ARTICLES 587 



soils which are waterlogged, and in bad tilth, the process 

 of gaseous interchange is much reduced, and conditions arise 

 favourable to the retention of the COg. In addition, the amount 

 of oxygen which enters the soil from the ordinary atmosphere 

 is necessarily less ; hence the composition of soil gases from 

 waterlogged soils shows a progressive diminution in the amount 

 of oxygen and a corresponding increase in the CO2 content. 

 The characteristic open structure associated with good tilth 

 promotes a free exchange between the soil gases and the ordinary 

 atmosphere, and certain investigators have been led to the 

 conclusion that this is the essential feature in healthy plant 

 growth. Howard and his collaborators, working mainly on 

 the indigo plant in India, have attributed extreme importance 

 to effective soil aeration. Some of their conclusions have been 

 questioned by Davis, who considers that the diseases of the 

 indigo plant are due to lack of phosphatic manures, and not to 

 defective soil aeration. Soil aeration is undoubtedly one of 

 the many constituent factors involved in good tilth, but its 

 degree of importance is not yet settled. 



Cultivation Implements 



Cultivation implements are used to lighten the labour of 

 preparing a good seed-bed and to maintain the soil in the best 

 condition for the plant at various stages of its development. 

 From the farming point of view the most important is the plough. 

 This instrument is of great antiquity. Rock carvings of a 

 plough and draught oxen have been found in Sweden and the 

 Alpes Maritimes, dating from 1500-900 B.C., and mural drawings 

 of the light plough used in Egypt during the same period may 

 be seen in the excavated tombs. The tapestries and illumi- 

 nated manuscripts of the Middle Ages show some of the essential 

 features of the modern implement. They all portray a rudi- 

 mentary and cumbersome form of mould-board, the purpose of 

 which is to invert the furrow-slice. Apparently the earliest 

 attempt, at any rate in England, to express in a definite way the 

 proper shape of the mould-board was made in 1 808 by Amos in 

 a Communication to the Board of Agriculture. He treated the 

 mould-board primarily as a system of two wedges : the first 

 horizontal, to pass under the furrow-slice and raise it slightly 

 after it had been cut by the plough-share ; the second vertical, 

 to turn the slice over on its side and partially to invert it. 

 Actually these two wedges merged insensibly into one another, 

 and, although Amos's treatment of the problem by the use of 

 solid geometry was somewhat cumbrous, the work deserves 

 to be regarded as a courageous attempt to reduce the traditional 

 design of the mould-board to a definite system. Id 1880 the 



