Response of carrots (Daucus carota L.) to applied phosphorus leaching on a Karrakatta sand, under two irrigation regimes

Abstract

The response of carrots (Daucus carota L.) to applied phosphorus (P) (0-320 kg/ha) and P leaching was investigated on a Karrakatta sand under 2 irrigation regimes (140% of pan evaporation in 2 or 4 applications per day). There was no significant effect of frequency of irrigation on response of carrots to P. Phosphorus at 157 � 57 kg/ha was necessary for 99% of maximum yield (95 t/ha) of carrots, which corresponded to maximum economic yield. For 95% of maximum yield 102 � 10 kg/ha was necessary. Applied P at up to 20 kglha resulted in a significant increase in the yield of medium (25-50 mm crown diameter) and large carrots (>50 mm diameter), a decrease in the yield of small (10-25 mm diameter) and very small carrots (<I0 mm diameter), and an increase in root to shoot ratios. Rejects were a constant percentage (9%) of total yield at all rates of applied P. The P concentration (dry weight basis) in youngest mature leaves at midgrowth required for 99% of maximum yield was 0.38 � 0.02%. Visual symptoms were not useful for diagnosing P deficiency in carrots because symptoms such as purpling of older leaves were only apparent on severely deficient plants. Phosphorus uptake by both roots and shoots increased with increasing rate of applied P; however, efficiency of P uptake (i.e. P uptake by shoots � roots/P applied, in per cent) decreased with increasing application rate from 27% of applied P at 20 kg/ha to 14% at 320 kg/ha. Eighteen per cent of applied P was taken up by the crop at the rate of P necessary for maximum yield and profit. There was no evidence of soil P leaching below 30 cm except at the highest application rate, and there was no effect of frequency of irrigation on P leaching. This supports evidence that Karrakatta sands have moderate P retention capacity and present a low pollution risk to water systems on the Swan Coastal Plain.