gricultureکشاورزی پایداربرای ماندگاری وتداوم کشاورزی
Organic farming for sustainable agriculture
Organic c and n: Standard treatment methods remove most of the organic matter from sewage. Consequently the typical COD BOD or TOC levels found effluents are much lower than in the original sewage and the quantities organic c added to soils through effluent irrigation are also relatively low for instance the amount of organic c applied through an annual additive of 1000mm of typical sewage effluent containing a total organic c level 50mg/l is only 500kg/ha. Since the amount of n in sewage effluents is high the c/n ratio for organic components is low (typical values are 5)and release of mine n from organic substances is favoured. The organic c/total N ratio is must lower than the organic C/organic N ratio. Application of sewage effluents having a high BOD value reduces the oxygen level in the soil and consist quently denitrification may be enhanced. Soil organic matter affects soil structure and its stability and is a sours fo nutrient elements essential for plan growth. The C cycle in soil cood sists of manifold reaction in which numerous microorganisms take plant (Alexander 1977).many reviews deal with C transformation in the soil plant system some of which are specific to soils treated with organic was (e.g.Gilmor et al.1977;Stevens The great concern for possible health hazards resulting from the present of trace organics in the relevant publications cussing various aspects of sewage effluent reuse .chang and page(1989) concluded that the risks from trace organics associated with the use in sewage effluent for irrigation are not greater than those presented by other sources of water. This is due to the redaction in the number of substance presents in the effluent and in their concentration following treatmer Moreover various soil reactions attenuation procenuation processes similar to thos occurring in pesticide – treated soils further reduce the level of trace organics adde through sewage effluents are usually smaller than those applied by means of standard pesticide treatment. Biodegradation is responsible for reduction in the level of trace organics in the soil The type of microorganism soil properties and the chemical properties of the organic substances involved determine the rate of decomposition . At low substrate concentrations degradation of organic substances follows a first-order reaction. The presence of available organic C derived from other sources enhances the decomposition of trace organics in the soil while adsorption reduces it rate . Suspended solids in secondary effluents are mainly of biological origin Their concentration and composition depend on the level of treatment and particularly on the degree of separation of sludge from the treated sewage in the final sedimentation process .Turbidity in excess of 20-30 mg/l (a common value for biological treatments ) may be found in over/or under-loaded system suspended particles in secondary effluent feaving thetreatment plant contain various microorganisms such as bacteria protozoa rotatoria and phytoflagellata ranging in size from colloidal to several hundred microns in size. Organic and mineralized flocs of 1mm or even large in size (usually of porous structure) as well as clay particles and other inorganic colloidal matter are also present. Irrigation with waste water Nutrients in municipal waste water and treated effluents are of particular advantage over conventional irrigation water sources and supplemental fertilizers are something not necessary. however additional environ mental and health must be taken into account when treate waste is the source of irrigation water . Irrigation methods Different methods are used by farmers to irrigate crops they range from watering individual plants from a can of water to highly automated irrigation through centrally controlled system. However from the point of wetting the soil these methods can be grouped under five headings namely.
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