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Wastewater irrigation may benefit agricultural crops with water and essential nutrients (mainly nitrogen), also affecting soil chemistry. The effects of effluent irrigation on yield, stalk nutrient uptake and on soil chemistry over 16 months were studied in a sugarcane (Saccharum spp.) crop growing on an Oxisol in Lins, State of São Paulo, Brazil. Irrigated plots received 50% of the recommended mineral-N fertilization and 100, 125, 150 or 200% of the crop water demand, while control plots received neither additional N nor water. The high sodium content of effluent resulted in Na inputs as high as 6.2 t ha -1 , along with 1497 kg N ha -1 and 628 kg K ha -1. All the effluent plots except the T125 treatment had higher yields (up to 247 t ha -1 ) than the control (153 t ha -1 ). Significant amounts of N (up to 597 kg ha -1 ) and K (up to 546 kg ha -1 ) were exported by the plant harvest. Additions of nutrients and Na via irrigation were not compensated by stalk growth, causing a low recovery of N, P, Ca, Na, and showing the relative over N fertilization of the crop. Changes in soil pH, H + Al, Ca, Mg and K were small, whereas Na accumulated over time with irrigation. The treated wastewater irrigation is expected to gain increased importance, requiring careful considerations involving the adequate balance between nutritional inputs via irrigation and optimal plant productivity requirements. Key words: Saccharum spp., water reuse, land disposal, nutrient cycling CÁTIONS TROCÁVEIS DO SOLO, PRODUÇÃO E EXTRAÇÃO DE NUTRIENTES PELA CANA-DE-AÇÚCAR APÓS IRRIGAÇÃO COM ÁGUA RESIDUÁRIARESUMO: A irrigação com águas residuárias pode beneficiar as culturas agrícolas com água e nutrientes essenciais (especialmente nitrogênio), afetando também a química do solo. Os efeitos da irrigação por 16 meses com efluente de esgoto na produtividade, extração de nutrientes pelo colmo, e nos atributos químicos do solo, foram estudados em um Latossolo cultivado com cana-de-açúcar (Saccharum spp.), situado em Lins, São Paulo. As parcelas irrigadas receberam 50% do N mineral recomendado e 100, 125, 150 ou 200% da demanda hídrica da cultura, enquanto o controle não recebeu N mineral e nem água. As elevadas concentrações de sódio do efluente ocasionaram um aporte de Na de até 6,2 t ha -1 , juntamente com até 1497 kg N ha -1 e 628 kg K ha -1 . Todas as parcelas irrigadas, com exceção do T125, apresentaram maior produtividade (até 247 t ha -1 ) do que o controle (153 t ha -1 ). Quantidades expressivas de N (até 597 kg ha -1 ) e de K (até 546 kg ha -1 ) foram exportadas através da colheita da cultura. As adições de nutrientes e de Na via irrigação não foram compensadas pelo crescimento da planta, ocasionando uma baixa recuperação de N, P, Ca e Na, evidenciando uma excessiva fertilização da planta (N). Alterações no solo de pH, H + Al, Ca, Mg e K, foram de pequena magnitude, enquanto houve acúmulo de Na trocável ao longo do tempo nos tratamentos irrigados. A irrigação com águas residuárias deverá adquirir importância crescente, exigindo atenção...

Summary The soils and sediments of the uplands in the Manaus region are described and analysed along a representative cross‐section. There are two broad types of features, lateritic and redoximorphic. Their formation is linked to two main processes acting under contrasted hydrological regimes. The first process, acting under well‐drained conditions, is lateritization. It has transformed strongly weathered sediment into soil and led to depletion of silica (mainly quartz) as well as to relative accumulation of both kaolinite and iron oxides (haematite and goethite). Crystallographic changes observed in the latter have resulted from alternating dissolution and crystallization cycles without significant transfer of iron and alumina. However, in the uppermost soil, dissolution of kaolinite has prevailed over crystallization, leading to depletion of clay and the formation of tiny crystals of gibbsite disseminated throughout the groundmass. The second process results from the development of reducing conditions in groundwater giving redoximorphic features in lateritic soils and sediments. In the sediments, iron has been depleted by regional aquifers to form a pallid zone. In the soil, large amounts of iron and minor amounts of alumina, mainly from aluminous goethite, have been mobilized at first in small patches, which with further mobilization and vertical transfer of these elements have increased in size and have led to the formation of bleached horizons over thin iron pans. Iron has crystallized predominantly as haematite in the iron pans and alumina as large crystals of gibbsite in soil voids. Formation of impervious iron pans holds up fluctuating perched groundwater in the overlying horizons depending on rainfall events. Neotectonic events (formation of uplifted blocks and small grabens) have markedly altered the hydrological regimes. In the uplifted blocks, the soil has been deeply truncated and iron loss has been checked in the uppermost sediment. By contrast, mobilization of iron has been initiated at various places in the soil of the small grabens. In this way tectonic events have checked mobilization of iron in sediments but activated it in soils, leaving spectacular fingerprints on the landscape.

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