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Effect of Cassava Legume Intercropping Systems on the Physicochemical  Properties of the Soil in Three Agro-Ecological Zones of Sierra Leone 

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*Corresponding author:  Augustine Mansaray 
E-mail: Tel +23278365421
Agro-ecological zone 
Available phosphorus 
Organic carbon 
Soil chemical properties 
Total nitrogen
The study aims at evaluating the effects of cassava-legume intercropping  systems on major soil nutrients across three agro-ecological zones of the  country. The experiment was arranged in a randomized complete block  design (RCBD) with three replications. The treatments consisted of four  cropping systems (sole cassava, cassava + cowpea, cassava + groundnut, and  cassava + soybean).The study shows a general decrease in soil pH by 1.48- 9.91% and 4.24-11.375% among the agro-ecological zones and cropping  systems respectively. Organic carbon increased by 28.8% in the savannah  woodland zone in Makeni but decreased by 9.69% and 40.37% in the  rainforest zone in Segbwema and the transitional rainforest zone in  Sumbuya respectively. It also decreased by 26.27%, 12.08%, and 0.92% for  the sole cassava, cassava-cowpea, and cassava-groundnut systems  respectively. It was however observed to increase by 10.97% for the  cassava-soybean system in the rainforest zone in Segbwema. The total  nitrogen, on the other hand, increased slightly by 1.11-1.73% across the  agro-ecological zones and 2.62-10.84% for the cropping systems. Total  nitrogen for the sole cassava was however observed to decrease by 14.31%.  Available phosphorus decreased by 47.35-59.02% and 36.23-72.89% for the  agro-ecological zones and the cropping systems respectively. In addition,  exchangeable potassium also decreased by 33.33-38.42% and 25.26-49.985  % for the agro-ecological zones and the cropping systems respectively. In  addition, the result shows strong, positive, and significant correlations  between pH with organic carbon, pH with total nitrogen, organic carbon,  and total nitrogen in the three agro-ecological zones. 


The sole production of cassava will impoverish the soil  rapidly unless the absorbed or lost nutrients are  replenished (Eke-Okoro et al., 1999). For example, an  average of 660 kg/ ha, 75 kg/ ha, and 450 kg/ ha of  Nitrogen, Phosphorus, and Potassium respectively, have  been lost from approximately 200 million hectares of  cultivated land in thirty-seven African countries (Smaling  et al., 1997) in the last thirty years. As a result of this, the  necessity to improve soil fertility through the inclusion of  

a very important issue in the development policy agenda  of most African governments due to the strong linkage  between soil fertility and food security on the one hand,  and the implications on the livelihood of the population  on the other (Mugwe et al., 2011). As a result, crop  scientists have recommended the inclusion of  leguminous crops into crop production systems as a way  to address the problem of declining soil fertility.

Several  studies have reported the advantages of cassava-legume  mixtures, especially in improving the nitrogen content of the soil through the fixation of atmosp heric nitrogen  (Aigh, 2007). Kurtz (2006) reported significantly higher  values of yield and yield components of cassava  intercropped with grain legumes than those of the yield  components of sole-cropped cassava.

However, although  legumes are known to fix nitrogen in the soil, studies have  shown that the amount of nitrogen fixed depends on the  species of the legume. For example, Peoples et al. (2009)  reported that (73 – 354), (168 –208), (72 – 124), (55 – 168), and (40 – 65) kg N ha-1 was fixed into the soil by  cowpea, pigeon pea, groundnut, and soybean  respectively. 

Of the sixteen essential plant nutrients needed for plant  growth, development, and reproduction, nitrogen is the  most important and the most easily limited or deficient  throughout the world, particularly in the tropics (Agbede,  2009).

The reason for the inadequate supply of nitrogen  is the fact that nitrogen exists in organic form in the soil,  which must be mineralized before it is used by plants  (Azam, 2002). As such, legumes can convert free  atmospheric nitrogen (N2) into ammonia (NH3) through  the process called biological nitrogen fixation (BNF) with the help of specific bacteria (Rhizobium) which reside in  the nodules of legumes. The plants will now thereafter  transform it into a usable form of plant nitrogen such as  amino acids and proteins. 

Despite the potential for cassava-legume intercropping  technology in addressing the soil nutrient depletion  problems of smallholder farmers in some parts of Africa,  this knowledge is lacking among the smallholder farmers  in Sierra Leone. To this end, this study was mainly aimed  at evaluating the effect of intercropping grain legumes  with cassava on the major soil nutrients.

It is  hypothesized that there is a net decrease in the  concentration of soil nutrients after the cultivation of sole  cassava than when intercropped with legumes such as  cowpea, soybean, and groundnut. 


Study area and soil 

The study was conducted between 2015-2017 cropping  seasons under rain-fed conditions in three agro ecological zones namely Sumbuya (N 08.040880 , W  011.4789550 ) in Bo district representing the transitional  rainforest, Makeni (N 08.87200 , W 012.03760) in  Bombali district representing the savannah woodland  and Segbwema (N 07.99300, W 010.952240) in Kailahun  district representing the rainforest region of the country  (Figure 1). 

Land preparation 

The land at the three zones was slashed with a cutlass,  burnt, destumped, and dug using a hoe and the plots  were laid out using a measuring tape, a garden line, and,  pegs. 

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Figure 1 Map of Sierra Leone showing trial sites of the  different agro-ecological zones in Sierra Leone 

Experimental design, treatments and planting 

The experiment was a factorial randomized complete  block design with three replications. The treatments  consisted of four cropping associations (sole cassava,  cassava + cowpea, cassava + groundnut, and cassava +  soybean) (Mansaray et al. 2022a). The plot size was 7m x  6 m. 

The cassava and the three legumes were planted on a flat  land in June of each year. Stem cuttings of about 25 cm  long with five nodes were used. Cassava was planted at  the spacing of 1 m x 1 m respectively; whilst cowpea and  groundnut were planted at the spacing of 50 cm x 20 cm  with two seeds per hole for cowpea and one seed per  hole for groundnut. On the other hand, soybean was  planted at the spacing of 50 cm x 10 cm with two seeds  per hole. The legumes were introduced in between the  rows of the cassava (Mansaray et al., 2022b). 

The cassava variety used was slicass 6 whilst, the cowpea,  soybean, and groundnut varieties used were IITA 573k-1- 1, Slibean 2, and Slinut 1 respectively. Weeding was done  with a hoe at one, three, and six months after planting  (Mansaray et al., 2021). Cassava was harvested at 12  months after planting whilst the three legumes were  harvested at their respective maturity dates. The haulms  of the harvested legumes were returned to the cassava  system (Mansaray et al. 2021). 

Soil sampling and laboratory analysis 

Prior to planting, initial soil samples were collected at  each of the three agro-ecological zones. In each zone, ten  core soil samples from the topsoil (0- 20 cm) depth were  collected in a “W” shaped design and mixed to form a  composite sample. Additional soil samples were also  collected per treatment at the harvesting stage of the  cassava. The samples were air-dried, crushed, and sieve  through a 2 mm sieve before analysis for chemical and  physical properties using standard laboratory  procedures. The soil properties determined were soil pH,  total nitrogen, soil organic carbon, available phosphorus,  exchangeable potassium, and soil particle sizes. 

The soil pH in water (1:1 soil: water ratio) was determined  using the pH meter; total nitrogen was determined using  the procedure described by Bremner and Mulvaney  (1982). The soil organic carbon was determined using the  modified Walkley and Black’s wet oxidation method as  outlined by Nelson and Sommers (1982) whilst the  available phosphorus was determined using the  procedure described by Bray and Kurtz (1945). The  exchangeable potassium was determined using the flame  photometer. The soil particle sizes were determined  using the hydrometer method described by Jones (2001). 

Data analysis  

Pearson correlation was performed among the soil  chemical properties in order to establish relationship  among them. 


Initial soil properties 

The soil in Makeni representing the savannah woodland  was sandy clay loam in texture with a pH of 4.50. Organic  carbon and total nitrogen were 67.60 t/ha and 8.06 t/ha  respectively. Available phosphorus and exchangeable  potassium were 6.60 mg/kg and 0.045 Cmol/kg  respectively (Table 1). In Sumbuya representing the  transitional rainforest, the soil was sandy loam in texture  with a pH of 5.20; whilst organic and total nitrogen  content was 114.40 t/ha and 10.67 t/ha respectively.

The  available phosphorus and exchangeable potassium were  24.50 mg/kg and 0.047 Cmol/kg respectively (Table 1).  For Segbwema representing the rainforest, the soil was  also sandy loam with a pH of 5.30. The organic carbon and  total nitrogen content were 132.60 t/ha and 11.70 t/ha  respectively. The available phosphorus and exchangeable potassium were 36.30 mg/kg and 0.054 Cmol/kg  respectively (Table 1). 

Table 1 The physicochemical composition of the soil in  the three agro-ecological zones before establishing the  trial. 

TreatmentAgro-ecological zones
Savannah  woodland  (Makeni)Transitional  rainforest  (Sumbuya)Rainforest  (Segbwema)
pH (Water) 4.50 5.20 5.30
Organic carbon (  t/ha)67.60 114.40 132.60
Total nitrogen  (t/ha)8.06 10.67 11.70
Available  phosphorus  (mg/kg)6.60 24.5 36.30
Exchangeable  potassium  (Cmol/kg)0.045 0.047 0.054
Electrical  conductivity115.00 266.00 66.00.00
Soil texture Sandy  clay loamSandy loam Sandy loam
Sand (%) 71.40 77.36 65.40
Silt (%) 10.00 4.00 12.0
Clay (%) 18.60 18.64 22.6

Changes in soil nutrient status at harvest of the cassava  at the three agro-ecological zones Soil pH 

The initial pH values for the agro-ecological zones in  Makeni, Sumbuya, and Segbwema were 4.50, 5.20, and  5.30 respectively (Table 2). The pH at harvest of the  cassava ranged from 4.30-6.60, 4.35-4.70, and 4.60-4.90  under all the cropping systems for the agro-ecological  zones in Makeni, Sumbuya, and Segbwema respectively.  At harvest of the cassava, the pH values were decreased  under all the cropping systems in the three agro ecological zones except for the cassava-soybean cropping  system in the savannah woodland in Makeni, which  recorded a slight increase in pH of 4.44% (Table 2).

The highest percentage decrease in pH was recorded in the  transitional rainforest zone in Sumbuya (12.77%)  followed by the rainforest zone in Segbwema and the  savannah woodland zone in Makeni (9.91%). 

Concerning the cropping systems, the highest percentage  decrease in pH was recorded for the sole cassava  (11.37%) followed by cassava-cowpea (9.00%), cassava groundnut (7.09%), and cassava-soybean system (4.24%). In general, the soil pH under all the cropping systems and agro-ecological zones was strongly acidic. 

Table 2 Effect of cropping systems on soil pH in the three agro-ecological zones over two cropping seasons 

Soil pH (1:1 H20)
Agro-ecological zone
Savannah woodland (Makeni) Transitional rainforest (Sumbuya) Rainforest (Segbwema) Mean  (%) 
Initi alFinal Cha nge%  ChangeInitial Final Chan ge%  ChangeInitial Final Chan ge%  Change
Cropping system
Sole  Cassava4.50 4.30 – 0.20-4.44 5.20 4.35 -0.85 -16.46 5.30 4.60 -0.70 -13.21 -11.37
Cassava cowpea4.50 4.40 – 0.10-2.22 5.20 4.50 -0.70 -13.46 5.30 4.70 -0.60 -11.32 -9.00
Cassava groundn ut4.50 4.40 – 0.10-2.22 5.20 4.60 -0.60 -11.53 5.30 4.90 -0.40 -7.54 -7.09
Cassava soybean4.50 4.60 0.20 4.44 5.20 4.70 -0.50 -9.61 5.30 4.90 -0.40 -7.54 -4.24
Mean -1.48 -12.77 -9.91

Soil organic Carbon 

Soil organic carbon increased at the harvest of the  cassava across all the cropping systems in the savannah  woodland zone in Makeni by 28.84% (Table 3). However,  it was observed to decrease harvest of the cassava by  40.37% and 9.69% in the transitional rainforest zone in  Sumbuya and the rainforest zone in Segbwema  

respectively. For the cropping systems, the percentage  decrease in soil organic carbon at harvest of cassava was  higher for the sole cassava (26.27%) followed by the  cassava-cowpea (12.08%), and the cassava-groundnut  system (0.92%) (Table 3). On the other hand, soil organic  carbon was on average higher by 10.92% at harvest of the  cassava for the cassava-soybean cropping system. 

Table 3. Effect of cropping system on soil organic carbon in three agro-ecological zones over two cropping seasons 

Soil organic carbon (t/ha)

Agro-ecological zone 

Savannah woodland (Makeni) Transitional rainforest (Sumbuya) Rainforest (Segbwema) Mean (%)
Initi alFinal *Chan ge%  ChangeInitial Final *Chang e%  ChangeInitial Final *Chang e%  Change
Cropping system
Sole  Cass ava67.6 070.2 02.60 3.84 114.40 59.80 -54.60 -47.77 132.60 85.80 -46.20 -34.84 -26.27
Cass ava cow pea67.6 088.4 020.80 30.76 114.40 62.40 -52.00 -45.45 132.60 104.00 -28.60 -21.56 -12.08
Cass ava gro und nut67.6 091.0 023.40 34.61 114.40 65.00 -49.40 -43.26 132.60 140.40 7.80 5.88 -0.92
Cass ava soy bea n67.6 098.8 031.20 46.15 114.40 85.80 -28.60 -25.00 132.60 148.20 15.60 11.76 10.97
Me an28.84 40.37 9.69

woodland zone in Makeni (1.11%) (Table 4). In the case of  

The total soil nitrogen content at harvest of the cassava  was observed to increase in the three agro-ecological  zones and the cropping systems except for the sole  cassava system (Table 4). The percentage increase in the  total soil nitrogen content was higher in the rainforest  zone in Segbwema (1.73%) followed by the transitional  rainforest zone in Sumbuya (1.24%) and the savannah  the cropping system, the highest increase in the total  nitrogen was recorded for the cassava-soybean system  (10.84%) followed by the cassava-groundnut system  (6.27%), cassava-cowpea system (2.62%), and the sole  cassava (14.31%) across the three agro-ecological zones  (Table 4). 

Table 4. Effect of cropping system on the total soil nitrogen in three agro-ecological zones over two cropping  seasons. 

Soil total nitrogen (t/ha)
Agro-ecological zone 
Savannah woodland (Makeni) Transitional rainforest (Sumbuya) Rainforest  (Segbwema)Mean (%)
Initi alFina l*Chang e%  ChangeInitial Final *Chang e%  ChangeInitial Final *Chang e%  Change
Cropping system
Sole  Cassava8.06 6.1 0-1.96 -24.32 10.66 9.36 -1.30 -12.19 11.70 10.9 5-0.75 -6.41 – 14.31
Cassava cowpea8.06 8.3 20.26 3.22 10.66 10.9 20.26 2.43 11.70 11.9 60.26 2.22 2.62
Cassava groundnu t8.06 8.8 40.76 9.42 10.66 11.1 80.53 4.97 11.70 12.2 20.52 4.44 6.27
Cassava soybean8.06 9.3 61.30 16.12 10.66 11.7 01.04 9.75 11.70 12.4 80.78 6.66 10.84
Mean 1.11 1.24 1.73

Soil available Phosphorus 

There was a general reduction in soil available  phosphorus concerning the cropping systems in the three  agro-ecological zones (Table 5). The percentage  reduction across the cropping systems ranged from  16.67-71.71%, 44.90-70.61%, and 47.77-76.86% for the  agro-ecological zones in Makeni, Sumbuya, and  Segbwema respectively. For the agro-ecological zones,  Sumbuya in the transitional rainforest (58.67%) recorded  

the highest reduction followed by Segbwema in the rain  forest (52.75%), and Makeni in the savannah woodland  (47.37%). Pertaining to the cropping systems, the highest  reduction in available phosphorus was recorded for the  cassava-soybean system (72.89%) followed by the  cassava-groundnut system (56.54%), cassava-cowpea  (54.39%), and the sole cassava (36.23%) (Table 5). 

Table 5 Effect of cropping system on the soil available phosphorus in three agro-ecological zonesover two cropping  seasons 

image 52
Effect of Cassava Legume Intercropping Systems on the Physicochemical  Properties of the Soil in Three Agro-Ecological Zones of Sierra Leone  6

Soil exchangeable Potassium 

Similarly, there was a general decrease in exchangeable  potassium across the cropping systems and agro ecological zones (Table 6). The percentage reduction was  higher in the rainforest zone in Segbwema (38.42%)  followed by the transitional rainforest zone in Sumbuya  (37.77%) and the savannah woodland zone in Makeni  (33.33%).

The percentage reduction in exchangeable potassium ranged from 22.22-48.88%, 27.66-51.06%, and 25.00-50.00% for the agro-ecological zones in Makeni,  Sumbuya, and Segbwema respectively. In the case of the  cropping systems, the highest mean percentage  reduction was recorded for the cassava-soybean system  (49.98%) followed by the cassava-groundnut system  (39.64%), the cassava-cowpea system (31.13%), and the  sole cassava (25.26%) (Table 6). 

Table 6 Effect of cropping system on the soil exchangeable potassium in three agro-ecological zones over two  cropping seasons 

Exchangeable potassium (Cmol/kg)
Agro-ecological zone 
Savannah woodland (Makeni) Transitional rainforest (Sumbuya) Rainforest (Segbwema) Mean  (%)
Initial Final *Chan ge%  ChangeInitial Final *Chan ge%  ChangeInitial Final *Chan ge%  Change
Cropping system
Sole  Cassava0.045 0.035 -0.010 -22.22 0.047 0.034 -0.013 -27.66 0.054 0.040 -0.014 -25.92 -25.26
Cassava cowpea0.045 0.034 -0.011 -24.44 0.047 0.032 -0.015 -31.91 0.054 0.034 -0.020 -37.03 -31.13
Cassava groundnut0.045 0.028 -0.017 -37.77 0.047 0.028 -0.019 -40.42 0.054 0.032 -0.022 -40.74 -39.64
Cassava soybean0.045 0.023 -0.022 -48.88 0.047 0.023 -0.024 -51.06 0.054 0.027 -0.027 -50.00 -49.98
Mean -33.33 -37.77 -38.42

Correlation among soil chemical properties across the  agro-ecological zones 

From the results, a strong, positive, and significant correlation was recorded between pH with soil organic  carbon in the savannah woodland zone in Makeni (P =  0.0017, r = 0.8023), the transitional rainforest zone in  Sumbuya (P = 0.0019, r = 0.8621), and the rainforest zone  in Segbwema (P = 0.0002, r = 0.8821) (Table 7).

For soil total nitrogen, a strong, positive, and significant  correlation was also recorded between pH with soil total  nitrogen for the agro-ecological zones in Makeni (P = 0.0014, r = 0.8246), Sumbuya (P = 0.0016, r = 0.8446), and  Segbwema (P = 0.0002, r = 0.8712) (Table 7).  In the case of the correlation between soil organic carbon  with total nitrogen, a very strong, positive, and significant  correlation was recorded for both agro-ecological zones  in Segbwema (P = 0.0069, r = 0.9930) and Sumbuya (P =  0.0443, r = 0.9257) whilst, a moderately strong, positive,  and significant correlation was recorded in the savannah  woodland zone in Makeni (P = 0.0045, r = 0.7245) (Table  7). 

Table 7 Correlation matrix among soil chemical properties in three agro-ecological zones 

Pearson Correlation Coefficients Prob> |r| under H0: Rho=0
Agro-ecological zone 
Soil Savannah woodland Transitional rainforest Rainforest
chemical  propertiespH SOC TN AP EP pH SOC TN AP EP pH SOC TN AP EP
pH 1.00 0.80 0.00 10.82 0.00 1-0.86 0.13-0.72 0.276 51.00 0.88 0.00020.87 0.000 2-0.32 0.670.23 0.761.00 0.86 0.00 20.84 0.00 20.2 1 0.7 8-0.04 0.95
SOC 0.80 0.00 21.00 0.72 0.00 5-0.96 0.03-0.59 0.400.88 0.00 021.00 0.99 0.00 7– 0.9 0 0.0 9-0.89 0.1010.86 0.00 21.00 0.92 0.04– 0.8 5 0.1 4-0.93 0.27

TN 0.82 0.00 20.72 0.00 51.00 -0.96 0.03-0.57 0.420.87 0.00 020.99 0.00 71.00 – 0.8 0 0.1 9-0.98 0.010.84 0.00 20.92 0.041.00 – 0.7 2 0.2 7-0.85 0.14
AP-0.86 0.13-0.96 0.03-0.96 0.031.00 0.76 0.23-0.32 0.67– 0.90 0.09-0.80 0.191.0 00.84 0.150.21 0.78-0.85 0.14-0.72 0.271.0 00.72 0.27
EP-0.72 0.27-0.59 0.40-0.57 0.420.76 0.231.00 0.23 0.76– 0.89 0.10-0.98 0.01 30.8 4 0.1 51.00 -0.04 0.95-0.93 0.06-0.85 0.140.7 2 0.2 71.00


Soil nutrients are essential for plant growth and  development. The results from the study show that  cassava-legume intercropping systems can affect the soil  in terms of pH, organic carbon, total nitrogen, available  phosphorus, and exchangeable potassium. 

The soil pH at harvest in the three agro-ecological zones  and cropping systems was very strongly acidic. There was  a decrease in pH of the soil concerning cropping systems  and agro-ecological zones, which could be attributed  probably to the removal of large quantity of nutrients  from the soil especially, bases by the component crops.  This result concord with the findings of Minhas et al.  (1995) who reported a reduction in the mean pH from  6.7-5.7 when cassava was intercropped with soybean. 

The reduction in pH was higher for the agro-ecological  zones in Sumbuya and Segbwema compared to Makeni  probably because higher yields of the component crops  were recorded in both zones compared to Makeni.

  Furthermore, the sole cassava recorded a higher  reduction in pH compared to the intercropping systems  probably because this system extracted more soil  nutrients in the form of bases from the soil than it added  to it. On the contrary, the slight increase in pH recorded  in the savannah woodland in Makeni for the cassava soybean cropping system suggests that intercropping  could improve soil pH.

The reason for the observed  increase according to Cong and Merckx (2005) could be  probably due to the transformation of nitrogen and the  release of metal ions resulting from the decomposition of  organic residues. Furthermore, Matusso et al. (2012) and  Owusu and Sadick (2016) argued that, the increase in soil  pH value in intercropping systems shows that  intercropping could decrease soil acidity as a result of  higher organic matter production. 

This observation concord with the findings of Esekhade and Idoko (2010) who reported higher soil pH in the  legume-cereal intercropping system compared to their  counterpart under the mono-cropping system. In  addition, Schoenerberger et al. (2002) reported changes  in soil pH from strongly acidic to slightly acidic in the  maize-legume intercropping system. 

The result further reveals a strong, positive, and  significant correlation between pH with soil organic  carbon, and between pH with total nitrogen; indicating  that the higher the pH the greater the availability of these  nutrients to the plant.

This further shows that the  availability of these two soil nutrients were strongly  affected by soil pH; as it determines the variation of soil  microorganism community structure and diversity  (Tripathi et al., 2018), which controls the process of  decomposition and mineralization of soil organic matter  and the subsequent released of nutrients to plants.  Furthermore, Rousk et al. (2010) showed that the relative  abundance and diversity of bacteria were positively  related to pH.

This effect impacts the mineralization  process leading to higher nitrogen content in soils with  higher pH. This result concord with the findings of Xu et  al. (2019) who reported positive correlations between  soil organic carbon and pH in central-eastern Europe. On  the contrary, Reisser et al. (2016) reported a general  negative correlation between organic carbon and  nitrogen with pH under natural conditions at various  sites.

This suggests that a relatively low pH favours the  accumulation of organic matter (Zhou et al., 2019). This  negative correlation shows that high pH values tend to  have lower soil organic carbon content and total nitrogen  whilst low pH tends to have higher soil organic carbon  and total nitrogen content.

The reason adduced by these  authors for the negative correlation is that soil organic  matter upon decomposition releases organic acids which  lead to low pH value. Soil pH is a major driver controlling nutrient availability for plants and thus, influences  biomass production indirectly (Bolan et al., 2003; Wang  et al., 2012). 

Soil organic carbon is one of the key attributes in  assessing soil health; it is generally positively correlated  with crop yield (Bennett et al., 2010). Murphy (2015)  opined that, important functional processes in soil such  as the ability to store nutrients especially, nitrogen, water  holding capacity, and aggregate stability are strongly  influenced by the organic carbon content in the soil.

It is also important for increased agricultural production  because organic matter helps improve soil structure and  cation exchange capacity and hold water; thus, it has a  positive impact on soil fertility (West and Post, 2002).  

From the result, soil organic matter was observed to  increase across cropping systems in the savannah  woodland in Makeni probably due to the decomposition  of a lot of biomass returned from cassava and component  crops during the growing season as reported by Ojeniyi  and Adetoro (1993) who noted an appreciable increase in  soil organic carbon following the decomposition of leaves  of Gliricidia sepium.

The authors adduced the increase in  soil organic carbon after cropping to the high rate of  mineralization informed by the fast rate of  decomposition of legume leaves due to their low C: N  ratio. Moreover, the increase in soil organic carbon at  harvest could also be because the cultivation of cassava  may have minimized erosion and microbial  decomposition rate considerably while maintaining  favorable soil moisture conditions. According to King et  al. (2019), regardless of the decomposition rates, where  organic inputs outweigh organic matter losses, soil  organic carbon should increase even though slowly. 

This result conforms to the findings of Matusso et al.  (2012) who reported higher soil organic matter with  intercropping. In addition, Ispandi (2002) reported an  increase in organic carbon of 12% and 56% when cassava  was intercropped with maize and groundnut respectively.  Similarly, Nath et al. (2003), Aulakh et al. (2004), and  Swain (2016) have also reported an increase in the  organic carbon content of orchard soil due to  intercropping practices in fruit orchards.  

On the contrary, there was a depletion of soil organic  carbon for the agro-ecological zones in Sumbuya and  Segbwema probably because of higher nutrient uptake by  component crops than the quantity of nutrients supplied  through the legumes (Jones, 2016). Another possible  

reason could be due to higher yields of component crops  reported for these agro-ecological zones. This result  concord with the findings of Yan et al. (2006) who  reported the possibility of rapid nutrient depletion under  intercropping systems due to the combined demands of  the individual intercrops for nutrients. 

Furthermore, soil organic carbon was observed to  decrease among the cropping systems except for the  cassava-soybean system which recorded an increase in  soil organic carbon. The rate of soil organic carbon  depletion was higher for the sole cassava probably  because cassava being a wide-spaced crop, most of the  soil was left vacant under the sole cropping system  resulting in a higher loss of soil organic matter by  oxidation and less addition of soil biomass.

The higher soil  organic carbon recorded concerning the cassava-soybean  system could be because a higher quantity of nutrients  was supplied to the system through the legumes than the  amount of nutrient that was taken up by the component  crops. This result agrees with the findings of Akinnifesi et  al. (2007) and Sebetha (2015) who reported an increase  in soil organic carbon under the cereal-legume  intercropping system.  

The result further reveals a strong, positive, and  significant correlation between soil organic carbon and  total nitrogen across the three agro-ecological zones.  This shows that an increase in soil organic carbon will be  followed by an increase in total nitrogen as reported by  Brevik et al. (2018). This result concord with the findings  of Sadovnikova et al. (1996) who reported a strong,  positive correlation between soil organic carbon with  total nitrogen. 

Pertaining to the total nitrogen, there was a general  increase in the total nitrogen content across cropping  systems in the three agro-ecological zones except for the  sole cassava, which recorded a decrease in total nitrogen  content at harvest of the cassava. The increase in total  nitrogen content across the agro-ecological zones was  generally slight, probably due to the excessive removal of  nitrogen by cassava for root formation in all the zones. 

The general decrease in the total nitrogen for the sole  cassava system could be because cassava removes a large  amount of nitrogen from the soil for root yield formation  as reported by Obigbesan (1977) and CIAT (1982). 

Furthermore, it has been reported by Padwick (1983) that  African soils show nutrient-deficiency problems after a  short period of cultivation, with nitrogen being the most rapidly depleted nutrient. Other possible reasons for the  observed depletion of nitrogen under the sole cassava  system could be because cassava is normally planted in a  wide spacing at the start of the rainy season when the soil  is exposed and has not been covered by a canopy and thus, susceptible to erosion. 

On the other hand, the observed increase in total  nitrogen content across the intercropping systems could  be because legumes have the ability to trap and fix  nitrogen into the soil through their root nodules as  reported by Crews and Peoples (2004). Furthermore, the  large amount of biomass produced by both the cassava  and the legumes after mineralization could have released  a large amount of nitrogen into the soil.

This result  corroborates the findings of Nweke (2016) who reported  significant levels of nitrogen in plots containing maize  that was intercropped with groundnut. Similar results of  an increase in the available nitrogen content of the soil  through intercropping in mango orchards have been  reported by Swain (2016). 

Furthermore, Nnadi and Haque (2017) have shown that  legumes might contribute about 30% N from the nitrogen  fixation process to other crops in intercropping and crop  rotation. Bundy and Andraski (2005) also reported that  the residues of corn returned to the field can contribute  50-100 kg N/ha where 5-20% of nitrogen residue can still  be used by the next crop (Bundy and Andraski, 2005). 

Concerning available phosphorus, there was a general  reduction across cropping systems in the three agro  ecological zones. The reason for the general decline in  available phosphorus could be related probably to the  fact that the component crops may have taken up a large  amount of phosphorus from the soil. The above  observation agrees with the findings by Onwueme and  Sinha (1991).

These authors reported that root crops take  up more Phosphorus and Potassium from the soil than  any other crop species. The reduction in available  phosphorus was higher for the agro-ecological zones in  Segbwema and Sumbuya probably due to the higher  yields of component crops reported in the two agro ecological zones. Furthermore, the reported decrease in  available phosphorus of the intercrops compared to the  sole cassava could be attributed to the excessive demand  and use of phosphorus by legumes for nitrogen fixation  and other physiological processes.

The result conforms to  the findings of Nweke and Emeh (2013) who reported  that legumes required an abundant amount of phosphorus in the soil for nitrogen fixation and growth.  This result is in contrast to the findings of Carel (2006)  who reported an increase in soil-available phosphorus  under intercropping involving legumes and adduced this  to the mineralization of organic phosphorus, which in  turn, results in the release of more phosphorus for crop  use. 

Similarly, a general reduction in exchangeable potassium  was recorded across cropping systems in the three agro ecological zones probably because there is always a high  demand for nutrients by component crops in  intercropping systems (Yahaya et al., 2014). Similar  observation was also reported by Kurt (1984).

The  depletion of exchangeable potassium was more severe in  the rainforest zones in Segbwema and the transitional  rainforest zone in Sumbuya compared to the savannah  woodland zone in Makeni probably due to the higher  tuberous root yield that was produced at these two zones  as potassium is the most limiting factor in cassava  nutrition. The indispensability of potassium in cassava  nutrition had been demonstrated by many studies (Nyi,  2014; Pypers et al., 2011). This result agrees with the  findings of Bharabwaj et al. (1994) who reported that the  uptake of potassium by crops generally increases with an  increase in crop yield. 

Furthermore, the higher rate of potassium depletion  recorded with respect to the intercropping system  compared to the sole cassava could be probably due to  the removal of potassium from the soil by both the  cassava and component crops. Another factor that can  be implicated in the decrease in exchangeable potassium  is the inability of the legumes to fix appreciable quantities  of potassium into the soil, unlike nitrogen, as legumes are  generally known for nitrogen fixation.

This result concord  with the finding of Yahaya et al. (2014) who reported  rapid nutrient depletion under intercropping systems due  to the combined demands of the individual intercrops for  nutrients. The decrease among intercropping systems  was higher for the cassava-soybean system probably  because of the higher root yield recorded for this  cropping system in the three agro-climatic zones. 


The result of this study has established that intercropping  cowpea, soybean, and groundnut with cassava decreased  soil pH among cropping systems in the three agro ecological zones except for the cassava-soybean system in the savannah woodland zone in Makeni. Soil organic  carbon increased in the savannah woodland zone in  Makeni but decreased in the rainforest zone in  Segbwema and the transitional rainforest zone in  Sumbuya among the cropping systems except for the  cassava-soybean system.

Soil total nitrogen increased  across cropping systems in the three zones except for the  sole cassava. Exchangeable potassium and available  phosphorus decreased under all cropping systems at all  three zones. Furthermore, a strong, positive correlation  was observed between organic carbon and total nitrogen  with pH on the one hand, and between organic carbon  and total nitrogen on the other. 


I wish to thank my supervisors for their support. 

Authors’ Contributions 

All authors contributed equally to the conception and design of the study. 

Competing Interests 

The authors have declared that no competing interests exist. 


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