160 AERATION AND AIR-CONTENT. 



grass is correlated with an accumulation of dead roots, leaves, and 

 other debris in the surface soil, which promotes the activity of soil 

 organisms. Rain-water percolating through such a layer of grass 

 would tend to lose its oxygen and to become heavily charged with 

 CO2. Pickering notes that when such ''toxic matter is exposed to 

 the air for 24 hours its toxic property is found to have entirely 

 disappeared." Exposure to the air would tend to make good a 

 deficiency of oxygen and to dissipate an accumulation of CO2 by 

 diffusion. Such an accumulation of carbon dioxid about plant roots 

 has been demonstrated by Leather for a number of plants. 

 HKing (1908 : 626) criticized at some length the assumption that 

 crops excrete and leave in the soil toxins which are the chief cause of 

 reduced yields and worn-out lands, and that rotation, manuring, and 

 fertihzing owe their good effects to destroying or removing the toxins 

 rather than to their ability to supply nutrients. The amount of 

 nutrients carried by the soil was discussed, and the conclusion reached 

 that it is impossible that a mere rotation of crops, coupled with good 

 tillage and adequate water-content, should indefinitely maintain 

 high yields, when the whole crop above ground is regularly and con- 

 tinually removed from the field. Figures were given to show the ex- 

 treme variation of the soil solution, and these indicated that there is 

 no good foundation for the contention that all soil solutions have 

 essentially the same composition and concentration, viewed from the 

 standpoint of their function in plant-growth. Evidence was also 

 given to show that there was a regular and corresponding increase 

 in the yield with each increase in the amount of nitrogen, phos- 

 phorus, and potassium recoverable from the soil, and Rothamsted 

 results were cited to prove that the soluble salt-content of soils is not 

 constantly maintained at a point sufficient to give good crop yields. 

 Experiments in which the toxic effect of organic compounds was de- 

 termined on the basis of transpiration or green weights were regarded 

 as misleading and indecisive, and particular objection was raised to 

 the short term, the small amount of solution, and the generally ab- 

 normal conditions of experimentation. In short, it is concluded 

 that nothing yet published by the Bureau of Soils or by others should 

 in any sense be regarded as proof that toxic excreta play an important 

 role in rendering soils unproductive. 



Hall, Brenchley, and Underwood (1913) have tested the conten- 

 tions of Whitney and Cameron, and reach the following conclusions 

 with respect to the soil solution and the growth of plants in it: 



"The composition of the natural soil solution is not constant as regards 

 phosphoric acid and potash, but varies significantly in accord with the com- 

 position of the soil and its past manurial history. Within wide limits the rate 

 of growth of a plant varies with the concentration of the nutritive solution, 

 irrespective of the total amount of plant food available. When other condi- 

 tions, such as the supply of nitrogen, water, and air, are equal, the growth 



