Phosphate rock (PR) is an alternative fertilizer to
increase the P content of P-deficient weathered soils. We eval-
uated the effects of fertilizer form on indicators of biological
cycling of P using an on-farm trial on a Rhodic Kandiudox in
western Kenya. Treatment plots were sampled after 13
cropping seasons of P applications as Minjingu phosphate
rock (PR) or as triple super phosphate (TSP) (50 kg P ha
−
1
season
−
1
), as well as a P-unfertilized control (0 kg P ha
−
1
season
−
1
). Soils (0
–
15 and 15
–
30 cm) were analyzed for mi-
crobial biomass P (P
mic
), activities of acid phosphomonoester-
ase, alkaline phosphomonoesterase, and phosphodiesterase,
and sequentially extractab
le P fractions. P additions as
Minjingu PR yielded 299% greater P
mic
than TSP at 0
–
15-
cm depth despite similar labile P concentrations in the two P
fertilization treatments and stimulated activities of acid phos-
phomonoesterase (+39%). When added in the soluble form of
TSP, a greater percentage of total soil P was present in
mineral-bound forms (+33% Fe- and Al-associated P).
Higher soil pH under Minjingu PR (pH 5.35) versus TSP
(pH 5.02) and the P-unfertilized treatment (pH 4.69) at 0
–
15-cm depth reflected a liming effect of Minjingu PR. The rm of P fertilizer can influence biological P cycling in
weathered soils, potentially improving P availability under
Minjingu PR relative to TSP via enhanced microbial biomass
P and enzymatic drivers of P cycling.

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