The role of plant residues in pH change of acid soils differing in initial pH( Soil Biology and Biochemistry,Volume 38, Issue 4, April 2006, Pages 709-719

Abstract: Reports on the effect of plant residues on soil pH have been contradictory. The conflicting accounts have been suggested to result from differences in compositions and types of plant residues and characteristics of soils. This incubation study examined the effect of plant residues differing in concentrations of N (3–49 g kg−1) and of alkalinity (excess cations) (220–1560 mmol kg−1) on pH change of three soils differing in initial pH (3.9–5.1 in 0.01 M CaCl2). The addition of plant residues at a rate of 15 g kg−1 soil weight increased the pH of all soils by up to 3.4 units and the pH reached the maximum at day 42 after incubation for Wodjil (initial pH 3.87) and Bodallin (pH 4.54) soils and day 14 for Lancelin soil (pH 5.1). The amount of pH buffering was decreased by residue addition in Wodjil soil, increased in Bodallin soil and remained unchanged in Lancelin soil, which closely related to changes of soil pH. Residue addition increased NH4+ concentration and the increase in NH4+concentration generally correlated positively with the concentration of residue N. The NH4+concentration increased with time, reached the peak at Days 42–105 for Wodjil soil, Days 14–105 for Bodallin soil and Days 14–42 for Lancelin soil, and then decreased only in Lancelin soil. The concentration of NO3− was kept minimal in Wodjil and Bodallin soils. In Lancelin soil, NO3− concentrations increased with incubation time from days 14–28. Irrespective of plant residue and incubation time, the amounts of alkalinity produced due to residue addition correlated highly with the sum of the alkalinity added as plant residues (excess cations) and those resulting from mineralization of residue N, with the slope of regression lines decreasing with increase of the initial soil pH. Direct shaking of soil with the residues at the same rate of alkalinity (excess cations) under sterile conditions increased the pH of the Wodjil soil but decreased it in the Lancelin soil. It is suggested that the decarboxylation of organic anions (as indicated by excess cations) of added plant residues and ammonification of the residue N causes soil pH increase whereas nitrification of mineralised residue nitrogen causes soil pH decrease, and that the association/dissociation of organic compounds also plays a role in soil pH change, depending initial pH of the soil. The overall effect on soil pH after addition of plant residues would therefore depend on the extent of each of these processes under given conditions.

In this regard an experimental result on ecological restoration of the natural grassland vegetation on Indian sodic soils from the year 1970 to 2006 can be cited. PDF is attached. Please see in the Section 3.1. It indicates that due to protection of natural vegetation on highly sodic soil for almost four decades showed that the soil pH and electrical conductivity of the grassland soil have been found to decrease from 10.6 to 7.7 and electrical conductivity from 22.34dS m-1 to 0.4 dS m-1, respectively.

Dr. Ali, the reports are contradictory. In some soils with the residues incorporation reported an increase in pH but it decreased in other soils. I am attaching some of the papers which may provide you some detailed information and the reasons.

The role of plant residues in pH change of acid soils differing in initial pH( Soil Biology and Biochemistry,Volume 38, Issue 4, April 2006, Pages 709-719

Abstract: Reports on the effect of plant residues on soil pH have been contradictory. The conflicting accounts have been suggested to result from differences in compositions and types of plant residues and characteristics of soils. This incubation study examined the effect of plant residues differing in concentrations of N (3–49 g kg−1) and of alkalinity (excess cations) (220–1560 mmol kg−1) on pH change of three soils differing in initial pH (3.9–5.1 in 0.01 M CaCl2). The addition of plant residues at a rate of 15 g kg−1 soil weight increased the pH of all soils by up to 3.4 units and the pH reached the maximum at day 42 after incubation for Wodjil (initial pH 3.87) and Bodallin (pH 4.54) soils and day 14 for Lancelin soil (pH 5.1). The amount of pH buffering was decreased by residue addition in Wodjil soil, increased in Bodallin soil and remained unchanged in Lancelin soil, which closely related to changes of soil pH. Residue addition increased NH4+ concentration and the increase in NH4+concentration generally correlated positively with the concentration of residue N. The NH4+concentration increased with time, reached the peak at Days 42–105 for Wodjil soil, Days 14–105 for Bodallin soil and Days 14–42 for Lancelin soil, and then decreased only in Lancelin soil. The concentration of NO3− was kept minimal in Wodjil and Bodallin soils. In Lancelin soil, NO3− concentrations increased with incubation time from days 14–28. Irrespective of plant residue and incubation time, the amounts of alkalinity produced due to residue addition correlated highly with the sum of the alkalinity added as plant residues (excess cations) and those resulting from mineralization of residue N, with the slope of regression lines decreasing with increase of the initial soil pH. Direct shaking of soil with the residues at the same rate of alkalinity (excess cations) under sterile conditions increased the pH of the Wodjil soil but decreased it in the Lancelin soil. It is suggested that the decarboxylation of organic anions (as indicated by excess cations) of added plant residues and ammonification of the residue N causes soil pH increase whereas nitrification of mineralised residue nitrogen causes soil pH decrease, and that the association/dissociation of organic compounds also plays a role in soil pH change, depending initial pH of the soil. The overall effect on soil pH after addition of plant residues would therefore depend on the extent of each of these processes under given conditions.