In vitro Inorganic Phosphate Solubilization Tests of Cowpea Root Nodule Bacteria from Ethiopia

Abstract

Background: Phosphorus is one of the limiting plant nutrients in most soil due to its fixation with metal ions both in acidic and alkaline soil. However, some soil bacteria have potential to solubilize the cation-fixed phosphorus and make it available to plants.Methods: Sixty-six cowpea root nodule bacterial species were tested for inorganic phosphate solubilization (PS). PS potential of the isolates from Ca3 (PO4)2, AlPO4 and FePO4 were tested on four different kinds of agar media. Solublization efficiency was determined as Solubilization Index (SI) on agar media and quantity of bioavailable phosphorous on broth media. Result: About 30% of the bacterial isolates, out of which 60% endophytes were Ca3 (PO4)2 solubilizers on Pikovskaya medium but did not grow on media containing AlPO4 and FePO4 as sole P sources. Isolates showed significant variation (p less than 0.05) in Ca3 (PO4)2 solubilization efficiency on the different culture media and ECE-21 (Pseudomonas putida) was versatile in solubilizing Ca3(PO4)2 from the four test media. However, ECE-26A (Bacillus subtilis) showed the highest PS in broth media (140.8 µg ml-1) indicating the moderate correlation (r=0.5578; p less than 0.05) of PS between solid and liquid media. Amounts of P solubilized and pH change of the culture filtrate was inversely correlated (r=-0.731; p less than 0.01). In general, ECE-21 was the most efficient P solubilizer (SI=2.67±0.1; P=87.74±7.4µg ml-1) solublizers and hence can be an ideal candidate as inoculants for the crop production in Ethiopia after field evaluation.