Soil-Plant Nutrient Dynamics and Fertilizer Use Efficiency in Maize & Desmodium Cropping System

Declining soil fertility remains a major constraint to agricultural productivity in sub-Saharan Africa (SSA), largely due to continuous cropping, minimal nutrient replenishment, and poor fertilizer management practices (Ntinyari & Gweyi-Onyango, 2021). Excessive, improper, and unbalanced fertilizer use, particularly of nitrogen (N), has raised concerns over environmental degradation and soil health deterioration. Efficient nutrient management, particularly of nitrogen (N) and phosphorus (P), is therefore crucial to improving crop yield and sustaining soil productivity. The 4R principle, which entails applying the right source (fertilizer with higher efficiency), right rate, right time, and right placement, is recommended to enhance fertilizer use efficiency and reduce nutrient losses (Hochmuth et al., 2014). Proper placement and timing improve nutrient uptake by roots, minimizing leaching and gaseous losses. A promising strategy to further enhance efficiency is reducing nitrification through the use of nitrification inhibitors, which slow the microbial conversion of ammonium (NH₄⁺) to nitrate (NO₃⁻), thereby reducing N₂O emissions and nitrate leaching (Coskun et al., 2017).

While chemical nitrogen fertilizers have significantly increased crop yields globally, their long-term excessive use has led to adverse effects, including soil acidification, ammonia volatilization, non-point source pollution, and elevated nitrous oxide emissions (Hu et al., 2023). Similarly, phosphorus fertilizers, such as triple superphosphate (TSP), play an essential role in root development, energy transfer, and nucleic acid synthesis. However, phosphorus is relatively immobile in soils and prone to fixation, particularly in iron and aluminum-rich soils such as Ferralsols and Nitisols. This fixation renders phosphorus unavailable to plants, necessitating higher application rates that can, in turn, cause eutrophication of water bodies when excessive Pis lost through runoff. Therefore, ensuring Pis applied at the right time and informs that minimize fixation is essential. Studies show that balanced fertilization combining organic and inorganic sources enhances P availability and utilization, resulting in improved yields and reduced environmental impacts (Ntinyari & Gweyi-Onyango, 2021). Organic materials such as farmyard manure (FYM) supply all the major nutrients (N, P, K, Ca, Mg, S)and essential micronutrients (Fe, Mn, Cu, Zn), acting as a comprehensive nutrient source. FYM application improves soil structure, enhances water holding capacity, stimulates microbial activity, and supports long-term soil fertility. Furthermore, FYM contributes to soil organic carbon (SOC) buildup, which enhances soil resilience, nutrient retention, and carbon sequestration potential. The combined application of FYM and mineral fertilizers has been shown to enhance soil fertility, reduce nitrogen losses, and increase crop productivity (Hu et al., 2023). For instance, (Zeyede et al., 2020) reported that combining manure with chemical fertilizers improved SOC by 2.45%, enhancing nutrient cycling and microbial activity. Although total organic carbon (TOC) changes slowly, labile organic carbon fractions, such as microbial biomass carbon (MBC), dissolved organic carbon (DOC), particulate organic carbon (POC), and easily oxidized organic carbon (EOC), respond rapidly to management interventions, serving as indicators of soil health (Z. Zhang et al., 2021).

Maize (Zea mays) is a staple crop in sub-Saharan Africa (SSA), where it accounts for up to 30% of the calorie intake of the population (Taylor & Tanumihardjo et al., 2010). Maize is also the most widely cultivated crop in the region, grown in 46 out of 53 countries in SSA (Abate et al., 2017). Despite the expansion in area under maize production, by 30.2% between 2007 and 2017, yield gains have been modest at only 8.5% (FAOSTAT, 2020). In Kenya, nitrogen and phosphorus deficiencies remain the most critical limitations to maize productivity (J. Kihara et al., 2016). High fertilizer costs and limited access (Nziguheba et al., 2016) have driven many smallholder farmers to rely on crop residues and manure to supply nutrients. However, these inputs often contain insufficient N and P to sustain high maize yields.

To address these challenges, intercropping maize with legumes has emerged as a promising strategy to improve soil fertility and productivity. Legume-based intercropping enhances nutrient cycling through biological nitrogen fixation, contributes organic matter, improves soil structure, and promotes more efficient resource use (Ndayisaba et al., 2021). Legumes can also increase phosphorus availability through the release of root exudates and organic acids that mobilize bound P, improving nutrient access for both crops in the intercrop.

Among potential intercrops, Desmodium (Desmodium silverleaf) stands out as a highly beneficial legume. It is a perennial, nitrogen-fixing forage crop known for its deep rooting system, high biomass production, and ability to improve soil fertility. Desmodium forms symbiotic associations with rhizobia to fix atmospheric nitrogen, enriching soil nitrogen pools. In addition, it exudes allelopathic compounds that suppress weeds like Striga hermonthica while enhancing phosphorus solubilization through organic acid secretion. These attributes make Desmodium an ideal intercrop with maize, improving overall nutrient efficiency, soil health, and system sustainability.

Efficient nutrient utilization is key to sustainable agricultural intensification. Nutrient use efficiency (NUE) reflects how effectively plants absorb and use applied nutrients for growth and yield. High NUE minimizes nutrient losses to the environment, while low NUE often results in nutrient accumulation in soils, leading to potential greenhouse gas emissions and environmental degradation. Optimizing NUE through balanced organic–inorganic fertilization is therefore critical for achieving sustainable yield gains while minimizing environmental footprints. This study aimed to: (i) Assess the impact of different organic–inorganic fertilizer combinations on soil chemical and physical properties in Keiyo North. (ii) Evaluate the nutrient use efficiency of maize and Desmodium under various fertilizer substitution ratios.