FOOTNOTES:
[33] This statement perhaps needs qualification. While the important rôle played by the physical qualities of the soil were in the early years of the science recognised, of more recent years the chemical composition of the soil has been engaging almost exclusive investigation. Physical properties of the soil have recently acquired a further importance in the eyes of the agricultural chemist, from the important influence they exert on what we have here called the biological properties of a soil—viz., the development of those fermentative processes whereby plant-food is prepared to a large extent.
[34] A good example of the absorptive capacity of a soil containing a large quantity of vegetable matter is furnished by peat-bogs, which, sponge-like, can absorb enormous quantities of water. (See Appendix, Note I., p. 98.)
[35] Jethro Tull, an early well-known agricultural writer, who lived about the middle of last century, propounded the theory, that as the food of plants consisted of the minute earthy particles of the soil, all that was required by the skilful farmer was to see that his soil was properly tilled. He accordingly published a work entitled 'Horse-hoeing Husbandry,' in which he advocated a system of thorough tillage. (See Historical Introduction, p. 10.)
[36] See Introduction, p. 55.
[37] See Introductory Chapter, p. 55.
[38] It is not exactly known why excess of water should prevent normal growth in the plant. Probably it is on account of the fact that free access of oxygen is hindered in such a case. The roots are thus not freely enough exposed to this necessary gas, and fermentative processes of the nature of nitrification are not promoted. It may be also due to the fact that the solution of plant-food is too dilute when such excess of water prevails.
[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, Fülles, 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 this kind have been investigated among others by Heraüs, Hueppe, and E. Wollny. According to the two first-mentioned investigators, certain colourless bacteria effect the formation in the absence of light from humus and carbonates a body resembling in its nature cellulose.
[60] Investigated by Springer, Gayon and Dupetit, Dehérain, and Marguenne.