Soil fertility changes in the long-term experimental plots at Kybybolite, South Australia. I. Changes in pH total nitrogen, organic carbon, and bulk density.

Abstract

Variously treated grassland plots at the Kybybolite Research Centre established in 1919 on solonetzic soils were sampled in 1957–58. Analyses of the soils and for pH and nitrogen were made and compared with earlier analyses made in 1938. Bulk density and organic carbon were also determined. Treatment with phosphorus fertilizers and grazing with sheep has resulted in large increases in the organic matter content of the surface soil. A large part of the increase has been concentrated in the 0–2 in. horizon. At the 6–8 in. level, increases have been small. When considered on the basis of pounds per acre, the increase in soil nitrogen in the surface 6 in. of some plots appears to be linear with time, and even after 39 years shows no signs of approaching an asymptotic value. However, there appears to be differential behaviour within the soil profile. The rate of increase of the 0–2 in. horizon in recent years shows signs of being much less than in the earlier years of the experiment. On the other hand, in the 2–4 and 4–6 in. horizons the recent rate of increase is similar to or greater than that in earlier years, which indicates that a deepening of the organic matter profile is occurring. There appears to be some relation between the increase in soil nitrogen and sheep-carrying capacity. The increase in soil nitrogen on the soils from the plots has been, on the average, equivalent to 14 lb of soil nitrogen per sheep per acre per year. Additional calcium applied with water-soluble phosphorus appears to result in higher levels of soil organic matter. Lime, in particular, appears to result in the highest levels of soil organic matter. Analysis of the soils shows that the increase in soil organic matter is associated with a slight decrease in pH. The soils also show a decrease in bulk density which is closely related to changes in organic matter content. Various aspects of organic matter accumulation are discussed, particularly the importance of the time factor in this process and the lack of a causal relationship between the amount of superphosphate applied and the increase in organic matter.