582 



Oxygen and hydrogen are taken up by the roots in the form of 

 water which is necessary for the conduction of the nutritive salts from 

 the soil to all parts of the plant. The bulk of this water is ultimately 

 given off through the pores of the leaves in the form of vapour. 



Nitrogen is the substance chiefly required for the building up of 

 young tissues and is always present in the protoplasm of the cell and 

 other albuminoids. Most plants obtain their nitrogen from the nitrates 

 and ammonic salts of the soil. Manures suitable for supplying fresh 

 nitrogen to soils are usually in the form of compounds containing 

 ammonia or other complex organic compounds. Leguminous plants 

 are, however, able to take up nitrogen through their leaves from the 

 atmosphere. In practice we make use of this peculiar property of 

 Leguminosce in green manuring our crops by sowing such plants as 

 Crotalaria striata, Tephrosia 'purpurea, Desmodium heterophyllum , 

 Desmodium triflorum, etc. etc., either in conjunction with crops such 

 as rubber or alone. 



If we examine the roots of any one of the above mentioned plants 

 and other Leguminosce which have been grown in poor soil we find the 

 roots bear numbers of curious tubercles. The tubercles are in greater 

 numbers the poorer the soil is in humus and soluble nitrogenous sub- 

 stances especially nitrates. In fact it has been proved that leguminous 

 plants shew a healthy development and yield a good crop in a soil al- 

 most devoid of nitrogen. 



These tubercles contain colonies of bacteria which by some pe- 

 culiar means assimilate the nitrogen obtained from the air and convert 

 it into plant food. These organisms multiply and inrich the soil in 

 which the plants grow, hence the custom of manuring by some legu- 

 minous green crop. 



This form of manuring crops has many points to recommend it. 

 The planter is saved the expense of weeding, his soil is always moist 

 and protected from the scorching sun ; wash by heavy rains is prevent- 

 ed etc., Crotalaria striata and Tephrosia purpurea are fairly tall grow- 

 ing plants and should be kept dwarf by periodically pruning their 

 tops to a reasonable height, say three feet. Desmodium heterophyl- 

 lum is the best of this genus for sowing as a green manure. It is not 

 a tall grower and does well in almost any soil. Desmodium triflorum 

 is often recommended but is comparatively rare. 



For other than Leguminous plants nitrogen in the form of nitrates 

 is generally regarded as the best form of nitrogenous food to apply to 

 the soil. The nitrogen for fertilizing purposes especially in farmyard 

 manure or decomposing vegetable matter is not in the form of nitrates 

 but either in the form of ammonia or other organic compounds and 

 before it is taken up by the growing plant the ammonia is changed 

 into nitric acid by means of ferments or micro-organisms in the soil. 

 The three conditions which exert a marked influence on this nitrifaction 

 are heat, air and moisture. This shews us the reason why thorough 

 tillage is essential to good cultivation. The loosening and breaking 

 up of the surface soil allow the admission of the necessary oxygen and 

 regulates the supply of heat and moisture. When the soil is saturated 

 with moisture or on the other hand is allowed to become very hard and 

 dry nitrification is retarded and is in danger of being permanently stopped. 



