AMELIORATION OF LAND 89 



physical effect, resulting in the improvement 

 of natural drainage, the raising of the tempera- 

 ture of the land owing to the water getting 

 more freely away, and the improvement of 

 the physical character as regards tilth and 

 freeness of working. The process of burning 

 also liberates a certain amount of potash, 

 which when the material is spread on the land 

 becomes available as plant food. Then, 

 again, low oxides of iron, which are injurious 

 to plants, are during the process of burning 

 converted into higher oxides, which are not 

 only innocuous but act as fixers of plant food 

 in solution, and may serve as food themselves. 

 These higher oxides are all redder in colour 

 than the low oxides, and this explains why clay 

 that has been burned is always of a much 

 brighter red, than the clay with which one 

 starts. It is for this reason, also, that in 

 many cases bricks are not of the colour of 

 the clay that has been used in their manu- 

 facture. It may also be mentioned that 

 the larvae of injurious insects, and the 

 seeds of injurious plants, are destroyed 

 in the process of burning ; but, on the other 

 hand, nitrogen present in the soil is likely to 

 be dissipated in the process. 



In former times extensive areas of light 

 land were marled, and in some parts of the 



