IX.] ACTION OF LIME 257 



On the lighter soils — the sands and gravels — lime 

 exerts a good effect by forming a weak cementing agent 

 and increasing the cohesion of the particles. As a rule, 

 however, it is not wise to apply quicklime in any 

 quantities to very light open soils, because oxidation of 

 the organic matter is pushed on too rapidly. Either 

 chalk or marl, or some form of calcium carbonate should 

 be used, or the quicklime should only be applied in 

 small quantities. 



From the chemical side the great value of carbonate 

 of lime in the soil lies in its power of maintaining 

 the neutral reaction necessary to the development 

 of those bacteria which oxidise the organic compounds 

 in the soil to the state of plant food. In the absence of 

 lime, organic matter by its decay gives rise to various 

 acid bodies which may be grouped as humic acid, and 

 the acidity thus produced inhibits the action of many of 

 the valuable groups of bacteria, such as the Azotobacter 

 which fix nitrogen, and the nitrifying bacteria which 

 convert ammonia into nitrates. It has been shown that 

 in soils that are acid through the accumulation of humic 

 acid, nitrification is at a standstill and bacterial life gener- 

 ally is repressed in favour of the growth of moulds and 

 micro-fungi, which compete actively with the crop for 

 the plant food in the soil. 



On all land which has been enriched by the 

 residues of past manuring or by the debris of previous 

 vegetation, lime is very necessary to promote the 

 oxidation of the nitrogen compounds and the formation 

 of nitrates for the crop ; consequently it is on bog or 

 peaty land, on old turf or reclaimed forest land, or on 

 old gardens, that liming exercises its maximum effect. 

 The following figures (Table LXXIX.) show the com- 

 parative crops on limed and unlimed plots, otherwise 

 manured alike, in an old hop garden at Farnham, 



R 



