Effects of organic and inorganic fertilisers on the growth of pseuderanthemum atropurpureum, soil fertility and leachate composition.

Abstract

The use of fertilisers in agricultural production systems, particularly nitrogen and phosphorous, has been shown to be one of the causes of eutrophication as a result of the excessive enrichment of freshwater systems through surface runoff and soil infiltration. The contamination of freshwater bodies from horticultural production systems in South Africa has, however, been rarely studied, although influx from such systems are considered highly polluting elsewhere. Eutrophication is particularly considered a major problem in areas with limited water resources. Phosphate is especially limiting in contributing to eutrophication in South African rivers and dams. The development of harmful algal blooms, particularly from cyanobacteria, has been a concern for a long time due to toxins introduced into freshwater systems from these algae. This study investigated whether the use of organic fertilisers compared with inorganic fertilisers was potentially less detrimental to freshwater systems as a result of leachate nutrient and algal microorganism composition; further it was examined, if organic fertiliser was more beneficial to plant growth of Pseuderanthemum atropurpureum. Liquid and soluble granular organic and inorganic N equilibrated fertiliser treatments were applied at low, medium and high concentrations based on recommended label rates. Plant growth parameters were determined from mean height, number of leaves, size of leaves, number of nodes, internode length and number of branches. The species was grown over a period of three months and the experiment was repeated three times. Leaf tissue was analysed for mineral nutrient content and chlorophyll a, b and total chlorophyll. Leachate was analysed for mineral nutrient content including total phosphate, orthophosphate and chlorophyll a. Growth media was analysed for total nitrogen, ammonium and nitrate. A phase contrast light microscope was used to identify larger algal microorganisms and a scanning electron microscope (SEM) to identify smaller algal microorganisms from growth media extracted leachate. One specimen of green algae and some diatoms were identified, including two which may be found in eutrophic waters, but would not pose a threat similar to some species of cyanobacteria, if leached into freshwater systems over a period of time. Further, results showed that total phosphate and orthophosphate concentrations were significantly higher in leachate extracts from bark-based growth media across all fertiliser treatments and at all rates of treatment compared with soil-based growth media. This may have been due to a lack of binding sites in soilless media such as bark. Nitrate concentrations from organic soluble granular treatments were higher in both growth media types, whilst other treatments were similar. Ammonium and leachate nitrogen concentrations were found to be also similar. This may explain why plant growth traits assessed together were similar across all parameters tested. No single fertiliser compared with any other, produced plants that were superior in all growth characteristics measured. It is, therefore, suggested that the fertiliser treatments used in this study be applied at the half rate and plants be rather grown in randles growth medium than gromor for the production of Pseuderanthemum atropurpureum.