Nutrient Bioavailability of Anthropogenic Dark Earth Soils and Surrounding Soils of Central Amazonian
Falcao, Newton Paolo de Souza; Souza da Silva, Ana Cristina; Franca Borges, Lillian; Comerford, Nicolas
A greenhouse experiment was carried out at the National Institute for Amazon Research, Manaus, AM, Brazil, to evaluate the nutrient bioavailability of Anthropogenic Dark Earth Soils and surrounding soils. Maize (Zea maiz L.) was planted in pots with two kg of air-dried soil to determine the relative fertility of the Dark Earth Soil and surrounding soil (Oxisol). Soil samples of surface layer (0-20 cm) was collected of four sites (Laranjal farm, Açutuba farm, Jiquitaia farm, Hatahara farm) and two soils per site (Dark Earth Soil and Oxisols) following a randomized factorial design (4X2) with 8 treatments and eight replication, totals 64 pots. Half the pots were watered with distilled water to field capacity as required. The other half was watered with a minus-P nutrient solution to field capacity as required. The nutrient solution was applied to supply nutrients minus P, giving a bioassay of P bioavailability for the different soil locations and depths. Entire plants were harvested at the end of 2 months and total dry matter of shoots, roots and total nutrients uptake by plant were measured. Additionally soil fertility variability and phosphorus fractionation was done after maize was harvested. The results showed that Dark Earth soils are inherently more fertile by contrasting the growth and nutrient accumulation in the water-only-pots. Dark earth soil phosphorus availability from Açutuba ranged from 236 mg kg-1 (minus nutrient solution treatment) to 227 mg kg-1 (treatment with nutrient solution); dark earth soil phosphorus availability from Rio Preto da Eva ranged from 284 mg kg-1 (minus nutrient solution treatment) to 189 mg kg-1 (treatment with nutrient solution) and dark earth soil phosphorus availability from Laranjal ranged from 367 mg kg-1 (minus nutrient solution treatment) to 305 mg kg-1 (treatment with nutrient solution). The total phosphorus in the shoots showed a slight decrease with all treatments with dark earth soil plus nutrient solution. The treatments with dark earth soil plus nutrient solution showed that the phosphorus amount in the shoots ranged from 3, 11 to 3, 79 g kg-1. On the other hand, the same treatment minus nutrient solution showed that the phosphorus concentration in shoots ranged from 3, 43 to 4, 92 g kg-1. The treatments with surrounding soil plus nutrient solution showed that the phosphorus amount in the shoots ranged from 0, 92 to 3, 01 g kg-1. On the other hand, the same treatment minus nutrient solution showed that shoots phosphorus concentration ranged from 1, 30 to 3, 86 g kg-1. The higher increment of biomass was got to dark earth soil plus nutrient solution ranged from 9,56 g/pot (minus nutrient solution) to 26,13 g/pot (plus nutrient solution), an increment of 273%. All dark earth soil treatment presented low amounts of exchangeable potassium, ranged from 0, 05 Cmolc kg-1 to 0, 14 Cmolc kg-1, after maize was harvested. Based on preliminary results presented above, we propose that the natural fertility of the Dark Earth Soil is relative since low levels of potassium are a restriction to crop growing. Not only the surrounding soils but also the dark earth soils in all sites presented aluminium phosphate plus iron phosphate higher than 65% of the total phosphorus pool, except the dark earth soil from the Hatahara farm that showed calcium phosphate higher then 60% of the total phosphorus pool.