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African Journal of Agricultural Marketing ISSN 2375-1061 Vol. 5 (1), pp. 001-006, January, 2017. Available online at 
www.internationalscholarsjournals.org © International Scholars Journals 

 

Author(s)                                           Author(s) retain the copyright of this article. 
 
 

 

Full Length Research Paper 

 

Measuring the benefits of biological nitrogen fixation of 
soybean (Glycine max (l.) Merrill) in cassava (Manihot 

esculenta crantz) and soybean intercrop 

 
Umeh S. I.* and Mbah B. N. 

 
Department of Crop Science, University of Nigeria, Nsukka, Enugu State, Nigeria. 

 
Accepted 16 September, 2016 

 
The measurement of the benefits of biological nitrogen (N) fixing properties of soybean in cassava and soybean 
intercrop was conducted at the University of Nigeria, Nsukka between 2000, 2001 and 2002. Changes in soil nutrient 
concentration were measured at 0 – 30 cm. The effects of N- fertilization on cassava tuber yield and soil-N were 
monitored for the three seasons using factorial in randomized complete block design. There was no apparent 
shading effect during the growth of the two crops. Soybean was harvested 110 days after planting, while cassava 
was harvested at 4, 8 and 12 months after planting in each year. Cassava tuber yield was highest at intercrop X 60 
and intercrop X 45 kg N ha

-1
 (30.0 and 29.9 t. ha

-1
) x tuber yield of intercrop X 45 kg N ha

-1
. The result indicates that 

60 kg (90 – 30) of applied nitrogen could be spared per hectare by intercropping cassava with soybean due to 
soybean nitrogen fixation. Grain yield of soybean may not be compromised by intercropping with cassava. Soil - N 
increased from 0.042 mg kg

-1
 before the experiment to 0.168 mg kg

-1
 in sole soybean system and 0.086 mg kg

-1
 in 

intercrop system at 8 months after planting and 0.150 mg kg
-1

 in the sole soybean and 0.085 kg mg
-1

 in intercrop 
system in 12 months after planting. The highest land equivalent ratio (LER) of 2.3 and area × time equivalent ratio 
(ATER) of 1.8 were obtained at 45 kg N ha

-1
 in the intercrop system. 

 
Key words: Soybean, nitrogen fixation, cassava, intercrop. 

 
 
INTRODUCTION 

 
Biological nitrogen fixation has great potential to 
compensate for the short falls in availability of fertilizers in 
African farming system (Danso, 1992). Some workers 
have investigated nitrogen fixed by legume crops and 
estimated how much nitrogen is contributed to the 
companion crop in intercropping systems (Giller, 1992). 
Intercropping low growing legumes with tall cereals has 
been a common practice in many tropical countries  
 
 
 
*Corresponding author. E-mail: simumeh@yahoo.com. Tel: 
08033509997. 
 
Abbreviations: MAP, Month(s) after planting; LER, land 
equivalent ratio; ATER, area × time equivalent ratio. 

 
 
 
 

 
(Mason and Leihner, 1988). Oforsu et al. (1995) reported 
nitrogen transfer to sorghum when intercropped with 
soybean and a higher dry matter in intercropped sorghum 
than in the mono crop. They noted the more the roots of 
companion crops were intermingled, the greater the 
amount of nitrogen transferred. Ayisi et al. (1997) 
intercropped canola with soybean in strips and reported 
reduction in fertilizer requirement due to the 
intercropping. Tijani and Akinnifesi (1996) determined the 
compatibility of soybean and cassava intercrop in the 
South Western Nigeria, and reported improved yield of 
cassava when intercropped with soybean.  

Cassava (Manihot esculenta Crantz) is a crop that is 
well adapted to a wide range of ecological conditions, 
being tolerant to low soil fertility, drought and pest (Tewe, 

file:///C:\Users\user\Documents\REPUBLICATION\AGRICULTURAL%20SCIENCES\AppData\Local\Temp\www.internationalscholarsjournals.org


 
 
 

 

1992) and has been reported as the 4th most important 
staple of the tropics, providing food and income for more 
than 750 million people annually (FAO, 1992).  

Since its introduction into Nigeria in 1908, soybean 
(Glycine max. (L) Merrill) has been grown primarily as a 
sole crop (Ogunwolu, 1991) yet it has been shown to be 

able to fix atmospheric nitrogen up to 417 kg N ha
-1

 
(LaRue and Patterson, 1981). Fertilizer-nitrogen 
requirement of soybean is generally low, ranging from 

15–30 kg N ha
-1

 (Mulongony, 1992) and nodulation is 
reduced with increased nitrogen fertilization (Shibles et 
al., 1975). From limited trials and observations, it has 
been shown that soybean and cassava can be  
intercropped based on their morphological 
characteristics. With careful selection of agronomic 
practices, methodologies of nitrogen supply to the 
intercrop, planting time and spatial arrangements, 
cassava should benefit from biologically fixed nitrogen of 
intercropped soybean. It is therefore the objective of this 
study to assess the benefit of biologically fixed nitrogen 
by soybean when intercropped with cassava and 
estimate the nitrogen fertilizer need of the system. 
 
 
MATERIALS AND METHODS 
 
Three experiment of cassava/soybean intercrop were conducted in 
2000, 2001 and 2002 at the University of Nigeria, Nsukka Utisol 
using factorial in randomized complete block design (RCBD), with 
two factors: nitrogen rates and cropping systems.  

Soybean (G. max (L) Merril) TGM 579 obtained from IITA Ibadan 
and cassava (M. esculeta Crantz) TMS 30572, obtained from 
cassava research unit of Soil Science Department, University of 
Nigeria, Nsukka were used. Fertilizer materials were urea (46% N) 
as nitrogen source, single supper phosphate (18% P2O5) as 
phosphorus source and muriate of potash (60% K2O) as potassium 
source.  

In experiments 1 and 3, twelve (12) plots of 4 × 4 m were 
replicated three times with 4 levels of nitrogen (0, 15, 30 and 45 kg 

N ha
-1

) respectively. Each plot contained two rows of soybean by 
the sides of the ridges at row spacing of 10.0 cm and a central row 
of cassava on the crest of the ridges at a plant distance of 1.0 m. 
There was no soybean by the sides of sole cassava ridges; also 
there was no cassava on the crest of sole soybean ridges. In 

experiment 2, nitrogen levels increased from 0 to 90 kg N ha
-1

 (0, 

15, 30, 45, 60, 75 and 90 kg N ha
-1

). Three cropping systems were 
maintained: sole cassava, sole soybean and cassava/soybean 
intercrop in all the experiments. There was uniform application of 

potassium and phosphorus fertilizers at the rates of 80 kg ha
-1

 of 

potassium and 50 kg ha
-1

 of phosphorus to all the plots through all 
the experiments. All the fertilizer materials were applied at 14 days 
after planting using row application method. Weeds were controlled 
manually.  

Soybean was harvested 110 days after planting while cassava 
was harvested 4, 8 and 12 months after planting. Soil samples at 0–
30 cm were taken from all the plots; composite samples were made 
for each treatment and were analyzed for total Kjeldahl nitrogen 
using macro Kjeldahl method for soil analysis (AOAC, 1984). 
Statistical analysis was done using statistical analytical system 
(SAS). Combined analysis of variance (ANOVA) was done using 
the general linear model procedure GLM to determine variations in 
yield between the system efficiency. 

 
 

 
 

 

RESULTS AND DISCUSSION 

 
The clay-loam soils before the treatment application in 
experiments 1, 2 and 3 were generally acidic with soil pH 

(H2O) ranging from 4.4 to 4.7% while the total nitrogen 
was in the range of 0.034 to 0.045%. 
 

 

Tuber yield of cassava 

 
Yield of fresh cassava increased with increasing rate up 

to 45 kg N ha
-1

 in 2000 and 2002 (Tables 1 and 2) and 

up to 90 kg N ha
-1

 in 1996 (Table 3) at 4 MAP in all the 
systems.  

The significant increasing rate to the highest nitrogen 

levels (45 and 90 kg N ha
-1

) (in sole cassava) over the 

lower nitrogen rates of applied nitrogen in the three 
years, showed that only applied nitrogen was available at 
4 MAP and did not satisfy the nitrogen need of cassava 
at all the levels of applied nitrogen at 4 MAP. This view is 
further suggested by the fact that sole cassava at 4 MAP 
produced more yield than intercropped cassava where 
competition for applied nitrogen was stiffer, biologically 
fixed nitrogen may not have become available to the 
intercropped cassava.  

At eight months after planting (8 MAP), fresh tuber 
yield of intercropped cassava was significantly higher 
than sole cassava and intercropped cassava with barrier 

at all nitrogen rates except at zero and 90 kg N ha
-1

 

(Tables 1, 2 and 3). Intercropped system yielded higher 
than sole crop probably because, fixed nitrogen has been 
made available with time (8 MAP) from the intercropped 
soybean, except in the intercropped system with barrier 
where cassava was barred from n-fixation and transfer. 
Similarly, at 12 months after planting, (Table 1), tuber 
yield was significantly higher at intercrop system without 
barrier than in sole at all nitrogen levels from zero 

nitrogen to 60 kg N ha
- 1

. The extra nitrogen was 

probably derived from nitrogen fixation of the associate 
legume crop. This finding agrees with Leihner (1988) who 
observed that the amount of nitrogen fertilizer 
recommended for cassava would be reduced if cassava 
were grown in association with efficient n-fixing legumes.  

At 8 MAP, (Table 4), the tuber yield on intercrop X 45, 

60, 75 and 90 kg N ha
-1

 did not differ among themselves 
but were significantly higher than at intercrop X 0, 15 and 

30 kg N ha
-1

. At 12 MAP, highest tuber yield was at 

intercrop X 45 and 60 kg N ha
-1

 N (29.9 and 30.0 kg ha
-1

, 
respectively). In the sole crop system the highest tuber 

yield was at sole X 90 kg N ha
-1

. This showed that the 
intercrop system gained extra nitrogen probably, the 
nitrogen fixed by the associate legume crop. This finding 
agrees with the finding of Bandyopadhyay and De 
(1986), Eaglesham et al. (1982) and Leihner (1988) who 
observed that some nitrogen was taken up by sorghum 
grown concurrently with nitrogen fixers. 



 
 
 

 

Table 1. Effect of applied nitrogen and cropping system on tuber yield of cassava (t. ha
-1

) at 4, 8 and 
12 (MAP) in 1995. 

 

Cropping system 
 Nitrogen level (kg N ha

-1
)  

 

0 15 30 45 Mean 
 

 
 

4 MAP      
 

Sole cassava 1.93 2.10 2.28 2.50 2.22 
 

Intercropped without barrier 1.38 1.58 1.54 2.20 1.84 
 

Intercropped with barrier 0.04 0.05 1.25 1.65 0.95 
 

Mean 1.24 1.40 1.81 2.12  
 

F-LSD0.05 for N = 0.23 CS = 0.02 N x CS = 0.40    
 

8 MAP      
 

Sole cassava 5.75 9.50 16.00 18.50 12.44 
 

Intercropped without barrier 9.00 12.75 18.25 26.00 16.50 
 

Intercropped with barrier 6.25 8.00 9.75 7.00 7.75 
 

Mean 7.00 10.08 14.67 17.17  
 

F-LSD0.05 for N = 2.42 CS = 2.10 N x CS = 4.19    
 

12 MAP      
 

Sole cassava 14.00 18.50 21.50 26.25 20.06 
 

Intercropped without barrier 15.25 22.00 27.00 35.25 24.88 
 

Intercropped with barrier 4.50 5.75 11.00 9.50 7.69 
 

Mean 11.25 15.4 19.83 2 .67  
  

F-LSD0.05 N = 3.42 CS = 3.01 N x CS = 6.02   
N = nitrogen; CS = Cropping system; N x CS = nitrogen x cropping system; NS = non significant. 

 

 

Table 2. Effect of applied nitrogen and cropping system on tuber yield (t. ha
-1

) of cassava at 
4, 8 and 12 (MAP) in 1997. 

 

 
Cropping system 

 Nitrogen level (kg N ha
-1

)  
 

 

0 15 30 45 Mean 
 

  
 

 4 MAP      
 

 Sole crop without barrier 0.29 0.44 0.53 0.63 0.47 
 

 Sole crop with barrier 0.33 0.22 0.18 0.11 0.21 
 

 Intercrop without barrier 0.46 0.57 0.60 0.87 0.63 
 

 Intercrop with barrier 0.17 0.2 0.26 0.36 0.25 
 

 Mean 0.39 0.36 0.39 0.50  
 

 LSD0.05 for N = 0.016 CS = 0.016 N x CS = 0.033     
 

 8 MAP      
 

 Sole crop without barrier 4.75 8.5 14.0 48.5 11.44 
 

 Sole crop with barrier 4.0 5.75 7.25 7.0 6.0 
 

 Intercrop without barrier 8.0 10.75 12.25 24.0 13.0 
 

 Intercrop with barrier 5.62 6.85 8.23 6.21 6.73 
 

 Mean 5.59 7.96 8.93 13.93  
 

 LSD0.05 for N = 1.55 CS = 1.55 N x CS = 2.54     
 

 12 MAP      
 

 Sole cassava without barrier 16.79 18.48 21.83 34.6 22.93 
 

 Sole cassava with barrier 9.88 10.64 10.89 11.7 10.78 
 



 
      

Table 2. Contd.       
       

Intercropped cassava without barrier 16.44 22.73 19.93 31.14 22.56  

Intercropped cassava with barrier 7.89 8.9 9.7 10.33 9.21  

Mean 12.79 15.19 15.51 21.94    
LSD0.05 for N = 2.82 CS = 2.82 N x CS = 5.64   

N = nitrogen; CS = Cropping system; N x CS = nitrogen x cropping system; NS = non significant. 
 
 
 
 

Table 3. Effect of applied nitrogen and cropping system on cassava tuber yield (t. ha
-1

) at 4, 8, and 12 MAP. 
 

Cropping system 
  Nitrogen level (kg N ha

-1
)   

 

0 15 30 45 60 75 90 Mean 
 

 
 

4 MAP         
 

Sole crop 0.49 0.48 0.58 0.58 0.67 0.87 0.91 0.65 
 

Intercrop without barrier 0.31 0.44 0.48 0.42 0.51 0.53 0.59 0.47 
 

Mean 0.40 0.46 0.53 0.50 0.59 0.70 0.75  
 

LSD0.05 for N = 0.025 CS = 0.015 N x CS =0.04       
 

8 MAP         
 

Sole crop 4.09 4.21 4.66 4.97 5.36 5.81 6.09 5.02 
 

Intercrop without barrier 4.23 5.61 5.83 6.82 6.89 6.87 6.32 6.02 
 

Mean 4.16 4.91 5.25 5.90 6.13 6.34 6.21  
 

LSD0.05 for N = 0.25 CS = 0.13 N x CS = 0.36       
 

12 MAP         
 

Sole crop 8.70 12.30 15.90 19.80 20.70 22.70 24.90 17.86 
 

Intercrop without barrier 11.70 13.80 24.60 29.90 30.00 26.10 20.70 22.40 
 

Mean 10.20 13.10 20.30 24.90 25.40 23.50 22.80  
  

LSD0.05 for N = 0.83 CS = 1.55 N x CS = 2.19  
 
 

 

Table 4. Effects of applied nitrogen and cropping system on Soil-N (mg.kg
-1

) after the harvest of cassava at 
4, 8 and 12 MAP. 

 

Cropping system 
  Nitrogen level (kg N ha

-1
)   

 

0 15 30 45 60 75 90 Mean 
 

 
 

4 MAP         
 

Sole cassava 0.032 0.034 0.034 0.034 0.064 0.037 0.037 0.039 
 

Sole soybean 0.157 0.154 0.149 0.164 0.089 0.120 0.109 0.135 
 

Cassava/soybean 0.101 0.134 0.138 0.131 0.090 0.091 0.099 0.112 
 

Mean 0.075 0.104 0.107 0.110 0.081 0.083 0.082  
 

LSD0.05 N = 0.018 CS = 0.043 N x CS = 0.065      
 

8 MAP         
 

Sole cassava 0.032 0.032 0.030 0.032 0.050 0.032 0.035 0.035 
 

Sole soybean 0.166 0.168 0.168 0.171 0.171 0.166 0.166 0.168 
 

Cassava/soybean 0.085 0.085 0.088 0.088 0.088 0.082 0.085 0.086 
 

Mean 0.094 0.095 0.095 0.097 0.103 0.093   
  

LSD0.05 N = NS CS = 0.052 N x CS = 0.075  



 
 
 

 
Table 4. Contd.  

 
12 MAP         

Sole cassava 0.030 0.030 0.030 0.030 0.030 0.030 0.030 0.030 

Sole soybean 0.157 0.157 0.157 0.157 0.158 0.160 0.160 0.150 

Cassava/soybean 0.078 0.080 0.088 0.088 0.086 0.086 0.090 0.085 

Mean 0.088 0.089 0.092 0.092 0.091 0.092 0.093   
LSD0.05 N = NS CS = 0.044 N x CS = 0.061   

N = nitrogen; CS = Cropping system; N x CS = nitrogen x cropping system; NS = non significant. 
 
 

 

The tuber yield at sole X 90 kg N ha
-1

 was equivalent to 

tuber yield at intercrop X 30 kg N ha
-1

 at 8 MAP. 

Similarly, the tuber yield at sole crop X 90 kg N ha
-1

 was 

equivalent to tuber yield at intercrop X 45 kg N ha
-1

 at 12 

MAP (Table 4), and tuber yield at 45 and 60 kg N ha
-1

 
under intercrop were statistically similar. Above 60 kg N 

ha
-1

, tuber yield decreased significantly with increasing 
nitrogen rate. This suggests therefore that cassava 

reached its response limit to nitrogen with 45 kg N ha
-1

 at 
intercrop system in the three consecutive cropping 
seasons: 2000, 2001 and 2002. At sole crop, the 

response to nitrogen continued up to 45 kg N ha
-1

 in 

2000 and 2002; and to 90 kg N ha
-1

 in 2001. This study 
therefore shows that the extra nitrogen available to 
cassava, which was derived from nitrogen fixation at both 
8 and 12 MAP, could be assumed to be equivalent to 60 

kg N ha
-1

 (90 - 30) at 8 MAP and 45 kg N ha
-1

 (90 - 45) at 
12 MAP of applied nitrogen in 3 months duration of 
soybean crop. This finding agrees with that of LaRue and 
Patterson (1981) who reported that soybean fixes 

between 45 - 217 kg N ha
-1

 annually. There was little or 
no additional benefit from added nitrogen when the 

applied-N was above 45 kg N ha
-1

 in soybean/cassava 
intercrop system. 
 

 

Soil nitrogen 

 

Soil nitrogen (Table 4) was lowest at sole cassava 
compared with all the cropping systems. At four months 
after planting there was no significant difference in soil 
nitrogen between sole soybean and intercrop system, but 
soil nitrogen under sole soybean (0.135 mg kg ) and 
intercrop system (0.112 m kg) where on the average 
three times higher than the soil nitrogen in sole cassava 
(0.039 mg kg). This suggests that although nitrogen has 
been fixed by soybean and was detectable in the soil, it 
has not been taken up by the cassava to make any 
difference since the yield of intercrop cassava (0.47 t.ha) 

was lower than sole cassava (0.62 t. ha
-1

) at 4 MAP 

(Table 3). At 8 and 12 MAP, soil nitrogen under intercrop 
has become significantly lower than that under soybean, 
suggesting that cassava the companion crop has taken 

 
 
 

 
up extra (fixed) nitrogen from the soil. At 8 months after 
planting (Table 4) soil nitrogen under sole soybean (0.168 

mg kg
-1

) and intercrop system (0.086 mg kg
-1

) were four 
times and two times higher than soil nitrogen under sole 

cassava (0.035 mg kg
-1

), respectively. While at 12 
months after planting (Table 4) sole soybean (0.150 mg 

kg
-1

) and intercrop (0.085 mg kg
-1

) had soil nitrogen 5 
times and 2.8 times higher than soil nitrogen under sole 

cassava (0.030 mg kg
-1

), respectively. These findings 
confirm that cassava benefited from nitrogen fixed by 
soybean when grown together. Soils under soybean and 
intercrop system had significantly higher nitrogen than the 
soil nitrogen before the experiment which was 0.042 mg 

kg
-1

 after 12 months of planting and subsequent crops 
grown on such soils should benefit from the residual soil 
nitrogen. 
 

 

System efficiency 

 
In 2000, the land equivalent ratio and area × time 
equivalent ratio of intercrop were above unity and were 

highest at 30 kg N ha
-1

. In 2002, LER and ATER 

increased with increasing nitrogen rate up to 45 kg N ha
-

1
. LER at all nitrogen rates was greater than unity while 

ATER X 90 kg N ha
-1

 was less than unity. 
 
 
Conclusion 

 

This study confirms that cassava/soybean intercropping 
system is a productive system, which has both economic 
and environmental benefits. Low nitrogen fertilizer 
requirement for the production of high yield of cassava in 
intercrop will result in reduction of the cost of 
procurement of fertilizer, labor input and higher land use 
advantage. Also the problem of leaching of excess 
nitrogen, which may occur as a result of fertilizer 
application to the system into ground water, will be 
reduced due to its full utilization by an accompanying 
cassava crop. The growth of a non nitrogen-fixing crop 
(cassava) intercropped with an efficient nitrogen-fixing 
legume (soybean) was compatible. There seems to be 



 
 
 

 

good utilization of photosynthetically active radiation with 
no apparent shading effect in the crop association. The 
cassava/soybean system is also a potentially sustainable 
system because it builds the soil by leaving it richer in 
nitrogen than before cropping. 
 

 
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