







































American International Journal of Agricultural Studies  

Vol. 3, No. 1; 2020 

ISSN 2641-4155   E-ISSN 2641-418X 

Published by American Center of Science and Education, USA 

 

14 

 

Influence of Inorganic Fertilizer (NPK 15:15:15) and Cacao 

(Theobroma cacao L) Leaf Litter Application Rates on Cowpea 

(Vigna unguiculata L.) Performance in the Tropics 
 

 

 

Onwuchekwa Ojimgba PhD 

Department of Soil Science 

Faculty of Agriculture, Abia State University, Uturu, Nigeria  

E-mail: onwuchekwao@yahoo.com 

 

 

Abstract 

Influence of inorganic fertilizer (NPK 15:15:15) and cacao (Theobroma cacao L.) leaf litter application rates on 

cowpea (Vigna unguiculata L.) performance in the tropics was studied for 24 months (January, 2017 – December, 

2018) at Cocoa Research Institute of  Nigeria (CRIN) Ibeku Sub-Station, Umuahia, Nigeria. Three 3 x 3 factorial 

experiments, each in a Randomized Complete Block Design (RCBD) with three replicates/blocks were used to 

study the responses of cowpea to three NPK  15:15:15 fertilizer rates 0 kg, 500 kg and 1000 kg ha-1 represented 

by (Factor F) F0, F1 and F2, respectively, and cocoa leaf litter rates 0 t, 10 t and 20 t  ha-1   which were also  

represented by Lo, L1 and L2  (Factor L), respectively. The results obtained from the study show that the 

application of either 10 or 20 t ha-1 cocoa leaf litter alone or complementary application of either  10 or 20 t ha-1 

cocoa leaf litter plus 500 or 1000 kg ha-1 NPK 15:15:15 fertilizer  significantly increased the various cowpea 

growth and yield components than the fertilizer rates alone. The treatment without fertilizer and leaf litter, i.e. 

topsoil only (F0L0) gave significantly the least cowpea growth and yield components. The highest values were 

obtained with the application of 1000 kg ha-1 NPK 15:15:15 fertilizer with 20 t ha-1 cocoa leaf litter (F2L2). The 

cowpea crop performance in terms of growth, yield and yield components over the study periods, 2017 and 2018 

were statistically similar. Therefore, the inclusion of leaf litter has greater potential for improving cowpea yield 

than when fertilizer was used alone.  Leaves are potential sources of valuable nutrients providing a high quality 

of organic matter, which should be returned to the soil. 

 

Keywords: Cowpea, Fertilizer, Cacao Leaf Litter, Application Rates, Performance, Tropics. 

 

1.  Introduction 

1.1 Background Information 

The increasingly dependence of modern agricultural technology on high-value inputs, on the one hand, and the 

deteriorating economic situation of most of the developing countries, on the other, have led to a renewed 

awareness of the productive and protective value of trees, and a realization of the potentials  of age-old 

conservation farming techniques (Ojimgba, 2019). Consequently, efforts are now being made to devise the most 

appropriate ways to integrate trees and other woody species with agricultural crops and/or livestock 

simultaneously from the same piece of land in a sustainable manner (Fernandes and Chen, 2008). Farmers in 

tropical West Africa rely mainly on traditional farming system, such as the bush fallow system, for the restoration 

of soil fertility after a cultivation cycle (Okeke and Omaliko, 1994). To meet the nutrient demand of crops 

especially for the resource poor farmers, the use of organic materials would be an inevitable practice for a long 

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time to come (Heerendra, 2017).  In traditional farming systems in Nigeria, a piece of land is cultivated for a few 

years and left to fallow in order to regain fertility.  

  

In the farming systems of the tropics, the bush fallow system, cheaply utilized for the maintenance of soil fertility, 

is still a dominant food production system (Kang et al., 1985).  Various crop production techniques, including 

alley cropping, have been tested in the humid tropics to replace the traditional bush fallow in order to increase 

food production (Kang et al., 1985).  However, large quantities of plant nutrients contained in crop residues, plant 

litter, poultry and cattle manure are often lost by wasteful methods of disposal, such as burning during land 

preparation (Prabhakaran  Nair and Gosh, 1984).  Thus, the efficient use of these resources to recycle plant 

utilizable nutrients, especially in agricultural system of developing countries beset by chronic shortages of 

fertilizer inputs, has attracted some studies recently (Nordmeyer and Ritcher, 1985; Prabhakaran Nair and Gosh, 

1984). 

 

 In forest with year’s long life cycle, litter is a major source of nutrient systematically enriching the soil. These 

forest tree leaf litters can be applied as an organic amendment for the production of agricultural crops. To meet 

the nutrient demand of the crops especially for the resource poor farmers, the use of organic materials would be 

an inevitable practice for a long time to come (Heerendra, 2017). 

Ojimgba (2019) found that the application of 1000 kg ha-1 of inorganic fertilizer (NPK 15:15:15)  with 20 t ha-1  

cacao leaf litter  gave significantly the highest maize grain yield, while topsoil  only (control) - 0 kg ha–1 fertilizer 

+ 0 t ha-1 leaf litter gave the least yield. Kang et al., (1985) also found that the addition of low rates of N and 

leucaena prunings in their alley cropping experiments significantly increased crop yields, compared to low crop 

performance where none of these was used.  Repeated applications of prunings, they further contended, 

maintained higher soil organic matter levels and increased soil moisture retention capacity, exchangeable K, Ca, 

Mg and NO3 levels. Omaliko and Agbim (1983) also obtained significantly higher yields of forage maize (Zea 

mays L.) with the application of either 75 or 150 t ha-1 of incubated rice waste than that with no waste application.  

They showed that yields of forage maize further increased as the waste was supplemented with N, P and K 

fertilizers applied together.  Similarly, Okeke and Omaliko (1994) found that the incorporation of either 75 or 

150 t ha-1 of D.barteri leaf litter into the soil significantly increased the growth and yield of maize.  Okeke and 

Omaliko (1994), however, cautioned that although the use of litter has potential for improving crop yields, there 

is yet little information on the various responses of different crops to the rates of incorporating litter in the soil 

and even the time-lag between the litter incorporation and sowing of the food crop. Therefore, this study looked 

at the influence of inorganic fertilizer (NPK 15:15:15) and cacao (Theobroma cacao L.) leaf litter application 

rates on cowpea (Vigna unguiculata L.) performance in the tropics and how best this leaf litter could enhance 

cowpea production. 

 



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2.  Materials and Methods 

       

The study was conducted on the experimental research farm of the Cocoa Research Institute of Nigeria (CRIN) 

Ibeku Sub-station at Ajata - Ibeku, Umuahia, Nigeria.  This study area is located on latitude 0.5° 29’ N and 

longitude 07° 33’ E in the rainforest ecological zone of South Eastern Nigeria and lies at a mean elevation of 122 

metres (400 ft) above sea level (climatic data for National Root Crops Research Institute (NRCRI) meteorological 

station, Umudike) (Odurukwe, et al., 1995) The soil is classified as Ultisol according to USDA and as an Acrisol 

according to FAO/UNESCO classification schemes, as summarized by Opara-Nadi (2000).  The soil is 

characterized by inherent constraints such as low organic matter, poor structural stability, low nutrient and water 

holding capacities, low clay activities and high susceptibility to soil erosion and drought stress (Opara-Nadi, 2000; 

Salau et al., 1992). The vegetation of the study area is typical of tropical rainforest vegetation.  The secondary 

bush which dominates the area is the remnant of the tropical rainforests which are fast disappearing in the area. 

 

2.1 Land Preparation and Layout 

This study was carried out on the experimental research farm of the Cocoa Research Institute of Nigeria (CRIN) 

Ibeku Sub-station at Ajata – Ibeku, Umuahia, Nigeria.  In early May of 2017, experimental plots were selected at 

the adjacent bush fallow site of the cocoa plantation.  Each of these plots had not been cultivated for at least 

eighteen (18) months thus ensuring that the topsoil had not been recently disturbed in order to collect soil samples.  

Thereafter, all the plots were cleared, ploughed and harrowed. 

 Three 3 x 3 factorial experiments, each in a Randomized Complete Block Design (RCBD) with three 

replicates/blocks were used to study the responses of cowpea to three NPK 15:15:15 fertilizer rates O kg, 500 kg 

and 1000 kg ha-1 represented by (Factor F) F0, F1 and F2, respectively, and cocoa leaf litter rates 0 t, 10 t and 20 

t ha-1   which were also represented by Lo, L1 and L2 (Factor L), respectively.  Possible treatment combinations 

are shown in Table 1 below i.e. 9 treatment combinations per replication. 

Field Application Rates 

1.   F0L0= No Treatment 

2.   F1L0 = 0.8 kg F + 0 kg L/16 m2,equivalent to 500 kg F + 0 kg L/ha  

3.   F2L0= 1.6 kg F + 0 kg L/16 m2, equivalent to 1000 kg F + 0 kg L/ha 

4.   F0L1= 0 kg F + 16 kg L/16 m2, equivalent to 0 kg F + 10,000 kg L/ha 

5.   F1L1= 0.8 kg F + 16 kg L/16 m2, equivalent to 500 kg F + 10,000 kg L/ha 

6.   F2L1= 1.6 kg F + 16 kg L/16 m2, equivalent to 1000 kg F + 10,000 kg L/ha 

7.   F0L2= 0 kg F + 32 kg L/16 m2, equivalent to 0 kg F + 20,000 kg L/ha 

8.   F1L2= 0.8 kg F + 32 kgL/16 m2, equivalent to 500 kg F + 20,000 kg L/ha 

9.  F2L2= 1.6 kg F + 32 kg L/16 m2, equivalent to1000 kg F + 20,000 kg L/ha 

Table 1.   Treatment Combinations – NPK 15: 15: 15 fertilizer and Cacao leaf litter rates 

 



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                F = fertilizer                    L =  leaf litter 

These treatment combinations were randomized within the plots.  The experiment was replicated three times.  

There were, therefore, a total of twenty – seven (27) treatments and plots.  Each replicate measured 45 m x 4 m 

(180 m2), while the plot measured 4 m x 4 m (16  m2) each.  The total area of the experimental site was 0.063 ha. 

Sources of Materials   

The cocoa leaf litter was collected from Cocoa Research Institute of Nigeria (CRIN) Ibeku sub-station, Umuahia, 

Nigeria, while the cowpea (test crop) was obtained from the National Seed Service, Umudike, Nigeria.  The 

fertilizer NPK 15:15:15 was bought from Umuahia main market.  

Application of Waste 

 The cocoa leaf litter was incorporated into the soil in early May and incubation period of four (4) months was 

allowed after the application before cowpea was planted early August. Inorganic fertilizer treatments were applied 

in split doses, at planting and one month after planting by incorporation into the soil. 

Planting and Observation 

 Planting of cowpea was done on a flat harrowed soil surface.  Cowpea was planted at 1m x 0.5 m spacing, which 

gave about 20,000 plants ha-1 that is 32 plants per plot of 16 m2.  Cowpea was planted in early August each year. 

This study lasted for a period of 12 weeks each year. 

Attributes Studied 

Measurements in each plot were done using twelve stands, three stands each from the four inner rows randomly 

selected, tagged and sampled. 

Cowpea Studies with Cacao Leaf litter + Fertilizer rates at 4, 8 and 10 W.A.P. 

i)Plant height (ii)  No of leaves   plant-1  (iii) Leaf diameter,(iv) cowpea grain yield plot-1 ha-1,(v) Leaf length (vi) 

Leaf  area (Leaf length x Leaf diameter x 0.6 i.e. K (Ambasht and Ambasht, 2006) ). 

 

 

 

 

Weed Control 

NPK 15:15:15  

Rates (kg ha-1) 

                      Leaf litter rates  ( t ha-1 )  

         LO                                L1                                              L2 

          0                             10                             20 

          

         FO       0 

                

              F1   500 

 

              F2  1,000 

  

          FOLO
 (1)                   FOL1

 (4)                                FOL2 
(7) 

        

          F1LO
  (2)                           F1L1

 (5)                                 F1L2 
(8) 

   

          F2LO
 (3)                             F2L1

 (6)                                F2L2 
(9) 



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No herbicide was applied to control weeds; rather, hand weeding was done using native hoe.  The weeds were 

not thrown away, but were spread within the plot from which they were removed and allowed to decompose to 

release the absorbed nutrients. 

Harvest and Yield Computation 

Harvest was done at 12 weeks after planting.  The matured pods were harvested from each plot, dried, shelled 

and weighed.  The grains were dried at 600C for 48 hours. 

 

2.2 Statistical analysis 

The data collected was analyzed statistically and significant differences between treatment means of various 

experiments were tested at P < 0.05 using the Fisher’s least – significant differences (F – LSD), standard errors 

and or Student’s “t” – test, according to the procedures of Steel and Torrie (1980). .  In all statistical analyses, P 

< 0.05 was used to test for significant differences between the treatments means of the various experiments. 

3.  Results and Discussion 

Chemical properties of the topsoil prior to fertilizer and cacao leaf litter applications at Umuahia, Nigeria 

 Table 2 summarizes the chemical properties of the topsoil of the study site prior to fertilizer (NPK 15:15:15) and 

cacao (Theobroma cacao L.) leaf litter applications at Umuahia, Nigeria. However, the topsoil of the study site 

had low pH, total nitrogen, organic carbon and consequently organic matter, available phosphorus, exchangeable 

cations (K, Na, Ca, Mg) and percentage base saturation. However the percentage aluminium saturation, effective 

cation exchange capacity (ECEC) and exchangeable acidity (EA) were high. These values are peculiar with soils 

of tropical rainforest ecological zone of southeastern, Nigeria which requires amendment for effective crop 

performance.    

 

Table 2. Chemical properties of the topsoil prior to fertilizer and cacao leaf litter applications at Umudike, 

Nigeria. 

   Attributes /Statistics  Units of measurement Mean values 

Soil pH and chemical properties  

pH  4.42 

Nitrogen % 0.03 

Organic C % 0.42 

Organic matter % 0.72 

Available P Ppm 3.40 

Exchangeable K C mol (+) kg –1 0.06 

       “      Na “ 0.12 

       “     Ca “ 0.70 



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       “     Mg “ 0.50 

       “     Al  “ 2.20 

Exchangeable H “ 0.60 

Exchangeable Acidity (EA) “ 2.80 

Effective cation exchange capacity (ECEC) “ 4.18 

Al Saturation  % 62.90 

Base saturation  % 39.00 

C:N  14:1 

 

 

Influence of fertilizer (NPK 15:15:15) and cacao leaf litter application rates on plant height of cowpea 

(Vigna unguiculata L.) 

Figures 1 and 2 summarize the influence of fertilizer and cacao leaf litter application rates on plant height of 

cowpea in 2017 and 2018, respectively. At 4 WAP significant differences existed between the application of 1000 

kg ha-1 NPK 15:15:15 along with cacao leaf litter (0,10,20 t ha-1) treatments over the years. However, the 

application 1000 kg ha-1 NPK 15:15:15 gave significantly higher plant height than the similar values of 500 kg 

ha-1 and 0 kg ha-1 in the following order: F2 > F1 = F0. Also, the applications of 20 t ha-1 of cocoa leaf litter gave 

significantly higher plant height than the values of 10 t ha-1, while 0 t ha-1 had the least result in 2017 and 2018, 

in the following order 20 > 10 > 0 t ha-1. There were no significant F x L interaction effects on cowpea plant 

height in both 2017 and 2018 (Figures 1 and 2)  

 

At 8 WAP, the plant height values of 2017 and 2018 followed the same trend. The application of 1000 kg ha-1 

NPK 15:15:15 gave significantly higher plant height than the similar results of 500 and 0 kg ha-1 NPK 15:15:15 . 

Also, the statistically similar plant height values of 20 and 10 t ha-1 cacao leaf litter gave significantly higher 

results than 0 t ha-1 (no leaf litter, L0). Generally, the application of fertilizer (F) plus leaf litter (L) rates gave 

similar plant height values at 8 WAP which were significantly greater than the plant height results of F0L0 and 

F1Lo (i.e. no fertilizer + no leaf litter and no fertilizer application along with 500 kg ha-1 NPK 15:15:15, 

respectively) in 2017. But in 2018, the application of various rates of fertilizer (500, 1000 kg ha-1 NPK 15:15:15) 

along with the cacao leaf litter (10, 20 t ha-1) gave statistically similar plant height values.  

 

However, at 10 WAP, NPK 15:15:15 fertilizer application rates had statistically similar cowpea plant height 

values in 2018, whereas in 2017 the application of 1000 kg ha-1 NPK 15:15:15 gave significantly  higher plant 

height than the statistically similar plant height values of 500 and 0 kg ha-1 NPK 15:15:15. The application of 

various rates of cacao leaf litter followed same trend in 2017 and 2018. The application rates of 20 and 10 t ha-1 

cacao leaf litter which gave statistically similar plant height values had significantly higher values than that of 0 

t ha-1. Generally, the application of higher rates of fertilizer and any of the two litter rates (10 or 20 t ha -1) gave 

statistically similar plant height values which were (p>0.05) significantly higher than the values of fertilizer rates 



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alone. The least plant height was obtained with the application of F0 and F1 without leaf litter.  The relative 

improvement of the final plant height of cowpea in 2008 is in the order:  F1L2 = F1L1 = F2L1 = F2L2 = F0L2 = F0L1 

> F2L0 > F1L0 > F0L0. The final plant height values of 2017 followed same trend. 

 

 
 

 
 

 

 

Influence of fertilizer (NPK 15:15:15) and cacao leaf litter application rates on leaf diameter of cowpea 

(Vigna unguiculata)   

Figure. 1 :  Influence of fertilizer and cocoa leaf  
litter application rates on cowpea plant height in 2017 

0 

20 

40 

60 

80 

100 

120 

4wks 8wks 10wks 

Weeks After Planting 

FoLo  

F1Lo 

F2Lo 

FoL1 

F1L1 

F2L1 

FoL2 

F1L2 

F2L2 

            
Plant 
Height 
(cm) 

 Figure. 2 Influence of fertilizer and cocoa leaf litter 
application rates on cowpea plant height in 2018 

0 

20 

40 

60 

80 

100 

120 

4wks 8wks 10wks 

Weeks After Planting 

FoLo  
F1Lo 
F2Lo 
FoL1 
F1L1 
F2L1 
FoL2 
F1L2 
F2L2 

Plant 
Height 
(cm) 



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Tables 3 and 4 summarize the effects of fertilizer, (NPK 15:15:15) and cacao leaf litter application rates on leaf 

diameter of cowpea in 2017 and 2018, respectively. Table 3 shows that in terms of fertilizer treatments at the 

weeks after planting (WAP) in 2017, the 1000 kg ha-1 NPK 15:15:15 had significantly higher leaf diameter than 

the 0 kg ha-1. However, at 8 WAP the results of 500 and 1000 kg NPK 15:15:15 were statistically similar. The 

cacao leaf litter rates significantly influenced cowpea diameter as follows: 10 = 20 > 0 t ha-1. The similar cowpea 

leaf diameter results of the 20.0 and 10.0 t ha-1 were significantly higher than that of the control (0 t ha-1). In 2017, 

there was no significant fertilizer (F) x leaf litter (L) interaction effects on cowpea leaf diameter in 4 and 8 WAP. 

However, in 10 WAP, the complementary use of F2L2, F2L1, F1L1 and F1L2 had statistically similar cowpea leaf 

diameter results which were significantly greater than the control (no fertilizer and no leaf litter i.e. F0L0), 500 or 

1000 kg ha-1 NPK 15:15:15 rates without leaf litter. The relative improvement in final leaf diameter values of 

2017 was in the order: F2L2 = F2L1 = F1L1 = F1L2 = F0L2 > F0L1 = F2L0 = F1L0 > F0L0. Generally, the addition of 

10 or 20 t ha -1 cocoa leaf litter with or without inorganic fertilizer NPK 15:15:15 rates significantly increased leaf 

diameter than no leaf litter rates with or without fertilizer application rates. 

The results in Table 4 show that the application rates of 1000 and 500 kg ha-1 NPK 15:15:15: had significantly 

higher leaf diameter than the 0 kg ha-1 at 4 WAP. However, the results of 500 and 1000 kg ha-1 NPK 15:15:15 as 

well as 0 and 500 kg ha-1 treatments were statistically similar at 4 WAP in 2018. The cacao leaf litter rates 

significantly influenced cowpea leaf diameter. This leaf diameter results at the 4th, 8th and 10th WAP had similar 

trends of cacao leaf litter treatment as follows: 20 > 10 > 0 t ha-1. There was no significant fertilizer (F) x leaf 

litter (L) interaction effects on cowpea leaf diameter in 2018.  



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Table 3.   Influence of fertilizer and cacao leaf litter application rates on leaf diameter of cowpea (Vigna unguiculata)  (2017) 

 

NPK Fertilizer 

(15:15:15) 

(Kg ha -1) 

4  Weeks 

leaf litter rates 

(t ha – 1) 

8 Weeks 

leaf litter rates 

(t ha – 1) 

10 Weeks                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        

leaf litter rates 

(t ha – 1) 

 0 10 20 Mean  0 10 20 Mean  0 10 20 Mean  

0 3.57 7.73 9.37 6.89 3.97 8.37 9.77 7.37 4.20 9.00 9.83 7.68 

500 5.43 8.80 9.60 7.94 6.20 9.77 10.30 8.76 7.00 10.50 10.43 9.31 

1000 7.07 9.73 9.97 8.92 7.07 10.17 10.60 9.28 7.77 10.53 10.80 9.70 

Mean  5.36 8.76 9.64  5.74 9.43 10.22  6.32 10.01 10.36  

     F-LSD(0.05) (t ha-1) 

Weeks 

    

    4 8 10      

   Fertilizer rates (F)     0.91     0.63       0.72      

   Leaf litter rates (L)     0.91     0.63    0.72      

   Interaction F x L     NS      NS    1.25      

             

 

 

Table 4.   Influence of fertilizer and cacao leaf litter application rates on leaf diameter of cowpea (Vigna unguiculata)  (2018) 

 

 NPK Fertilizer 

(15:15:15) 

(Kg ha -1) 

4  Weeks 

leaf litter rates 

(t ha – 1) 

8 Weeks 

leaf litter rates 

(t ha – 1) 

10 Weeks                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        

leaf litter rates 

(t ha – 1) 

 0 10 20 Mean  0 10 20 Mean  0 10 20 Mean  

0 4.17 7.67 9.73 7.19 4.60 8.37 10.37 7.78 4.73 8.77 10.70 8.07 

500 5.70 8.43 10.17 8.10 5.93 10.43 10.93 9.10 6.37 11.07 11.40 9.61 

1000 6.80 9.63 10.30 8.91 7.43 11.17 12.33 10.31 8.33 12.10 12.83 11.09 

Mean  5.56 8.58 10.07  5.99 9.99 11.21  6.48 10.64 11.64  

     F-LSD(0.05) (t ha-1) 

Weeks 

    

    4 8 10      

   Fertilizer rates (F)   0.92     1.09    0.43      

   Leaf litter rates (L)   0.92     1.09    0.43      

   Interaction F x L   NS      NS     NS      

             

 

 

 



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Influence of fertilizer NPK 15:15:15 and cacao leaf litter application rates on the number of leaves plant -1 

of cowpea 

Tables 5 and 6 also summarize the effect of fertilizer, NPK 15:15:15 and cacao leaf litter application rates on the 

number of leaves plant-1 of cowpea in 2017 and 2018. Generally, the application of fertilizer, NPK 15:15:15 and 

cacao leaf litter increased the number of leaves plant-1 more than the control. However, at 4 WAP in 2017 and 

2018, the complementary use of 500 or 1000 kg ha-1 plus either 10 or 20 t ha-1 leaf litter gave similar number of 

leaves plant-1. At 8 WAP in 2017, 1000 kg ha -1 NPK 15:15:15 gave significantly the highest number of leaves 

plant-1 as follows:   1000 > 500 > 0 kg ha-1 NPK 15:15:15. But in 2018, the applications of 1000 and 500 kg ha-1 

which have similar cowpea number of leaves plant-1 were significantly greater than the 0 kg ha-1 (no fertilizer). 

Cacao leaf litter application rates (20, 10 and 0 t ha-1) followed the same trend. However, in 2017 and 2018 study 

periods, the use of 20 t ha-1 leaf litter alone or the complementary use of either 500 or 1000 kg ha-1 NPK 15:15:15 

with either 10 or 20 t ha-1 leaf litter gave similar leaf number plant-1 values. These results, however, were 

significantly higher than those of the other treatments. The control, again gave the least result, At 10 WAP in 

2017 and 2018, the number of leaves plant-1 values obtained followed the same trend with those of 8 WAP. 



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Table 5.  Influence of fertilizer and cacao leaf litter application rates on the number of leaves plant-1 of   cowpea  (Vigna unguiculata) (2017) 

 

NPK Fertilizer 

(15:15:15) 

(Kg ha -1) 

4  Weeks 

leaf litter rates 

(t ha – 1) 

8 Weeks 

leaf litter rates 

(t ha – 1) 

10 Weeks                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        

leaf litter rates 

(t ha – 1) 

 0 10 20 Mean  0 10 20 Mean  0 10 20 Mean  

0 3.27 4.40 4.62 4.09 3.83 5.86 6.40 5.36 3.87 6.83 6.68 5.80 

500 3.60 4.51 4.72 4.28 4.46 5.86 6.51 5.61 4.83 6.53 6.73 6.03 

1000 3.78 4.72 5.06 4.52 5.54 6.48 6.70 6.24 6.61 7.00 7.11 6.90 

Mean  3.55 4.54 4.80  4.61 6.07 6.54  5.10 6.79 6.84  

     F-LSD(0.05) (t ha-1) 

Weeks 

    

    4 8 10      

   Fertilizer rates (F)   0.14     0.22    0.26      

   Leaf litter rates (L)   0.14     0.22    0.26      

   Interaction F x L    NS     0.38    0.44      

             

 

Table 6. Influence of fertilizer and cacao leaf litter application rates on the number of leaves plant-1 of   cowpea  (Vigna unguiculata) (2018)  

 

NPK Fertilizer 

(15:15:15) 

(Kg ha -1) 

4  Weeks 

leaf litter rates 

(t ha – 1) 

8 Weeks 

leaf litter rates 

(t ha – 1) 

10 Weeks                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        

leaf litter rates 

(t ha – 1) 

 0 10 20 Mean  0 10 20 Mean  0 10 20 Mean  

0 3.21 4.32 5.04 4.19 3.83 6.48 5.94 5.42 3.95 6.65 6.19 5.60 

500 3.51 4.42 4.86 4.26 5.42 6.21 6.52 6.05 5.75 6.45 6.67 6.29 

1000 3.56 4.79 5.19 4.51 5.48 6.35 6.81 6.21 5.72 6.63 6.91 6.42 

Mean  3.43 4.51 5..03  4.91 6.35 6.42  5.14 6.58 6.59  

     F-LSD(0.05) (t ha-1) 

Weeks 

    

    4 8 10      

   Fertilizer rates (F)    NS     0.40    0.45      

   Leaf litter rates (L)   0.34     0.40    0.45      

   Interaction F x L    NS     0.69    0.78      

             



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Influence of fertilizer NPK 15:15:15 and cacao leaf litter application rates on cowpea leaf length 

Tables 7 and 8 summarize the effect of fertilizer NPK 15:15:15 and cacao leaf litter application rates on cowpea 

leaf length in 2017 and 2018, respectively. The results in the Tables show that the applications of fertilizer NPK 

15:15:15 as well as cacao leaf litter significantly influenced cowpea leaf length. At the 4 th, 8th and 10th WAP in 

2017 and 2018, the application of 1000 kg ha-1 NPK 15:15:15 fertilizer significantly gave higher leaf length than 

either 500 or 0 kg ha-1 (control i.e. no fertilizer). In terms of cacao leaf litter treatment, the similar results of 20 

and 10 t ha-1 gave significantly higher leaf length  than the 0 t ha-1 as follows; 20 = 10 > 0 t ha-1  

 

However, at 10th WAP in 2017 and 2018, there was no significant fertilizer (F) x leaf litter (L) interaction effects 

on cowpea leaf length. The 4th and 8th WAP had similar trends of F x L interaction effects on cowpea leaf length. 

Generally, the addition of 10 or 20 t ha-1 cocoa leaf litter with or without inorganic fertilizer (NPK 15:15:15) rates 

more significantly increased leaf length than no leaf litter rates with or without fertilizer application rates. The 

complementary use of F2L2, F1L2 and F0L2 had statistically similar cowpea leaf length results which were 

significantly (P ≤ 0.05) greater than those of the control treatment (no fertilizer and no leaf litter). The order of 

improvement in 2018 was as follows:                                       F2L2 = F2L1 = F1L2 = F0L2 = F1L1 = F0L1 > F1L0 = 

F2L0 > F0L0. This means that the use of 10 t ha-1 cacao leaf litter without fertilizer gave significantly higher cowpea 

leaf length than either 1000 or 500 kgha-1 fertilizer when used alone.



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Table 7. Influence of fertilizer and cacao leaf litter application rates on leaf length of  cowpea (Vigna unguiculata)  (2017)  

 

NPK Fertilizer 

(15:15:15) 

(Kg ha -1) 

4  Weeks 

leaf litter rates 

(t ha – 1) 

8 Weeks 

leaf litter rates 

(t ha – 1) 

10 Weeks                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        

leaf litter rates 

(t ha – 1) 

 0 10 20 Mean  0 10 20 Mean  0 10 20 Mean  

0 4.00 7.50 7.73 6.41 3.90 9.17 7.93 7.00 4.00 8.17 7.97 6.71 

500 6.17 7.67 9.67 7.83 7.30 9.80 10.17 9.09 7.50 9.87 10.17 9.18 

1000 6.67 10.00 10.83 9.17 7.93 10.63 9.50 9.36 8.37 10.77 10.00 9.71 

Mean  5.61 8.39 9.41  6.38 9.87 9.20  6.62 9.60 9.38  

     F-LSD(0.05) (t ha-1) 

Weeks 

    

    4 8 10      

   Fertilizer rates (F)   0.54     0.84     1.26      

   Leaf litter rates (L)   0.54     0.84     1.26      

   Interaction F x L   0.94     1.46      NS      

             

 

Table 8. Influence of fertilizer and cacao leaf litter application rates on leaf length of cowpea (Vigna unguiculata)  (2018)  

 

NPK Fertilizer 

(15:15:15) 

(Kg ha -1) 

4  Weeks 

leaf litter rates 

(t ha – 1) 

8 Weeks 

leaf litter rates 

(t ha – 1) 

10 Weeks                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        

leaf litter rates 

(t ha – 1) 

 0 10 20 Mean  0 10 20 Mean  0 10 20 Mean  

0 4.43 8.00 9.07 7.17 4.93 9.40 9.77 8.03 5.17 9.53 9.00 7.90 

500 6.83 7.33 8.73 7.63 7.67 9.50 10.00 9.06 8.13 9.83 10.63 9.53 

1000 6.33 9.33 9.23 8.30 6.93 10.63 10.63 9.40 7.20 11.00 10.87 9.69 

Mean  5.87 8.22 9.01  6.51 9.84 10.13  6.83 10.12 10.17  

     F-LSD(0.05) (t ha-1) 

Weeks 

    

    4 8 10      

   Fertilizer rates (F)   0.41     0.56    1.04      

   Leaf litter rates (L)   0.41     0.56    1.04      

   Interaction F x L   0.71     0.96     NS      

             

 



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             F2L1                          F1L0                                      F1L2                              F2L0                                  F0L2 

                       

                                          
                                                                                 F2L2 

Cowpea plants at 6 weeks after planting (WAP) treated with various rates of NPK 15:15:15 fertilizer (F) and 

cocoa leaf litter (L) 

 

 Influence of fertilizer  NPK 15:15:15 and cacao leaf litter application rates on the leaf area (cm2) of cowpea in 

2017 and 2018 

Table 9  summarizes the effects of fertilizer NPK 15:15:15 and cacao leaf litter application rates on cowpea leaf area 

at the 10th week after planting (WAP) in 2017 and 2018. The results from the table show that the applications of the 

various rates of fertilizer had significant (P>0.05) effect on leaf area of cowpea. However, the application of 1000 

kg ha-1 NPK 15:15:15 gave significantly (P>0.05) higher cowpea leaf area than the 0 or 500 kg ha-1 in the following 

significant order : F2 > F1 > F0. Cacao leaf litter application rates followed same trend as that of the fertilizer in the 

order : L2 > L1 > F0. 

 

Table 9 also shows that fertilizer NPK 15:15:15 (F) x cacao leaf litter (L) interaction had significant effects on cowpea 

leaf area at the 10th week after planting (WAP) in 2017 and 2018.  However, cowpea plants with leaf litter application 

gave significantly higher leaf area than those without leaf litter application. Also, cowpea plants treated with 1000 

kg ha-1 NPK 15:15:15 plus 20 or 10 t ha-1 cacao leaf litter (i.e. F2L2 or F2L1) gave higher leaf area than the rest 

treatments. The treatment without fertilizer and leaf litter, i.e. the topsoil only (F0L0), gave significantly the least leaf 

area values in 2017 and 2018. The relative improvement of the leaf area in 2018 was in the following significant 

order : F2L2 > F2L1 > F1L2 > F1L1 > F0L2> F0L1 > F2L0 > F1L0 > F0L0.    

 

 

 

 

 

 



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Table 9.  Influence of fertilizer and cacao (Theobroma cacao L.) leaf litter application  rates on  cowpea (Vigna 

unguiculata) leaf area (  cm2)   

 

NPK Fertilizer 

(15:15:15) 

(Kg ha -1) 

2017 

leaf litter rates 

(t ha – 1) 

2018 

leaf litter rates 

(t ha – 1) 

 0 10 20 Mean      0 10 20 Mean    

0 10.1 44.1 47.0 33.7   14.6 50.2 57.8 40.9   

500 31.5 62.2 63.6 52.4   31.1 65.3 72.7 56.4   

1000 39.0 68.1 64.8 57.3   36.0 79.9 83.7 66.5   

Mean  26.9 58.3 58.5    27.2 65.1 71.4    

            F-LSD(0.05) (cm2) 

                Year 

    

    2017 2018        

Fertilizer rates (F)                  0.55 0.27        

Leaf litter rates (L)    0.55 0.27        

Interaction F x L    1.70 1.24       

 

 

Influence of fertilizer NPK 15:15:15 and cacao leaf litter application rates on cowpea grain yield in 2017 and 

2018 

 Figure 3 summarizes the influence if various NPK 15:15:15 fertilizer and cacao leaf litter application rates on cowpea 

grain yield in 2017 and 2018. The significant performances of the various fertilizer rates were as follows; 

a. 1000 = 500 > 0 kg ha-1 for 2017 and   

b. 1000 > 500 > 0 kg ha-1 for 2018.                                                                                   

 

However, the influence of various cacao leaf litter rates on grain yield of cowpea for 2017 and 2018 were as follows: 

20 > 10> 0 t ha-1. Thus, the 20 and 0 t ha-1 cacao leaf litter gave significantly the highest and least cowpea grain 

yield, respectively in 2017 and 2018. 

 

There was no significant F x L interaction effects on cowpea grain yield in 2017 (Figure 3). However, in 2018 the 

application of 1000 kg ha-1 NPK 15:15:15 + 20 t ha-1 cocoa leaf litter (F2L2) gave significantly the highest cowpea 

grain yield  than the control – 0 kg ha-1 fertilizer plus 0 t ha-1 leaf litter. Again, the application of 20 t ha-1 leaf litter 

with either 1000 kg ha-1 or 500 kg ha-1 NPK 15:15:15 gave significantly higher cowpea grain yield than those of the 

10 t ha-1 leaf litter plus either 1000 kg ha-1 or 500 kg ha-1  NPK 15:15:15 . However, the applications of various rates 

of  NPK 15:15:15 (1000, 500 and  0 kg ha-1) with no cacao leaf litter application rates, that is F2L0, F1L0 and F0L0, 

gave significantly lower cowpea grain yield results than those of the fertilizer application rates plus either 20 or 10 t 

ha-1 cacao leaf litter (F2L2, F2L1, F1L2 or F1L1). 



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 (a)  Cowpea Growth Parameters 

Generally, the cowpea growth components followed the same trend with those of maize growth components as 

indicated by Ojimgba (2019). The addition of 10 or 20 t ha-1 cacao leaf litter with or without inorganic fertilizer NPK 

15:15:15 rates more significantly increased the cowpea plant height, leaf diameter, number of leaves plant-1, leaf 

length and leaf area than no leaf litter rates with or without fertilizer application rates. This improvement according 

to Young (1989) could be as a result of the return in litter and pruning of some 100 – 300 kg N/ha/yr under cocoa 

and coffee plantations with shade trees. Ouda and Mahadeen (2008) added that shoot fresh and dry weights of 

broccoli plants tended to increase by increasing dose of organic manure. They further stated that number of leaves 

per plant showed a little increase with increased organic manure dose. That each increase in inorganic fertilizer dose 

tended to increase number of leaves per plant compared with control. They confirmed that plots receiving a 

combination of organic and inorganic fertilizers produced slightly higher (P>0.05) fresh and dry weights of shoot. 

These results are in accordance with Magnusson (2002) on chinensis cabbage (B. chinensis) and Abdelrazzag (2002) 

on onion (Allium cepa). Furthermore, organic manure activates many species of living organisms, which release 

phytohormones and may stimulate the plant growth and absorption of nutrients (Arisha et al., 2003). Such organisms 

need nitrogen for multiplication. This is plausible reason that use of organic manure with inorganic fertilizer showed 

a beneficial effect on dry matter accumulation. Organic manure can serve as alternative practice to mineral fertilizers 

(Naeem et al., 2006) for improving soil structure (Dauda et al., 2008) and maize growth Mbagwu, 1992). However, 

the litter fall is major recognized avenue for addition of organic matter (nutrients) to the soil from the trees. Therefore, 

Figure.3  Influence of fertilizer and cacao leaf litter application  

rates on cowpea grain yield in 2017 and 2018 

0 

500 

1000 

1500 

2000 

2500 

F0L0 F1L0 F2L0 F0L1 F1L1 F2L1 F0L2 F1L2 F2L2 

Treatment combinations 

Cowpea Grain 
Yield (kg/ha) 

2017 

2018 



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the increase in cowpea growth components might be due to the combinations of the cacao leaf litter with high organic 

matter status and inorganic fertilizer NPK 15:15:15, nitrogen being one of the major nutrient elements. 

 

(b) Cowpea Grain Yield 

The results contained in Figure 3 shows that the applications of various rates of NPK 15:15:15 fertilizer (i.e. 1000, 

500 and 0 kg -1) without cacao leaf litter application rates gave significantly lower cowpea grain yield results than 

those of the fertilizer application rates plus either 20 or 10 t ha-1 cacao leaf litter. However, the applications of the 

highest dose of inorganic fertilizer (1000 kg ha-1, F2) with the highest dose of cocoa leaf litter (20 t ha-1, L2) (i.e. F2L2) 

which produced the highest yield of cowpea grain yield was similar to results obtained by Ouda and Mahadeen (2008) 

and Ojimgba (2019). They confirmed that the application of the highest dose of inorganic fertilizer (60 kg ha-1) with 

the highest dose of organic manure (80 t ha-1) produced the highest yield of main heads of broccoli and maize, 

respectively. Furthermore, Anikwe et al., (2003) added that the cumulative results of maize plots amended with 

manure + NPK had an over 8-fold increase in grain yield relative to the control. They added that other treatments 

(mulch + NPK and NPK alone) did not statistically affect grain yield of maize relative to the control (unamended 

plots). According to them, in groundnut plots amended with manure + NPK, cumulative data show that grain yield 

also increased (P<0.05) by 123% relative to the control. Ojimgba et al., (2008) reported similar results and concluded 

that the reason for this is that organic wastes had about 90% higher mineralization rate than the unamended control or 

inorganic fertilizer. 

 

The poor yield obtained in unamended control plots may be associated with the acidity which is a major problem in 

many tropical soils (Ojimgba and Mbagwu, 2007). Muller-Samann and Johannes (1997) confirmed this statement that 

on soils with pH values under 5, the exchange particles elements are coated with hydrogen and nutrient cations (Mg, 

K, Ca) and often also with aluminum (Al) ions which may lead to aluminum toxicity and phosphate fixation through 

the formation of Al-phosphates. Also, only at pH values exceeding 5 does aluminum become hydroxide, so that 

toxicity is no longer a problem. The signs of aluminum toxicity are reduced, stunted growth and disrupted Ca and P 

metabolism, which was the problem with the control and fertilizer treated plots as well as their high Al saturation. The 

little yield of the unamended control plots of cowpea might be due to nitrogen fixed by the cowpea which is the 

characteristics of all leguminous crops.  

 

4.  Conclusion and Recommendations 

The research has documented basic information about the potentials and uses of cacao in an agroforestry farm at the 

Cocoa Research Institute of Nigeria (CRIN) Ibeku Sub-station, Umuahia, Nigeria. The study also revealed that cowpea 

(Vigna unguiculata L.) can be successfully grown with cacao leaf litter incorporated in the soil without the use of 

costly inorganic fertilizers. The results also pointed to the fact that the leaf litter improved highly the grain yield of 

cowpea in the eroded soils, whereas inorganic fertilizer had no such effect. 

The following recommendations are made based on the results of the study: 

i)   Studies on litter production and decomposition effects on crop growth and yield should be carried out in different 

agroforestry ecosystems of the humid and sub-humid (including the arid and semi-arid) zones of West Africa. 



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ii)   I, also recommend that farmers should plant cocoa trees along the contour bunds or boundaries as alley, prune 

them periodically or allow the leaves to fall and decay to improve the nutrient status and structure of the soils. 

It is hoped that the results of this study will encourage farmers embark on the establishment of cacao plantations using 

economically viable landuse management system such as agroforestry. The results will encourage greater production 

of cocoa beans as well as encourage the establishment of small-scale industries to process the products. The 

establishment of cocoa plantations and the processing of the products from the plantations could boost the income of 

farmers and the nation’s foreign exchange reserve via exportation of the processed products.  It is also hoped that the 

results of the study will encourage the farmers to profitably grow cowpea with little or no inorganic fertilizer 

application.  

 

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