Soil water movement retention and release properties of selected soils of Kenya

Abstract

The study evaluated the Soil Moisture. Retention, Release and Saturated Hydraulic Conductivity, (Water Movement), in 6 Kenyan soils that included: 1 Andosol, 1 LuvisoljAcrisol, 1 Arenosol, 2 Nitosols and 1 Vertisol. Further investigations were carried out to establish the relationship between the three parameters and the selected soil characteristics. Simple linear correlations showe d that Saturated Hydraulic Conductivity was mainly a: function of the Fine Silt and Medium Sand fractions of the soils studied and that the Total Sand and Coarse Silt Fractions became important down the profiles. However, ':L'otal Sand had a negative effect in the 30-36 cm depth. Ksat was mainly reduced by the Clay fraction, particularly the Montmorillonitic type. Moisture Retention was mainly increased by the finer textural fractions of Clay, Medium Sand and Medium Silt. oran t c Matter also was associated with high Moisture Retention. The coarser textural fractions namely, Total Sand and Fine Sand, reinforced by the Bulk Density were associated with Lower Moisture Retention at

specified suctions. Soil Moisture Storage between Specified reference suctions was positively related to Total Silt, Medium Silt and Organic Matter Content. Low Soil Moisture Storage was related to high Bulk Density, Total and Fine Sand fractions. At very low suctions (0.1-1 bar), the Moisture Storage was positively correlated with Total and Fine Sand fractions whereas, at higher suctions, (0.3-1 and 1-15 b ar ranges), the Total Sil, and nedium 5=-1::. enhanced SMse. The overall Moisture Release, (RSe O _1S)' was positively influenced by the Fine Silt, Fine Sand and Total Silt in the 0-36 cm depth whereas the Total and Fine 'Sand were dominant in the 60-66 cm depth. The Clay fraction and Organic Matter had negative effects on the. RSeO_lS with such magnitudes that increased down the profiles. The Moisture Release low suctions, (0-1 bar), was a function of Total Sand and Fine Sand where as at higher suctions, (1-15 bars), it was a function of the Total Silt, Medium Silt, Organic Matter and Fine Silt. Lower RSe in the same range was associated with high Bulk Density and the Sand fractions. High gradients of the moisture retention curves were attributable to high Sand fractions reinforced by Bulk Density while the lower gradients were related to higher Clay, Organic Matter and the Silt fractions. Moisture release curves with high gradients were often associated with high Total Sand and Fine Sand at low suctions while at high suctions the Silt and Organic Matter content dominated. Bulk density and the Sand fractions led to lower gradients at high suctions. The Clay Mineralogy variations did not feature very prominently in the study and most observations could be attributed to Texture, Organic Matter and Bulk Density, with strong indications of the structural influence. Subdivision of the main textural fractions of Sand and Silt improved the soil moisture evaluations in this study. In the overall soil moisture status evaluation) K sat was considered a vital link

between the SMse and Rse as far as the potential moisture availability to both plants' and other varied soil life forms is concerned. Emanating from the study was the possibility of basing soils' potential productivity evaluations on their ability to release moisture.