sufficient to recover in some cases nearly all the K added and in most cases more than what was added.
These results prove that K fixation and release can take place in these soils. The next step will be to assess the importance of this process under field conditions.
K Buffering
Results of the K-buffering analyses are shown in Figure 1. The Kbuffering power is related to the inverse of the slope of the isotherms. The soils that underwent smaller changes in delta K at a given K addition have higher K-buffering power. The relatively important difference between the slopes of the isotherms of Y-103A and Y-103B indicates that factors other than the level of exchangeable K need to be considered when the K status of these soils is evaluated.
The relationship between the NH4OAc-extractable K level and the K concentration of the soil solution is shown in Figure 2. The upper line corresponds to soils of low K-buffering power and the lower line to soils of higher K-buffering power. Each of the Yurimaguas soils falls into a different category. The figure indicates that any given level of NH4OAcextractable K level will be related to higher levels of K in the soil solution for soils of low K-buffering power. Therefore, the availability of exchangeable K will be greater in these soils (low CEC soils). Critical exchangeable K levels should then be different for soils of high and low Kbuffering power.
The laboratory experiences described above are to be tested under field conditions at the Yurimaguas station on soils of high (Y-103C) and low (Y103D) K-buffering power. The effect of "small" amounts of 2:1 clay minerals in the fixation/release reactions will also be evaluated under field conditions (Y-103C, Y-103D).
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