Abstract:
Low phosphorus availability is a significant constraint in the calcareous soils of Ethiopia,
particularly in the farmlands of the Metehara Sugar Estate. This limitation arises due to
the reaction between calcium from calcium carbonate and soluble phosphorus, resulting in
the formation of insoluble calcium phosphate compounds that reduce phosphorus
availability to plants. To address this issue, an incubation experiment was conducted on
abandoned farmland at the Metehara Sugar Estate to evaluate the combined effects of
inorganic phosphorus fertilizer and compost on phosphorus availability over an eight
week period. The experiment involved factorial combinations of four rates of P fertilizer
(0, 46, 92, and 138 kg P2O5/ha) and four rates of compost (0, 5, 10, and 15 t/ha),
arranged in a completely randomized design (CRD). Each treatment was mixed with 2 kg
of air-dried soil in a 3 kg plastic container and incubated at 25°C in a greenhouse,
maintaining moisture at 43.2% of field capacity. Data were analyzed using two-way
analysis of variance (ANOVA) with SAS software. The results showed that both compost
and inorganic fertilizer significantly increased phosphorus availability (p < 0.001), with a
more pronounced synergistic effect observed when the two were applied together. The
combination of 138 kg P₂O₅ ha⁻¹ inorganic fertilizer (TSP) and 15 t/ha compost yielded the
highest available phosphorus concentration, reaching 23.4 mg kg-1 by week 5, before
gradually declining. In contrast, the control treatment maintained a low and stable
phosphorus concentration around 4 mg/kg throughout the incubation period. These results
demonstrate that the combined application of inorganic phosphorus fertilizer and compost
enhances phosphorus availability in calcareous soils more effectively than either
amendment alone. Therefore, it is recommended that field trials involving actual crops be
conducted to evaluate the effects on plant growth and phosphorus uptake under natural
conditions and to better understand the time-dependent availability pattern of phosphorus
to crop plants in such soils. Additionally, testing the fertilizer and compost combination on
other soil types is advised to determine if similar benefits occur beyond calcareous soils.
Key words: Available phosphorus, Incubation experiment, Interaction effect, triple
superphosphate (TSP
