Highly weathered soils of the humid tropics generally provide a poor mineral reserve of potassium (K), but evidence has been found which indicates that even in such soils non-exchangeable forms of K can be made plant available and this warrants further investigation. The objective of this study was, therefore, to determine the extent to which K can be released from poorly available reserves over a long period of time. The focus was on an Oxisol in southern Brazil cultivated for 32 years with a rotation of soybeans (Glycine max L.), maize (Zea mays L.), wheat (Triticum aestivum L.), and oats (Avena strigosa L.) with and without K fertilization. Mineral sources of K were identified by X-ray diffraction and by sequential chemical extraction from the clay fraction. The amounts of K-bearing mineral species and the amounts of total and plant-available K were quantified, then the effects of the long-term K-fertilization regime on these values were evaluated. The clay fraction was dominated by hematite, gibbsite, and phyllosilicates such as kaolinite. These minerals were unaffected by the K deprivation in the cropping systems, but in the clay fraction the absence of K fertilization for 32 years reduced the structural order of the 2:1 phyllosilicates associated with K reserves. This effect was most prominent in the root zone of the soil. Deprivation of K for more than three decades decreased the crystallinity of 2:1 phyllosilicates, which could be better evaluated from XRD patterns after the removal of kaolinite and Fe (oxyhydr)oxides. The K-free cultivation reduced the amounts of total soil K by increasing the depletion of K from pools that typically are poorly accessible to plants.

中文翻译：

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已施肥三年的热带土壤粘土部分中的钾储量

潮湿热带高度风化的土壤通常提供较差的钾 (K) 矿物储备，但已发现的证据表明，即使在此类土壤中，不可交换形式的钾也可用于植物，这值得进一步调查。因此，本研究的目的是确定在很长一段时间内从可用储量不足的储量中释放钾的程度。重点是在巴西南部种植了 32 年的 Oxisol，轮作大豆 (Glycine max L.)、玉米 (Zea mays L.)、小麦 (Triticum aestivum L.) 和燕麦 (Avena strigosa L.)并且没有施钾。K 的矿物来源是通过 X 射线衍射和从粘土部分的连续化学提取来确定的。对含钾矿物种类的数量以及总钾和植物可利用钾的数量进行量化，然后评估长期施钾制度对这些值的影响。粘土部分以赤铁矿、三水铝石和页硅酸盐如高岭石为主。这些矿物质不受种植系统中钾缺乏的影响，但在粘土部分，32 年没有施钾，降低了与钾储量相关的 2:1 层状硅酸盐的结构顺序。这种影响在土壤的根区最为显着。剥夺 K 超过三十年会降低 2:1 层状硅酸盐的结晶度，这可以在去除高岭石和 Fe（羟基）氧化物后从 XRD 图案中更好地评估。