132 AERATION AND AIR-CONTENT. 



Ruprecht and Morse (1917) state that the positive presence of 

 soluble salts of iron, aluminum, and manganese in soils which have 

 been repeatedly dressed with ammonium sulphate without adding 

 lime; the formation of one or more of these salts in soils that were 

 extracted with solutions of ammonium sulphate; and the positively 

 injurious action of manganese sulphate, iron sulphate, and aluminum 

 sulphate form a chain of facts which clearly indicates that the in- 

 jurious effects of sulphate of ammonia when used freely without the 

 accompaniment of lime are due to the formation of these soluble 

 salts in the soils of the fields so dressed. 



Hartwell and Pember (1918 : 276) have determined that the unlike 

 response of rye and barley to acid soil is due to active aluminum. 

 Treatment of an acid soil with phosphoric oxide or acid phosphate 

 reduced the amount of active aluminum in the soil, and large addi- 

 tions of the latter caused remarkable growth in plants in which pre- 

 viously growth was impossible. This was in spite of the fact that 

 the acid phosphate greatly increased the acidity of the soil ; however, 

 it much decreased the active aluminum. It is suggested that the 

 practical advantage of phosphate and lime may be due to the pre- 

 cipitation of active aluminum, quite as much as to the value of the 

 first as a nutrient and the second as a reducer of acidity. 



In similar studies of the toxicity of aluminum, Mirasol (1920 : 153) 

 has shown that its salts are directly concerned in the unproductivity 

 of acid loam soils. In soils with sufficient calcium, toxic aluminum 

 salts may never be formed, but in acid soils, where the bases are 

 deficient, such salts are largely the end-products of sulfofication and 

 nitrification. The toxicity of aluminum salts is corrected by calcium 

 carbonate or by acid phosphate through their precipitation as in- 

 soluble calcium aluminate or aluminum phosphates. 



In studies of an acid soil in Assam, Meggitt (1914) has concluded 

 that the acidity is partly due to specifically toxic organic compounds, 

 arising in consequence of reduced oxidation in the soil. Such toxic 

 organic compounds are destroyed by oxidation, and this is promoted 

 by lime, nitrates, or phosphates, whether it is carried on by the roots 

 themselves or by other organisms. 



Harris (1914 : 14) concludes that the behavior of the soil toward 

 neutral salts is not due to insoluble humic acids or to the presence of 

 organic matter, but to inorganic compounds, probably hydrated 

 silicates. The reaction of so-called acid soils of the sandy-loam type 

 is due to the selective adsorption by the soil of the basic constituents 

 of the neutral salt solution, and is not caused by double decomposi- 

 tion with adsorbed acids or insoluble "humic acids." The "acidity" 

 of soils of this type probably arises from the formation of soluble 

 salts through the interaction of weak acids (C 2 H 2 4 , C0 2 , etc.) in 

 the soil solution and the basic material naturally held absorbed by 



