Soil pH changes associated with lupin and wheat plant materials incorporated in a red–brown earth soil

Both alkalization and acidification of soil occurred when shoot and root materials from lupin and wheat were incubated in a red–brown earth soil, but with three different starting pH values, during a 70-day period. The response of soil pH change to the addition of organic matter depended on the type of plant materials and starting pH. The net effect of addition of lupin and wheat shoots to acid soils (pH<5) caused soil pH to increase, the addition of lupin roots to soils caused soil pH to decrease slightly, whilst with a higher pH soil (6.5) the wheat straw and lupin shoots raised pH and pH was unchanged for soil with addition of lupin roots. The ash alkalinity of plant materials and the mineralization of organic N are major reasons for the soil pH increase, and the nitrification of mineralized N results in soil pH to decrease. Whilst the data given here would suggest the likelihood of soil acidification occurring, particularly on poorly buffered soil given the inevitable influence of legume root materials, the overall directions of soil pH change in a cropping system that is legume-based will be very much influenced by the balance of many factors associated with the soil and plant system.