An ecological evaluation of three systems of farm management using field beans

Abstract

An evaluation and comparison of the flow of geochemicals through three systems of farm management is made, using Vicia faha L. as a crop phytometer. Organic manures form the only nutrient addition to one system (Organic), artificial fertilizers alone are applied to the second (Stockless), and the third system receives a mixture of the two nutrient sources (Mixed). The influence of the farm management upon each of the main components of the agricultural ecosystem is determined, and these are discussed separately in the five main sections of the text. Crude balance sheets are constructed to demonstrate the magnitude of the annual flow of geochemicals through each system. -The soil of the three farms is compared by chemical analysis. It is shown that where organic manures have been used, the content of available plant nutrients is significantly higher than in the soils where only inorganic fertilizers have been applied. The flow of water through the ecosystem is determined by the use of lysimeters. A study of the chemical composition of the leachate shows that the system receiving both manures and fertilizers provides the greatest threat of eutrophication. The maximum loss of plant nutrients from the soil is shown to occur during the season of the greatest rainfall. The rate of the fixation of energy into the system is determined by a study of the growth and yield of the crop phytometer. It is found that the long-term practice of growing separate stocks of seed on each farm, (imprinting), does not cause any physiological evolution to occur by which each plant type could become better suited to its own farm type. It is demonstrated that, the magnitude of the growth of the bean plants does not show any significant effect of farm management, despite the differences in nutrient availability from the three soils. It is suggested that this lack of growth response is due to the high background level of nutrients in all the soils. The only effect of nutrient availability is shown by the size of the bean seed yield, but this effect is seen to be drastically modified by weed competition. The flow of geochemicals into the standing crop is investigated by detailed chemical analysis of the plants throughout the season. Corresponding to the lack of growth response to the farm management, no significant effect of soil treatment can be shown to be influencing the chemical composition of the crop. There is no evidence to show any eutrophication of the crop by nitrates, but a marked death reaction is demonstrated to be causing high concentrations of lead in the bean seeds. Due to a greater biomass on the Mixed section, it was this management which incurred the greatest flow of geochemicals into the standing crop. The phenomenon of symbiotic nitrogen fixation by the phytometer is investigated. A close relationship between the rate of fixation, the extent of nodulation and the availability of soil nitrogen is determined. The former two characteristics are show to provide an accurate assay of the level of nitrogen in the soil. Calculation of the probable annual fixation of nitrogen shows that this is maximal on the farm where no organic manures are supplied. From the construction of the balance sheets, and the discussion in the text, it is indicated that the management system, which is the most geo- chemically and economically viable, is the Mixed system. This farm produces the highest yield of bean seeds, and permits a reasonable level of nitrogen fixation by the legume, but also poses the most serious threat of eutrophication to other ecosystems. Even so, the concentration of nitrates in the leachate never exceeded the maximum safe limit suggested by the World Health Organization.