1. Introduction
Coffee is grown in at least seven of Kenya’s eight provinces, making it a popular and important crop [
1,
2]. Coffee has long been one of the leading strategic cash crops contributing to Kenya’s economic growth. In 2019, Kenya had approximately 28 million hectares of agricultural land [
3], out of which, coffee occupied about 119,000 hectares, with about 204 million coffee trees. Approximately 6,000,000 people are directly and indirectly employed in the sector, and about 70% of Kenya’s coffee is produced by small farmers. An estimated 800,000 producers, mostly smallholders, are involved in growing coffee [
4], employing roughly 5 million people, or 30% of Kenya’s agricultural workforce, at different stages of the production chain. More than 800,000 rural households’ economic status and means of subsistence are directly impacted by this. Additionally, it is significant in the agricultural gross domestic product (GDP) of the nation, accounting for approximately 10% of the total agricultural exports [
5,
6]. Yet, due to a number of problems, e.g., soil erosion, climate change, low profits from smallholding plantations, and high cost of production, Kenya’s coffee-planted area has declined by 30% in recent years [
7].
Despite the great importance of coffee to the Kenyan economy, it has not been possible to have sustainable production due to various challenges cited in this paper. Due to the degradation of soils, production has been declining, leading to losses for most of the farmers, forcing them to either seek other revenue sources or diversify by intercropping with other crops, mostly fruits. There is also the problem of soil erosion occurring in the farms, leading to the loss of fertile top soil. This does not only increase the need for more fertilizer application but also lowers the net income of the farmer. Reversing the declining trend of coffee plantation, which is both a pillar for Kenya’s economy and a backbone supporting smallholders, requires combined approaches of intervention, e.g., pests and disease control, consolidation of land fragmentation, stabilizing of coffee prize fluctuations, curtailing soil and water erosion, and improving the functionalities of the soil etc. [
8,
9]. Among the identified intervention tasks, an urgent need exists for a combined strategy to prevent soil and water erosion, increase soil fertility, and improve soil functionality and outputs. This can be achieved by minimizing management practices that negate soil fertility such as the excessive use of herbicides, compaction of soil, and depletion of the top soil layer.
Developing or finding principles for sustainable orchard farming can be undertaken by resorting to conservation agriculture (CA) principles. CA, with its three pillars (no till, soil cover, crop rotation), has evolved into a number of principles and practices in the last decades. Examples of these practices include ridge tillage [
10,
11], cover crops [
12], straw mulching [
12], contour tillage [
13,
14], basin tillage [
15,
16], strategic tillage [
17], sub-soiling [
18], etc. The selection of each practice or combination of individual ones has been proved to be site-specific and the impacts of the interventions were found to be influenced by not only the local farming system, but also the soil condition, climate, and the geology.
By lowering production costs, preserving soil quality, lowering labor input costs for herbicide and weeding, and lowering emissions of greenhouse gases, conservation agriculture (CA) has been promoted for the sustainability of agricultural productivity and the environment [
19,
20]. Preservation farming has likewise been found to further develop the total solidness of soil that upgrades nutrient maintenance and lessens soil disintegration, subsequently adding to soil fertility and promoting air permeability, water infiltration, and nutrient cycling [
21,
22]. One of the engineering methods used to control runoff on steep slopes is ridge tillage, which prepares a seedbed higher than the average land surface of the field. It has been proposed as a superior option in contrast to zero tillage, as it upgrades soil fertility, further enhances management, lessens water and wind erosion [
23], it can also allow for planting of more than one crop, promotes proper root development, and advances pest controlling [
24,
25]. These benefits can be tapped into when correctly incorporated into coffee farming which suffers from frequent soil erosion since most of the farms are on steep slopes.
Poor land management, including land use without the installation of appropriate erosion control measures and the export of nutrients from farms, is directly linked to the depletion of soil resources because most coffee farms are located on steep slopes that are prone to runoff. Using herbicides to control weeds between rows for an extended period of time not only increases production costs but also lowers environmental quality [
26]. Washing away top soil by frequent runoff has led not only to the depletion of nutrients from the farms but also to the high and frequent use of inorganic fertilizers in the production of coffee. This raises the cost of production and also leads to water pollution downstream by phosphorus coupled with the release of greenhouse gases such as N
2O and CO
2 to the atmosphere [
27]. Therefore, the traditional approach of shifting cultivation, applying mineral fertilizers and/or manure in inadequate amounts, and incorporating grain legumes into cropping systems is insufficient to meet these challenges. To deal with this problem in coffee farms, new farming techniques need to be explored and studied for adoption by local farmers. The ties that exist between social problems and soils are the engine behind the recently developed concept of soil security. This is centered on issues such as food security, sustainability, climate change, carbon sequestration, emissions of greenhouse gases, and degradation caused by erosion and the loss of organic matter and nutrients [
28,
29].
A number of interventions have been used to help reduce soil degradation with varying levels of success. Most farmers use improved practices such as compost manure application, mulching, intercropping, and row planting [
30]. Because of the hilly terrain and steep slopes in the Cherara area, most farmers are using terraces to prevent the loss of soil due to surface runoff; this has not been very successful due to the labor demand in coming up with the terraces. A lack of appropriate machinery to mechanize the operations has largely affected the adoption of these technologies. The steep slopes are less workable with machines without modifications on either the machinery or the terrain. The introduction of ridging into the coffee farms will allow the planting of the crops on relatively flat beds that would allow for some level of mechanization. This study was carried out to evaluate the effects of the use of leguminous cover crops (common beans) and ridging singly and in combination on the physical properties of the soil. The common bean was chosen because it is one of the most popular crops in the area, and most farmers would readily adopt it for their farming operations. It is a staple food in Kenyan communities, used in dishes such as “githeri” and stews [
31]. Additionally, it has good vegetative characteristics that make it a good cover crop. These interventions were introduced into already established coffee plantations for ease of data collection and to form a basis for more focused future work into the implementation of the chosen strategy.
This study assumes that the introduction of cover crop and ridge conservation tillage practices will increase the soil organic matter (SOM) within the coffee farms and also reduce compaction. This increase in the SOM is assumed to have a direct influence on the porosity and, thus, on the water holding capacity. This change in the water holding capacity would in turn affect related water hydraulic processes through the soil moisture. In most instances, changes in soil moisture affect lateral runoff and leaching, and affect the infiltration rate. Soil moisture affects crop growth and yield since it determines how much water is accessible to plants. The presence of cover crops above the soil increases soil moisture by facilitating water infiltration. Additionally, surface cover captures some of the precipitation, reducing the amount of water reaching the soil’s surface and reducing soil evaporation, thereby reducing wasteful water losses to the atmosphere. Overall, we made an assumption that the data obtained on the physical properties of the soil were solely affected by the interventions under this study, neglecting the changes that would occur due to natural processes since the work was not performed in a controlled environment. For this study, the following hypotheses were proposed: 1. Introducing combined ridge tillage and cover crops to coffee plantations can increase the soil moisture content (MC), reduce the bulk density (BD), increase the infiltration rate (IR), and increase aggregate stability. 2. By using cover crops or ridge tillage as intervention methods in coffee plantations, the physical properties of the soil will improve, whereas the fields without intervention will have poor soil properties because of a lack of soil cover or changes to slope gradients. As such, this study’s main objective was to determine how the physical properties of the soil (MC, BD, IR, and AS) change when ridge tillage and cover crops are combined, or when cover crops or ridge tillage are introduced individually to be able to propose an intervention for the long-term study and adoption in coffee plantations in Kenya.
4. Conclusions
The results of ridge tillage, cover crops, and combined ridge tillage and cover crop treatments showed differences in the response of the moisture content of the soil, bulk density, infiltration rate, and aggregate stability. Soils under combined cover crop with ridge tillage and cover crop alone practices showed a significant difference in the effect on the bulk density when compared with the control. When practiced independently, soils under either ridge tillage or cover crop systems had intermediate values of moisture content, bulk density, infiltration rate, and aggregate stability of wet soil. A cover crop–ridge tillage system practiced together performed significantly better on the bulk density and infiltration rate. According to our findings, practices involving short-term cover crops–ridge tillage, if introduced to a farming system, may maintain soil moisture while simultaneously increasing soil aggregation and the proportion of soil pores. With or without ridge tillage, cover cropping tends to increase the moisture content, decrease the bulk density, and increase the infiltration rate. It appears to be especially effective when combined with ridge tillage to improve the aggregate stability. The results showed that when we consider soils that need a low infiltration rate, especially where water is added through irrigation, then no intervention could be better as compared to other treatments. We discovered that cover crops and ridging systems altered the aggregate stability values, indicating an improvement in the soil’s structure. These results show that the treatments had a significant effect on the bulk density and infiltration rate but no significant difference in the response on the moisture content and aggregate stability. As a result of these studies, we have validated our hypotheses that ridge tillage with cover crops, or ridge tillage alone, or cover crops alone, in the short-term, can increase the soil’s moisture content (MC), reduce the bulk density (BD), increase infiltration (IR), and improve the aggregate stability in a field without treatment. As a result, ridge tillage and cover crops can be combined for the purpose of mitigating the soil erosion challenges on Kenyan coffee plantations. The implications of these interventions’ long-term effects on the physical properties of the soil may be further explored in future studies with larger sample sizes and a wider range of soils and climates over longer periods.