386 



i:he agricultural news. 



December 2, 1916. 



to the cultivation of Indigo. This crop is largely 

 cultivated on the higher levels in rice-growing 

 districts. The occ4sion;il flooding of these higher 

 levels due to the rise of the rivers is the cause of the 

 low yields obtained in India compared with those 

 obtained under drier conditions in Java. 



The two crops just referred to are of course 

 leguminous, and the detrimental effect of insufficient 

 air is partly due to the limited supply of nitrogen 

 available for fixation by the nodules on the roots. But 

 that is only partly the reason; bad aeration has a 

 general retarding influence uporn root development. 

 Howard has noticed this even in|. the case of wheat, 

 which is a crop that can be successfully grown on very 

 heavy land. Experiments conducted at Pusa show 

 that the best-grown wheat can be raised only on soil 

 that is w'ell aerated. Lastly, rice which grows in 

 swamps is unable to thrive without a supply of oxygen 

 for root development. This is obtained through the 

 surface film of algae and other green organisms on 

 these soils. Certain cultural operations after harvest 

 also help to conserve a store of oxygen in the soil 

 subsequent to the arrival of the ra.ins. 



Other crops in other parts of the world are 

 ecpuilly susceptible to anaerobic conditions in the soil. 

 In regard to cotton, we know that this plant thrives 

 best on soils of open texture, and that the principal 

 cause of boll shedding is root asphyxiation, proved by 

 Balls in Egypt and fully supported by observations in 

 the West Indies. Cacao is extremely sensitive to clay. 

 I'hat may be because cacao is naturally a deep-rooting 

 plant and the clay offers mechanical resistance to the 

 extension of the roots; but it is also likely to be due to 

 the fact that a clay soil contains less air than a light 

 .soil. It is not merely a clay subsoil but a clayey 

 surface soil that has an adverse effect on the growth 

 of cacao. 



Coco nut trees ai-e very sensitive to inadequate 

 aeration. They will thri\o only on land that is well 

 drained either naturally or artificially. No harmful 

 effect is produced on the roots by the presence of water; 

 coco-nuts will thrive in saturated soil provided the 

 water is continually moving. This is a very significant 

 fact concerning the physiological importance of soil 

 aeration. 



In view of all these facts, which come within the 



range of observation of the plantfer himself, it will be 



admitted that soil aeration demands greater attention 



than it has received. The significant fact i.s, that air 



' is the limiting factor to the efficiency of water-supply. 



Beyond a certain point, water is wasted in the soil if 

 it is not aerated. 



Turning more particularly to the physiology of 

 roots, it is very desirable to know more concerning 

 their respiration. Respiration has been studied almost 

 exclusively in regard to the parts of the plant above 

 ground, and the generalizations have been extended to 

 apply to the roots. But it does not seem justifiable to 

 assume that the manner in which roots breathe under 

 the complex conditions, both chemical and physical, 

 of soil environment is the same, and follows the same 

 laws as those parts of the plants exposed to the com- 

 paratively simple environment of the atmosphere. 

 There is probably a difference in the rate of respiration 

 of the roots of certain plants, and as already suggested in 

 this article, the growth of certain crops might be stimu- 

 lated by theartifical introduction of air into the soil. 



Constructive Soil'' Ventilation as an established 

 branch of agricultural engineering presupposes success- 

 full researches into the air requirements of roots and 

 soil organisms. The desirable conditions in different 

 cases having been determined, it sh(Mild then be 

 possible to establish them. 



The methods of effecting soil aeration artificially 

 would come within the province of the eno-ineer. 

 Possibly one metlmd would be to lay down porous 

 ventilation pipes through which air could be introduced 

 if necessary, under pressure. In orchard cultivation 

 vertical tubes might be introduced near the trees and 

 air pumped down them periodically. In the lit^ht of 

 soil aeration better use might be made of soil explosions 

 with dynamite, to aerate clayey subsoils especiallj-. 



Investigation might show tiiat an alteration in the 

 percentage composition of the soil atmosphere would 

 prove advantageous in souie circumstances: for 

 example, a high proportion of oxygen might prove 

 beneficial, or in some cases the removal of an excess of 

 carbon dioxide. There is also the (juestion of the possible' 

 value of introducing gases other than those that norm- 

 ally constitute the air of the soil. A matter for specula- 

 tion is whether licjuid air could be usefully employed as 

 a soil fertilizer. 



Finally, more might be done to bring about a better 

 state of aeration in certain circumstances b}' means of 

 ntethods of cultivation. The forking of orchard soils is 

 still a matter of some controversy, and the t^rue value of 

 this operation recjuires investigation. The ploughing of 

 the soil in coco-nut plantations gives good results, but 

 its relation to soil aei'ation is not generally recognized. 



