Plant residue mulch increases measured and modelled soil moisture content in the effective root zone of maize in semi-arid Kenya

Difficulties in efficient utilization of seasonal precipitation cause limitations in yields and even total crop failure on rainfed farms in semi-arid East Africa. The objective of the present study was to find out if covering the soil with plant residue mulch at a semi-arid site could retain water in the soil between precipitation events and build dry spell resilience by reducing soil water evaporation and increasing infiltration to deeper soil. Covering soil with plant residue mulch was studied at a smallholder farm in semi-arid Kenya by continuously measuring volumetric soil moisture content with soil sensors at multiple depths in bare soil and in maize (Zea mays L.) plant residue mulched soil. A physically based one-dimensional soil moisture model was calibrated and used to estimate the effect of plant residue mulch on soil moisture over a two-year period (multiple growing seasons). The modelled multiyear time series provides an estimate of the effect residue mulches of different thicknesses have on soil moisture over time. The simple soil moisture model was able to estimate soil moisture in the effective root zone of maize. By comparing measured data from mulched and uncovered soil and by model prediction, it was demonstrated that maize residue mulch conserved soil moisture over time in the effective root zone of maize compared to bare soil. During the two-year period mulching increased the total amount of days when measured relative soil moisture (s) exceeded water stress limit of maize (s*) by 24%–46%. Moisture accumulated in the mulched profile, especially in the deeper layers of the effective root zone. Calculations indicated that further increasing mulch thickness (δm) from 1 to 3 cm would have increased the total days when s > s* 59%. Furthermore, increasing δm from 3 to 5 cm would have resulted in 25% increase in total days when s > s*. According to our calculations mulching (δm > 1 cm) could have maintained s > s* throughout a 19 days dry spell that occurred during the measurement period. The demonstrated moisture conserving effect of mulch increases with δm, but availability of plant residue may set limits on mulch application rates. The results suggest that maize residue mulching is as an accessible and feasible method for conserving soil moisture in the effective root zone in dryland smallholder systems in East Africa.