Phosphorus nutrioperiodism in rubber

Abstract

Mature rubber tree does not respond to application of phosphorus fertilizers consistently. In majority of the earlier fertilizer experiments on mature rubber, there was no response to application of phosphorus fertilizers, in terms of growth and yield. Trees continued to maintain leaf P status without application of P even when the soil P status was low. An experiment was initiated at the Rubber Research Institute of India, Kottayam in 1997 in an area planted in 1984 to study the response of mature rubber to phosphorus application. From planting to till the commencement of the experiment all trees were given uniform quantities of nutrients. Treatments were imposed from 1998 onwards. Detailed investigation was carried out in this experiment during 2002-2004 to explore the possibility of reducing the present dose of P fertilizer (30 kg P2O5 ha-1 year-1) for mature rubber. Phosphorus was applied at five levels (0,10,20,30 and 40 kg P2O5 ha–1 year¬1). Nitrogen and potassium were applied @ 30 kg ha-1 year¬1 at uniform rates. None of the growth characters studied, viz., girth, girth increment, length of tapping panel and bark thickness were influenced by the application of P. The yield and dry rubber content were also not influenced by levels of applied P. Trees which did not receive any P fertilizer continued to maintain leaf P status at comparable levels with that of the P applied treatments. Magnesium content of the leaves was generally higher in the P applied treatments after fertilizer application. Content of other nutrients (N, K, Ca, Fe, Mn, Zn and Cu) in the leaves was not affected by the different treatments. The accumulation of P and other nutrients in the dry matter was not influenced by application of P. The content of P and other nutrients (N, K, Ca, Mg, Fe, Mn, Zn and Cu) in the latex and their annual removal were not influenced by the levels of applied P. The quantity of litter added and the litter nutrient content were also not affected by the different treatments. Removal of P through latex was slightly less than addition through litter. Before defoliation the litter P content was comparatively less in the treatment which did not receive any P fertilizer indicating high P utilization efficiency of trees. Application of P did not influence the content of different P fractions (saloid P, Fe-P, Al-P, Ca-P, reductant P, occluded P and organic P) in soil. Organic P constituted more than 50 per cent of the total P in the soil. Total P content was significantly higher in the treatments which received higher levels of P. Soil available P status was higher in P applied treatments. In most cases, a decline in micronutrient content was observed in the treatments which received higher levels of P. More than 85 per cent of the roots were colonised by arbuscular mycorrhizal fungi (AMF), but no relation was observed between the levels of applied P and root colonization. Population of phosphorus solubilizing bacteria (PSB) in the rhizosphere was suppressed after application of P whereas the population of other microorganisms (total bacteria, phosphofungi, total fungi and actinomycetes) was not influenced. The trees which were not supplied with P improved their P acquisition by several processes. Secretion of acid phosphatase, an enzyme which hydrolyzes organic P, was found to be higher from the roots of trees which did not receive P fertilizers. Soil phosphatase activity was also higher in the rhizosphere of trees which did not receive any P fertilizer. Activities of phosphoenol pyruvate carboxylase (PEPC) and malate dehydrogenase (MDH), enzymes involved in the biosynthesis of organic acids were higher in the roots of trees which were not supplied with P fertilizers indicating the possibility of more exudation of organic anions and H+ from these trees. Rhizosphere pH was generally lower in the treatment which did not receive P fertilizers. Compared with non-rhizosphere soil pH, rhizosphere pH was comparatively higher in most cases. Roots of mature rubber trees explored a large area of soil (89.38 m2). More than 60 per cent of the fine roots explored the surface 10 cm layer indicating that the trees have a high P acquisition efficiency. A spatial difference in fine root production and distribution was observed between treatments. Trees which were not supplied with P produced more fine roots than trees supplied with P. Root hair production was also higher in the control trees. All these adaptations contributed to a high P acquisition efficiency of rubber trees which were supplied with P during active growing phase. The results of the present study indicate the self sustainability of phosphorus cycle in a well managed mature rubber plantation which was fertilized during the active growing phase of the plant suggesting the possibility of skipping P application in trees of that age group. This situation is likely to continue during the later phase also, however if a need is indicated through leaf P status, application of P can be resumed.