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Potassium Status and Exchange Characteristics of Some Lowland Soils in Luzon
Thesis Abstract:
The study evaluated the potassium (K) status and exchange characteristics of 10 soils representing important lowland areas in Luzon. The K content of the soils was inventoried using the following extracting solutions: hydrogen flouride (HF) for total K, sodium tetraphenylboron (NaTPB) for nonexchangeable K, and ammonium acetate (NH4OAC) for exchangeable K. Quantity/Intensity (Q/I) parameters derived from the Q/I curves were tested against K availability indices to assess their value in characterizing K status. Q/I data described K status by relating the intensity (I) of K to the amount (Q) of labile K in the soil.
A greenhouse experiment was conducted to determine the influence of K addition and K uptake on the Q/I parameters of selected lowland soils and at the same time assess their K supplying capacity. A high soil to plant ratio was used to exhaust available K supply in a short period.
Eight of the soils studied contained 2:1 expanding type of clay minerals. Soils from Bantog and Tarlac were dominated by vermiculite. X-ray diffraction patterns of the sand fractions of all the soils showed the presence of quartz and feldspars. The soils varied in their chemical characteristics, which included pH, cation exchange capacity, organic matter content, exchangeable K, NaTPB-extractable K, ARKe, ΔKo , and PBCK.
The exchangeable K contents (NH4OAc extractable K) of eight soils were found to be be low satisfactory level of exchangeable K for rice. Only San Manuel I and Bantog had exchangeable K level higher th an 0.2 me/100g. Significant correlation were found between exchangeable K and ARKe (r = 0.77) and ΔKo(r = -0.96). Sodium tetraphenylboron extractions gave higher values for K than NH4OAc extractions since it was able to extract even the "difficulty available" K.
Although the soils contained low exchangeable K, most of the soils, with the exception of La Paz, Tarlac, and Buenavista, have high buffering capacities (PBCK). Significant corre lation between PBCK and clay content (r = 0.81 ) and CEC (r = 0.87) were recorded.
The Q/I relations of the soils used in the greenhouse experiment (Maligaya 1 and San Manuel 1) redetermined after K condition and after the soils have been depleted of K by crop uptake were transposed vertically on the direction expected.
The Q/I parameters (ARKe, ΔKo, Kx, and KL) increased with K additions but decreased with time of incubation. K uptake decreased ARKe and ΔKo with time. Significant correlation between the increase in K uptake and decrease in ΔKo was obtained.
The intensive cropping technique employed caused a rapid decrease in ARKe in a short period and the release of nonexchangeable K to plant.