EFFECT OF DEPTH AND LIME OR PHOSPHORUS-FERTILIZER APPLICATIONS ON THESOIL SOLUTION CHEMISTRY OF SOME NEW-ZEALAND PASTORAL SOILS

Thirteen trails were sampled to investigate the effects of depth, or t he surface application of lime and phosphorus (P) fertilizer, on solut ion composition. Soil solutions were extracted by centrifuge from fiel d moist soils within 24 h of sampling. Solution Ca, Mg, Na and K, Al, Mn and Fe concentrations generally decreased and Al, Mn and Fe concent rations generally increased with depth; although exceptions occurred. The largest decrease occurred in the first 25-50 mm of soil. Higher so lution Al concentrations occurred in a band at a depth of between 50 a nd 100 mm in some soils. Lime generally increased solution pH and solu tion Ca, Mg and HCO3 concentrations, and reduced solution Al, Fe and M n concentrations in the topsoils. In one soil (Matapiro silt loam) 2 y ears after lime was applied, lime increased solution pH down to a dept h of 100 mm, Ca and HCO3 down to 75 mm and Mg down to 50 mm. Lime also decreased solution Al and Mn down to 75 mm and Fe down to 50 mm. In o ne series of trials, lime increased solution Ca concentrations at a de pth of 50-100 mm 4 years after application in six out of the eight sit es. In the same trial series, the application of P fertilizer increase d solution P concentrations at 0-50 mm from a mean of 5 muM in the no- added P plots up to a mean of 56 muM at the highest P rate. The highes t solution P concentration recorded was 194 muM. The increase in solut ion P concentrations for a given application of fertilizer P varied fr om 0.05 to 1.03 muM P per kg P ha-1 applied. Maximum pasture yield and 90% maximum yield occurred when solution P concentrations were about 28 and 14 muM respectively. Solution P concentrations determined from P adsorption isotherms were not a good indicator of solution P concent rations measured in soil. Solution pH was higher than soil pH (1:2.5 s oil:water ratio, 2 h equilibration) with a solution pH of 6.0 correspo nding to a soil pH in water of about 5.2.