Phosphate sorption indices as affected by the calcareousness of soils

Abstract

An experiment was carried out to study the effects of calcareousness on
phosphate sorption indices of soils using three representative calcareous soils,
namely Sara (Aquic Eutrochrept), Gopalpur (Aquic Eutrochrept), and Ishurdi
(Aeric Haplaquept) series of Bangladesh. Three non-calcareous soils, namely
Belabo (Typic dystrudepts), Sonatala (Aeric Endoaquepts) and Ghatail (Aeric
Haplaquept) series were also selected for comparison purposes. Phosphate
sorption indices of soils were calculated using Langmuir and Freundlich
isotherms. Isotherms were constructed taking one gram of air-dried sieved (< 2
mm) soil into a 50 ml centrifuge tube, and subsequently adding seven initial P
concentrations, namely 0, 1, 10, 25, 50, 100 and 150 μg/ml to each centrifuge tube
employing a soil/solution ratio of 1 : 20 (w/v). According to the Langmuir
equation, the amount of phosphate sorbed followed the order: Sonatala > Ghatail
> Sara = Gopalpur > Ishurdi > Belabo. The abundance of amorphous iron rather
than the calcareousness was putatively responsible for the high phosphate
sorption capacity of soils. Maximum phosphorus buffering capacity (MPBC) of
the calcareous soils ranged from 33.4 - 62.8 l/kg. Langmuir and Freundlich
equations produced different values for soil P requirements (SPR) at 0.2 and 1.0
mg P/l. Calcareous soils would require 27 - 44 mg P/kg soil to attain 0.2 mg P/l
soil solution, which is deemed sufficient for crop growth. The soils would require
32 - 58 mg P/kg soil to reach 1.0 mg P/l soil solution, which is regarded to be safe
for soils in terms of susceptibility to P losses. The calculated Langmuir constant b
values were higher than the threshold value of 0.07 l/mg for two of the
calcareous soils. Therefore, even though the non-calcareous soils sorbed more
phosphate, higher bonding energy of P sorption for calcareous soils makes them
less vulnerable to loss via surface runoff.