Characterisation and Mapping of Soils in Major Coffee Growing Regions of Uganda

There is limited information on the soil nutrient status, site specific fertilizer and agronomic recommendations for coffee in Uganda hence limiting its production and productivity. Therefore, this study was undertaken to characterize the soils under coffee farms and provide fertilizer and land management recommendations for coffee farming in selected districts in Uganda. 717 soil samples were collected from 45 coffee growing districts that were purposively selected to represent the major coffee growing regions of Eastern, Northern, Western, Mid-west and West Nile. 35 districts were sampled from Robusta coffee growing areas and 10 districts from Arabica coffee growing areas. Parameters considered in the laboratory analysis included; pH, total organic carbon, total nitrogen, available P, exchangeable Calcium (Ca), Sodium (Na), Magnesium (Mg), and Potassium (K) and micro elements: Iron (Fe), Zinc (Zn), Copper (Cu), Manganese (Mn) and Boron, soil texture, bulk density and hydraulic conductivity. Soil nutrient levels distribution maps for Robusta and Arabica coffee growing regions were generated in ArcGIS for the entire country. The soil chemical and physical properties were subjected to analysis of variance using Genstat 14 th edition. The soil mapping results showed that, the overall average soil macro-nutrients concentrations were significantly different across regions (p < 0.05) with Eastern having the highest levels of macro elements (CEC of 19.28 meq/100 g, Base Saturation (BS) of 43.40%, pH of 5.78, N of 0.20%, K of 1.64 meq/100 g, P of 107.68 mg/kg and OM of 3.31%) followed by Western region. No significant difference (p > 0.05) was recorded for the micronutrients across the different regions, except Zn (p < 0.05). This study showed that Phosphorus and Potassium were generally moderate to high levels in most of the regions sampled while Nitrogen and organic matter were moderate to low. Results from this study provide a general picture of the nutrient status across all coffee growing regions in Uganda and highlight the required modifications for increased production and productivity.


Introduction
Coffee is the second largest valued commodity in international trade and most widely traded tropical agricultural commodity after petroleum (UCDA, 2021;ICO, 2019).It plays a leading role in the livelihoods of Ugandans and contributes substantially to the national economy (Nahanga et al., 2015).Nearly 42% of farming households grow coffee which has contributed on average 30% of the country's foreign exchange earnings over the past 20 years (UCDA, 2015).Coffee continues to be one of the main cash crops of Uganda, playing a role in providing foreign exchange (NCP, 2013).The crop is grown on an estimated 353,907 hectares of land by about 1.7 million smallholder farmers (a quarter of them being female-headed) and 90% of these smallholder farmers owning gardens ranging between 0.5 and 2.5 hectares in size (Hill, 2005;NCP, 2013;Mugoya, 2018).Uganda grows two types of coffee; Robusta coffee occupies 80% and Arabica coffee, takes the remaining 20%.Contribution of Coffee to Uganda's export and foreign exchange earnings has increased from 8.2% in 1991/1992 to 23% in 2017/2018 Coffee Year in quantity and 14% to 28% in US$ 492 million value respectively (UCDA, 2019).For example, coffee exports for the coffee year (2022/23) totaled 6.14 million bags worth US$ 940.36 million (UCDA, 2023).More than 9 million people in Uganda are estimated to derive their livelihood from coffee-related activities along the value chain (NCP, 2013).It therefore has a very high employment potential, and poverty reduction effect on the smallholder farming households.The Government of Uganda National Development Plan III (NDP III) therefore prioritizes coffee as one of the six commodities for value addition for increased household incomes, exports earnings and import replacement.
In the last decades, smallholder coffee farmers have been experiencing declining agricultural productivity, mostly due to soil fertility depletion (Woniala & Nyombi, 2014), pests and diseases (Jonsson et al., 2015;Merga & Alemayehu, 2019;Nzeyimana et al., 2013) and poor land management (Pender, 2003;Okubal & Makumbi, 2000).Other main contributing processes to soil fertility decline are soil erosion, a decline in organic matter and soil biological activity, degradation of soil structure and loss of other soil physical qualities, reduction in availability of major nutrients (N, P, and K) and micro-nutrients, and an increase in toxicity due to acidification or pollution (Zake et al., 1997;NARO & FAO, 1999;NEMA, 1999).The underlying causes include; population pressure, poverty, land tenure insecurity, bad policies and institutions, poor infrastructure and services, and farmers' lack of knowledge about soil conservation methods (Pender et al., 2001).
Decline in soil fertility is the most limiting constraint particularly to coffee production in Uganda (Bekunda et al., 2002), where most agro-ecosystems remove more nutrients than are provided by external inputs (Edmeades, 2003).Farmers' decisions on the use of fertilizers in production requires an understanding of the expected crop yield which is a function of crop nutrient needs, nutrient supply from indigenous sources and the fate of the applied fertilizers (Bidraban et al., 2015).However, major gaps still exist on the status of nutrients, supply of nutrients from indigenous sources, site specific fertilizer recommendations and the general understanding of soil fertility in coffee ecosystems in Uganda (Wang et al., 2015).Soil mapping was therefore undertaken in 45 districts (35 that grow Robusta coffee, 10 that grow Arabica coffee).The objective was to characterize the soils under coffee farms and understand the regional specific soil nutrient status across coffee farms in Robusta and Arabica growing areas to guide site specific coffee nutrient recommendations.
Robusta is the major type of coffee grown in Uganda, accounting for about 80% of production.It grows in most low altitude areas of Uganda, covering Central, Eastern, Mid North, West Nile, Western and South Western Uganda that are within 900-1,500 m above sea level.Robusta has Lake Victoria Crescent as its native habitat (UCDA Robusta Coffee Manual, 2019).(2) Laboratory Analysis Soil samples were air-dried, ground and sieved through a 2 mm sieve and then analysed using the standard methods (Okalebo et al., 2002).The analysis included: pH, total organic carbon, total nitrogen, available P, exchangeable Calcium (Ca), Sodium (Na), Magnesium (Mg), and Potassium (K) and micro elements Iron (Fe), Zinc (Zn), Copper (Cu), Manganese (Mn) and Boron, soil texture, bulk density and hydraulic conductivity.
Soil pH was measured using a pH meter (1: 2.5 soil: water); SOM and total N were determined using Walkley and Black method and Kjedhal method, respectively.Soil texture was determined using Bouyoucos hydrometer method (Bouyoucos, 1960) while their textural classes were determined according to the FAO classification (FAO, 1990).Copper, zinc, iron and manganese were measured by atomic absorption spectrophotometer method.Exchangeable bases were determined by the procedures described by Okalebo et al. (2002).Five (5 g) of air-dried soil, 100 ml of ammonium acetate was added in a clean plastic bottle and shaken for 30 minutes.The contents were filtered through number 42 Whatman paper and this constituted the extract for measuring Na, K, Ca and Mg.Sodium and K were measured on a flame photometer while Ca and Mg were determined by atomic absorption spectrophotometer at wave length of 422.7 nm and 285.2 nm respectively.
In addition to the procedures used to extract total micro-nutrient (Okalebo et al., 2002), standard solutions for micronutrients (Zn, Cu, Fe, Mn) were prepared in Diethylenetriamine Penta-Acetate (DTPA) extraction solution using Certified Reference Material (CRMs).DTPA metal-extraction principle and protocol was as described by Lindsay and Norvell (1978).Diethylenetriamine Penta-Acetate (DTPA) was used as chelating agent which can effectively bind with water-soluble and weakly adsorbed exchangeable metals in soil.The chelation reactions are slow and required weeks or months to attain the equilibrium state.Therefore, DTPA quantity in solution-to-soil ratio (2:1) was adjusted at a level that can chelate metals equal to 10 times of atomic weight of respective metals.This reduces the competition between metals ions to bind with chelating agent.Calcium Chloride (CaCl 2 ) maintains higher CO 2 level in soil and avoids the release of metals bonded with CaCO 3 by inhibiting dissolution of CaCO 3 in calcareous soils.
Soil particle analysis to estimate the percentage sand, silt and clay (texture) was determined using the hydrometer method.The hydrometer method of determining the proportions of sand, silt and clay depends on particle size of the differential settling velocities in a water column.The settling velocity is a function of liquid temperature, viscosity and specific gravity of the falling particle.Saturated hydraulic conductivity and bulk density were also determined in all plots surveyed.Saturated hydraulic conductivity was determined using the inverse auger-hole method, while bulk density was determined using the core method.

Statistical Analysis
Data collected on both soil physical and chemical properties during the study was entered into excel and subjected to various types of analyses.Both descriptive and inferential statistical analytical tools were used in the study.Summary statistics (means, variances) were used to compare soil properties from different geographical locations.Analysis of variance was performed on soil chemical properties such as CEC, N, C, P, K and soil physical properties like texture (% sand, silt and clay) was carried out using GenStat 14th Edition statistical package; and significant means were separated using Least Significant Difference (LSD) at the 0.05 probability level.

Characteristics of the Soils Under Coffee Farms in Selected Districts in Uganda
Table 1 shows the overall average soil macro-nutrients concentrations across the different regions.These concentrations were significantly different across regions (p < 0.05).Generally, the Eastern region had higher levels of CEC of 19.28 meq/100 g, BS of 43.40%, pH of 5.78, N of 0.20%, K of 1.64 meq/100 g, P of 107.68 mg/kg, Na of 0.32 meq/100 g, Ca of 10.32 meq/100 g, Mg of 3.94 meq/100 g, OC of 1.92% and OM of 3.31% compared to other regions except for P which was 107.68.This was followed by Western Region which had levels of CEC of 14.32 meq/100 g, BS of 38.63%, pH of 5.75, N of 0.25%, P of 173.38 mg/kg, K of 1.11 meq/100 g, Na of 0.15 meq/100 g, Ca of 8.86 meq/100 g, Mg of 4.19 meq/100 g, OC of 2.33% and OM of 4.02%.

Organic Carbon
Organic carbon is related to soil organic matter.The latter affects nutrients in several ways.It is very vital in improvement of the soil structure which is vital for root development.Organic matter is responsible for moisture and nutrient retention.A good soil should have SOM of at least 3%.Figure 2 shows how organic carbon (OC) in the country was distributed.The organic carbon concentration ranges from 1.27 to 6.56 mg/kg with an average of 2.51 mg/kg in the country.The organic carbon concentration in the soil is generally moderate throughout the country.The only patches of high organic carbon concentration were on the western shores of Lake Victoria (Kabarole, Mityana, Masaka and Bukomansimbi districts).The low concentrations of organic carbon in soils were found in west Nile districts, Busia in the east and central districts while the rest of the regions had moderate levels of organic Carbon.The low level of N and carbon in other areas could be due to continuous mining in these sites and its removal through harvest and biomass transfer as observed by Stoorvogel and Smaling (1990) and Gachimbi (2002) while working on nutrient flows and balances.The variability of soil fertility in the land uses depends on the land use, level of nutrient (organic or inorganic) application, type of crop grown which is in most times based on farmers' perception of soil quality in farm unit. jas.ccsenet.

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Nitrogen, i is a buildi which help are the bui plant cells

Recommendations
Based on the findings of this study, regional specific recommendations can now be calculated and applied.Farmers can also apply the following fertilizer formulations and land management practices to improve coffee production in their regions.
P (mg/kg) 6.31 > 15 Application of fertilizers that contain Phosphorus e.g.DAP is recommended.Addition of organic matter inform of compost will also help buffer soil pH while also adding P.

West Nile
Figure 6 sh concentrat exchange observed Patches of north-west Soils with plant uptak CEC (Zha in clayey a with low nutrients.cations wh management involving adding manure, inorganic fertilizers and growing legumes with coffee will help add nitrogen.It is also recommended to add coffee specific fertilizers in particular NPK 22:6:12 + Mg + S + Zn + B) applied in two splits.contain Phosphorus such as DAP and TSP.Apply manure and intercrop with cover crops like Mucuna, Brachiaria brizantha.Addition of coffee specific fertilizers in particular NPK 22:6:12 + Mg + S + Zn + B). contain Phosphorus such as DAP and TSP.Apply manure and intercrop with cover crops like Mucuna, Brachiaria brizantha.Addition of coffee specific fertilizers in particular NPK 22:6:12 + Mg + S + Zn + B).

Table 1 .
Soil macro-nutrients content by region

Table 2 .
Soil micro-nutrients content comparison by region

Table 3 .
Site specific fertilizer and land management practices for different coffee growing regionsThe moderate nitrogen concentration levels in Central Uganda will require incorporation of compost manure.Furthermore, addition of coffee specific fertilizers to such soils in particular NPK 22:6:12 + Mg + S + Zn + B) is encouraged.Incorporation of agroforestry and cover crops such ascrotalaria (Crotalaria spectablis), cowpea (Vigna unguiculata L. Walp.), and Brachiaria brizantha (Urochloa brizantha) can also help.These can be ploughed back into the soil at harvest to incorporate nitrogen Application of fertilizers that contain Phosphorus such as Di Ammonium Phosphate (DAP) and Tri Super Phosphate (TSP) is suggested.Manure should also be applied if available.
Application of fertilizers that contain Phosphorus such as Di Ammonium Phosphate (DAP) and Tri Super Phosphate (TSP) is suggested.Manure should also be applied if available.