Available K increased in the 0–0.05 m layer in most of the
treatments, especially with the triticale/millet rotation. In the
triticale/chisel + fallow treatment, K content decreased in this
layer and increased in the 0.05–0.10 m layer (Table 5). The K
content in upper soil layers can increase over time due to the
deposition of fertilizer a few centimeters below the surface, the
release of nutrients from plant tissues on the soil surface,
decreased nutrient movement to lower layers via leaching, and/
or decreased immobilization and export by plants. Garcia et al.
(2008) also found increased K levels in superficial soil layers under
NT, and attributed this finding to the cover crops’ ability to recycle
nutrients, taking up non-exchangeable K from the soil and then
returning the nutrient in available forms through rain washing.
Franchini et al. (2003) attributed the K accumulation at the
uppermost soil layers in systems with high organic residue inputs
to the preference of organic anions for polyvalent cations, which
are leached to deeper layers instead of K, and thus increase the
percentage of K in the soil CEC of the superficial layers.