Salt stress has been a major problem in the soils of the world. Semi-arid and arid soils are the most affected and bring about adverse crop productivity and yield especially in susceptible species. Salinity is usually denoted with electrical conductivity (EC) equal to or greater than 4 deci Siemens per meter (dS/m). With expanding areas affected by salinity, especially with increasing sea water level and incursion of land, rise in salty water table is eminent as consequences of climate change. Sustainable remediation of soil salinity is becoming a major concern, especially in the field of agriculture and climate change mitigation. Remediation of saline soil with application of organic amendments has shown positive results with the added advantage of carbon build up in the soil. Recently with the focus on biochar as one of the recent soil conditioners and amendment, many researchers have tried to find out its remediating capacity on soil salinity. This article will focus on how biochar affects soil salinity with much emphasis on cation exchange capacity (CEC) and anion exchange capacity (AEC). Results from researchers showed that biochar remediates soil salinity due to its inherent characteristics like high CEC and possession of AEC that deprives both cations and anions that form salt in the soil. This positively favour the thrival of crops under salt stress by boosting K⁺ availability, high salt sorption capacity, increased plant water intake, accelerated transpiration, lowered soil EC, increased soil accessible nutrients, increased soil moisture saving, optimizes the water-salt balance of the soil during saline irrigation, extended life span of plant which all favour crop performance. Future studies should explore synergistic effects of combining biochar with other soil management practices like drainage systems to enhance salinity mitigation and crop performance. Additionally, nano-biochar represents a promising direction in biochar development, with more possible potential for salinity amelioration due to higher surface area compared to biochar. While biochar has the potential to mitigate this issue, research on its application in African contexts is limited. More localized studies are needed to develop effective biochar solutions tailored to specific African environments and agricultural practices. Graphical abstract

Agricultural application of anaerobic digestates can play an important role in plant nutrition. The effects of digestate treatment were studied in pot experiments, using sandy and loamy textured soils with distinct characteristics. Three different treatments were studied and compared: control, digestate, and irrigation treatments. Nitrogen loading was calculated on the bases of the digestate analysis. The same amounts of digestate and irrigation water were applied in the different treatments before sowing and at the V4–V6 stages of maize. Besides the conventional soil chemical analysis, invertase, dehydrogenase, catalase, and the abundance of some cultivable microbes were measured in two consecutive years. According to our results, irrigation and digestate treatments had greater impact in the case of sandy soils than in loamy textured soils. Digestate provides more effective phosphorus and potassium sources than nitrogen. Based on the results of discriminant analysis, the digestate application had a greater influence on soil chemical properties, followed by microbiological parameters.

The carbon, nitrogen, phosphorus, and sulfur (CNPS) ratios of soils are known to be relatively stable parameters, characterizing different land uses. We hypothesized that the long-term application of sewage sludge compost (SSC) would not change these ratios but would increase the concentration of these elements and change the quality of organic matter (OM), as well as soil enzyme activities. Hence, soil chemical and microbiological properties were studied in a 20-year long-term experiment. The plots were grouped into five blocks and treated every third year with SSC at the rates of 0, 9, 18, or 27 t ha⁻¹. Three plants, in a crop rotation, were tested and sown every year as follows: rye, rye with hairy vetch, and maize. The results showed that basic soil parameters (pH, OM content, E4/E6 ratio, NO₃-NO₂-N, AL-P₂O₅, and soil moisture content) were increased, along with the SSC doses in soil for the rye. Similar trends were found in CNPS concentrations, β-glucosidase, and alkaline phosphatase activities, while the acidic phosphatase activity was reduced. The C:N, C:S, and N:S ratios were not affected by the compost application. The main factors of treatment effects were plant-available phosphorus (ammonium lactate (AL)-soluble P₂O₅), total P, and NO₃-NO₂-N, based on principal component analysis. The canonical correspondent analyses revealed that phosphatase activities were affected by C:N, C:P, and N:P ratios and β-glucosidase was correlated with P forms and the E4/E6 ratio, while the soil pH strongly affected all soil enzymes. Based on the alkaline and acidic phosphatase activities, the role of microbes became more important with increasing compost doses in phosphorus mobilization. We conclude that the addition of SSC could improve soil health through increasing the pH, OM, nutrient content, and microbial activity. Also, some elemental ratios have an important role in the regulation of soil enzyme activities.

Understanding how elevation gradients and soil depths influence soil organic carbon stocks (SOCS) and total nitrogen stocks (TNS) is essential for sustainable forest management (SFM) and climate change mitigation. This study investigated the effects of elevation and soil depth on SOCS and TNS in the Mount Kenya East Forest (MKEF). A stratified systematic sampling approach was applied, involving collection of 38 soil samples from two depths (0–20 cm and 20–40 cm) across three elevation zones: Lower Forest (1700–2000 m), Middle Forest (2000–2350 m), and Upper Forest (2350–2650 m). Samples were analysed for bulk density (BD), pH, texture, soil organic carbon (SOC), and total nitrogen (TN), using standard laboratory methods. In topsoil (0–20 cm), SOCS ranged from 109.28 ± 23.41 to 151.27 ± 17.61 Mg C ha⁻¹, while TNS varied from 8.89 ± 1.77 to 12.00 ± 2.46 Mg N ha⁻¹. In subsoil (20–40 cm), SOCS ranged from 72.03 ± 19.90 to 132.23 ± 11.80 Mg C ha⁻¹, with TNS varying between 5.71 ± 1.63 and 10.50 ± 1.90 Mg N ha⁻¹. SOCS and TNS increased significantly with elevation (p < 0.05), exhibiting the following trend: Lower Forest < Middle Forest < Upper Forest. Topsoil consistently stored significantly higher SOCS than subsoil (p < 0.05), emphasizing the critical role of surface soils in carbon sequestration. Regression analysis revealed a significant positive relationship between SOCS and TNS (R² = 0.84, p < 0.001). Both SOCS and TNS were positively correlated with elevation, SOC, TN, and total annual precipitation (TAP), but negatively correlated with BD and mean annual temperature (MAT). These findings provide baseline data for monitoring SOCS and TNS in the MKEF, offering insights into sustainable forest management strategies to improve soil health and enhance climate change mitigation efforts.

The smallholder farming systems in Sub-Saharan Africa (SSA) are highly diverse and heterogeneous in terms of biophysical and socio-economic characteristics. This study was conducted in upper Eastern Kenya (UEK) to categorize farm households and determine the influence of socio-economic characteristics (SeC) and soil fertility management practices (SFMP) on soil fertility across farms. Conditioned Latin hypercube sampling (cLHS) was performed to determine 69 soil sampling sites within Meru and Tharaka Nithi counties. From each household (whose field soil sample was obtained), data relating to resource endowment and soil fertility management were collected through a household questionnaire survey. Standard laboratory procedures were used to analyse soil samples. Data reduction was performed using categorical principal component analysis (CATPCA) (for SeC and SFMP) and standard principal component analysis (PCA) (for soil properties). Two-step cluster analysis identified three distinct farm categories or farm types (FT), namely, low fertility farms (FT1), moderately fertile farms (FT2), and fertile farms (FT3). The correlation of clusters against soil properties was significant across pH, soil organic carbon (SOC), cation exchange capacity (CEC), available P, plant available K, and exchangeable bases. FT1 had low SOC, pH, CEC and available P (soil characteristics), low usage of fertilizer and manure (soil fertility management), and smaller household size, lower income, and smaller farm size (socio-economic). FT2 had lower SOC (compared to FT3) and available P. In terms of soil fertility management, FT2 had higher cases of fallowing and composting with moderate fertilizer usage. Households in this category had moderate income, family size, and land size (socio-economic). FT3 had relatively high SOC, pH, CEC, and mineral nutrients. This farm type was characterized by high fertilizer use (soil fertility management) as well as larger household size, higher income, and larger farm size (socio-economic). The results indicate the importance of nutrient management in enhancing soil quality. Delineation and characterization of farms based on the various parameters including resource endowment reveal imbalanced farm resource flows, suggesting a need for locally tailored interventions suited for location-specific conditions to facilitate improved targeting of soil fertility-enhancing technologies and sustainable crop production regimes. While fertilizer is one of the most critical inputs for enhancing agricultural production, it is a major contributor to nitrous oxide emissions from agriculture and can have negative environmental effects on soil biota and water sources. Farmers’ knowledge on the use of fertilizer is thus necessary in developing strategies (such as integrated approach) to promote its efficient use and minimize its detrimental impacts.