Phosphorus Relationships in Water- logged Soils of Southern Nigeria as Influenced by Aluminum and Manganese Toxicity

Aim: To determine the phosphorus relationships in mangrove swamp with particular reference to Al and Mn contents in acid sulphate soils.

Place and Duration of Study: The study was carried out in Mbiabet in Akwa Ibom State, Nigeria, in 2014 and 2015, respectively.

Methodology: Soil samples were collected in the order of 0-2, 2-4, 4-6, 6-8, 8-10, and 10-15 cm in depths. Treatments consisted of potassium di-hydrogen phosphate added to the swamp mud, cat-clay, and mud-clay in equal doses of 244 kg/ha P2O5, fitted into Latin square of 36x5 m swamp, except for the control plot. Limestone (CaCO3) was applied to both fertilized and unfertilized plots at the rate of 50 kg/ha to reduce the activity of aluminum.

Results: The results showed that phosphorus in the unfertilized local acid sulphate soils was largely in the organic and occluded forms. It is apparent that the bulk of the phosphate in the fresh mud was held within 2 cm of the mud surface, where 54-70% of the total phosphorus present was in organic and occluded forms, and remainder chiefly in association with Mn and Al. In spite of the fact that both organic and occluded phosphorus decreased in amount with soil profile depth, 94-97% of the total phosphorus present is in these forms at depths greater than 2 cm from surface mud. It was also found that the mud-clay had relatively high extractable manganese content of 7,501 cmolkg/ha and 8,591 cmolkg/ha of aluminum. Unlike the cat-clay, the exchange between aluminum-bound phosphorus and manganese and calcium – bound phosphorus was masked by the occlusion of inorganic forms on drying through the formation of manganese oxide films on the surface of the sulphate particles. Occluded phosphate increased in quantity and in the mud-clay the net loss to the occluded fraction amounted to 235 mg/kg. Difference between the behavior of the unfertilized and fertilized pond – mud on drying, suggests that the amount of phosphate occluded may be insufficient to mask the exchange reactions, if sufficiently large amounts of phosphate are present to combine with the extractable manganese. All the extractable aluminum and manganese were available to fix phosphorus. The precipitation of Al by excess P was quantitative at pH 5.0-5.5. The results are discussed in light of phosphorus relationships in mangrove swamp with particular reference to aluminum and manganese toxicity.

Conclusion: The addition of acidic phosphate in the soil aggravated manganese toxicity levels, and the use of non-aluminum phosphate showed greater effect on Mn toxicity. In screening plants for Al-toxicity, some important factors to consider include soil pH, Al, P, and Ca levels of the soil. Tolerance of plants to Al-toxicity, could be enhanced by high Ca than low Ca. Al-toxicity may probably be more important than Mn-toxicity but because some areas may contain toxic levels of both Al and Mn and rice plant tolerance to the two factors may not necessary coincide, it may be necessary to develop genotypes with high tolerance. Both Al and Mn toxicity are important growth limiting factors in plants.