothamsted soils 
have provided two leading cases. I have shown that the accumulation of 
fertility in grass-land left to itself and neither grazed nor mown, so that 
virgin conditions were being re-established, was due to the action of the 
organism called Azotobacter, which fixes free nitrogen from the atmosphere, 
and was indirectly determined by the presence of calcium carbonate in the 
soil, without which the Azotobacter cannot function. Examination of typical 
examples of black soils from all parts of the world, the prairies of North 
America, the steppes of Russia and the Argentine, New Zealand and Indian 
soils, showed in all of them the Azotobacter organism and a working pro- 
portion of carbonate of lime. Now, as we know, all virgin soils are not rich, 
and only in a few parts of the world are to be found those wonderful black 
soils that ere often several feet in depth and contain 10 to 20 per cent. of 
organic matter and 3 to 5 parts per thousand of nitrogen. These soils are all 
calcareous, they occur in regions of a moderate rainfall inducing grass-steppe 
or bush conditions, and the annual fall of vegetation provides the organic 
matter which the Azotobacter requires as a source of energy in order to fix 
nitrogen. Non-calcareous soils under similar climatic conditions do not ac- 
cumulate nitrogen and become rich; in the absence of carbonate of lime 
the nitrogen-fixing organisms are not active, and the soil only receives from the 
annual fall of vegetation the nitrogen that was originally taken from it. There 
is but a cyclic movement of nitrogen from the soil to the plant and back 
again, whereas in the calcareous soils there is also continuous addition of fresh 
