Scientific Agriculture. 



272 



[March. 1909, 



can endure for any length of time the loss 

 of mineral substances which go to form 

 part of the crop, without being replenish- 

 ed, it is necessary to replace these by 

 applying manures. 



It has been shewn by chemical analyses 

 that only a small number of substances 

 are necessary for the growth of culti- 

 vated plants. The essential nutritive 

 substances which must always be present 

 in sufficient quantity and in soluble form 

 are hydrogen, nitrogen, oxygen, sulphur, 

 phosphorus, carbon, potassium, iron, 

 calcium, magnesium, and probably chlo- 

 rine. In addition to these there are 

 several subsidiary substances which are 

 always present in the tissues of plants 

 but not indispensable for growth, such as 

 sodium and silica, also such admixtures 

 as zinc, copper, cobalt, aluminium, man- 

 ganese, etc., according to the nature of 

 the soil. 



All plant tissues contain oxygen, hydro- 

 gen and carbon. 



Carbon is the chief constituent of all 

 vegetable tissues forming about one-half 

 of their dry weight. The leaves absorb 

 the carbon from the atmosphere in the 

 form of carbonic acid which is decom- 

 posed in the cells of the leaves under the 

 influence of light, giving back oxygen to 

 the air. 



Oxygen and hydrogen are taken up by 

 the roots in the form of water which is 

 necessary for the conduction of the 

 nuritive 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 re- 

 quired for the building up of young 

 tissues and is always present in the pro- 

 toplasm of the cell and other albumin- 

 oids. 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 

 Lequminosce in green manuring our 

 crops by sowing such plants as Crotalaria 

 striata, Tephrosia purpurea, Desmodium 

 heterophylium, 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 

 Leauminosce which have been grown in 

 noor 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- 



tances especially nitrates. In fact it 

 has been proved that leguminous plants 

 shew a healthy development and yield 

 a good crop in a soil almost devoid of 

 nitrogen. 



These tubercles contain colonies of 

 bacteria which by some peculiar means 

 assimilate the nitrogen obtained from 

 the air and convert it into plant food. 

 These organisms multiply and enrich the 

 soil in which the plants grow, hence the 

 custom of manuring by some leguminous 

 green crop. 



Tnis form ol 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 prevented, etc., Crotalaria striata 

 and Tephrosia purpurea are fairly tall 

 growing plants, and should be kept 

 dwarf by periodically pruning their 

 tops to a reasonable height, say three 

 feet. Desmodium heterophylium 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 nitrogon for fertilizing purposes, 

 especially in farmyard manure or decom- 

 posing 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 grow- 

 ing 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 in- 

 fluence on this nitrification 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 mois- 

 ture. 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 dan- 

 ger of being permanently stopped. 



The nitrifying organisms in the soil can- 

 not develop in the preseuce of a free acid, 

 and it often happens that a soil becomes 

 sour being overcharged with nitric acid. 

 In cases of this kind the application of 

 lime will counteract the acidity and 

 restore the soil to a healthy state. Most 

 soils, however, contain a sufficiency of 

 lime for this purpose, and it needs only 

 to be applied in rare instances. 



