This research examined the effects of different types of organic amendments at different rates of addition and with different quantities of irrigation water on two major soil types of Bahrain. The aim was to quantify potential improvements in soil quality, the accumulation of trace metals in the soil, and the release of metals and nutrients to drainage waters, and leaching of nutrients and metals to ground water. Organic amendments, cow manure, chicken manure and sewage sludge were analysed for various physical and chemical properties. Pot experiments, field soil and water analysis in areas of organic amendment were carried out. Drainage waters close to intensive agricultural production, where mineral fertilizers and organic amendments were used, were analysed for
nutrients and heavy metals. Run-off water from major farms and local ground waters were also analysed.
From the pot experiments it was found that soil water holding capacity did not change much after the addition of organic amendments except in the case of sewage sludge addition. Total organic carbon and total kjeldhal nitrogen content Increased in the 0-5 cm layer. Low TDS water and sewage sludge applications improved aggregate stability to some extent.
Extractable phosphorus was enhanced by the chicken manure treatment more than any others. Addition of different organic amendments did not affect exchangeable cations. pH values did not show appreciable changes and soils were neutral.
Among the trace metals Investigated, cadmium, lead, zinc and copper, all were found in the 0-5 cm layer and none were present at toxic levels in soils. Zinc and Copper were the only metals showing a tendency to leach to the lower soil layer. In all cases, the metal concentrations in the surface soil were proportional ‘to the quantities added in the amendments. Concentration of trace metals in the leachate from pot experiments were very low.
Field studies indicated that, use of organic amendments resulted in increased N, P and C in the surface soil. Nitrogen, phosphorus and potassium concentrations in drainage waters near agricultural areas were higher than at other non-agricultural areas. Ground water nutrients did not show any correlation between fertilizer addition and their proximity to the agricultural land except at Al-Akr. Nitrogen, phosphorus and potassium levels in the soil profile indicated that nutrients are present mainly at the 0-5 cm levels and downward movement was minimum.
Application of organic manure before the fertilizer addition apparently retarded the leaching process. Soil surface run-off water from agricultural land also showed high concentrations of nitrate (NO3). All fertilizer amended plots had lower metal concentrations than the two sites amended with sewage sludge.The sewage sludge treated sites contained elevated levels of heavy metals at the 0-5 cm layer and plants grown on the sludge treated sites had higher concentrations of heavy metals than the control sites using inorganic fertilizers. Cadmium content of plant materials grown on sludge application sites exhibited higher levels than allowed in plants meant for feeding the animals.