Avaliação da Fertilidade de Solos Antrópicos (Terra Preta de Índio) com ênfase na Biodisponibilidade do Fósforo.

Abstract

Upland soils in the humid tropics such as in the Amazon are highly weathered and therefore possess low plant available nutrient content. However, Amazonian Dark Earths (ADE) of prehistoric origin are differentiated from Oxisol and Ultisol by their darker color, higher organic matter content, higher pH, greater total phosphorus (P) content, greater exchangeable calcium (Ca) and magnesium (Mg), and increased minor element concentrations. A greenhouse experiment was carry out at the National Institute for Amazon Research, Manaus, AM, Brasil, to evaluate the fertility variability and the phosphorus bioavailability of the Amazon Dark Earth and surrounding soils. Corn (Zea mays L.) and Cowpea (Vigna unguiculata L. (Walp) were planted in continnuos crops in pots with four kg of soils collected from three sites (Açutuba Cost (Lat. 3º 30 S e Long. 60º 20 WGr.); Laranjal Cost (Lat. 3º 30 S e Long. 60º 40 WGr.); and Rio Preto da Eva (Lat 3º S e Long. 59º WGr.) farmers), two soils per sites (Amazon dark earth and Surrounding soil), and watered with minus phosphorus nutrient solution and watered with distilled water to field capacity, following a randomized factorial design (3x2x2) with 12 treatment and four replication, totaling 48 pots. The nutrient solution was applied to supply the nutrient minus P, giving a bioassay of P bioavailability for the different soil location and depths. Entire plants were harvested at the end of 45 days and total dry matter of shoots, roots and total nutrients uptake by plants were measured. Additionally soil fertility variability and phosphorus fractionation was done after Corn and Cowpea harvested. The results showed that Amazon Dark Earth soils are inherently more fertile by contrasting the growth and nutrient accumulation in the water only pots. However, The treatment Amazon Dark earth watered with only distilled water presented some limitation concerning the potassium, and zinc, in some cases affecting the plant growing, and also the soil fertility analysis presented nutritional desequilibrium. In on another hand, those treatment that received nutrient solution minus phosphorus showed increased on growth and nutrient uptake by Corn only. The results also showed that part of P available in the soil was, probably, immobilized by some minor elements such as zinc, iron of the nutrient solution. Regarding the phosphorus soil pool was observed high amounts of aluminium phosphate in all treatment studied, this results suggested that the higher sources of p available and ca exchangeable would be the calcium phosphate.