Integrated Soil Fertility and Water Management Practices for Enhanced Agricultural Productivity

Declining agricultural productivity has been a challenge worldwide and especially in Sub-Saharan Africa (SSA). Low agricultural productivity has been attributed to factors such as poor farm management practices, soil moisture stress, soil infertility, and soil degradation, among others. Te nations in the SSA are prone to insufcient crop yields due to their inadequate capacity to adapt to good agricultural practices that support crop productivity such as integrated soil fertility and water management (ISFWM) practices. Tis lowers the farmers’ capacity to improve crop productivity, thus contributing in jeopardizing the food and nutritional security in SSA. Past research has shown that ISFWM strategies have not been properly adopted probably due to the lack of adequate awareness among the farmers about them. In addition, there is limited documentation on the importance of ISFWM in enhancement of soil fertility, water use efciency, and sustainable crop production in SSA. Tis paper discusses some of the key ISFWM options that have the potential to enhance soil fertility, improve water use efciency, and consequently increase agricultural productivity. Te practices include intercropping, use of tied ridges, minimum tillage, mulching, and combined use of organic and inorganic fertilizers.


Introduction
Te two interrelated problems that impact all nations are the rising population growth rate and the demand for food. Te population of Sub-Saharan Africa (SSA) is projected to reach 2.7 billion people by the year 2060 [1]. Due to the upsurge in population, there is a need to maintain good productivity to be able to curb the insufcient food production [2]. Most of the arable land in SSA countries is utilized for agricultural production [3]. Additionally, in most areas, especially the highland agro ecosystems, land is a scarce resource due to competition in land use systems [3]. Diferent unmanageable land uses have led to a decline in agricultural productivity due to the loss of soil fertility as a result of increased soil degradation arising from soil erosion and poor farm management practices [4]. One of the strategies that should be embraced by the farmers is the adoption of integrated soil fertility and water management (ISFWM) practices. Tese practices encompass the adoption of soil conservation, soil fertility enhancement, and water conservation practices. Te main aim of ISFWM is to improve soil productivity, improve the efciency of external inputs, and improve on farm outputs [5]. Terefore, low adoption of ISFWM practices is one of the major contributors to low crop productivity [6].
According to the reports from earlier studies, the main biophysical root causes of decreasing crop output are attributed to persistently low soil fertility and low adoption of ISFWM [7][8][9]. Soil infertility is mostly caused by soil nutrient erosion. Ideally, conventionally tilled plots' have declining nutrient reserves which causes the soil structure to be destroyed throughout the land preparation [10], thus causing the depletion of nutrient reserves [11]. According to Kiboi et al. [12], continuous ploughing of lands has also been a key factor in nutrient depletion with farmers adding less or no organic amendments back to the soil. Over the years, most small-scale farmers in SSA, particularly in Kenya, have had a tendency to limit the use of soil amendments and organic fertilizers [5]. Tis results in the collapse of the soil structure and continuous depletion of available nutrients due to continuous uptake by the plants without adequate replenishment [13]. Lack of sustainable soil management practices has led to immense land degradation and the loss of nutrients through soil erosion [14] and leaching. In order to increase soil fertility, reduce soil depletion rates, and improve agricultural productivity, integrating soil fertility and soil and water conservation practices in the farming systems would be desirable.
Over the years, population growth in Kenya has been on the rise [15] and crop productivity has been on the decline [16,17]. Tis underscores the global need to increase agricultural productivity. In SSA, about 30% of the population experiences food insecurity with the majority being resource-poor smallholder farmers in the rural settings who depend on rain-fed farming [18]. In Kenya, a huge number of the population is dependent on agricultural production, with 60% of the population working in the agricultural sector [19]. Poor soil management practices that lack adaptive strategies to climatic uncertainties intensify low agricultural productivity [20]. Such disruptions and incumbent practices result in a signifcant decline in yields [13]. In addition, unpredictable rainfall that results in extended dry spells and frequent droughts contributes further to the decline in agricultural productivity [21,22]. Adoption of ISFWM practices by farmers can assist greatly in increasing agricultural productivity in a sustainable manner.
Agricultural production in most smallholder farming systems is mostly rain fed [23]. Tis therefore means that it is mostly in the rainy seasons when farmers can ultimately utilize the rainfall and produce agricultural crops [24]. In Kenya, it is estimated that 70% of the marketable produce is obtained from rain-fed agriculture [25], indicating that a huge number of farmers do not use irrigated water to grow their crops. According to Akinnifesi et al. [26], continuous variations in rainfall patterns caused by climatic changes are another major problem faced by farmers in rain-fed production systems. Tis therefore causes either prolonged droughts or foods that result in reduced agricultural production. Failure of farmers to efciently manage the available rainwater has also contributed to soil moisture stress, thus reducing agricultural productivity [27]. Te low availability of soil moisture as a result of scarce and erratic rainfall has also been documented by Kabubo-Mariara and Kabara [24] and Nathan et al. [25]. Additionally, even the drought-tolerant crops such as sorghum are afected by moisture stress during low amounts of rainfall and interseasonal dry spells thus reducing their productivity [28]. It is therefore evident that these changes in rainfall patterns directly impact agricultural productivity. Farmers, therefore, need to take action by adopting soil and water conservation practices to reduce the efects of soil moisture stresses.
Previous research shows that adoption of ISFWM practices has been extremely efective in reducing land degradation, improving soil fertility, increasing productivity, attaining agricultural sustainability, and increasing household income of farming households in Kenya [25,29]. Unfortunately, even with ISFWM strategies being in existence for decades, the adoption rates are still low [6,11]. In the central highlands of Kenya, only about 44% of the farmers are able to adopt integrated soil and water management practices [11,30]. Some of the ISFWM practices that should be promoted for adoption by the farmers include suitable replenishment of soil nutrients using organic and inorganic fertilizers, use of tied ridges, minimum tillage, and mulching among others. Tis review documents some of the key benefts of these ISFWM options.

Knowledge Gaps
Food security has been on the decline over the years, and this has been attributed to various causes such as low soil fertility, water scarcity, and poor farm management practices. A majority of farmers, particularly those that operate on a small scale, have limited knowledge of soil and water management techniques that can support sustainable agricultural production. Farmers are more focused on providing subsistence crops when, on the contrary, they can economically upgrade to medium-scale or even large-scale commercial farming through proper adoption of ISFWM technologies. Although various ISFWM technologies have been in existence, their benefts are not well understood by the small-scale farmers, and hence they have not been adequately exploited to curb the low crop productivity especially in the SSA. In addition, most farmers do not understand the interactive benefts of the integrated soil fertility management (ISFM) and soil and water conservation (SWC) practices, and hence they do not apply ISFWM practices in full. Terefore, there is a need to document the pros and cons of ISFWM practices to enable the farmers to make informed adoption decisions for enhanced crop productivity.

Methodology
Multiple databases were used as sources of information in this review. Te most used platform was Google Scholar, where initial samples of the articles were taken. A number of broad search terms were used in Google Scholar to establish a list of relevant peer-reviewed research articles. Te literature search targeted the efects of the following practices in crop production: integrated soil fertility management practices; soil and water conservation; tied ridges; manure and fertilizer impacts; soil organic matter enhancement; and intercropping. From the list obtained from Google Scholar, a predefned list was obtained when using other databases such as research gate and university repositories. In addition, the snowball method was used in identifying articles in line with the purpose of this literature review.
Te sources of the literature were analysed using two criteria. Te frst and the main one was that the source had to be appropriate and relevant to the purpose of this literature review. Secondly, the sources had to be from credible peerreviewed journals. Information from other credible sources that aligned with this literature review was also included. Where possible, the search targeted mainly recent publications that were not more than 5 years old. In addition, only articles that are well written with proper research procedures, valid and reliable literature, and verifable methodology were used.

ISFWM and Teir Efects on Agricultural Productivity.
Integrated soil fertility and water management may be defned as the integration of organic and inorganic sources of soil nutrients together with other innovative methods of preventing soil degradation and improving the soil structure to facilitate fertilizer and water use efciency for enhanced crop production [31,32]. Tere are numerous strategies that may constitute the ISFWM package. Tese practices are set to reduce soil erosion, maximize soil water, replenish soil nutrients, reduce nutrient loss, increase soil fertility, and improve crop productivity [5]. Tis paper addresses some of the key strategies that may be interlinked with the ISFWM paradigm particularly the following: (i) Intercropping (ii) Tied ridges (iii) Combined use of organic and inorganic fertilizers (iv) Minimum tillage with mulch application 3.1.1. Intercropping. Intercropping refers to the practice of growing many crop varieties in close proximity on the same plot of land so as to maximize resource utilization. Intercropping has been practiced over the years by knowledgeable and unknowledgeable farmers [33]. Te utmost importance of intercropping is to boost the crop yields and reduce losses for the farmers. Tis is achieved when the intercropped crops make use of the available resources in the soils such as nutrients that could not have been utilized by a single crop [25]. Intercropping has many benefts which include improving soil cover and enhancing soil physical properties [34], weed controlling, minimizing pests attack on plants, controlling of diseases, promoting soil water conservation, and increasing water use efciency among plants [25,35]. Consequently, intercropping helps in improving soil fertility, decreasing soil nutrients loss and increasing crop yields. For example, the efects of intercropping in Zimbabwe on the smallholder farmer's feld depicted signifcant variation between the pure cereal feld and the intercropped feld. Te statistics showed that pure maize felds productivity was 1 t/ha, while intercropped feld productivity was 1.3 t/ha [36]. Intercropping of common bean and maize was found to increase productivity of common bean by up to 2.5 t·ha −1 in Tanzania [37].
Tere have been diferent types of intercropping systems that farmers have been practicing over the years including mixed intercropping, row intercropping, relay intercropping, and strip intercropping among others. Mixed intercropping has been the most practiced form of intercropping. Most farmers do not even tend to distinguish what crops should be mixed on the same piece of land, and they mix the crops unsystematically in the little spaces available in their farms. Tere is therefore a need to educate farmers on efective forms of mixed intercropping whereby their preliminary benefts will be realized and help in raising the productivity levels [6]. Row intercropping is the type of intercropping whereby two or more crops are grown simultaneously with each crop type being grown in a discrete row arrangement [38]. Schulz et al. [39] and Mousavi and Eskandari [40] acknowledged some advantages of row intercropping such as increasing the crop production levels, reducing crop failure risk, and controlling weeds. Relay intercropping has been the latest form of intercropping practiced by farmers whereby two or more crops are planted on the same piece of land, but with the second crop planted after the frst one has reached physiological maturity. Te estimates of the yields increment due to implementation of relay intercropping are 17%-45% for cowpea and 12%-80% for pasture, all compared to conventional cowpea and pasture, respectively [41]. Another cropping practice known as strip-intercropping whereby an early-season crop is relayplanted with a late-season crop in strips on the same feld has been identifed as one of the most efective practices in enhancing water use efciency [42]. Strip-intercropping was shown to increase water use efciency by 5.64% and sorghum yield by 0.98 t·ha −1 under sorghum-cowpea intercrop [42]. Farmers in SSA have also been practicing temporal intercropping where they intercrop a very fast growing crop with a slow growing crop [43]. In this case, the fast growing crop is harvested before the slow growing crop matures, thus improving yields per unit area [44]. For example, in East Africa, cereals intercropped with pigeon pea increased their yields by 71-282% [45].
Increased food production, higher household incomes, a broader variety of food crops, and better risk management are just some of the ways in which intercropping can help farmers become more resilient [25]. For instance, soil fertility depletion which is a major contributor to reduced crop productivity worldwide and in Sub-Saharan Africa [46] can be reduced by intercropping with legumes. Te legumes are usually helpful in the enhancement of soil fertility since they are capable of forming a mutually benefcial connection with rhizobia and other nitrogen-fxing soil bacteria [47]. Additionally, intercropping promotes nutrient acquisition levels as compared to mono-cropping [48]. Köpke and Nemecek [49] concluded that in phosphorus defcient soils, crops with varying characteristics can explore numerous organic P sources. For instance, higher P uptake can be attained in cereal/legume combinations on such soils compared to similar monocultures in the same type of soils. Barley-legume intercrops show 10-70% P accumulation estimates and an increment in biomass accumulation of 0-40% compared to monocultures [50]. Such systems lead to improved soil fertility while enhancing water use efciency. A study by Fu et al. [51] showed that intercropping maize and chickpea increased soil organic matter (SOM) by 9.4% as compared to maize monocrop. In the central highlands of International Journal of Agronomy Taraka-Nithi County, intercropping in combination with manure and fertilizer has shown great performance in increasing the soil water content, increasing infltration, enhancing water use efciency, and increasing yields [25,35,52]. Tese empirical evidence confrm that intercropping is efective in promoting soil and water conservation by reducing the amount of soil and water lost through runof and regulating both water infltration and retention, thus increasing the usage of available soil water [25].

Tied Ridges.
Tied ridges are small rectangular basin-like structures formed within the furrows of cultivated land to help the soil retain more water [6,53,54]. Tied ridges have a tendency to improve soil infltration as well as soil retention since rainwater is stored in the basin like furrows for the plants to utilize [52]. Te use of tied ridges helps in increasing soil water in the rhizosphere by 30% compared to nonridge sites [55]. Te use of tied ridges led to an increase in rainwater use efciency and grain yields in Northern Ethiopia of at least 40% compared to nonridged farms [53]. Te ridges have been shown to store more water than no-till lands. Tey reduce soil water erosion thus giving time for the rainwater to infltrate and also increase moisture retention [53]. Te retained soil water in the ridges provides enough moisture to the plants and therefore improves crop productivity by generally improving water use efciency. It has therefore been established that tied ridges have a higher ability to conserve soil moisture as compared to other water conservation practices [12]. Wolka et al. [56] reported the possibility of increasing maize yields by 7.5 to 87% when ridges are used as compared to the conventional felds.
Erratic rainfall, unreliable, insufcient, and seasonal rainwater are a signifcant challenge for crop production globally [28,30]. Tis has resulted in soil moisture stress, a challenge that has caused about 15% crop losses globally [55]. As it is evident, agricultural production greatly relies on rain-fed agriculture, and thus soil moisture availability to crops is a major factor for increased productivity [19,23,57]. Low availability of soil moisture can be a result of scarce and erratic rainfall caused by climatic changes [24,25]. Rainfall variability, scarcity, and erratic rainfall patterns have a continued efect on rainfalldependent agriculture in the world at large [58]. Soil moisture provides optimum wet conditions for crop growth. Both drought tolerant and intolerant crops tend to dry and eventually die when faced with a minimal moisture supply [25]. In addition, low amounts of rainfall and interseasonal dry spells subject even the drought tolerant crops such as sorghum to moisture stress, thus declining their potential productivity [28,59]. In cases when rainfall is sufcient, crop growth may be afected by the inefcient use of the available rainwater [27], hence the need to integrate soil moisture conservation with other ISFWM strategies. According to Zhao et al. [60], an integration of tied ridges and other soil and water conservation practices coupled with soil fertility practices can curb soil moisture stress and improve crop productivity.

Combined Use of Organic and Inorganic Fertilizers.
Integration of organic and inorganic fertilizers combined with soil and water conservation measures has resulted in increased soil fertility and improved crop productivity [6]. Organic fertilizers are materials that have a very defnite chemical composition and are able to add analytical value and supply nutrients to plants in their available form while inorganic fertilizers are synthetic or mineral chemicals manufactured artifcially to provide essential nutrients for improved crop growth [33]. Continuous soil fertility depletion is one of the primary concerns threatening crop output and food security globally [61]. In response to the population growth and increase in food demand, land degradation has been exacerbated by the expansion of agriculture without efective land management or external inputs [62,63]. Tere has been a tremendous annual decline in the nutrients in Africa by about 22 kg N ha −1 , 2.5 kg P ha −1 , and 15 kg K ha −1 [64]. Tis can be attributed to inadequate management and continuous use of nutrients without replenishment as characterized by smallholder farmers in SSA, which is the main cause of soil nutrient depletion. Continuous cropping has also led to nutrient mining [11]. Farmers do not add back to the soil the organic amendments that have been mined through continuous ploughing [65]. Due to soil erosion and nutrients defciency in the soils, most land in SSA has been degraded [56], thus resulting in low soil fertility. Te lack of adoption of integrated soil fertility and water management practices, has caused a loss of 50 kg·ha −1 of nutrients equivalent to US $4 billion in SSA [66]. Another main cause of low soil fertility is the fact that fertilizer prices have been increasing rapidly in Africa which also discourages its use among smallholder farmers [67].
Soil fertility can be increased by the application of watersoluble inorganic fertilizers that are easily absorbed by the plants, hence providing almost all the necessary nutrients required for plant growth [68]. Te other form of fertilizers is the organic ones which include animal manure, farmyard manure, and compost manure. Tey are derived mainly from biodegradable organic compounds. Organic fertilizers add nutritional values to the soils by rightfully distributing nutrients to the soil [69]. Tey also improve the soil structure and contribute to a steady plant growth [44]. Organic fertilizers also contribute to a higher moisture absorption capacity and thus provide a platform for soils to absorb the available water and reduce the soil water erosion [70]. Contrary to inorganic fertilizers, organic fertilizers release their nutrients slowly, making them available to the soil and plants efectively and for the longest period of time, from cultivation to harvesting [71]. Organic fertilizers have the ability to improve the soil structure, soil physiochemical properties, and soil qualities while inorganic fertilizers mainly improve the soil fertility. Organic fertilizers also help reduce the negative impacts on the environment caused by inorganic fertilizers [72].
Combining organic and mineral (inorganic) fertilizers can be utilized to boost crop output and nutrient utilization efciency [19]. Tis strategy increases the soil's physiochemical and biological conditions which enhances the environment for crop growth hence high yield productivity [73]. Te approximate increase in production under the combination of organic and inorganic fertilizers has been 4 International Journal of Agronomy reported to be 7-15% [74] due to provision of all the needed nutrients to the plants [25]. Combined use of organic and mineral fertilizers has also been identifed as a water saving strategy since they improve the soil structure thus increasing infltration and minimizing surface runof [15]. Tey also increase the water holding capacity levels resulting in improved water use efciency [75]. Asmamaw et al. [76] estimated an increase in the water holding capacity by 37 to 64% with the implementation of a combination of manure, lime, and inorganic fertilizers compared to a controlled feld under Nitisols in Ethiopia. Integration of organic and inorganic fertilizers have the most sustainable increase in crop yield per unit of water used [31]. Tey improve the nutrients content in the soil, thus improving soil fertility and contributing largely to agricultural sustainability by increasing the productivity of crops [14]. In addition, the combination has a synergistic efect, which results in the soils improving their nutrient release and the nutrients uptake by the crop [77]. Te combined fertilizers also help in improving soil infltration rate, aggregation, bulk density, hydraulic conductance, and efcient water utilization [14], thus improving the soil status and rebuilding the soil health status [78]. Te strategy also helps in reducing nutrient loss in the feld as reported by Martínez-Mena et al. [14] who estimated an associated 45% reduction in nutrient losses. Integrated use of organic and inorganic fertilizers is therefore an important strategy to meet the food and feed demands brought about by an increasing population, dwindling land area, and soil infertility [25].

Minimum Tillage with Mulch.
Minimum tillage is defned as a soil conservation system where agronomic practices are applied with very minimal soil disturbance [79]. Te main goal of minimum tillage is to reduce land manipulation by reducing soil disturbances [80]. In contrast to intensive tillage whereby soil nutrients are being washed of the farming land, minimum tillage retains the bare ground cover that immensely adds to the reservoir of soil organic matter [12]. Te soil structure is therefore improved while increasing the aggregate stability [11,81,82]. Trough minimal land tillage, the undisturbed soil ground cover decreases soil erosion since the soils are less exposed to erosive forces such as runof water or wind [83]. On the other hand, mulching is the practice of surrounding crops with a protective soil covering made mostly of organic materials such as crop residues, grass, cereal, or legume straw in order to control weeds, retain soil moisture, and lessen soil erosion caused by water or wind processes [14,84]. Mulching enhances moisture availability through the reduction of moisture evaporation. Integrating minimum tillage with mulching raises soil organic carbon, boosts soil fertility, and lowers soil erosion [79]. As mulch decomposes, benefcial nutrients are added to the soil, hence improving soil fertility [85]. Minimum tillage with mulch has shown to have many benefts including fertilizer usage efciency, less surface runof, moderated soil temperatures, and improved moisture retention and infltration, all of which culminate in increased crop yields [86]. Adoption of minimum tillage revealed an increase in sorghum yields by 11% in Kenya [87]. On the contrary, conventional tillage disturbs the soil causing more indirect soil pores that results in reduction in the infltration rate and encourages surface runof losses [25]. Due to soil erosion, there is an estimated annual soil loss of about 22 kg N ha −1 , 5 kg P ha −1 , and 15 kg N ha −1 [88]. Tis results in soils of poor quality that insufciently support crop production. Practicing minimum tillage with mulch can reduce these losses by enhancing soil aggregate stability, bulk density, and soil fertility, hence improving the agricultural productivity [89][90][91].

Conclusion
Tis paper confrms that the decline in soil fertility and soil moisture retention can be signifcantly improved through the adoption of various types of ISFWM practices, thus resulting in improved crop production. Implementation of minimum tillage with mulch and the use of organic fertilizers contribute to the accumulation of adequate soil organic matter which results in increased water infltration and improved water holding capacity, thus reducing nutrients loss from the soil through reduction of runof and leaching. Te use of tied ridges helps greatly in water conservation and prevention of soil erosion, thus improving water use efciency while preventing soil degradation. Intercropping reduces the cost of production signifcantly by ensuring efcient use of production resources including water, fertilizer, land, labour, and time. Terefore, adoption of ISFWM can potentially transform agricultural production especially in SSA where production resources are scarce. However, the ISFWM technologies have not been widely adopted by the small-scale farmers and hence their benefts have not been properly exploited. Tere is therefore need to sensitize the small-scale farmers to the interactive benefts of various ISFWM technologies in order to promote their adoption as a package for increased agricultural productivity. Te technical capacity of the farmers for the implementation of various ISFWM practices should also be enhanced.

Data Availability
No numerical data were used to support this study.

Conflicts of Interest
Te authors declare that there are no conficts of interest.