The relationship between the pH in a 1:5 soil/water suspension and the pH in 0.01 m CaCl2 has always been considered to be linear, but recent findings favour a non-linear relationship. This trend was confirmed with a set of 1342 surface soils from acid to neutral red duplex and gradational soils from the Lachlan Shire in central western New South Wales. The linear relationship was consistent with that found by earlier workers, but inclusion of the second and third powers of pH(w) accounted for significantly more of the variation in pH(ca) resulting in a sigmoidal curve. It is suggested that the deviation from linearity is due to the buffering effect of Al at low pH and to the presence of carbonate at high pH. Several other attributes were shown to influence pH, namely the ionic strength as indicated by the electrical conductivity of the 1:5 soil/water suspension, exchangeable cations (Na, K, Ca,Mg and Al) and Al and Mn extracted with 0-01 m CaCl2, and their effect varied at different pH levels.
Soil acidification and related land degradation issues are assuming increasing
importance in Australia and challenging the concept of sustainability of
current land management systems. In this study, the impacts of tree
plantations of 2 species and permanent pasture on soil chemical properties are
compared. Soil samples were collected from the top 50 cm (0–5,
5–10, 10–15, 15–20, 20–30, and 30–50 cm depths)
from 3 adjacent sites carrying pasture and monocultures of
Pinus radiata (radiata pine) and
Quercus suber (cork oak) on a deep-surfaced yellow
podzolic soil, and differences in soil pH and other soil chemical properties
were examined. In the surface 0–5 cm, pH was similar at all 3 sites.
Below that depth, soil pH was significantly lower and exchangeable Al greater
under the cork oak stand than at the other 2 sites. Consistent with a decrease
in soil pH there was significantly less exchangeable Ca under cork oak. Also,
less clay was observed under the cork oak stand and this is taken as evidence
of the degradational impact of soil acidification.
An estimate of Ca in the top 50 cm of the soil implies considerable loss of Ca
under oak, probably by leaching and loss of litter down the slope. Evidence is
presented to show that there has been more Fe and Al movement under oak than
under pasture and pine, this being ascribed in part to the greater Al and Fe
mobilising capacity of the water-soluble component extracted from freshly
fallen leaf litter of oak. The Fe and Al composition of the oxalate extract
from concretionary material at 10–30 cm under oak is consistent with a
process similar to podzolisation. Pseudogleying of Fe and Al may have
accompanied the leaching of bases from the system and a reduction of pH.
A leaching procedure employing 1N potassium chloride was found to yield satisfactory estimates of exchangeable aluminium in subsoil samples. The use of 1N ammonium acetate solutions as extractants was not recommended. When buffered at pH 4.8 these solutions gave low results, whereas at pH 4.0 there was some attack on hydroxy-aluminium polymers, which led to erroneous results, particularly in sesquioxidic soils. Satisfactory estimates of exchangeable aluminium in subsoils were obtained by potentiometric titration of soil suspensions in 1N potassium chloride with 0.1N sodium hydroxide, provided allowance was made for the contribution of pH-dependent charge between pH 4.0 and 5.5. This procedure gave values consistently lower than those obtained by leaching with 1N potassium chloride, and of the order of 81 % of these. This was consistent with the formation of an aluminium compound with the formula Ak6(OH)153+ whose existence has been postulated by other workers.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.