Evaluation of phosphorus availability in a lateritic gravelly, coconut grown and long term phosphate fertilizer applied soil in Sri Lanka with an indicator plant

Abstract

The residual phosphorus availability and response of Ginger (Zinigiber officinale) to residual available phosphorus in (lateritic gravelly) Red Yellow Podzolic soil (RYP - Rhodustults) of Andigama soil series from Rathmalagara estate, Madampe was evaluated after 10 years of continuous application of 03 sources of phosphates to young coconut palms. A pot experiment was established using Andigama soil series which was collected from field experimental site where coconut palms were treated with 03 levels of phosphate with 03 different phosphate sources i.e., Eppawela Rock Phosphate (ERP, 30 % P2O5), Imported Rock Phosphate (1RP, 27.5% P205) and Triple Super Phosphate (TSP, 46 % P205) from the seedling stage up to the age of 10 years. The initial rates of phosphate applications were P205 equivalent of 50, 100 and 200 g palnrf'year'1 respectively which were annually increased by 5, 10 and 20 g respectively. Ginger was used as an indicator plant as the growth parameters, yield and the total P content in the ginger can be taken as an indication of the response to different treatments. Ginger rhizomes were planted in pots filled with 5 kg of phosphate treated soil and all the plants were treated with a basal dose of nitrogen (N) and potassium (K.) at every 1 V2 month intervals for 6 months. Number of tillers per bush, height of the plants and number of leaves in bush in each pot were recorded. Six months after planting, ginger plants were uprooted and fresh and dry weight of shoots of the plants, roots and rhizomes were recorded. All the plant parts were analysed for total P. Soil samples were collected before planting the ginger and analyzed for available P by 2.5 % acetic acid method, Olsen’s bicarbonate and Bray and Kurtz 1 method. The overall result showed that the extractable P in soil treated with all 03 levels of ERP, IRP and TSP by 2.5 % HAc-P, Olsen - P and Bray - P values were above the sufficiency soil P values of 7.8 mg kg-1, 8.5 mg kg-1 and 5.8 mg kg-1 respectively. The highest level of TSP, IRP and ERP treatments were significantly effective in increasing the dry weight of rhizomes and the shoots of ginger. It indicated that residual phosphorus availability of TSP, IRP and ERP was high and its effectiveness on response to ginger was more or less similar. This might be due to low dissolution of ERP and fixation of TSP in lateritic gravel soil. According to the findings of this experiment, P fertilizers can be taken out from the coconut fertilizer recommendation for a period of time by monitoring the effectiveness of the residual P status in soil and coconut leaf. Findings of this experiment can be included into site specific fertilizer recommendation programmes of the other perennial crops. By monitoring the availability of P in soil, intercrops can be grown in coconut plantations without application of P fertilizers. COCOS Vol. 20: 59-70 (2013)