vails.

[39] See Appendix, Note II., p. 98.

[40] Some experiments by E. Wollny show this. He found, when
experimenting with _summer rape_, that the best results were obtained
when the soil contained only 40 per cent of its total water-holding
power; when the amount was either lessened or increased the results
obtained fell off. The effect of either too little or too much water is
seen in the development of the different organs of the plant as well as
on its period of growth, much water seeming to retard the growth. The
quality of the plant seems also to be influenced by this condition.
Experiments on cereal grains by Wollny show that not merely is the
texture of the grain influenced, but that much moisture lessens the
percentage of nitrogen. Wollny is of the opinion that for crops
generally, the best amount is from 40 to 75 per cent of the total
water-holding capacity of the soil.

[41] See Appendix, Note III., p. 99.

[42] See p. 55.

[43] The effect of the temperature of the soil on the development of the
plant is most important. This is especially marked at the period of
germination, but is felt at subsequent periods of growth. Up to a
certain temperature the warmer the soil the more rapid the plant's
development. In this country the temperature most favourable to growth
is rarely exceeded, or indeed reached.

[44] See Chapter on Farmyard Manure.

[45] As will be seen further on, the fermentation of organic substances
is caused by the action of micro-organic life.

[46] See Appendix, Note IV., p. 100.

[47] Of course it must be remembered that a large amount of carbonic
acid in soils comes from the decay of vegetable matter. Soils are twenty
to one hundred times richer in carbonic acid than the air.

[48] See Chapter III., p. 119.

[49] See Introduction, p. 40.

[50] See Introductory Chapter, p. 54.

[51] See pp. 44 and 135.

[52] Occasionally also _lime_.

[53] See Appendix, Notes V. and VI., pp. 100, 101.

[54] Note VI., p. 101.

[55] Note VII., p. 107.

[56] Even larger estimates of the number of germs in a gramme of soil
have been made--from three-quarters to one million (Koch, Fuelles, and
others).

[57] These organisms consist of molds, yeast, and bacteria, the
last-named being most abundant. In the surface-soil, among the bacteria,
bacilli are most abundant. Micrococei are not abundant.

[58] Investigated by Winogradsky, Olivier, De Rey Pailhade, and others.

[59] Organisms of thi