lOOOi 



800- 



X 

 V. 



07 



J 6001 



z 

 o 



< 



UJ 



y 



5 

 u 



200- 



103 



14 6 



493 



! 



% 58 



854 



Sodium 



CZZZ3 



Magnesium 



236 



23 



JZZL 



1 



tM» 



Calcium 



[XXX] 



1.5 

 DRYING PERIOD (months) 



3.0 



Figure 4. Effect of soil drying on Na, Mg, and Ca concentrations. 



Urease Activity and Gypsum Additions 



Gypsum (CaS0 4 »2H 2 0) is used as a source of soluble Ca for reclaiming sodic soils, preventing 

 soil crusting, and ameliorating water quality (Gobran et al. 1982; Oster, 1982; Gobran and 

 Miyamoto 1985). A favorable positive interactive effect of gypsum and NPK fertilization on saline 

 soils was reported by Rankov (1967). He stressed that gypsum + dung + NPK increased the 

 numbers of ammonifying and nitrifying bacteria and also increased ammonification and nitrification. 

 Similar effects were reported by Carter et al. (1978) on the combined application of ammonium 

 nitrate and gypsum. 



In the work reported here (Figure 5), gypsum additions did not result in any significant increase 

 of urease activity of soils. Nonetheless, there was a 50% increase in urease activity of soil treated 

 with 20 mg/ha gypsum compared with the control. Earlier findings of Rankov (1967) showed that 

 application of gypsum alone, particularly at the rate of 25 mg/ha, tended initially to decrease the 

 number of nitrifying bacteria and inhibit ammonification and nitrification. Bower (1969) reported 

 that application of gypsum followed by leaching is the most common method to replace adsorbed 

 Na. Leaching with successive dilutions of highly saline water having 30% or more Ca + Mg salts 

 is an effective means of removing adsorbed Na while maintaining soil permeability. 



93 



