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Influence of lime and farm yard manure

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par Dieudonne MUGOBOKA
ISAE Busogo - Bachelor's Degree 2008

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2.2. Generality on Andosols

2.2.1. Definition

Andosols are soils that are dominated by amorphous (or short-range order) Aluminium Silicates and/or Al-organic matter complexes. They usually have an Ah - BW - C horizon Sequence. The Ah horizon is dark-coloured and normally very high in organic matter (Often more than 10%) stabilized by Aluminium. The B-horizon is usually dominated by amorphous aluminium silicates (allophane, imogolite). Andosols form mainly on volcanic ashes, but can also be found on other highly weatherable rock such as Amphibolites, arkoses, etc. Andosols have many peculiar properties, such as a high phosphate fixation, aluminium toxicity, irreversible drying, high water retention with low water availability, and high erosion resistance. They usually have sedimentary stratification, with the most unweathered materials occurring on top.

One of the most important characteristics of Andisols is their high capacity to immobilize (Fix) phosphorus (P) on the surface of the amorphous minerals. This is perhaps the principal chemical constraint of Andisols. It seems that the P fixing capacity varies with the type of clay mineral, affecting the residual value of phosphate applications. (Dahlgren et al., 2004).

2.2.2. Phosphorus Fixation Mechanisms in Andosols

Initially, P fixation in Andosols was considered to occur only on the active surface of allophane and imogolite. Fixation mechanisms include chemiadsorption and displacement of structural silicon (Si). The importance of Al complexes in the P fixation processes has attracted attention. Soil humus in Andosols readily forms metal complexes with transition metals like Al.

Furthermore, hydroxyl groups attached to the complexed Al enter into ligand exchange reactions with phosphates (HPO4-- and H2PO4-).

Formation of allophane and imogolite is restricted by the accumulation of humus and the subsequent formation of humus-Al complexes.

The strong complexation of Al with humus limits the possibility of co precipitation of Al and Si released from the weathering of volcanic ash. This process is common in Andosols of high altitude. Accumulation of organic matter is higher in volcanic soils located at higher altitude (more than 2,000 m above sea level).

The clay fraction of Andosols is dominated by allophane and imogolite (amorphous, short range ordered minerals) which come from the weathering of pyroclastic material produced from recent volcanic depositions. Research conducted in the last 20 years has demonstrated that humus-aluminum (Al) complexes also play a significant role in Andosols chemical behavior. Phosphate fixation potential of Andosols appears to be related to the presence of different materials in the clay fraction as a result of different weathering conditions of volcanic ash. Soil dominated by humus-Al complexes seems to have higher P fixing potential, which is apparently difficult to satisfy. (Juo and Valverde, (1996).

2.2.3. Volcanic soils and agriculture

The volcanic soils are considered among the most productive soils of the world for agriculture. According to Shoji et al. (1993b), the agricultural areas most productive of the world are localized close to volcano, and support a strong population density in general.

Between the principal chemical limitations of agricultural productivity of Andosols is the strong capacity of fixing of phosphorus a strong acidity and thus, the possibility of aluminum. However, the volcanic soils are generally easy to plow and the well developed, and is relatively resistant to erosion and compaction (Dahlgren et al., 2004).

The unique Chemical and physical characteristics of Andosols, in particular organic matter contents, porosity, apparent density and capacity of retention of water, influence largely their productivity (Dahlgren et al., 2004).

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