& Danilo dos Santos Rheinheimer (5) RESUMOA absorção de K pelas plantas, em geral, é maior que a quantidade inicial de K trocável, indicando que formas não-trocáveis contribuem no suprimento deste nutriente às plantas. O trabalho objetivou avaliar a depleção de formas de K por cultivos sucessivos em um solo com textura superficial arenosa, com ou sem histórico de adubação potássica. O experimento foi realizado na casa de vegetação do Departamento de Solos da Universidade Federal de Santa Maria, Santa Maria, RS. Amostras superficiais (0-0,10 m) de um Argissolo Vermelho distrófico arênico, sem ou com histórico de adubação potássica, foram submetidas à aplicação total de 0, 90 e 270 mg kg -1 de K e a cinco cultivos sucessivos com plantas em vasos. Na colheita, a parte aérea das plantas foi cortada rente à superfície do solo e seca em estufa, sendo determinados a matéria seca e o K. Uma amostra de solo na camada de 0-10 cm foi coletada, seca, moída e passada em peneira de malha de 2 mm; a extração do K trocável foi feita pelo extrator Mehlich-1; do K não-trocável, pelo tetrafenil de sódio (Na-TFBo) e HNO 3 1 mol L -1 fervente; e do K total, pelo HF. A capacidade de suprimento de K do solo às plantas depende mais de suas características próprias que do seu histórico de adubação potássica. A absorção de K pelas plantas desencadeia um processo contínuo de depleção de diferentes formas de K, a qual é mais acentuada quando os teores de K disponíveis são mais baixos.Termos de indexação: adubação potássica, formas de potássio, extratores de potássio.(1) Trabalho realizado com recursos do CNPq e FAPERGS. Recebido para publicação em julho de 2006 e aprovado em junho de 2007. (2) Professor colaborador do

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Purpose The dynamics and availability of potassium (K) in soils depend on many factors, including the mineralogy of fractions and the soil geochemical conditions. The objective of this work is to quantify K in distinct lowland soil compartments in the south of Brazil, relating them to particle size fractions and mineralogical constitution. Material and methods The soil samples (0-20 cm depth) were collected in six different lowland areas of flooded rice fields in southern Brazil. A mineralogical characterisation was performed. A semi-quantitative analysis was performed on the clay fraction using X-ray diffractogram (XRD) decomposition technique. The K concentration in the different soil fractions was determined using the following methods: potassium total (Kt) by the hydrofluoric acid total digestion method, nonexchangeable K (Kne) extracted by sodium tetraphenyl borate (NaTPB) after a 24-h contact, exchangeable K (Ke) extracted by ammonium acetate at pH 7, and available K extracted by the Mehlich-1 method. Potassium desorption curves from the soil were obtained by successive extractions with NaTPB.Based on the contents found in the different soil forms and compartments, the contribution of each Kne and Kt particle size fraction could be mathematically evaluated. Results and discussion Silt fraction mineralogy may vary depending on the soil and the parent rock. The K-feldspar minerals were observed in the XRD of three soils, explaining the high Kt contents. The following mineral species were observed in the clay fraction: kaolinite, illite, smectite/vermiculite or smectite/vermiculite with a hydroxy-Al interlayer. The Kt and Kne (R 2 =0.99) values are correlated, and both show a low correspondence with the Ke (R 2 <0.20). The silt and clay contributed with values from 9% to 37% and 63% to 91% of the Kt, respectively, for the different soil samples. The Kne contribution, however, ranged from 10% to 62% and 62% to 90% for the clay and silt fractions, respectively. Conclusions Lowland soils in the south of Brazil include large non-exchangeable K reserves that are associated with the presence of K-feldspar and mica in the silt fraction, as well as smectite, vermiculite and poorly crystallised illite in the clay fraction. For this reason, large quantities of nonexchangeable K are available for desorption by a NaTPB extractor, which represents a potential source of release of K to plants. The use of mineralogy to delineate the K dynamics in soil compartments was quite effective and should be adopted in fertility diagnosis and potassium fertilisation programmes for rice crops.

In a system in which fertilization is recommended, diagnosis of soil K availability and the establishment of critical levels are made difficult by the possibility of a contribution of non-exchangeable forms of K for plant nutrition. Due to its magnitude, this contribution is well diagnosed in long term experiments and in those which compare fertilization systems with positive and negative balances in terms of replacement of the K extracted by plants. The objective of this study was to evaluate K availability in a Hapludalf under fertilization for sixteen years with the addition of K doses. The study was undertaken in an experiment set up in 1991 and carried out until 2007 in the experimental area of the Soil Department of the Federal University of Santa Maria (Universidade Federal de Santa Maria -UFSM), in Santa Maria (RS), Brazil. The soil was a Typic Hapludalf submitted to four doses of K (0, 60, 120 and 180 kg ha -1 K 2 O) and subdivided in the second year, when 60 kg ha -1 of K 2 O were reapplied in the subplots in 0, 1, 2 and 3 times. As of the fifth year, the procedure was repeated. Grain yield above ground dry matter and total K content contained in the plant tissue were evaluated. Soil samples were collected, oven dried, ground, passed through a sieve and submitted to exchangeable K analysis by the Mehlich-1 extractor; non-exchangeable K by boiling HNO 3 1 mol L -1 and total K by HF digestion. Potassium fertilization guidelines should foresee the establishment of a critical level as of which the recommended dose should accompany crop needs, which coincides with the quantity exported by the grain,