[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.