SOIL MANAGEMENT. 41 



or no harmful effect ; but combinations of several toxic salts in 

 solution often show greatly reduced toxicity, apparently due 

 to antagonism between different ions, e.g., highly toxic solu- 

 tions of magnesium salts may be rendered innocuous by the 

 addition of a small quantity of a soluble calcium salt. The 

 absence of pronounced toxic effects in many brak soils is pro- 

 bably due to such antagonism among the various salts, and 

 perhaps partly to adsorption of soluble salts by the soil. 



B. Injury to Soil. — (6) Structure. — Carbonates of 

 sodium and potassium are strong deflocculants. The presence 

 of appreciable quantities of these salts in the soil then leads 

 to puddling with all its characteristic harmful effects upon 

 mechanical properties. Surface crusts may form of such 

 hardness that seedlings cannot push through. Most of the 

 other common brak salts are flocculants and do not injure 

 structure, except in so far as they may be changed in the soil 

 to the harmful carbonates rcfen-ed to, e.g., repeated applica- 

 tion of nitrate of soda to lands is frequently observed to cause 

 deterioration in physical condition, due not to the nitrate it- 

 self, but to the residue of sodium carbonate which is apparently 

 left in the soil by this fertiliser. 



(7) Hardpan Formation. — The presence of brak in soils 

 seems to facilitate the formation of hardpan. 



(8) Movement of Soil Moisture. — Tn general capillary 

 movement is retarded by the presence of brak salts, though in 

 low concentrations this effect is very small. Sodium carbon- 

 ate appears to differ from the neutral salts in that it gives a 

 much greater capillary rise than the neutral salts in equal 

 concentration. 



(9) Evaporation of Moisture. — Evaporation from the soil 

 is lessened by the presence of brak salts, as these reduce the 

 vapour tension of the soil solution. For this reason, brak 

 spots in soils tend to remain moist much longer than the 

 surrounding normal soil. 



C. Injury to Micro-Organisms. — (10) While a great 

 deal of the present evidence is conflicting, it seems to indicate 

 that decreased bacterial activity is an important phase of the 

 injury to crop production caused by brak salts. Ammonifica- 

 tion, and nitrification in particular, are greatly retarded, in 

 some cases at least. 



Reclamation of Brak Soils. — Observations in regard to 

 the growth of crops, coupled with chemical analysis, will 

 clearly indicate the presence of brak. The limit of the 



