Test of guidelines for field estimates of water retention properties of soils derived from volcanic materials in northern Tanzania

Soil water properties were estimated in the field, using guidelines established in Canada, for five volcanic-derived soils in Tanzania. Many of the estimates for horizons near the surface were satisfactory but those for water retained at − 1500 kPa by horizons below a depth of 1 m were far too low. These low estimates combined with underestimates of air porosity at − 10 kPa for many of the samples resulted in very poor estimates of available water capacity. The guidelines were not directly applicable for estimating water properties of most of the soils studied. The samples had bulk densities of 0.9–1.3 Mg m−3, air porosities at − 10 kPa ranging from 3 to 27%, and available water capacities of 280–116 mm to depths of 140 cm or to a petrocalcic layer. X-ray analysis indicated weakly crystalline clays in all of the soils. Tiron extracted from 7 to 38% of the material from the < 2 mm samples tested but the amounts of extracted material were not closely related to anomalously high water retention values at − 1500 kPa. Specific surface areas, but not clay contents, of these samples were significantly related to their measured water retention values at − 1500 kPa. A high proportion of porous particles in the silt fraction of a sample from a depth of 2 m in one soil indicated that highly porous non-clay sized material may account, in part, for the anomalous water retention and specific surface data. The effect of wetting and drying on the water retention properties of these soils may be important but remains to be clarified. Key words: Available water, air porosity, amorphous material, field estimate