Effect of Farmyard manure on soil surface sealing and crusting of disturbed topsoils - a case study of West Pokot, Kenya
The effects of two farmyard manure application rates (5 and 10 t ha-1) on infiltration, runoff, soil loss and soil crust strength of four disturbed topsoil samples from luvisols (FAD/UNESCO, 1974 classification) were investigated under laboratory conditions for 60 days. The experiment was conducted in three experimental phases of 20 days each. The four topsoil samples were packed in soil trays and subjected to simulated rainfall of a constant intensity of 360 mm h-1 for three minutes on four consecutive days. Each of the three treatments of 0, 5 and 10 t ha-1 manure application rates was replicated three times and was subjected to similar experimental procedures as other replicates. The investigations on the topsoil samples were confined to the top 5 cm layers from which time to runoff, infiltration capacity, runoff and soi 1 loss were measured on each day of rainfall simulation. Crust strength and moisture contents were monitored over a period of two weeks after the last simulation day of each experimental phase. Crust organic matter content and the degree of soil aggregation were determined after each experimental phase. Results show that soil aggregation changed slightly with time and with manure application. There were increases of between 2 and 14.4 Y. in the percentage of aggregates of sizes less than 250 ~m and those greater than 2 mm. Aggregates of sizes ranging between 250 ~m and 2 mm were generally reduced with a maximum decrease reaching 10.5 Y.. Crust organic matter content decreased by the end of the 60 day period of study between 0.36 % and 1.17 % for all the four topsoil samples. The 5 t ha-1 treatment gained in organic matter content between 0.12 Yo to 0.46 Yo while the 10 t ha-1 treatment gained between 0.50 Y.to 0.86 %. At 0.05 probabi 1ity level, time to runoff and the amount of surface runoff were signi ficantly influenced by the day of simulat ion, topsoi 1 type and soi I surface condi tions. Time to runoff decreased with time while differences in the amount of runoff between treatments were insignificant. There were significant increases in soil loss (at 95 Yo confidence level) with the addition of manure to the topsoil samples from chromic and plinthic luvisols especially during the first experimental phase. Influences of topsoil type on soil 1055 were noted with the clay loam topsoil sample from orthic luvisol (sample D) having significantly more soil loss than other topsoil samples. least soil losses were from the sandy clay loam topsoil sample from orthic luvisol (sample B) and the sandy loam topsoil sample from chromic luvisols (sample A). The moisture content of crusts varied with topsoil type. Clay loam topsoi I from orthic luvisol showed some significant crust moisture increase with manure application rate. Crust strength increased significantly when the crust moisture content was below 4 Yo. Thus, there was a decrease in crust strength with any increase in moisture content as was expected. The investigations showed that adding 10 t ha-I farmyard manure to the four topsoil samples of the luvisols studied increased soil loss. This could be attributed to improved soil aggregation and lowered soil bulk density due to the significant increase in soil organic matter content. Generally, there was an increase in crust moisture content, improved soil aggregation and decreased crust strength with the increase in farmyard manure appIication. Surface runoff and infiltration capacities were not significantly affected by the treatments at 95% confidence level.