The effect of secondary salinisation on soil physical Properties of salt affected soils in Makindu, Kenya
In part I of the study, three different kinds of farms were studied in Muhindi farm, Makindu in Makueni district. The farms were; i) Virgin land (ii) Semi abandoned land/land under production being irrigated and (iii) land completely abandoned due to salinity. These farms were selected such that they are adjacent to one another and represent three different kinds of treatments. The physical and chemical properties were determined in the laboratory. Water samples used for irrigation were analysed to determine their chemical characteristics and their suitability for irrigation. As far as irrigation is concerned all the waters can be regarded as being unsuitable for irrigation on heavy textured soils unless drainage is applied. The study indicated that poor quality irrigation water has little impact on light textured soils. Relationships were drawn between infiltration rates and selected physical parameters namely; Ksal, bulk density, texture, organic carbon and antecedent moisture. Infiltration rates varied significantly between the three treatments. K„,, organic carbon, and sand fractions correlated poorly with infiltration rates whereas silt, clay fractions, bulk density and antecedent moisture correlated negatively with infiltration rates. Similar relationships drawn between Ksat and the selected physical parameters indicated that Ksal correlated positively with sand fraction and organic carbon. It was significant in the first horizon for organic carbon and highly significant for sand in the second horizon. Clay was negatively and highly significantly related to Ksal in the second and third horizons. Bulk density, silt and antecedent water correlated negatively significant with Ksat. A salt leaching experiment carried out in the completely abandoned area showed that about 41 % of the soluble salts were leached from the 0-30cm depth on application of 30cm of water flood if applied in two equal parts. Salt leaching was seen to be not effective in reclaiming the area. The drain spacing experiment carried out in the completely abandoned area where drain spacing was calculated using Hooghoudt equation found out that the spacing of the drains should be between 21.50 metres to 39.80 metres for the water table to be lowered below the root zone. This spacing is for most crops that do well around the area like tomatoes, water melon, pepper etc. In part II of the study, the effects of irrigation water quality on water flow was studied for selected soil types. The soils whose Ksat were studied were by FAO, classification; vertic luvisol from Makindu, Ferric lixisol from Makindu and Haplic lixisol from Kibwezi. The water flow was studied through salts accumulation using sixteen water qualities. The sixteen water qualities were synthesized by the blending procedures described by Ayers (1985). Tap water (SARndj 1.62) and Magadi water (SAR0flj 382.21) were mixed at several proportions to give blends of SARadi values 4.01, 5.38, 7.99, 9.48, 12.42, 14.87, 24.45, 31.11, 36.60. 48.05, 64.53, 82.46, 92.86 and 145.35. The physical and chemical properties were determined in the laboratory from the soil samples obtained from the studied places. The cations and anions determined were the only ones that have significant importance in irrigation water quality studies. The K,.„ at increasing SAR condition were found to be in very low to moderate Ksa, class (xvii) and varied from 0.05 to 5.75 cm/hr for the Vertic luvisol. In low to very high Ks„ class from 0.60 to 27.42 cm/hr for the Ferric lixisol from Makindu and in a moderate to very high K,.„ class varying from 1.0 to 52.04 cm/hr for the Kibwezi’s Haplic lixisol.