Index

• Introduction
• Materials and Methods
• Results and Discussions
• Conclusion
• References

Abstract

This incubation trial was carried out to determine the effect of urea amendment of cocoa pods husk (CP) and coffee wastes (CW) on their ability to improve some physico-chemical properties and potassium availability in a Ferralsol of the Western region of Cameroon. It was conducted using a completely randomized design consisting of seven treatments and three replications. Modalities included three doses of urea (0; 0.003 and 0.006g per 100g of dry soil corresponding to 0, 75 and 150 Kg per hectare) combined with 0.225 g corresponding to 5 t.ha-1 for each organic residue in addition to the control. The study was conducted under aerobic conditions, at a temperature of 28°C and in darkness for 90 days. The Changes in the studied parameters was taking every 30 days. Compared to the control, the urea amendment of the bio-waste studied reduced the alkalinity and the negative net charge of the soil. On the other hand, it improved organic carbon mineralization and potassium availability (0.81 cmol+/kg with 5 t.ha-1 of CP + 75 to 150 kg.ha-1 of urea compared to 0.48 cmol+/kg for the control). Urea amendment of cocoa pods husk and coffee wastes can therefore improve the availability of potassium and the mineralisation of organic carbon from these residues in ferralsols.

Keywords: Availability of potassium, Cocoa pods husk, Coffee wastes, Mineralization, Urea amendment, Western region of Cameroon.

Received: 9 March 2023 / Revised: 17 April 2023/ Accepted: 28 April 2023/ Published: 12 May 2023

Contribution/ Originality

This study contributes to existing literature that urea amendment of cocoa pods husk and coffee wastes can improve their mineralization and therefore increase their ability to affect some physico-chemical properties and potassium availability in ferralsols.   

1. INTRODUCTION

In the West Cameroon region, Ferralsols are the most represented soils [1]. These soils generally have an acidic pH of around 5 and are subject to significant mineralization of organic matter and increased nutrient leaching due to high rainfall and high temperatures. In addition to this, the common practice of farmers is clear cutting and burning to clear the plots. This situation predisposes the soil to degradation, decrease in organic matter, reduction in the amount of nitrogen in the soil, and the rapid depletion of other nutrients such as potassium [2-4]. In agriculture, the most pronounced effects are reduced yields of the main food crops. For example, Sanginga, et al. [5] noted a reduction in cassava yields from 11 t.ha-1 to less than 2 t.ha-1 for a reduction in the duration of fallow in the humid savannah zones of Nigeria and Benin to less than two years. Despite mineral fertilization, crop yields tend to decrease from one cropping season to the next due to mineral leaching [6]. Hence the need to use amendments such as organic matter to improve the properties of these soils and to make the use of mineral fertilizers profitable [7] as they are expensive and not widely available, are not affordable for most farmers [8].

The main sources of organic matter in agricultural soils are crop residues and root systems, droppings, manure, slurry or recycling of organic wastes from effluents, composts and sludge. When these organic materials are incorporated into the soil, there are three main reactions: CO2 production, humification and mineralization [9]. Mineralization of organic matter results from its use as an energy source by soil micro-organisms [10]. The organic compounds are then degraded with the production of carbon dioxide. Several studies argue that the presence of exogenous mineral nitrogen in the soil can modify the mineralization rates of organic inputs. Along these lines, Schomberg, et al. [11] have shown that decomposition of nitrogen-rich, lignin-poor plant residues is rapid, whereas decomposition of nitrogen-poor, lignin-rich compounds requires mineral nitrogen that is immobilized by soil microorganisms to satisfy their "thirst for nitrogen". According to Monsanto [12] the carbon to nitrogen ratio  (C/N)  varies from one plant residue to another, but soil microbes prefer a C/N ratio around 10. Several studies have examined the potential to enhance soil fertility through the use of plant by-products with or without mineral fertilizers [13-16] but the use of cocoa husks and coffee residues with or without urea amendments has not yet been widely studied. These potassium-rich residues [17] which are available in Cameroon [18] are rarely used for soil fertilization. They are generally abandoned in the fields either in pits or in the form of piles, especially in the case of cocoa. This is mainly due to their low decomposition rate and the inoculation of cryptogamic diseases, especially Phytophtora infestans infection [19].

 Hence the present study, which was conducted to test the effect of cocoa pods husk and robusta coffee wastes amended or not with urea on some physico-chemical properties and potassium in a Ferralsol of West Cameroon.

2. MATERIALS AND METHODS

2.1. Soil Used

The soil used (0-20 cm deep) for this study came from an experimental plot of the Faculty of Agronomy and Agricultural Sciences (FAAS) of the University of Dschang, located in the Western Highland of Cameroon at 5°20'N, 11°45'E and an average altitude of 1410 m above sea level. The area has a Cameroonian equatorial-type climate modified by altitude characterized by monomodal annual rainfall. The dry season runs from mid-November to mid-March and the rainy season runs from mid-March to mid-November with an average annual rainfall of 1500 to 2000 mm. Temperatures vary between 10 and 25°C for an average of 20°C. The soil used was according to the World Reference Base (WRB) 2015 a Mollic Geric Rhodic Ferralsol (Aric, Orthoeutric). Table 1 shows the initial soil characteristics determined at the FASA soil analysis laboratory using the methods described by Pauwels, et al. [20].

2.2. Used Crop Residues and Chemical Fertilizer Material

The crop residues used in this study were cocoa pods husk and robusta coffee wastes (parchment and pericarp) both in the form of grindings (ø < 2mm). The cocoa husks were collected from a family farm in the town of Santchou and the coffee residues were collected from a hulling factory in the same town. Once obtained, we dried these by-products in the open air for two weeks and then dried them in a ventilated oven at 70 °C until a constant mass was obtained. Finally, these residues were crushed and sieved (ø < 2 mm). The chemical characteristics of these waste products were also determined in the soil analysis laboratory of FASA (Table 2). The inorganic fertilizer used in this experiment was urea at 46% nitrogen bought in a phyto-sanitary shop.

2.3. Experimental Design

The trial was conducted using a completely randomized design consisting of seven treatments and three replications. For each waste tested, 0.225 g corresponding to 5 t.ha-1 was combined with three doses of urea (0; 0.003 and 0.006g per 100g of dry soil) corresponding to 0, 75 and 150 Kg per hectare. Subsequently, the combinations thus obtained were mixed with soil samples (100 g each) for comparison with an unamended control.

The treatments were as follows:

• T0 : Unamended soil (control) ;
• TA : 0.225 g of cocoa pods husk (5 t.ha-1) without urea ;
• TB : 0.225 g of cocoa pods husk (5 t.ha-1) + 0,003 g of urea (75 kg.ha-1) ;
• TC : 0.225 g of cocoa pods husk (5 t.ha-1) + 0,006 g of urea (150 kg.ha-1) ;
• TD : 0.225 g of coffee wastes (5 t.ha-1) without urea;
• TE : 0.225 g of coffee wastes (5 t.ha-1) + 0,003 g of urea (75 kg.ha-1);
• TF: 0.225 g of coffee wastes (5 t.ha-1) + 0,006 g of urea (150 kg.ha-1).

2.4. Conduct of Incubations

The procedure applied in this study was based on those used by Bouajila, et al. [15] and Mboua, et al. [19]. In 125 mL jars, 100 g of dry treated or untreated soil was introduced. Throughout the trial, weekly weigh-overs were used to maintain the moisture content of the mixtures at field capacity. The jars thus formed were also kept in the dark at a temperature of 28°C in a thermo-regulated and ventilated chamber for a period of 90 days. The potassium dynamics and other physico-chemical parameters studied in this experiment were monitored every 30 days after the waste products were added. These were actual acidity, total acidity, net soil charge, organic carbon and electrical conductivity.

2.5. Physico-Chemical Analysis

In this study, grain size was determined by the Robinson-Köhn pipette method and bulk density on undisturbed samples after oven drying at 105 °C taken with metal cylinders. The actual acidity was determined using a sample-distilled water suspension, while the total acidity was determined using a normal potassium chloride (KCl) sample suspension with a ratio of 1/2.5 (w/v). The water holding capacity was determined by the amount of water held in the soil versus the dry weight of soil sample 24 hours after saturation and organic carbon content was determined by Walkley and Black potassium dichromate titration method. Total nitrogen was determined by the Kjeldalh method and available phosphorus by the Bray 2 method. Cation exchange capacity (CEC) and Exchangeable Ca2+, Mg2+, K+ and Na+ were determined by the leaching of ammonium acetate method.  

2.6. Statistical Analysis

Data entry and diagram design was done using Microsoft EXCEL software. Using SPSS 23 (statistical package for social sciences), the normal residue test (Shapiro-Wilk test) and the one-factor analysis of variance (ANOVA) were performed. This software was also used for multiple comparison tests at the 5% level of significance according to Duncan's model and for the analysis of the correlation between some of the parameters studied.

3. RESULTS AND DISCUSSIONS

3.1. Initial Physico-Chemical Characteristics of the Soil 

The initial physico-chemical properties of the soil in this study are shown in Table 1. Analysis of the results showed that the soil was loamy sand with an average clay content of 12.24%. According to Beernaert and Bitondo [21] it was moderately acidic (5.3 < pH < 6) with a negative ∆pH (pH KCl-pH H2O = -0.9) giving it a negative net charge. Its organic carbon content was very high (4.63%) but of very poor quality (C/N > 20) with an average total nitrogen content (0.125 < N < 0.225%). The sum of bases was high and CEC7 moderate with a high base saturation rate (78.89%). The Ca2+ level was low (2 < Ca2+ < 5cmol+/kg) while Mg2+, K+ and Na+ were high, medium and very low respectively. Finally, the level of available phosphorus was low (7 < Pass < 16 ppm). 

Physico-chemical characteristics	Means and standard deviations
Texture Clay (%) Total silt (%) Total sand (%) Textural class (USDA)	11.71 ± 0.69 06.10 ± 1.49 82.19 ± 1.76 LS
Acidity pH H2O pHKCl	5.4 ± 0.10 4.5 ± 0.20
Organic matter Organic carbon (%) Organique matter (%) Total nitrogen (%) C/N ratio	04.63 ± 0.17 07.99 ± 0.08 00.18 ± 0.02 25.36± 1.71
Exchangeable cations Calcium (cmol+/kg) Magnesium (cmol+/kg) Potassium (cmol+/kg) Sodium (cmol+/kg) Sum of exchangeable cations (S)	03.68 ± 0.41 05.76 ± 0.18 00.58 ± 0.00 00.07 ± 0.01 10.09 ± 0.24
Cation exchange capacity CEC with 1N ammonium acetate pH7 (cmol+/kg) ECEC (S + exchangeable acidity in cmol+/kg) Saturation rate (%)	12.84 ± 0.43 11.69 ± 0.07 78.89 ± 3.58
Assimilable phosphorus « P » Bray 2 (ppm)	07.01 ± 0.54
Electrical conductivity (mS/cm)	00.05 ± 0.01
Bulk density (g/cm3)	01.11 ± 0.03
Field capacity (% of dry soil)	63.44 ± 2.23

Note:

Mean ± standard deviation; n=3; USDA: United State department of agriculture; LS: Loamy sand; ECEC: Effective cation exchange capacity.

3.2. Chemical Characteristics of Cocoa Pods Husk and Coffee Wastes

The chemical characteristics of the by-products used in this work are listed in Table 2. The analysis of this table shows that cocoa pods husk has a C/N ratio (23) and a total phosphorus content (0.11%) almost twice as high as those of coffee residues and a similar organic matter content. Also, the potassium contents of these two residual products are high (2.1 and 1.5% respectively) in relation to the contents of calcium, magnesium and sodium.

Chemical characteristics	Cocoa shells	Coffee residues
Ash (%)	06.60	06.60
Organic carbon (%)	46.70	45.45
Organic matter (%)	80.51	78.36
Total N (%)	02.03	03.57
C/N ratio	23.00	12.73
Total phosphorus (%)	00.11	00.05
Ca (%)	00.68	00.88
Mg (%)	00.56	00.32
K (%)	02.11	01.51
Na (%)	00.23	00.16
CEC pH7 (cmol+/kg)	22.24	23.04
Fe (ppm)	2035.90	5861.07
Electrical conductivity (mS/cm)	03.19	04.16

Note:

Nb: Contents are expressed as a percentage of dry matter (DM).

3.3. Effect of Different Treatments on Soil Acidity and Net Soil Charge

Figure 1 and 2 show, respectively, the results of the effect of the different treatments on actual and total acidity as a function of incubation periods. Analysis of these figures after 90 days of incubation reveals that, compared to the initial state (pH water = 5.4), the actual acidity was lower for soils treated only with plant residues (pH water ≥ 5.50) and higher for soils treated with combined inputs of organic fertilizers and increasing doses of urea (pH water ≤ 5.27). For pH KCl, the general trend was increasing (pH KCl ≥ 4.57) with all treated soils and only the control showed a decrease (pH KCl = 4.40) from the initial state (pH KCl = 4.50). Significant (P < 0.05) and highly significant differences (P < 0.01) were observed after 60 and 90 days of incubation respectively between the different treatments for actual acidity, whereas only at 60 days of incubation were significant differences (P < 0.05) observed between the different treatments for potential acidity.

After 90 days of incubation, the TA modality predominated for actual acidity (pH water = 5.53) and the TB modality predominated for potential acidity (pH KCl = 4.67). In this study, the effects were greatest at 30 and 60 days and the pH values were lowest at 90 days. These results corroborate those of Kaho, et al. [16]; Tabitha, et al. [22] and Nyami, et al. [23] which obtained decreases in pH when increasing rates of mineral fertilizers (especially urea) were applied in combination with biochar, quail manure and Tithonia diversifolia leaves. The decrease in pH in soils with increasing doses of urea is thought to be due to the leaching of base cations such as Ca2+ and Mg2+ and their replacement by acidifying NH4+ ions resulting from the decomposition of urea in the clay-humic complex. On the other hand, the increase in pH in soils under the application of crop residues alone would be due to the chelation of toxic cations (Fe and Al) by organic matter which reduces their acidifying power [19, 24].

Note:

* = Significant differences (P < 0.05); ** = highly significant differences (P < 0.01). T0: Control; TA: 5 t.ha-1 of cocoa pods husk without urea ; TB : 5 t.ha-1 of cocoa pods husk + 75 kg.ha-1 of urea ; TC : 5 t.ha-1 of cocoa pods husk + 150 kg.ha-1 of urea ; TD : 5 t.ha-1 of coffee wastes without urea; TE : 5 t.ha-1 of coffee wastes + 75 kg.ha-1 of urea ; TF : 5 t.ha-1 of coffee wastes + 150 kg.ha-1 of urea; a and b: means with the same letters on the same dates are not significantly different

Figure 2. Effect of different treatments on total soil acidity according to incubation periods.

Note:

* = Significant differences (P < 0.05) ; T0: Control; TA: 5 t.ha-1 of cocoa pods husk without urea ; TB : 5 t.ha-1 of cocoa pods husk + 75 kg.ha-1 of urea ; TC : 5 t.ha-1 of cocoa pods husk + 150 kg.ha-1 of urea ; TD : 5 t.ha-1 of coffee wastes without urea; TE : 5 t.ha-1 of coffee wastes + 75 kg.ha-1 of urea ; TF : 5 t.ha-1 of coffee wastes + 150 kg.ha-1 of urea; a and b: means with the same letters on the same dates are not significantly different.

Figure 3 shows the effect of the different treatments on the net soil charge as a function of incubation periods. Throughout the trial, the ∆pH remained all negative, giving a negative net soil charge. After a considerable increase in the negative net soil charge after 30 days, the general trend is towards a decrease in this parameter for all treatments except those based on plant residues alone. It was at 90 days after incubation that highly significant differences were observed between the different treatments. At the end of the incubations, the highest pH difference was obtained with TA (∆pH = -0.97) and the lowest with TF (∆pH = -0.5).

Thus, the net charge of all treatments remained negative throughout the trial with a decrease in negativity for the organic treatments and an increase for the organo-mineral treatments. These results corroborate those of Djeke, et al. [25] and can be explained by a higher release of acidifying cations (NH4+) from crop residues combined with urea compared to plant residues applied alone [19, 26].

Figure 3. Effect of different treatments on the net soil charge according to incubation periods.

Note:

* = Significant differences (P < 0.05) ; T0: Control; TA: 5 t.ha-1 of cocoa pods husk without urea ; TB : 5 t.ha-1 of cocoa pods husk + 75 kg.ha-1 of urea ; TC : 5 t.ha-1 of cocoa pods husk + 150 kg.ha-1 of urea ; TD : 5 t.ha-1 of coffee wastes without urea; TE : 5 t.ha-1 of coffee wastes + 75 kg.ha-1 of urea ; TF : 5 t.ha-1 of coffee wastes + 150 kg.ha-1 of urea; a, b, c and  d: means with the same letters on the same dates are not significantly different.

3.4. Effect of the Different Treatments on the Organic Carbon Content (%) of the Soil

Very highly significant differences (P < 0.001) were observed between the different treatments at all incubation periods (Table 3). After 90 days, the organic carbon content was increased in soils treated exclusively with cocoa pods husk (TA = 3.88%) and coffee residues (TD = 3.84%) compared to the control (T0 = 3.74%). This level was rather lower in soils that received combined inputs of organic and mineral fertilizers. Compared to the initial state, the general trend was a decrease in organic carbon content for all treatments after 90 days of incubation. Thus, the greatest rates of decline were recorded between 30 and 60 days. This period would probably correspond to the time when the rate of organic carbon mineralization was at its optimum. Thereafter, between 60 and 90 days, these rates of decline tend to stabilize, reflecting a decrease in the importance of this process.

The changes in organic carbon content observed in this study are in agreement with the results of Nyami, et al. [23] as well as those of Khan, et al. [27] who reported similar observations with the use of increasing doses of mineral nitrogen fertilizers combined or not with crop residues. According to Khan, et al. [27] exclusive nitrogen, phosphorus and potassium (NPK) fertilization as opposed to organic fertilization depletes soil organic matter stocks in the long term despite considerable crop residue inputs. The latter has also shown that the use of increasing rates of nitrogen in the soil is correlated with increases in the rate of humus mineralization. Increases in organic carbon levels for soils treated only with cocoa pods husk and coffee residues corroborate those of Mboua, et al. [19]; Tabitha, et al. [22] and Kabirinejad, et al. [28]. These results show that the exclusive use of these residues led to an increase in more humified compounds in the soil, which according to Hartemink and O'sullivan [29] are likely to slow down decomposition.

Treatments	Organic carbon (%)
	0 Day	30 Days***	60 Days***	90 Days***
T0	4.24±0.01 (a)	4.15±0.02 (b)	3.95±0.02 (a)	3.74±0.02 (c)
TA	4.26±0.01 (a)	4.23±0.02  (a)	3.93±0.02 (a)	3.88±0.00 (a)
TB	4.24±0.05 (a)	4.14±0.04  (b)	3.24±0.04 (c)	3.11±0.02 (e)
TC	4.22±0.01 (a)	4.04±0.02 (a)	2.64±0.02 (d)	2.53±0.01 (g)
TD	4.24±0.04 (a)	4.12±0.01 (b)	3.94±0.04 (a)	3.84±0.03 (b)
TE	4.25±0.02 (a)	4.16±0.03 (b)	3.25±0.02 (c)	2.95±0.02 (f)
TF	4.29±0.01 (a)	3.73±0.03 (c)	3.35±0.03 (b)	3.15±0.03 (d)

Note:

*** = Very highly significant differences (P < 0.001); T0: control; TA: 5 t.ha-1 of cocoa pods husk without urea ; TB : 5 t.ha-1 of cocoa pods husk + 75 kg.ha-1 of urea ; TC : 5 t.ha-1 of cocoa pods husk + 150 kg.ha-1 of urea ; TD : 5 t.ha-1 of coffee wastes without urea; TE : 5 t.ha-1 of coffee wastes + 75 kg.ha-1 of urea ; TF : 5 t.ha-1 of coffee wastes + 150 kg.ha-1 of urea; a, b, c, d, e, f and g : means with the same letters in brackets on the same dates are not significantly different.

3.5. Effect of the Different Treatments on the Exchangeable Potassium Content of the Soil   

The results presented in Table 4 illustrate the evolution of potassium content in the soil during the trial. They show that there are very highly significant differences (p < 0.001) between the different treatments at 60 and 90 days of incubation.

After 90 days of incubation, the treatments resulting from the combination of cocoa pods husk and urea showed the highest levels of exchangeable potassium (0.75 ≤ K ≤ 0.81 cmol+/kg) while the treatments resulting from the combination of coffee residues and urea gave lower and similar levels of potassium (0.75 ≤ K ≤ 0.76 cmol+/kg). The lowest level at the end of the trial (0.48 cmol+/kg) was obtained with the control. Between the dates for each treatment, there was a small increase between 0 and 30 days, then a large increase between 30 and 60 days, and finally stability. Also, between 30 and 60 days, soils treated with amended crop residues showed greater increases in exchangeable potassium than other soils. The highest rate (50%) was obtained with the TC modality, while the lowest rate (0.02%) was obtained with the control.

These results corroborate those of Luciens, et al. [30] and Kaho, et al. [16] who obtained increases in exchangeable potassium contents of soil respectively with poultry manure and Tithonia diversifolia leaves combined or not with mineral fertilizers. These observations highlight the improving power of organic matter on soil chemical properties and confirm that it is a source of nutrients for crops [31]. The predominance of urea-combined cocoa pod husk treatments over urea-combined coffee wastes treatments is thought to be related to the higher content of this element in these residues [32].    

Treatments	Exchangeable potassium (cmol+/kg of soil)
	0 Day	30 Days	60 Days***	90 Days***
T0	0.42±0.03 (a)	0.45±0.05 (a)	0.46±0.04 (d)	0.48±0.03 (c)
TA	0.44±0.04 (a)	0.45±0.05 (a)	0.64±0.01 (c)	0.75±0.01 (a)
TB	0.43±0.05 (a)	0.53±0.05 (a)	0.74±0.02 (b)	0.81±0.01 (a)
TC	0.43±0.04 (a)	0.54±0.04 (a)	0.81±0.04 (a)	0.81±0.04 (a)
TD	0.46±0.02 (a)	0.44±0.05 (a)	0.63±0.03 (c)	0.75±0.02 (b)
TE	0.42±0.04 (a)	0.53±0.04 (a)	0.71±0.01 (b)	0.75±0.01 (b)
TF	0.43±0.04 (a)	0.52±0.03 (a)	0.75±0.01 (b)	0.76±0.03 (b)

Note:

*** = Very highly significant differences (P < 0.001) ; T0: Control; TA: 5 t.ha-1 of cocoa pods husk without urea ; TB : 5 t.ha-1 of cocoa pods husk + 75 kg.ha-1 of urea ; TC : 5 t.ha-1 of cocoa pods husk + 150 kg.ha-1 of urea ; TD : 5 t.ha-1 of coffee wastes without urea; TE : 5 t.ha-1 of coffee wastes + 75 kg.ha-1 of urea ; TF : 5 t.ha-1 of coffee wastes + 150 kg.ha-1 of urea; a, b, c and  d : means with the same letters in brackets on the same dates are not significantly different.

3.6. Effect of the Different Treatments on the Electrical Conductivity of the Soil

Table 5 presents the evolution of the soil electrical conductivity during the experiment. Very highly significant differences (P < 0.001) were observed between the different treatments throughout the incubations. At the end of the trial, all treated soils showed higher electrical conductivity values than the control (0.08 mS/cm). The highest value was obtained with the TB, TC and TF treatments (0.15 mS/cm) and the lowest value for treated soils was obtained with the TA, TD and TE treatments (0.12 mS/cm). Nevertheless, the values obtained at the end of the incubations were all below the critical value (2 mS/cm) beyond which favorable plant growth is compromised in the study area [20].

These results are in agreement with those of Bryla, et al. [33] as well as those of Antil and Singh [34] who obtained increases in the electrical conductivity of the soil with mineral nitrogen fertilizers combined or not with organic fertilizers. They show that the addition of fertilizers to the soil considerably modifies the extent of solubilization, sorption and mineralization/immobilization processes in the soil. Organic and mineral inputs therefore have a major influence on the quantities of ions that determine the electrical conductivity of the soil [14].

Treatments	Electrical conductivity (mS/cm)
	0 Day***	30 Days***	60 Days***	90 Days***
T0	0.05±0.01 (c)	0.07±0.01 (e)	0.07±0.01 (e)	0.08±0.01 (c)
TA	0.06±0.01 (c)	0.09±0.01 (d)	0.09±0.01 (cd)	0.12±0.01 (b)
TB	0.10±0.01 (ab)	0.12±0.01 (bc)	0.14±0.01 (a)	0.15±0.01 (a)
TC	0.11±0.01 (a)	0.15±0.01 (a)	0.11±0.01 (b)	0.15±0.01 (a)
TD	0.06±0.01 (c)	0.08±0.01 (de)	0.10±0.01 (bc)	0.12±0.02 (b)
TE	0.08±0.01 (b)	0.11±0.01 (c)	0.10±0.01 (bc)	0.12±0.01 (b)
TF	0.11±0.01 (a)	0.13±0.01 (b)	0.08±0.01 (de)	0.15±0.01 (a)

Note:

*** = Very highly significant differences (P < 0.001) ; T0: Control; TA: 5 t.ha-1 of cocoa pods husk without urea ; TB : 5 t.ha-1 of cocoa pods husk + 75 kg.ha-1 of urea ; TC : 5 t.ha-1 of cocoa pods husk + 150 kg.ha-1 of urea ; TD : 5 t.ha-1 of coffee wastes without urea; TE : 5 t.ha-1 of coffee wastes + 75 kg.ha-1 of urea ; TF : 5 t.ha-1 of coffee wastes + 150 kg.ha-1 of urea; a, b, c, d and e : means with the same letters in brackets on the same dates are not significantly different.

3.7. Correlation between the Soil Physico-Chemical Properties Studied

Table 6 presents the correlation coefficients of the relationships between the soil physico-chemical properties studied. These coefficients ranged from -0.53 to 0.53, with most of them being less than 0.5. In half of the cases, these correlations were significant (P < 0.05) or highly significant (P < 0.01). Also, just over half of the correlation coefficients obtained were positive. The highest correlation coefficient values were found between pH H2O and pH KCL (0.53**), between ΔpH and organic carbon (-0.52**) and between ΔpH and organic matter (-0.53**). Exchangeable potassium and organic carbon were negatively and highly significantly correlated (-0.51**). The lowest correlation coefficient (0.11) was obtained between ΔpH and electrical conductivity.

Soil characteristics	Kech	pHwater	pHKCl	ΔpH	CO	Na	EC
Kech	1
pHwater	0.14	1
pHKCl	0.35	0.53**	1
ΔpH	0.23	-0.52**	0.38*	1
CO	-0.51**	0.31	-0.12	-0.53**	1
Na	0.26	-0.46*	0.05	0.52**	-0.41*	1
EC	0.38*	0.17	0.38*	0.11	-0.30	0.20	1

Note:

* = Significant differences (P < 0.05); ** = Highly significant differences (P < 0.01); Kech : Exchangeable potassium, OC : Organic carbon, EC: Electrical conductivity; ΔpH : delta pH.

These results are in agreement with those obtained by Tabi, et al. [35] and Kozak, et al. [36] on the correlations between different soil chemical parameters (pH H2O, pH KCl, CO, MO and K among others) in areas with high nitrogen fertilization. The correlation coefficients between exchangeable potassium and the other soil properties studied were generally less than 0.5 and not significant except with organic carbon, organic matter and electrical conductivity. According to Kozak, et al. [36] potassium is a less important exchangeable cation compared to calcium and magnesium, but soil acidification is the result of the leaching of all exchangeable cations. This therefore justifies the positive correlation between pH and exchangeable potassium. In addition, the increase in soil pH causes the dissociation of H+ ions from the functional groups of organic matter, which creates negative charges that justify the positive correlation between exchangeable potassium, pH and electrical conductivity.

4. CONCLUSION

 The results obtained from this study suggest that urea amendment of cocoa pods husk and coffee wastes may be a solution to increase mineralization of organic matter, potassium availability and electrical conductivity of the soil. On the other hand, the organo-mineral combinations tested decreased alkalinity and the negative net charge of the soil. In addition, the exchangeable potassium content of the soil was positively correlated with pH H2O, pH KCl, ΔpH, sodium and electrical conductivity.  Our results finally indicated that exchangeable potassium content was negatively correlated with organic carbon content of the soil.  

Funding: This study received no specific financial support.

Competing Interests: The authors declare that they have no competing interests.

Authors’ Contributions: All authors contributed equally to the conception and design of the study.

Acknowledgement: The authors appreciate the institutional support from the Soil Analysis and Environmental Chemistry Research Unit, of the University of Dschang, Cameroon.

REFERENCES

[1]         B. P. K. Yerima and E. Van Ranst, Major soil classification systems use in the tropics: Soils of Cameroon. Victoria, Canada: TRAFFORD Publishing, 2005.

[2]         F. Kaho, M. Yemefack, R. Yongue-Fouateu, J. Kanmegne, and P. Bilong, "Potentials of Calliandra calothyrsus Meissner for improving soil fertility and Maize performance in the forest-savannah transition zone of Cameroon," Nigerian Journal of Soil and Environmental Research, vol. 7, no. 1, pp. 33-44, 2007.  https://doi.org/10.4314/njser.v7i1.28416

[3]         M. Yemefack, V. G. Jetten, and D. G. Rossiter, "Developing a minimum data set for characterizing soil dynamics under shifting cultivation systems," Soil & Tillage Research, vol. 86, no. 1, pp. 84-98, 2006.

[4]         L. R. Ahuja, "Quantifying agricultural management effects on soil properties and processes," Geoderma, vol. 116, no. 1-2, pp. 1-2, 2003.  https://doi.org/10.1016/s0016-7061(03)00090-9

[5]         N. Sanginga et al., "Evaluation of symbiotic properties and nitrogen contribution of mucuna to maize grown in the derived savanna of West Africa," Plant and Soil, vol. 179, no. 1, pp. 119-129, 1996.  https://doi.org/10.1007/bf00011649

[6]         S. Adjei-Nsiah, T. W. Kuyper, C. Leeuwis, M. K. Abekoe, and K. E. Giller, "Evaluating sustainable and profitable cropping sequences with cassava and four legume crops: Effects on soil fertility and maize yields in the forest/savannah transitional agro-ecological zone of Ghana," Field Crops Research, vol. 103, no. 2, pp. 87-97, 2007.  https://doi.org/10.1016/j.fcr.2007.05.001

[7]         M. Mucheru-Muna, D. Mugendi, J. Kung’u, J. Mugwe, and A. Bationo, "Effects of organic and mineral fertilizer inputs on maize yield and soil chemical properties in a maize cropping system in Meru South District, Kenya," Agroforestry Systems, vol. 69, no. 3, pp. 189-197, 2007.  https://doi.org/10.1007/s10457-006-9027-4

[8]         A. N'Goran, "Integration of legumes in maize cultivation as a means of maintaining soil fertility and control such as grass cover," Report of the Second Meeting of the WECAMAN Research Committee, USAIS, IITA, 1995.

[9]         UNICE, "Land degradation in the world," Retrieved: http://unt.unice.fr/uoh/degsol/matiere-organique.php . 2008.

[10]       M. Tuomela, M. Vikman, A. Hatakka, and M. Itävaara, "Biodegradation of lignin in a compost environment: A review," Bioresource Technology, vol. 72, no. 2, pp. 169-183, 2000.  https://doi.org/10.1016/s0960-8524(99)00104-2

[11]       H. Schomberg, J. Steiner, and P. Unger, "Decomposition and nitrogen dynamics of crop residues: Residue quality and water effects," Soil Science Society of America Journal, vol. 58, no. 2, pp. 372-381, 1994.  https://doi.org/10.2136/sssaj1994.03615995005800020019x

[12]       Monsanto, "Processes of crop residue breakdown. Agronomic Spotligth," Retrieved: https://www.corn-states.com/app/uploads/2018/07/processes-of-crop-residue-breakdown.pdf . 2015.

[13]       R. K. Sikder, M. Rahman, S. W. Bari, and H. Mehraj, "Effect of organic fertilizers on the performance of seed potato," Tropical Plant Research, vol. 4, no. 1, pp. 104-108, 2017.

[14]       D. L. Do Carmo, L. B. de Lima, and C. A. Silva, "Soil fertility and electrical conductivity affected by organic waste rates and nutrient inputs," Brazilian Journal of Soil Science, vol. 40, p. e0150152, 2016.  https://doi.org/10.1590/18069657rbcs20150152

[15]       K. Bouajila, F. B. Jeddi, H. Taamallah, N. Jedidi, and M. Sanaa, "Chemical and biochemical composition’s Effect of crop residues on their decomposition in semi-arid," Journal of Materials and Environmental Science, vol. 5, no. 1, pp. 159-166, 2014.

[16]       F. Kaho, M. Yemefack, P. Feujio-Teguefouet, and J. Tchantchaouang, "Combined effect of Tithonia diversifolia leaves and inorganic fertilizers on maize yields and properties of ferralitic soil in central Cameroon," Tropicultura, vol. 29, no. 1, pp. 39-45, 2011.

[17]       E. I. Moyin-Jesu, "Use of plant residues for improving soil fertility, pod nutrients, root growth and pod weight of okra (Abelmoschus esculentum L)," Bioresource Technology, vol. 98, no. 11, pp. 2057-2064, 2007.  https://doi.org/10.1016/j.biortech.2006.03.007

[18]       INS, "Statistical yearbook of Cameroon," 2015 ed. Yaoundé, Cameroon: Ministry of Scientic Research and Innovation, 2016, p. 248.

[19]       E. Mboua, D. Bitondo, and F. O. Tabi, "Effect of the incorporation of crushed cocoa shells and robusta coffee residues on some chemical properties and on the availability of potassium in the ferralsols of West Cameroon," Soil Study and Management, vol. 27, no. 1, pp. 209-220, 2020.

[20]       J. Pauwels, E. Van Ranst, M. Verloo, and A. Mvondo Ze, Soil laboratory manual: Methods of soil and plant analysis, equipment, stock management of glassware and chemicals. Bruxelles: Publications Agricoles No. 28, AGCD, 1992.

[21]       F. Beernaert and D. Bitondo, Simple and practical methods to evaluate analytical data of soil profiles. Dschang, Cameroon: Department of soil Soil Science, University Center of Dschang, 1992.

[22]       K. Tabitha, T. Wilson, and P. Joseph, "Effect of organic and inorganic fertilizer on the soil chemical properties of the rhizosphere in Kiambu county, Kenya," Asian Research Journal of Agriculture, vol. 7, no. 4, pp. 1-13, 2017.  https://doi.org/10.9734/arja/2017/37081

[23]       B. L. Nyami, C. K. Sudi, and J. Lejoly, "Effect of biochar and leaves of Tithonia diversifolia combined with mineral fertilizer on the cultivation of maize (Zea mays L.) and the properties of a ferralitic soil in Kinshasa (DRC)," Biotechnology, Agronomy and Society Approximate, vol. 20, no. 1, pp. 57-67, 2016.

[24]       P. L. Woomer, A. Martin, A. Albrechdt, D. V. S. Resck, and H. W. Scharpenseel, "The importance and management of soil organic matter in tropics. In: Woomer P.L. and Swift M.J., Eds., Biological Management of Tropical Soil Fertility." Chichester, UK: John Wiley and Sons, 1994, pp. 47-75.

[25]       M. D. Djeke, P. K. T. Angui, and J. Y. Kouadio, "Decomposition of shredded cocoa shells in ferralitic soils in the Oumé area, Center-West of Côte d'Ivoire: Effects on the chemical characteristics of the soils," Biotechnology, Agronomy and Society and Environment, vol. 15, no. 1, pp. 109-117, 2011.

[26]       G. Ritchie and P. Dolling, "The role of organic matter in soil acidification," Soil Research, vol. 23, no. 4, pp. 569-576, 1985.  https://doi.org/10.1071/sr9850569

[27]       S. Khan, R. Mulvaney, T. Ellsworth, and C. Boast, "The myth of nitrogen fertilization for soil carbon sequestration," Journal of Environmental Quality, vol. 36, no. 6, pp. 1821-1832, 2007.

[28]       S. Kabirinejad, M. Kalbasi, A. H. KhoshgoftarmanesH, M. Hoodaji, and M. Afyuni, "Effect of incorporation of crops residue into soil on some chemical properties of soil and bioavailability of copper in soil," International Journal of Advanced Biological and Biomedical Research, vol. 2, no. 11, pp. 2819-2824, 2014.

[29]       A. E. Hartemink and J. O'sullivan, "Leaf litter decomposition of Piper aduncum, Gliricidia sepium and Imperata cylindrica in the humid lowlands of Papua New Guinea," Plant and Soil, vol. 230, no. 1, pp. 115-124, 2001.  https://doi.org/10.1023/A:1004868502539

[30]       M. Luciens et al., "Improvement of the physical and chemical properties of the soil under the combined contribution of bio-waste and mineral fertilizers and influence on the behavior of maize (Zea mays L. variety Unilu," Journal of Applied Biosciences, vol. 74, pp. 6121-6130, 2014.  https://doi.org/10.4314/jab.v74i1.7

[31]       C. Mulaji, "Use of household bio-waste composts to improve the fertility of acid soils in the Province of Kinshasa (Democratic Republic of Congo)," Doctoral Thesis, Gembloux Agro-Bio Tech, University of Liège, Belgium, 2011.

[32]       Y. U. Sikuzani et al., "Improving the quality of acid soils in Lubumbashi (Katanga, DR Congo) by applying different levels of chicken manure compost," Journal of Applied Biosciences, vol. 77, pp. 6523-6533, 2014.  https://doi.org/10.4314/jab.v77i0.3

[33]       D. R. Bryla, A. D. Shireman, and R. M. A. Machado, "Effects of method and level of nitrogen fertilizer application on soil pH, electrical conductivity, and availability of ammonium and nitrate in blueberry," in VI International Symposium on Mineral Nutrition of Fruit Crops 868, 2008, pp. 95-102.

[34]       R. S. Antil and M. Singh, "Effects of organic manures and fertilizers on organic matter and nutrients status of the soil," Archives of Agronomy and Soil Science, vol. 53, no. 5, pp. 519-528, 2007.

[35]       F. Tabi, M. Omoko, A. Boukong, A. Mvondo Ze, D. Bitondo, and C. Fuh-Che, "Evaluation of lowland rice (Oryza sativa) production system and management recommendations for Logone and Chari flood plain–Republic of Cameroon," Agricultural Science Research Journals, vol. 2, no. 5, pp. 261-273, 2012.

[36]       M. Kozak, M. Stêpieñ, and A. Joarder, "Relationships between available and exchangeable potassium content and other soil properties," Polish Journal of Soil Science, vol. 38, no. 2, pp. 179-186, 2005.

Views and opinions expressed in this article are the views and opinions of the author(s), Current Research in Agricultural Sciences shall not be responsible or answerable for any loss, damage or liability etc. caused in relation to/arising out of the use of the content.