Agro-Economic Implications of Combined Application of Organic and Mineral Fertilizers on Maize

Continuous cultivation without adequate 
fertilizer application is responsible for plant nutrient depletion and yield 
decline of major food crops in sub-Saharan Africa. Field experiments were conducted 
on umbric Cambisol in Babungo and ustic Oxisol in Bansoa in the Western Highlands 
of Cameroon over two years to evaluate the effects of combined application of leaf 
biomass of agroforestry species and mineral fertilizers with their economic returns 
on maize yield performance. Ten treatments encompassing sole mineral fertilizer, 
sole leaf biomass of Calliandra, Leucaena, Acacia and Cassia, leaf biomasses combined 
with mineral fertilizer and absolute control were laid in a randomized complete 
block design and replicated three times. Data collected on maize grain yield was 
subjected to analysis of variance using XLSTAT version 2014 statistical software. 
In addition, an economic analysis was performed to evaluate the profitability of 
the different treatment combinations. The results showed significant differences 
(P ≤ 0.05) amongst 
the different treatments on maize yield on both soil types. The highest maize yields 
which stood at 5.93 t/ha and 6.01 t/ha were recorded in plots treated with Acacia and Cassia plus half recommended mineral fertilizer on umbric Cambisol and 
ustic Oxisol respectively. Economic analyses showed that Acacia and Cassia plus half 
recommended mineral fertilizer gave the highest net benefit and return to labor 
of 286500 FCFA and 5.1 on umbric Cambisol and 296051 FCFA and 5.3 on ustic Oxisol 
respectively. On both soils, 
all the treatments except Calliandra with 
half recommended mineral fertilizers were economically profitable with benefit to 
cost ratios that ranged from 2.02 to 2.92. Combined application of agroforestry tree species leaf 
biomass and mineral fertilizers is an appropriate technology to enhance maize grain 
yield and is economically profitable except Calliandra with half recommended mineral fertilizers and is therefore recommended for degraded Calliandra and Oxisols 
in the Western Highlands of Cameroon.


Introduction
Maize (Zea mays L.) is a quick growing C 4 plant of the Poaceae family. Among cereals, it is an important food and feed crop which ranks third after wheat and rice in [1]. According to Ragasa et al. [2], the bulk of maize produced in sub-Saharan Africa (SSA) goes into food consumption making it an important crop for food security. Because of its expanded use in the agro-industries, it is recognized as a leading commercial crop of great agro-economic values. Despite the entire benefits and availability of improved germplasm, average maize yield in SSA including the Western Highlands of Cameroon (WHC) remains one of the lowest (1.6 t/ha) in the world [3].
In most parts of SSA including Cameroon, the inherent poor fertility status, continuous cultivation without adequate supply of nutrients, soil erosion, lack of soil fertility restoring practices and nutrient mining especially N and P are major reasons for plant nutrient depletion and yield decline of major food crops including maize [4] [5]. Unsustainable farming activities have led to nutrient depletion and the overall result has been a decline in land productivity and massive environmental degradation all contributing to malnutrition and food insecurity [6]. In order to obtain high maize yield and sustain productivity, the soil's nutrient status needs to be increased [7].
Application of inorganic fertilizers is widely used globally to overcome nutrient deficiencies and dramatic increases in yields of several crops have been obtained [8]. However, their use remains very low in SSA including the WHC and ineffective for sustaining crop production and maintaining soil fertility because they are not affordable and accessible to poor resource farmers [9]. Moreover, continuous use of inorganic fertilizers acidifies the soil, promotes cation loss and depletes macronutrients through losses in gaseous form and by leaching especially when recommended doses are not respected [8]. Since resource-limited farmers cannot afford inorganic fertilizers because of high prices and consequently low profit margins, it is necessary to seek for affordable and less risky nutrient management practices that contain necessary ingredients for superior performance [10] [11]. Greater yield benefits can be achieved following the combined application of organic resources and mineral fertilizers compared to either resource applied singly [11] [12]. Combined application of organic materials and inorganic fertilizers enhanced carbon storage in soils and reduced emissions from nitrogen fertilizer use while contributing to high crop productiv-  [13]. According to Abdelzaher et al. [12], integrated application of organic and inorganic fertilizer increased the growth and yield of maize than when any of the fertilizer was used alone. Improved application methods targeting inorganic fertilizers and organic inputs do not only conserve nutrients in the soil, but also increase nutrient uptake efficiency [14] [15] and the long-term build ecologically sound and viable crop production and farming systems [8] [ 16]. Organic fertilizers can improve soil physicochemical properties, increase yield for major food crops and favor nutrient cycling through mineralization or immobilization turnover of added matter [17] [18] [19]. Organic resources play an essential role in soil fertility management in the tropics by their short-term effects on nutrient supply and long-term contribution to soil organic matter formation [20].
Leaf biomass of Calliandra calothyrsus, Leucaena leucocephala, Acacia angustissima and Cassia spectabilis hereafter referred to as: Calliandra spp., Leucaena spp., Acacia spp. and Cassia spp. respectively are some organic resources used in soil management practices in the WHC [4] [5] even though at very low application rates. Although, recommendations of the use of agroforestry tree biomass for soil fertility restoration and climate change adaptation abound, adaptation in the WHC remains low.
In the study area, apart from works carried out to examine the effects of green manure and NPK fertilizer on the growth and yield of maize in the mount Cameroon region by Egbe et al. [4] and the effect of tree hedgerow pruning on maize yield by Neba et al. [5], information on the effect of application of these organic materials and inorganic fertilizers into the soil on maize yield performance as well as their economic returns is scarce. However, adoption of any new technology depends on farmer's perceptions of financial benefits, particularly when additional labor is required in the establishment and management of these technologies [18].
It is against this backdrop that this study was therefore established in the two main maize growing areas of the WHC in 2018 and 2019 with the main objective of bringing closer, feasible soil nutrient replenishment technologies to resource-limited farmers to increase soil fertility and enhance food security through maize production. More specifically, the study was designed to:  Establish the interactive effect on the combined use of leaf biomass of Calliandra spp., Leucaena spp., Acacia spp. and Cassia spp. and inorganic fertilizers into soils on maize yield;  Evaluate the economic returns of combined application of leaf biomass of Calliandra spp., Leucaena spp., Acacia spp. and Cassia spp. and mineral fertilizer with maize as test crop in the WHC.

Location and Description of Study Sites
Agronomic experiments were conducted on-farm under rain-fed conditions N. E. Banye et al. during two cropping seasons (2018 and 2019) in two sites: the experimental plot of the Institute of Agricultural Research for Development (IRAD), Babungo in the North-West region and the Teaching and Research farm of the Department of Agronomy at Bansoa in the West region, Cameroon. The growing period covered end-March to mid-September. Babungo is located between longitudes 15˚25'E and 15˚30'E and between latitudes 5˚4' and 5˚10'N and situated at an altitude of about 300 meters above sea level while Bansoa is located between longitude 10˚15'E and latitude 05˚25'N and situated at an altitude of about 1414 meters above sea level. The areas have mean monomodal annual rainfall and temperature of approximately 1600 mm and 27.2˚C for Babungo and 1936 mm and 20.4˚C for Bansoa respectively. The soil of Babungo which is sandy clay loam is broadly classified as umbric Cambisol while that of Bansoa is sandy loam and classified as ustic Oxisol [21] [22]. The results of soil analyses before the start of experiments showed that they were acidic, high in organic matter, low in total N and available P as shown in Table 1.

Plant Material, Organic and Mineral Fertilizers Used in the Study
Plant material used in the study was maize composite seed variety (Cameroon Highlands Composite) CHC 202 with yellow flint due to its adaptability to acidic soil of the western highlands agro-ecological zone purchased from IRAD Dschang. According to inquiries with agronomist and farmers in the study areas, it is a variety of great agronomic performance since it is tolerant to leaf blights, Weeding was done at 4 and 12 weeks after planting. Stem borers in maize were controlled by preventive spraying of Caillemas Blue pesticide. Crop residues from the first major rainy season cropping were totally removed from the fields to reduce the confounding or residual effects of organic residues of the maize plants for the subsequent cropping major rainy season.

Agronomic Data Collection
Four weeks after planting (4 WAP), 4 maize plants in the middle rows with similar characteristics were randomly selected from each plot and tagged and data on maize yield was collected on ustic Oxisol and umbric Cambisol for each cropping season as described by Agba et al. [23] with slight modification. To determine grain yield, maize plants from each replicate were manually harvested at physiological maturity (20 WAP), peeled and oven-dried at 70˚C inside envelopes for three days and hand-threshed. Dry maize grains were weighed on a scale balance to obtain the total yield (t/ha) and grain yield was computed in tons per hectare based on the population of plants per hectare (62,500) used in  (1) Agba et al. [23] below: where, GY ha = Grain yield per hectare; Y p = Average grain yield per plant and P ha = plant population per hectare.

Economic Analysis
Detailed data on labor requirements were collected every season for each of the field operations (land preparation, planting, fertilizer application, weeding, pest control and harvest). The time taken to perform every activity was recorded and the labor was costed at the local wage rate of Cameroon FCFA per working day (8 hours) in the study area. Other input and output prices, derived from the farm gate prices and wages in the area, and costs used in the economic analysis are presented in (Table 2). However, since all the organic amendments were collected near the experimental plots, only the labor for harvesting, transportation and incorporation were taken into consideration [14]. A benefit cost analysis was done to evaluate the profitability of leaf biomass of agroforestry tree species and mineral fertilizer used in the experiments. The benefit to cost ratio was calculated following the procedure described by Boukong et al. [22] with slight modification based on the following costs: Total cost of labor (TCL) was calculated according to Equation (2): where: CLP = Labor cost for land preparation, CPM = Labor cost for planting maize, CAF = Cost for applying fertilizers, CAP = Labor cost for applying pesticide, CW = Labor cost for weeding and CSL = Labor cost for harvesting maize. Total cost of production (TCP) was calculated as shown by Equation (3): The benefit to cost ratio (B/C) was calculated using Equation (5): where: RCSY = Return cost of supplementary yield and RCPF = Return cost of fertilized treatments. Profitability (RT) was calculated as shown in Equation (6): where: RCSY = Return cost of supplementary yield and RCPF = Return cost of fertilized treatments.
Net benefit (NB) was calculated based on Equation (7) where: TCP = Total cost of production and TCL = Total cost to labor.

Statistical and Economic Analyses of Data
Analysis of variance was conducted using XLSTAT 2014 statistical software to compare treatment effects on maize yields. Means were separated using the student Newman keuls' Test at 5% level (P ≤ 0.05). In order to conduct an econom-ic analysis of the different treatments, partial budgeting analysis techniques was employed to evaluate the various costs and corresponding benefits of technologies used in the experiments.

Effects of Combined Application of Organic and Mineral Fertilizers on Maize Grain Yield on Umbric Cambisol and Ustic Oxisol over 2 Years
The results of ANOVA indicated that maize grain yield varied significantly (P ≤ 0.05) among treatments on umbric Cambisol and ustic Oxisol and between soil types. However, the test used in mean separation (student Newman keuls' Test) showed that there were no significant differences among sole biomass treatments as well as those combining leaf biomass and mineral fertilizer. Furthermore, the results over 2 cropping seasons showed that, 1½ t/ha Acacia + (50 N + 37.  Table 3.

Economics of Combined Application of Organic and Mineral Fertilizers on Maize on Umbric Cambisol and Ustic Oxisol over 2 Years
The results of the economic analysis varied according to the technology used in soil management practices. On the average across the two cropping seasons, 1½

Effects of Combined Application of Organic and Mineral Fertilizers on Maize Grain Yields on Umbric Cambisol and Ustic Oxisol over 2 Years
Grain yield is the final result of many complex morphological and physiological processes taking place during the growth and development of crops [24]. The application of organics alone or in combination with mineral fertilizers led to increased maize grain yield compared to the control. Sole application of leaf biomass of Calliandra spp. and Acacia spp. (umbric Cambisol) and Leucaena spp. and Cassia spp. (ustic Oxisol) generated higher grain yields than the control treatments while the combination of leaf biomass of Calliandra spp. and Acacia spp. (umbric Cambisol) and Leucaena spp. and Cassia spp. (ustic Oxisol) and inorganic fertilizers generated higher grain yields than other treatments ( Table  3). The higher yields from organic treatments could be attributed to positive effects of organic materials on soil physicochemical properties [14]. The differences in fertilizer technologies that generated higher yields on umbric Cambisol and ustic Oxisol might be related to their nutrient release patterns. These technologies might have released nutrients proportionately to crop demand during growth and development stages which led to greater maize yields. Soil characteristics might have influenced nutrient release patterns of organic residues added to the soil [23].
According to Huang et al. [8], the leafy pruning incorporated into the soil at the beginning of the season decomposed and released nutrients especially nitrogen, which enhanced crop performance and soil characteristics. The integration of organic and inorganic fertilizers might have resulted into synergy and improved synchronization between nutrient release and uptake by plants leading to higher yields [25]. According to Vanlauwe et al. [26] and Afe et al. [24], the production of grain yield might be due to better growth, development and dry matter accumulation with proper supply of nutrients to plant and increase in the availability of other plant nutrients with the respective source of nitrogen application or extra crop produce can often be observed due to positive direct or indirect interactions between fertilizer and organic inputs as suggested by Magda et al. [27]. The inorganic fertilizer provides most of the nutrients and the organic fertilizer increases soil physicochemical properties [17]. The findings of this study are similar with those of Mugendi et al. [28] who found that Leucaena biomass combined with mineral fertilizer gave higher crop yields as compared to sole use of mineral fertilizer or sole Leucaena biomass. These results corroborate with those of Mucheru-Munaet al. [18] who found that Leucaena and Calliandra biomass combined with mineral fertilizer gave higher maize yields as compared to sole use of inorganic fertilizer or Leucaena and Calliandra biomass. These results equally corroborate with those of Zhao et al. [29] who reported that farmyard manure combined with chemical fertilizer resulted in higher increase in maize yield. Similarly, Dilshad et al. [30] and Makinde and Ayoola [31] stated that high and sustainable crop yields are only possible with integrated use of mineral fertilizers with organic matter. Maize plants in the control plots which did not receive any treatment but submitted under the same ecological conditions (light, amount of water, constant weeding and treatment of insecticides) showed low results as far as grain yields are concern. The relatively poor maize grain yields results in the control plots could be partly due to low doses of mineral elements compared to optimal doses used in the experimentation. It could equally be due to the absence of phosphorus in the soil as lack of phosphorus has been particularly identified as a factor limiting the growth of plants. Besides, Baraloto [32] showed that most tropical zones are situated on soils chemically poor, especially in phosphorus and nitrogen. Since the organic materials may often not be available in large amounts that are required for sole application, farmers are encouraged to adopt combined application of leaf biomass and inorganic fertilizers is a better option for increasing fertilizer use efficiency and providing a more balanced supply of nutrients and ultimate yield otherwise higher N and P application rates are required to attain better yield in maize [17] [33].

Economic Returns of Combined Application of Organic and Mineral Fertilizers on Maize on Umbric Cambisoland Ustic Oxisol over 2 Years
Averagely across the two cropping seasons, 1½ t/ha Acacia + (50 N + 37. Boukong et al. [22] or lower labor required compared to the sole applications [18]. This study equally showed thatall the treatments that received fertilizers were economically profitable as their benefit to cost ratios were greater than 2 but treatments with sole leaf biomass recorded the higher benefit to cost ratios compared to combined leaf biomass and inorganic fertilizers. This could be attributed to the fact that the cost of organic fertilizer and the labor cost of applying it was lowercompared to combined leaf biomass and inorganic fertilizers. These findings are similar with those of FAO [34], who reported that any treatment that receives fertilizer in an environment where water is not limiting is considered profitable only when its B/C ratio is equal to or greater than 2. These findings are in agreement with those of Mucheru-Muna et al. [18] who reported that Leucaena gave the highest benefit to cost ratio while manure and Tithonia with half recommended rate of inorganic fertilizer gave the lowest. Though Mutuo et al. [35] indicated that organics have high labor costs, the results on both soils indicated that treatments that received sole organics and those combining organic and mineral fertilizers (except Calliandra with half recommended rate of mineral fertilizer) recorded a return to labor greaterthan 2.0, the minimum acceptable for small holder farming activities. Some of the organic materials like Calliandra and Leucaena could be more economically attractive when used as animal feed protein supplement for dairy cattle [5] and the manure returned to the farm. Maximum maize yield, net benefit and return to labor and benefit to cost ratio obtained on ustic Oxisol compared to umbric Cambisol could be at- tributed to the soil texture. The differences observed in maize yield, net benefit and return to labor and benefit to cost ratio on umbric Cambisol and ustic Oxisol could be attributed to the type of soil, the sandy soil favored more the N mineralization and take up processes than the clayey loam or soil nutrient status [18] [36].

Conclusion and Recommendations
Results from this study divulged that there was a great increase in maize grain yield with the integrated application of organic and inorganic fertilizers relative to the sole use of either of them or the control. Over two years of application of leaf biomass of agroforestry tree species and inorganic fertilizers, 1½ t/h Acacia and Cassia + (50 N + 37.5 P 2 O 5 + 45 K 2 O) treatments respectively gave the highest maize grain yield, net benefit as well as return to labor while the control gave the lowest yield, net benefit and return to labor on umbric Cambisol and ustic Oxisol. On the other hand, 3 t/h Acacia spp. and Cassia spp. treatments respectively gave the highest benefit to cost ratios on umbric Cambisol and ustic Oxisol. Maximum maize yield, net benefit, return to labor and benefit to cost ratio were obtained on ustic Oxisol. This work also showed that all treatments that combined leaf biomass and mineral fertilizers were economically profitable except Calliandra with half recommended mineral fertilizers on ustic Oxisol. Farmers are encouraged to adopt combined application of leaf biomass of agroforestry tree species and mineral fertilizer on degraded Cambisols and Oxisols in the Western Highlands of Cameroon to enhance maize grain yields, sustain production at a profitable rate and consequently mitigate climate change. In order to come up with comprehensive recommendations, further research should be conducted considering different varieties, different fertilizer rates and locations.