RESUMO A cinza de casca de arroz apresenta como uma de suas principais utilizações a incorporação na agricultura, com potencial para substituir a calagem e fornecer cálcio e potássio. Entretanto seu efeito residual no solo ainda é desconhecido. Este trabalho teve como objetivo avaliar a velocidade de reação e o efeito residual da cinza de casca de arroz nos parâmetros de acidez do solo. Para tal, foi realizado um experimento de campo constituído por cinco tratamentos: quatro doses de cinza de casca de arroz 0, 30, 60 e 120 t ha-1 e um tratamento com adubação e calagem recomendada (calcário dolomítico para atingir pH 6, 150 kg ha-1 P2O5 como superfosfato simples e 80 kg ha-1 K2O como cloreto de potássio) onde foram realizadas cinco amostragens de solo, nas camadas de 0.00-0.10 m e 0.10-0.20 m, aos 15, 211, 400, 517 e 804 dias após a incorporação da cinza. Os atributos químicos analisados foram: pH em água, porcentagem de saturação de bases, teores de cátions trocáveis (Ca, Mg, K e Na) e capacidade de troca de cátions (CTC) a pH 7. Os resultados indicaram que, a velocidade de reação da CCA é muito rápida quando comparada ao calcário. Quanto maior for a dose de cinza de casca de arroz aplicada ao solo, maior é o efeito residual no pH. E, como corretivo de acidez, o efeito residual da cinza de casca de arroz resume-se ao tempo necessário para ocorrer o processo natural de reacidificação e lixiviação dos cátions básicos, cerca de 33 meses para solos e condições climáticas similares ao deste trabalho.

ABSTRACT The rice husk ash has been applied in agricultural land, with potential of replace limestone and, supply phosphorus and potassium. However, its residual effect in soil is still unknown. This investigation aimed to evaluate the reaction rate and residual effect of rice husk ash in soils acidity parameters. A field experiment was conducted with five treatments: four rice husk ash dosages 0, 30, 60 and 120 t ha-1 and one treatment with recommended soil lime and fertilizer (dolomitic limestone to reach pH 6, 150 kg ha-1 P2O5 as single superphosphate and 80 kg ha-1 K2O as potassium chloride) where five soil samples. Soil samples were collected in the layers 0.00 - 0.10 m and 0.10 - 0.20 m at 15, 211, 400, 517 and 804 days after ash incorporation. Chemical attributes were determined: soil pH (pH), soil base, exchangeable cation values (Ca, Mg, K and Na) and cation exchange capacity (CEC) at pH 7. Results showed that reaction rate of rice husk ash is faster when compared to liming. As greater was rice husk ash dosage applied in soil, higher is the residual effect in pH. As corrective of soil acidity, the residual effect of rice husk ash is just the required time to occur the natural process of reacidification and leaching of basic cations, about 33 months for soils and weather conditions similar to this work.

RESUMO Os solos de várzea, sazonalmente alagados para o cultivo do arroz, apresentam alternância nas condições de oxidação e redução, que determinam modificações intensas na fase sólida mineral do solo e na dinâmica de elementos reativos, como o fósforo. Este estudo teve como objetivo avaliar o efeito da drenagem de dois solos de várzea, (Planossolo Háplico e Chernossolo Ebânico), após período de alagamento sobre a capacidade máxima de adsorção de fósforo (CMAP). O experimento foi conduzido em casa de vegetação, o fator sistema foi composto por dois níveis: 1) solos alagados por 63 dias seguido de drenagem e após manteve-se a umidade em 16% Ug por 184 dias; 2) solos mantidos com umidade de 16% Ug durante todo o período do experimento. Após a drenagem foram feitas coletas de solo nos tempos zero, 36, 66, 96 e 184 dias para determinação da CMAP. A determinação da CMAP foi feita por meio do ajuste de isotermas de adsorção segundo o modelo de Langmuir. O comportamento da CMAP após a drenagem do solo foi diferente para as duas classes de solo avaliadas. O Planossolo demonstrou tendência de redução imediata da CMAP após a drenagem do solo, no caso do Chernossolo, houve tendência de manutenção de valores elevados da CMAP logo após a drenagem, com posterior início da redução. A drenagem dos solos após período de alagamento aumenta a capacidade máxima de adsorção de fósforo, sendo que este efeito perdura por 163 dias para o Chernossolo e 121 dias para o Planossolo.

ABSTRACT Lowland soils, seasonally flooded for rice cultivation, feature alternate oxidation and reduction conditions, which determine intense modifications in the soil mineral solid phase and in the dynamics of highly reactive elements, such as phosphorus. This study aimed to evaluate the effect of drainage of two lowland soils (Albaqualf and Argiaquoll) after a flooding period on the maximum phosphorus adsorption capacity (MPAC). The experiment was conducted in a greenhouse, and the system factor was composed of two levels: 1) flooded soil for 63 days, followed by drainage, and then humidity remained at 16% for 184 days; 2) soil moisture maintained at 16% throughout the trial period. After drainage, soil samples were collected at 0, 36, 96 and, 184 days for MPAC determination, which was performed by the adjustment of adsorption isotherms according to the Langmuir model. The behavior of MPAC after soil drainage was different for both soil classes evaluated. An immediate reduction of maximum phosphorus adsorption capacity was seen after drainage in the case of Albaqualf, while for Argiaquoll, there was a tendency to maintain higher values of MPAC after drainage, with subsequent reduction. Soil drainage after a flooding period increases de maximum phosphorus adsorption capacity, and this effect remains for approximately 163 days in Argiaquoll and 121 days in Albaqualf soils.

ABSTRACT The combustion of rice husk generates a partially burnt mixture called rice husk ash (RHA) that can be used as a source of nutrients to crops and as a conditioner of soil physical properties. The objective of this study was to evaluate the effect of RHA levels on the hydro-physical properties of a Typic Hapludult. The experimental design was composed of random blocks with four replications, which comprised plots of 24 m2 and treatments with increasing RHA rates: 0, 40, 80 and 120 Mg ha-1. Undisturbed soil samples were collected in the soil layers of 0.00-0.10 and 0.10-0.20 m after nine months of RHA application, using steel cylinders (0.03 m of height and 0.047 m of diameter). These samples were used to determine soil bulk density (Bd), total soil porosity (TP), soil macroporosity (Ma), soil microporosity (Mi) and the available water capacity (AWC). Disturbed soil samples were collected to determine the stability of soil aggregates in water, mean weight diameter of water stable aggregates (MWD), and soil particle size distribution. The results show that, as the RHA rate increased in the soil, Bd values decreased and TP, Ma and MWD values increased. No effect of RHA was found on Mi and AWC values. The effects of RHA on the S parameter (Dexter, 2004), precompression stress and compression index (Dias Junior and Pierce, 1995) values are consistent those shown for density and total porosity. Rice husk ash was shown to be an efficient residue to improve soil physical properties, mainly at rates between 40 and 80 Mg ha-1. Rice husk ash reduces bulk density and increases total porosity, macroporosity and soil aggregation, but does not affect microporosity, field capacity, permanent wilting point, and available water capacity of the soil. The effect of rice husk ash on the S parameter, precompression stress and index compressibility coefficient values are consistent with those observed for the bulk density and total porosity.