Effect of phosphorus on the yield, quality and petiolar phosphorus concentrations of potatoes (cvv Russet Burbank and Kennebec) grown in the krasnozem and duplex soils of Victoria

The response of cvv. Russet Burbank and Kennebec potatoes to different levels of residual and applied phosphorus (P) was compared on krasnozem and duplex soils in the Central Highlands of Victoria. Field experiments were conducted at 12 sites over 3 years to examine the effects of applied P (banded at planting) at rates up to 475 kg/ha on yield, petiolar P concentrations, tuber size, number of tubers/plant and specific gravity. The relationships between yield response and fertiliser P required to optimise yields with Olsen P and P adsorption isotherms were also determined. Application of P significantly (P < 0.05) increased yields at 11 of the 12 sites for cv. Russet Burbank and at 6 of the 9 sites for cv. Kennebec. Based on data for all sites, there was a significant (P < 0.001) Mitscherlich relationship between yield response to applied P and Olsen P. This relationship was significantly (P < 0.05) different for each cultivar. The critical Olsen P concentration was 27 mg/kg for cv. Kennebec. However, for cv. Russet Burbank the relationship had not plateaued, although our Olsen P concentrations ranged from 5-46 mg/kg. For cv. Russet Burbank there was a significant (P < 0.05) negative linear relationship between the amount of applied P required to achieve 95% of maximum yield and Olsen P concentrations. However, there was no significant (P > 0.05) relationship between the amount of applied P required to achieve 99% of maximum yield and Olsen P concentrations. For these krasnozem and duplex soils, the predictive models of either yield response or the amount of applied P required to optimise yield, were not significantly (P > 0.05) improved by including P sorptivity measures. At yield-responsive sites there were significant (P < 0.05) changes in the proportion of tubers > 280 g for both cultivars. However, the only quality parameter adversely affected by applied P was specific gravity. There were significant (P < 0.05) Mitscherlich relationships between relative yield and petiole P concentrations for both cultivars. The following critical petiole P ranges have been proposed to assist in the assessment of the P status of cv. Russet Burbank crops in Victoria: 0.45-0.57% at a tuber length of 5-10 mm; 0.35-0.47% at a tuber length of 35-45 mm and 0.21-0.26% at a tuber length of 75-85 mm. The use of petiole sampling for assessment of the P status of potatoes before the 5-10 mm tuber stage is not recommended. The critical P range for cv. Russet Burbank was higher than the critical P range for cv. Kennebec at a 5-10 mm tuber length. The practical implications from this work are that Olsen P is a good indicator of a site's potential yield response to applied P fertiliser. However, at those soil P concentrations where a yield response is expected, Olsen P is of little value to predict the amount of P fertiliser required to achieve maximum yields. We have established that separate calibration curves are required for cvv. Russet Burbank and Kennebec to define their critical Olsen P concentrations on krasnozem and duplex soils. This is the first work in Australia showing critical nutrient ranges for petiolar P over time in cv. Russet Burbank.