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The adequate amounts of potassium (K) fertilizer application may play an essential role in the growth and tolerance of plants against the drought stress. In this study we investigate the effectiveness of potassium fertilization on the growth and tolerance of soybean plants [Glycine max (L.) Merrill.] to drought stress. Treatments were arranged in a randomized block design in a 2 × 3 factorial: two potassium fertilizer levels [40 mg kg-1 of K (low) or 160 mg kg-1 of K (high)] and three irrigation regimes [100% of pot capacity - PC (well watered control), 50% of PC (moderate stress) and 25% of PC (severe stress)] with four replicates. Leaf relative water content, cell membrane stability, plant growth, and morphophysiological indexes were recorded after 18 days of exposure to drought stress. The appropriate supply of potassium fertilizer improved leaf membrane stability and minimized the water loss from leaf tissue of soybean plants exposed to drought stress. The adverse effects of drought on leaf abscission and pod abortion rate could be mitigated by adequate K supply. The appropriate supply of potassium fertilizer alleviates the negative effects of drought stress and maintain shoot growth and the water status soybean plants, and therefore, the proper management of potassium fertilization may confer greater drought tolerance. [ABSTRACT FROM AUTHOR]

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Mineralogy is a major factor in the definition of rocks for use in agriculture. Phonolite rock, located in the Poços de Caldas plateau, southern Minas Gerais, has 7.4% (w/w) of K2O and its main constituent is feldspar. Studies with fresh rocks showed that top K released was about 1% of the total K, in Mehlich 1 solution. Currently, pure or mix rocks are studied after chemical and/or heat treatments, in order to increase the availability of K contained in the rock when applied to soil. In this study was investigated the thermal behavior of the phonolite for identify the mineralogical phases formed with increasing temperature. Rock samples were calcined at 300, 600, 900, 1.000, 1.100 and 1.200°C and analyzed by XRD, IR and SEM. At 600°C the diffraction peaks of analcime are not observed, while at 900°C occurs the formation of leucite. At 1.200°C was obtained amorphous materials that confirm total loss of crystal structure of the mineral. According literature, these structure changes are favorable for potassium availability as a nutrient for plants. [ABSTRACT FROM AUTHOR]