Effects of Organic and Inorganic Inputs on Soil Chemical Properties, Organic Carbon Fractions and Maize Yield in Mbeere South Sub-county, Kenya

Abstract

Declining soil fertility affects farmers both globally and in Kenya. Small-scale farmers are experiencing declining agricultural productivity and land degradation due to nutrient and soil carbon depletion. Soil erosion, high temperatures, and poor farming practices are some of the factors that lead to low soil organic matter (SOM). This study investigated the effects of organic and inorganic inputs on soil chemical properties, organic fractions, and maize yield. The research was conducted on a long-term soil fertility field experiment established in 2004 in Kenya's relatively arid Mbeere South sub-county. A randomized complete block design was used for the experimental setup. The Integrated Soil Fertility Management (ISFM) treatments included 60 kg ha-1 nitrogen (N) from goat manure (GM60); 30 kg ha-1 inorganic N fertilizer (IF30); 60 kg ha-1 inorganic N fertilizer (IF60); GM30+IF30; 90 kg ha-1 inorganic N fertilizer (IF90); 60 kg ha-1 N from lantana (Lantana camara) (LC60); LC30+IF30; 60 kg ha-1 N from mucuna beans (Mucuna pruriens) (MP60); MP30+IF30; 60 kg ha-1 N from Mexican sunflower (Tithonia diversifolia) (TD60); TD30+IF30, as well as a control treatment. The carbon (C) compositions of ground soil samples and organic amendments were analyzed using 13C solid-state nuclear magnetic resonance (NMR). The GM60, GM30+IF30, LC60, and TD60 treatments had much higher Alkyl and O-Alkyl C SOM functional groups than the control and other treatments. Analysis of variance (ANOVA) was performed on the data. The relationship of grain yields against soil N or soil C was evaluated using a bivariate Pearson Correlation to produce a correlation coefficient. Differences between treatment means were separated using the least significant difference (LSD) at p=0.05. The average soil C for the control was 7.47 mg kg-1 and ranged from 5.03 to 7.37, 9.57 to 18.77, and 7.03 to 14.50 mg kg-1 for inorganic fertilizers, organic fertilizers, and organic + inorganic fertilizers, respectively. The mean grain yield for the control was 0.56 Mg ha-1 and ranged from 1.51 to 1.99, 1.94 to 4.16, and 2.98 to 4.60 Mg ha-1 for inorganic fertilizers, organic fertilizers, and organic + inorganic fertilizers, respectively. The results showed that applying sole organic fertilizers or combined with inorganic fertilizers increase maize yield and promotes soil C sequestration potential. The increase was attributed to high Alkyl and O-Alkyl C SOM functional groups. It is crucial to consider and understand the C fraction content of organic inputs when selecting the most appropriate management techniques to enhance soil fertility and maintain or increase crop yields. The findings of this study have practical implications for smallholder farmers, as they can adopt and make informed decisions regarding suitable and sustainable soil management options to improve soil and crop productivity in their areas. Additionally, this research contributes valuable knowledge on local strategies for dealing with climate variability, further benefiting farmers in the study areas.