TILLAGE OPERATIONS AND MANURING in 



sulphides and lower oxides of iron, are converted by oxygen 

 into harmless sulphates and higher oxides. In a soil which 

 has been loosened by digging, water percolates more freely. The 

 exposure of fresh surfaces to the action of atmospheric oxygen, 

 carbonic acid, and water results in bringing into solution small 

 quantities of mineral salts necessary to plant life, and this is 

 assisted by the mechanical effect of alternations of heat and 

 cold on the soil particles. This mechanical effect is especially 

 marked in the case of frost action, and for this reason autumn 

 digging is of great importance. Water expands about one- 

 twelfth of its volume on freezing, and the result is that any 

 compound particle of soil containing interstitial water is, on 

 exposure to a temperature at or below the freezing-point of water, 

 subjected to a powerful disintegrating force far exceeding any 

 that can be directly exercised by an implement of tillage. The 

 results of this disintegration are most marked on strongly cohesive 

 clay particles, which after exposure, first to frost and subsequently 

 to a higher temperature, break down and form a tilth exceed- 

 ingly suitable for a seed bed. It is important that, when such a 

 surface has been produced, there should be no subsequent tillage 

 which will destroy the tilth. The surface should be touched 

 only with the rake or hoe. It is desirable in autumn tillage to 

 expose as large a surface of the soil as possible to the action of 

 frost, and hence ground dug at this period of the year with the 

 intention of leaving it until the spring should be as rough as 

 possible. The more lumpy it is the greater will be the area 

 exposed to frost action. For rather heavy soils the process of 

 ridging in the autumn may be adopted. This operation is carried 

 out by means of the spade, and consists in digging out a series 

 of parallel rectangles equal in width to twice that of the spade, 

 and laying the soil thus dug out in a series of parallel ridges 

 occupying the centre line of each rectangle. Only a moderate 

 depth of soil should be thus dealt with, otherwise the ridges will 

 be too large to be raked down in the spring. Apart from the 

 mechanical result obtained by ridging, the exposure of a large 

 volume of surface soil to a low temperature will result in the 

 destruction of many grubs of hurtful insects. 



In considering the mechanical effect of change of temperature 



