EFFECTS OF N P K FERTILIZATION ON THE CHEMISTRY OF A YELLOW PODZOLIC SOIL UNDER PINUS-RADIATA

The effects of a large quantity of fertiliser containing N P K applied either once in solid form (spring 1983) or gradually (weekly in solution during the growing season, and repeated for 3 consecutive years) on the chemical properties of a yellow podzolic soil supporting a 10-year-old stand of Pinus radiata near Canberra, Australia, are described. Prior to fertilisation the soil was low in organic matter, slightly acidic with a low proton buffering capacity, and contained no KCl-extractable Al. Application of solid fertiliser (400 kg ha-1, 200 kg ha-1 and 100 kg ha-1 of N, P and K, respectively) resulted in significant uptake (166 kg year-1), net immobilisation (128 kg ha-1) and leaching (58 kg ha-1) of N during the following year. Proton balance was highly positive (approximately 30 kmol ha-1) during this period and the pH of surface (0-20 cm) soils declined by up to 0.8 units in KCl. The pH of soils which had received liquid fertiliser for 3 years fell to 3.6-3.8 in KCl, below the critical value of 4.2 at which Al begins to be released onto exchange sites. Reduction in pH also reduced the amount of soil surface charge and hence the cation retention capacity which was already very low. Application of solid fertiliser resulted in no net change in levels of exchangeable Ca, a significant loss (67 kg ha-1) of Mg and a slight gain in K in the 0-40 cm soil. Phosphorus was leached to a depth of 30 cm within 4 months after addition of 200 kg P ha-1 as superphosphate, reflecting very low soil P fixing capacities and a wet growing season. After this, little further downward movement occurred but there was progressive reduction in the amount of inorganic P in the forms examined. Temporal changes were greatest for resin P in the surface (0-10 cm) soil layers. This study suggests the importance of management practices which conserve or enhance organic matter levels in yellow podzolic soils.