Integrated soil fertility, residue and tillage management for improved soil quality and maize (Zea mays, L.) production in urban agriculture in Harare and Chitungwiza

Abstract

Crop production has been affected by poor soil fertility and persistant droughts in urban agriculture (UA) in Zimbabwe. This gave rise to the use of livestock manure and organic municipal wastes as basal fertilizers despite their nutrient relesease patterns and health risks associated with their use being largely unknown. This study was carried out to determine the interactive effects of soil fertility, tillage and residue management on soil quality and maize yield under UA farming conditions. Additionally, nutrient mineralization patterns of sewage sludge (SS), cattle manure (CM), pig manure (PG) and poultry manure (PM) and their potential use as basal fertilizers in maize production were determined. Mineralization patterns of organic soil fertility amendments were determined in a 56-day incubation experiment. Maize biomass production, nutrient- and heavy metal (Cd, Cu, Ni and Zn) uptake were determined in a glasshouse pot experiment. Mineral fertilizer (N:P2O5:K2O = 7:14:7), PG, PM, SS and CM were applied at 40 kg N ha-1. After harvesting, a second maize crop was grown without adding fertility amendments. Each time, aboveground biomass was harvested at 4 weeks after emergence. A 2-year field experiment was also carried out in sandy (Chitungwiza) and red clay soils (Harare) to determine short term effects of integrated soil fertility, residue and tillage management on soil quality, maize nutrient uptake and yield. Treatment combinations that included mineral fertilizer, CM or PG + mineral N, 0 or 5 t ha-1 maize stover mulching and reduced (RT) or conventional tillage (CT) were imposed on sandy and red clay soils. Soil fertility amendments were applied at 0 and 120 kg N ha-1. After two cropping seasons, soil samples were taken from the top 30 cm depth profile for determination of selected soil chemical and physical properties. Results showed significant (p < 0.05) differences in carbon (C) and N mineralization patterns. Carbon mineralization was in the order PG>PM>CM>SS and PM≈PG>CM>SS for nitrogen (N) and phosphorus (P). In the glasshouse, maize grown in PM and PG amended soil significantly (p < 0.05) produced higher biomass and had higher N and P uptake than maize grown in mineral fertilizer amended soil in the first maize crop. The second maize crop grown in soil amended with organic soil fertility amendments significantly (p < 0.05) produced higher biomass and had higher N and P uptake than maize grown in mineral fertilizer amended soil. Heavy metal uptake was significantly (p < 0.05) highest in SS amended soils. From field trials, soil organic carbon (SOC) was significantly (p < 0.05) improved in the top 15 cm by at least 22% after manure + mineral N and mulch application. RT improved SOC (6-13%) in surface soils (0-5 cm) in both red clay and sandy soils. However, RT reduced SOC (17-63%) compared to CT in the 5-15 cm depth. Soil pH, CEC, total soil N and available P were also improved by manure + mineral N, mulching and RT. Compared to sole mineral fertilizer application, manure + mineral N improved soil moisture retention (SMR) and macro-aggregate stability (MAS) by 3-6% and 11-15% in sandy soils and 8-11% and 11-15% in red clay soils, respectively in the top 15 cm. Manure + mineral N also improved stead state infiltration rate (SSIR) by 17-23% and 18-22% in sandy and red clay soils respectively. Mulching improved SMR and MAS by 10-12% and 12-28% in the top 15 cm in both soils. RT improved (0-5 cm) and reduced (5-15 cm) SMR and MAS in both soils. It also reduced soil bulk density by 5-7% in the top 15 cm. Maize nutrient (N and P) uptake, grain and stover yields followed the order; PG + mineral N > CM + mineral N > sole mineral fertilizer > no fertilizer and PG + mineral N ≈ CM + mineral N ≈ sole mineral fertilizer > no fertilizer, respectively in red clay soils. In sandy soils, both maize grain and stover yields iii followed the order; CM + mineral N > sole mineral fertilizer > no fertilizer. Manure + mineral N, maize stover mulching and RT significantly improved soil chemical and physical properties which ultimately improved maize nutrient uptake and yields. However, when the different soil quality parameters were classified, they remained low to medium. It was concluded that CM, PM and PG can substitute mineral fertilizers as basal fertilizers and that, nickel toxicity is likely when SS is used as a soil fertility amendment in maize production. It was also concluded that manure + mineral N, maize stover mulching and RT improved soil quality and ultimately maize nutrient uptake and yield in UA. It was however, recommended that urban farmers should lime their soils to reduce soil acidity. Long term effects of integrated soil fertility, residue management and tillage are recommended under urban farmer conditions since the practice has shown potential in improving soil quality and maize yield in the short term.