In Ethiopia, the soils derived from basalt and even limestones under sub-temperate climate often show high C: N ratio. Apparently, the soil organic matter is strongly under immobilization. High biological activity is indicated by common, fine, open or filled channels or holes called krotovinas. Seldom are termite mounds found in the vicinity of these soils. Ingestion of soils by insects may influence the association and chemical character of inorganic/organic constituents in these soils. Generally, these soils are low in total nitrogen, but indicate appreciable amounts of available phosphorus. They are further characterized with high clay contents (> 60%), prominent slickensides, clay skins, absence of gilgai (1, 2), and with smectite, corrensite, attapulgite and iron-rich clay minerals. They are moderately well drained (3). It is probable that this suite of minerals form complexes with organic matter during biological activity and/or during organic matter decomposition.

Morphologically, they have strong features of mollic, nitic and even vertic horizons with soil depth. The surface horizons normally have granular to crumb structure, whereas sub-surface zones show blocky or prismatic structure of different grades. The vertic features of the surface soils remain suppressed due to the granular and/or crumb structure. Such a situation complicates the grouping of these soils into any of the recognized reference soil groups of the world. Philosophically, one may look for the proper soil grouping based on dominant criteria for management. In this respect, the dominant attribute of importance relates to organic matter immobilization. For this reason, we propose a new reference soil group called Immobisol, reflecting organic matter immobilization. The form together with state of organic matter in such soils plays a dominant role leading to immobilization. Even if the soil has adequate total nitrogen, its availability to plants appears to be restricted due to protected soil organic materials by clay minerals or even amorphous materials. This might explain significant crop response to applied nitrogen fertilizer, even though the total soil nitrogen is 0.4%.

If the soil contains more total nitrogen, the actual amount of nitrogen to be added to soil for N-enrichment for crop requirement is more than any recommended level. In one study, where the soil was derived from volcanic materials, out of 1300 mg kg-1 total nitrogen (0.13% total N), only 4.10 mg kg-1 was NH4-N and 10.44 mg kg-1 was NO3-N. This means that the remaining 98.9% of the total N was in organic forms. However, it is not quantified as to what amount of the soil organic matter is decomposed and how much is really protected physically and chemically. Such protected organic materials can hardly be utilized by microorganisms involved in decay process of soil organic matter. This basic restriction in soil organic matter decomposition results in immobilization. It is proposed that the existing climatic condition and agro-ecology play a role in developing such soils which are potentially very productive with proper management. The proposed Immobisol at the Unit level (Level 2) may be vertic, mollic, nitic and even haplic. Further sub-division would be based on local factors influencing directly the management options of such soils. Ethiopia has, by and large, appreciable area covered with Immobisols.

References:

1. Heluf Gebrekidan and Mishra, B.B. 2005. Pedogenic characterization of the soils of Hirna watershed, Ethiopia. Agropedology, 16, in press. 2. Heluf Gebrekidan and Mishra, B.B. 2005. Relevance of organic farming in Ethiopia: Mission and commitment with the second green revolution. Indian J. Fert. submitted. 3. Mishra, B. B. and Heluf Gebrekidan. 2005. Soil mineralogy:II. Clay fractions of soils of Hirna watershed, Ethiopia, Catena, submitted.