Effect of ammonium, organic amendments, and plant growth on soil pH stratification

Abstract

We studied the development of soil layers of different pH (strata) within the surface 10 cm of soil. Substrates which affect nitrogen transformations (ammonium sulfate, sucrose, and lucerne hay) were added to the soil to determine the effect of amendment on the development of pH stratification.

A glasshouse experiment used soils of 3 different initial pH(0·01 CaCl₂; pH_Ca) values: 4, 5, and 6. At the end of the experiment, soil was sampled in depth intervals of 0·5 cm between 0 and 2 cm depth and then 1-cm intervals between 2 and 10 cm depth. Soil pH_Ca decreased over time and pH_Ca changes could be explained by variations in the concentration of ammonium and nitrate, which were brought about by changes in the rate of mineralisation, nitrification, and plant uptake of nitrogen. Little stratification in soil pH_Ca was found within the 0–10 cm layer. This minimal stratification was considered to be due to the soil in the pots being a closed system where nitrate was not lost through leaching.

A field trial was rotary hoed to a depth of approximately 10 cm to achieve adequate mixing of soil and amendments. Soil pH_Ca decreased over the 6 months of the growing season, ranging from 0·1 pH_Ca units in the control to 0·4 pH_Ca units in the hay amendment. Between the initial and final sampling times there was little change in soil pH_Ca in the surface 0–2 cm. The pH_Ca decreased in the layers between 2 and 10 cm, thus producing the stratification of soil pH_Ca. The growth of oats had little effect on the development of pH_Ca stratification. Decreases in the soil pH_Ca from the 0–2 cm to the 8–10 cm layer in the absence of plants were 0·22, 0·27, 0·30, and 0·51 for the control, added sucrose, added ammonium sulfate, and added hay amendments, respectively. The processes of mineralisation and nitrification were considered to be the major causes of change in soil pH_Ca. Stratification of pH_Ca in the field trial was attributed to nitrification followed by nitrate leaching in the open system.