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African Journal of Agricultural Marketing ISSN 2375-1061 Vol. 5 (10), pp. 001-005, October, 2017. Available online at 
www.internationalscholarsjournals.org © International Scholars Journals 

 

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

 

Full Length Research Paper 

 

Economic analysis of soil fertility restoration 
options in potato – bean of gardening intercropping 

in South – rift, Kenya 

 
Ngeno Vincent*, Langat B. K., Rop Wendi, Chepngeno W. and Kipsat M. J. 

 
Department of Agricultural Economics and Resource Management, Moi University, Kenya. 

 
Accepted 23 May, 2017 

 
Soil fertility management options to restore soil fertility depletion have been developed and these consist of 
inorganic and organic fertilizer applications. However, the adoption rates by smallholder farmers for these soil 
fertility management options are negligible. The overall aim of this study was to evaluate side-by-side economic 
aspects of different soil fertility improvement options, which have been tested in the region. Field experiments were 
carried out to evaluate soil fertility management options on potato-bean production in five districts in South-rift, 
Kenya (Narok, Bomet, Bureti, Kericho and Konoin). It appeared that application of organic residues is a more 
feasible and sustainable alternative to the recommended fertilizers. However, for long term yield improvement, 
fertilization with these recommended fertilizers would only be profitable if applied seasonally. 

 

Key words: Fertility depletion, restoration option, economics. 
 

 
INTRODUCTION 

 
Soil fertility depletion, in many areas of sub-Saharan 
Africa, is reported to be the fundamental root cause of 
declining per capita food production (Sanchez and Jama, 
2002). In East Africa rates of nutrient depletion are huge 
and highest especially in areas with favourable climate for 
crop production and high population density (Stoorvogel 
et al., 2003). Yields of crops are decreasing on 
continuously cropped land without nutrient inputs, and 
fallowing is no longer practical due to the high population 
pressure (Cleaver and Schreiber, 2007). The use of 
fertilizers for soil fertility restoration/replenishment is often 
constrained due to lack of capital by farmers especially 
after the removal of government subsidies in Kenya and 
other African countries (Franzel, 2009; Jaetzold and 
Schmidt, 2003). The net result is low crop productivity of 
land that is being further degraded by erosion and the 
nutrient removals from crop harvests.  

To replenish these soils, various soil fertility restoration 
options have been tested by the National and  
 
 
 
*Corresponding author. E-mail: vcngeno@gmail.com. 

 
 
 
 

 
International organizations (Landon, 2004). However, the 
options tend to be site and institution specific in 
effectiveness and applicability. Furthermore, soil fertility 
restoration options or recommendations are normally 
formulated to cover broad areas with diverse soils in their 
particular fields (Tisdale et al., 2005). These soil fertility 
restoration options, and often some of the “best-bets” 
which have been tested in various parts of Rift Valley 
Kenya are:- Organic inputs, Farmyard manure, FURP 
(fertilizer use recommendation project), PRE-PAC 
(phosphate rock evaluation project package), MBILI 
(managing beneficial interactions in legume intercrops). 
Efforts to compare soil fertility management options side-
by-side are rather minimal in Kenya with research 
institutions preferring to test one technology at a time.  

Therefore, this study focuses on the side-by-side 
evaluation of the economic returns of the tested options, 
so that the recommended soil fertility management 
options fit the ecological and socio-economic 
circumstances of the farmers. The ultimate goal is 
therefore to contribute towards attainment of food security 
and reduction of poverty through adoption of productive 
soil fertility management options by smallholder 

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


 
 
 

 

farmers in South-Rift, Kenya. 
 

 
MATERIALS AND METHODS 
 
Data sources 
 
The study used on-farm experimental data and qualitative 
information gathered by interviewing participating farm household in 
different districts namely Narok, Bomet, Bureti, Konoin and Kericho 
districts in South-Rift Kenya. Production data and prices were 
collected for three consecutive growing seasons, beginning in the 
long rains of 2009; since some sites were established in the long 
rains of 2009 while others were in the short rains of 2009. 

 

Experimental design 
 
The field experiment comprised of six treatments during the initial 

installation (long rains 2001) and additional two treatments in the short 

rains 2001. The variation in the treatments was due to 

recommendations obtained from collaborating institutions in the region. 

The experiment was laid out in a randomized complete block design 

and replicated two to four times depending on land availability. The plot 

size was 5 x 5 m based on recommendations by Okalebo and Woomer 

(2006). This plot size was considered to represent the widespread low 

soil fertility patches common at on-farm level and was considered 

adequate to show treatment differences in the field particularly to 

smallholder farmers, the expected beneficiaries of the soil fertility 

management options. Information on the existence and applicability of 

this treatment was obtained from research publications and brochures in 

libraries from research institutions namely, Kenya Agricultural Research 

(KARI), International Centre for Research in Agroforestry (ICRAF), 

Sustainable Agricultural Centre for Research Extension and 

Development in Africa (SACRED-AFRICA), Kenya Forestry Research 

Institute (KEFRI, and Moi University, the treatments hereafter were 

referred to as soil fertility management options. Several economic 

indicators were developed and used to compare the benefits of 

producing potatoes/beans under the different soil fertility management 

options. The indicators were: 

 

Enterprise budgets 
 
Enterprise budgets were developed and used to compare costs and benefits 

accruing from food production under the different soil fertility management 

options. Rather than take an average of the costs and benefits for the 

cropping seasons, costs and benefits were compared across the seasons. 

This approach was preferred because it allowed a comparison of residual 

effects of the different options over time and assessment of incremental net 

benefits of using these options over typical farmer practices: 
 

 =                      ………………………………(1) 
 
Where: 
 

 = the gross value of output in season i, for a given 

management option in Kshs ha
-1

.  
 = the total output produced under a given management option 

in season i, in Kshs ha
-1

.  

 = the average price obtained for the output (maize or beans) 

for season i, in Kshs ha
-1

. 
 
It is an average of the price obtained immediately after harvest and 
that obtained 2 months after the harvest: 

 
 
 
 

 

 =                     …………………………… (2) 
 
Where: 
 

 = gross value of output in season i, produced under a given 

management option, 

 = gross costs incurred in season i, with that option in Kshs 

ha
-1

 and,   
= the net benefit obtained in season i in Kshs ha

-1
. 

 
Incremental net benefit 
 

 =               ……………………………………… ( 3 ) 
 
Where: 
 

 = the incremental net benefit of option j over the farmer's 

practice in: season (i), in Kshs ha
-1

. 

 = the net benefit of option j, in season i, in Kshs ha
-1

.  

= the net benefit from the farmer's practice (either absolute 

control option or farm yard manure) in season i, in Kshs ha
-1

. 

 
Return to land 
 
Return to land was devised from the net present value (NPV) of 
each treatment summed over the cropping seasons (2 to 3 
seasons). In general, the return to land is more appropriate as an 
an economic indicator in situations where land is relatively scarcer 
than labour or where there are fewer opportunities for the farmer to 
hire out labour or to engage in off-farm employment. Under such 
circumstances, land is viewed as the most limiting resource and 
hence farmers should strive to optimize return to land. For farm 
households who have ample opportunities for off-farm employment 
and/or have relatively high land-labour ratios, returns to labour 
would be the relevant indicator because of the relatively high 
population pressure, availability of hired farm labour and limited off-
farm employment opportunities in the study areas  
 
 

 
……………..……… ( 4 ) 

 
Where: 
 

 = The net present value from applying management option j,  

The discount rate estimated at 

10%, and,  = The time in seasons. 

 

The marginal rate of return of investing in the technology 

 
The marginal rate of return (MRR) is used to show how net benefits 

accruing from an investment increase as the amount invested 

increases. The MRR was calculated by dividing the marginal net 

present value by the marginal cost, as a farmer shifts from an option 

with low net benefit to one with the highest net present value. The first 

step in calculating the MRR was to identify the option with the highest 

NPV (that is, return to land). The MRR was then 



 
 
 

 
Table 1. Marginal rate of return (MRR) for adopting 
different soil fertility management options in Narok.  

 
 Technology MRR (%) 

 Control to FYM 509 

 PRE-PAC to FYM 20 

 MBILI to FYM 208 

 MBILI to PRE-PAC 491 

 Compost to FYM 760 

 MBILI to FURP 109 
 

 
calculated by dividing the marginal return (difference between the 
option with the highest return to land and any other option, such as 
farmers’ practice, that is absolute control or farm yard manure) by 
the marginal costs (difference between the gross costs of the option 
giving the highest return to land and the one being compared with) 
times 100%. 
 

 

RESULTS AND DISCUSSION 

 
Return to land 

 

The returns to land from food production under the 
different soil fertility management options were estimated 
by the NPV of the net benefits over the two seasons. The 
results show negative returns to land for all the five soil 
fertility management options in Bureti and Sotik sites. The 
negative returns to land are explained by the relatively 
low yield levels especially in the second season. In 
Bomet site, because of relatively high production costs, 
only use of FYM produced a positive return to land. Both 
PREP-PAC and FURP registered negative returns to 
land. This finding implies that rather than shift to new soil 
fertility management options such as use of organic 
inputs, FURP or PREP-PAC, farmers may be better off 
applying FYM to their potato and bean crops. In Kericho, 
all the technology options recorded negative returns to 
land, perhaps because of the low yield response 
observed and the relatively high production costs, 
especially with PREP-PAC, MBILI and FURP. Even 
organic inputs and farmyard manure, which recorded 
some positive net benefits in the second season, on the 
aggregate had negative returns to land.  

Although results of the economic analysis appear 
daunting, an examination of the yield data suggests that 
MBILI and farmyard manure might be more promising as 
soil fertility augmenting and soil fertility management 
options in Kericho, if something could be done to reduce 
the production costs, particularly labor costs. The highest 
return to land in Narok of Kshs. 61,4087- was obtained 
from use of FYM, a typical farmer practice in the area. 
The second highest return to land was obtained from 
MBILI. In Konoin, fortified compost gave the highest 
return to land (Kshs. 15,4807- per ha per year), followed 
by MBILI with Kshs. 1,4197. All the other soil fertility 
management options had negative returns to land. This 

  
  

 
 

 

finding implies that farmers in Konoin can improve their 
net incomes and overall return to land by shifting from 
current practices such as zero input or little farmers' FYM 
to use of fortified compost or MBILI as soil fertility 
replenishment strategies. 
 

 

Marginal rate of return 

 

In general, the marginal rate of return (MRR) is another 
indicator of what the farmers can expect to gain, or on 
average, in return for their investment when they decide 
to change from one practice (or a set of practices) to 
another. Returns to land suggest that in Narok, the typical 
farmer practice of applying some FYM is by far the most 
profitable soil fertility option to use in production. 
However, three other technology options gave higher 
returns to land than the absolute control. These 
technology options were, MBILI (Kshs. 59,0667-), FURP 
(Kshs.51,5907-) and PREP-PAC(Kshs.46,3137-). Table 1 
shows the results of marginal analysis, depicting what 
farmers can expect to gain at the margin, in return to their 
investment when they decide to change from one of 
these technology options to another. As the table shows, 
the highest rate of return to investment is obtained from a 
shift from absolute control and fortified compost to farm 
yard manure. A shift from MBILI to FYM or to PRE-PAC 
or FURP also can result in rates of return to investment of 
over 100%. The empirical evidence has shown that rates 
of return above 100% are above the minimum acceptable 
to farmers. This suggests that all things being constant, it 
is safe to recommend to farmers in Narok, farm yard 
manure, and FURP as soil fertility augmenting soil fertility 
management options with which to produce food crops 
such as potatoes and beans. 
 

 

Marginal rate of return 
 

In general, the marginal rate of return (MRR) is another 
indicator of what the farmers can expect to gain, or on 
average, in return for their investment when they decide 
to change from one practice (or a set of practices) to 
another. Returns to land suggest that in Narok, the typical 
farmer practice of applying some FYM is by far the most 
profitable soil fertility option to use in production. 
However, three other technology options gave higher 
returns to land than the absolute control. These 
technology options were, MBILI (Kshs. 59,0667-), FURP 
(Kshs. 51,5907-) and PREP-PAC (Kshs.46,3137-). Table 
1 shows the results of marginal analysis, depicting what 
farmers can expect to gain at the margin, in return to their 
investment when they decide to change from one of 
these technology options to another. As the table shows, 
the highest rate of return to investment is obtained from a 
shift from absolute control and fortified compost to farm 
yard manure. A shift from MBILI to FYM or to PRE-PAC 
or FURP, can also result in rates of return to 



 
 
 

 
Table 2. Incremental net benefits (INB) from farmers practice (Kshs) in Konoin district. 

 

 
Incremental net benefits 

1
st

 season 1
st

 season 2
nd

 season 2
nd

 season Total 
 

 
INB discounted INB INB discounted INB discounted INB  

  
 

 From absolute control to compost 24800.43 22543.59 15359.50 12686.95 35,230.54 
 

 From absolute control to PRE-PAC 8153.58 7411.60 16655.55 13757.48 21,169.09 
 

 From FYM to compost 18790.50 17080.57 5812.10 5812.10 21,881.36 
 

 From FYM to PRE-PAC 2143.66 1948.58 7108.15 5871.33 7,819.91 
 

 

 
Table 3. Incremental net benefits (Kshs) for various soil fertility management options in Narok. 

 

 
Technology 

1
st

 season 1
st

 season 2
nd

 season 2
nd

 season Total 
 

 

INB Discounted INB INB Discounted INB Discounted INB  

  
 

 FYM 1115.15 1013.67 22369.42 18477.14I 19490.81 
 

 FURP -755.15 -686.43 12541.57 10359.34 9672.91 
 

 PREP-PAC -13040.80 -11854.10 19672.92 16249.83 4395.74 
 

 MBILI -14516.90 -13195.90 36736.72 30344.53 17148.67 
 

 

 

investment of over 100%. The empirical evidence has 
shown that rates of return above 100% are above the 
minimum acceptable to farmers. This suggests that all 
things being constant, it is safe to recommend to farmers 
in Narok, farm yard manure, PRE-PAC and FURP as soil 
fertility augmenting soil fertility management options, with 
which to produce food crops such as potatoes and beans. 
 

The evaluation of the five technology options tested in 
Konoin so far suggest that it would be profitable for 
farmers to adopt use of fortified compost or PRE-PAC as 
soil fertility management strategies. The results of the 
marginal analysis adopting show that adopting fortified 
compost implies a 1639% rate of return to farmers who 
currently use minimum FYM in potato and bean 
production. For farmers who currently apply nothing to 
augment soil fertility in potato and bean production, 
adoption of fortified compost. The technology that offered 
the highest aggregate output and return to land, implies 
the highest rate of return for farmers currently applying 
minimum FYM rate or not applying any inputs. Since 
experience and empirical evidence have shown that the 
minimum rate of return acceptable to farmers falls 
between 50 to 100%, it is safe to recommend fortified 
compost to farmers in Konoin District. 

 

Incremental net benefit (INB) 
 
Since all the technology options had negative net benefits 
in Sotik, Bomet, Bureti and Kericho, it was not sensible to 
determine the incremental net benefit of shifting from 
farmers' practices to other soil fertility management 
options. Table 2 shows the potential incremental net 
benefits that farmers in Konoin District can gain by 
changing from their current soil fertility management 
practices to use of either fortified compost or using PRE-
PAC. The highest incremental income is 

 

 

obtained by a shift from farmer practice of not applying 
anything, to use of fortified compost. The results show, 
for example, that a shift from no inputs to use of fortified 
compost can increase the net benefits (net income) by 
about Kshs. 25,000/- in the first season and by Kshs. 
15,000/- in the second season.  

The incremental net benefits of switching from farmer 
practice to new technology options in Narok could not be 
extensively analyzed since the farmer practices seemed 
to perform better than the alternative new options. 
Nevertheless, a move from no inputs to use of farmer's 
farm yard manure promises an incremental net benefit of 
nearly Kshs. 20,000/- in the two seasons, most of which 
is achieved in the second season (Table 3). 
 

 

CONCLUSION AND RECOMMENDATION 

 

On average, the performance of the soil fertility manage-
ment options in terms of yield response particularly the 
residual effect in subsequent seasons and the net 
benefits was inferior than anticipated in most sites. The 
sites in Bureti, Bomet and Sotik showed the poorest 
performance with soil fertility management options 
showing negative net benefits and return to land. 
Nevertheless, at some locations, the data was indicative 
of the possible technology options that could be 
recommended to farmers. In Narok, the farmer practice 
of use of FYM gave the highest net benefits and return to 
land from food production, followed by PRE-PAC and 
FURP. Results of marginal analysis further supported 
promotion of these technology options to farmers in the 
area because they showed rates of return to investment 
of over 100%.  

Thus, based on the available data, it is safe to urge 
farmers in Narok to continue use of FYM and to promote 
recommended use of PRE-PAC and FURP. In Konoin, 



 
 
 

 

the analysis revealed that fortified compost had the 
highest potential of improving net benefits and returns to 
land from food production, and therefore should be 
promoted to farmers in Konoin was PRE-PAC, although 
the return to land was a modest Kshs. 1,418/- compared 
to over Kshs. 15,000/- from fortified compost. In general, 
this study had a limitation of five soil fertility amelioration 
soil fertility management options at five districts and two 
cropping seasons. There is need to extend the scope of 
the study to include a wide range of nutrient depleted 
soils in Kenya. Nonetheless, guidelines on the usefulness 
of soil fertility management options have been obtained. 
 

 
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agriculture and environment nexus in sub-Sahara Africa. World Bank 
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Landon JR (2004). Booker tropical soil manual. A handbook for soil 
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Tisdale SL, Werner LN, Beaton JD (2005). Soil fertility and fertoizers. 
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