138 AERATION AND AIR-CONTENT. 



(6) its effect on the S5niibiotic nitrogen-fixing bacteria, (c) and its toxic or 

 destructive effect on the root tissues of plants. 



"The supply of available calcium in all forms becomes less as soils become 

 acid, but usually there is still sufficient present to furnish that needed as 

 direct plant-food material. Since the symbiotic nitrogen-fixing bacteria live 

 in the nodules, soil acidity can not affect them directly, except before they 

 enter into symbiosis, when it may lessen their activity and delay the time of 

 infection. Since the relation of non-legumes to soil acidity runs parallel with 

 the relation of the legumes, it follows that the direct influence is not on the 

 legume bacteria but on the plants themselves. 



"That this direct effect on the plants is not often due to a destructive 

 action of the acidity on the root tissues is evident from the fact that experi- 

 ments have shown that plant roots are unaffected by solutions of a higher 

 acidity than that of most acid soil solutions. This is further substantiated 

 by the fact that the acidity (H-ion concentration) of the sap of most plants 

 is of the same order as that of the soil solution of most acid soils, indicating 

 that similar processes are probably at work in the two cases, as a result of 

 certain analogous conditions which exercise a regulatory function in this 

 respect. It is undoubtedly largely proteins in the case of plants, and colloidal 

 organic and inorganic matter, especially silicates, in the case of soils which act 

 as "buffers" and thus bring about this regulation of acidity to a considerable 

 extent, preventing rapid changes and unusually high degrees of soluble 

 acidity. 



"In most cases it thus appears that the main specific harmful influence of 

 soil acidity on certain plants is not due to any of the three suggested reasons, 

 but to its influences in preventing these plants from getting at a sufficiently 

 rapid rate the calcium as the carbonate or bi-carbonate which is needed 

 to neutraUze and precipitate certain acids in the plants themselves, which 

 are probably largely by-products, produced as the result of certain vital 

 reactions in the growth of plants. If calcium in these forms is not furnished 

 at a sufficiently rapid rate, then the rate of these reactions is lowered accord- 

 ingly as is also the rate of plant growth. 



"Each species of plant has a certain Hme requirement which must be 

 satisfied for maximum plant growth and this Hme requirement is defined by 

 the writer as follows: The expression 'hme requirement' of a plant refers 

 to the actual hme needs of the plant itself, especially as to the ease and rate 

 at which lime must be secured from the soil by the plant for normal growth. 

 Thus if a plant has a high hme requirement, then the solution and dehvery 

 must be rapid and easy in order to meet the needs of the plant. 



"The three main factors which determine the hme requirement of a plant 

 are : (a) hme content, (b) rate of growth, and (c) feeding power of the plant 

 for lime. The first two factors operate in one direction while the third operates 

 in the opposite direction. That is, the higher the hme content and the rate 

 of growth, the higher will be the Hme requirement, and conversely. Also, 

 the higher the feeding power for Hme, the lower will be the Hme requirement, 

 and conversely. The resultant of these three gives the Hme requirement 

 of the plant. A simple method of expressing these factors and obtaining the 

 resultant is described in this article. 



"A table is also given in which are expressed the Hme requirements of 62 

 species of plants as obtained by this method. These Hme requirements are 

 compared with corresponding figures which represent the relative response 

 of these plants to the Hming of acid soils, or reciprocaUy to their abihty to 

 grow on acid soils. The comparison reveals a close correspondence and hence 

 substantiates the theory which has been proposed that, usually, the main 

 specific injury of soil acidity is that it prevents plants, especially those with 



