Soil salinity, a chemical form of soil degradation, involves an excess of salts in the soil solution. This condition is a manifestation of chemical soil degradation, specifically characterized by an excess of salts present within the soil solution. This condition is termed salinization when there is an excess of divalent salts like calcium and magnesium. Conversely, when the sodium ion (Na+) dominates the soil composition, it is called sodic soil or solonetz. The specific process by which this sodium ion dominance occurs is known as sodization. Excessive salt levels are harmful to plants, the soil, and the wider environment. Plants, while requiring salts for their growth and metabolic processes, only need them in limited quantities. When these levels become excessive, they pose significant harm not only to the plants themselves but also to the overall soil health and the broader environmental ecosystem. In Burundi, both sodic and saline soils have been identified in the Rusizi plain, particularly in the Imbo-Centre and the Gihanga-Gatumba-Ruvyagira-Rukaramu corridor. Soils in the lower Rusizi, specifically in Gihanga and Gatumba, are saline, with certain areas exhibiting a dominance of sodicity. Several hectares of the Mugerero rice-growing perimeter have been abandoned due to these salinity or sodicity issues, noting the direct impact on agricultural productivity.
Causes and Manifestations
Plants require salts, but only in limited quantities. Excess sodium in soil can create issues for plant health and soil structure, leading to dispersion of clay and organic matter. This process, where excess sodium causes problems for both plants and the structural integrity of the soil, results in a loss of structure, a muddy appearance when wet, reduced porosity, poor aeration, and decreased soil microbiology. When dry, sodic soil forms an unbreakable crust on the surface, further impeding agricultural activities and plant growth.
Salinization and sodization can stem from natural processes, known as primary salinization, or human activities, termed secondary salinization. This distinction between natural and anthropogenic causes is key for understanding and addressing the problem. Primary salinization occurs due to rock weathering and inadequate drainage, which promotes evapoconcentration. In desert or semi-desert regions, where high evaporative demand outweighs precipitation, salts from the water table can ascend to the soil surface, contributing to natural salinization.
Human actions can significantly contribute to salinization through poor irrigation practices. These include using low-quality water with high salt content or applying insufficient water amounts that do not allow for continuous salt flushing. Such practices exacerbate the accumulation of salts in the soil profile. Employing large quantities of water when the water table is saline and close to the surface can elevate the water table, bringing salts to the surface. Low-quality irrigation water characteristically contains high levels of salts or a disproportionately high amount of sodium compared to calcium and magnesium. Another indicator of low-quality water, which can contribute to salinization and sodization, is a high content of bicarbonate and carbonate anions relative to calcium and magnesium cations. This chemical imbalance in irrigation water can severely degrade soil quality over time.
Visual indicators of saline soils include the appearance of whitish saline efflorescence or powders on the soil surface during the dry season. This visible salt crust is a clear sign of the problem. Poor growth of cultivated plants is another clear indicator of saline soil conditions, often manifesting as stunted growth, wilting, or reduced yields, even with adequate water and nutrient supply.
Burundi's Saline and Sodic Soils
Burundi's geological and climatic conditions contribute to the presence of both saline and sodic soils, affecting agricultural productivity and land use in several areas. These environmental factors create a predisposition for salt accumulation in various regions. Some plants exhibit resistance to excessive salts, including Cyperus platicaulis, locally known as indava, Typha domingensis, or umubere, and Cynodon, referred to as urucaca. These species can indicate areas where high salt concentrations are present, serving as bio-indicators for soil degradation.
Sodicity in soils manifests through colloid dispersion, a process that can impart a brown color to the soil if organic matter is present. The soil solution, particularly water observed in puddles, may appear murky or resemble 'purin' due to this dispersion. This visual characteristic is a strong indicator of sodic conditions. The presence of sodic soils has been specifically identified in the Mugerero rice-growing perimeter, impacting cultivation efforts there and leading to significant land abandonment.
Beyond agricultural implications, saline efflorescence, or salt crusts, are extracted in large quantities by herders at Gatumba for livestock consumption, indicating a significant local presence of salts. This practice shows the widespread nature and local recognition of salt-affected areas. Pockets of salinity also extend to Cibitoke, located near the Rusizi River, further illustrating the widespread nature of the issue across the region.
The origins of salinity in the lower Rusizi region are complex, stemming from multiple geological and hydrological factors. These include Kivu volcanism in the Democratic Republic of Congo (DRC), which contributes mineral-rich deposits, hydrothermal activity from thermal waters within the watershed, and hydrothermalism associated with Lake Tanganyika, which can bring dissolved salts to the surface. Additionally, the weathering of rocks from the Congo-Nil crest contributes to the salt content in these soils, releasing minerals that accumulate in the lower elevations. Localities situated near Burundi's northern lakes are also suspected to have saline soils, a hypothesis supported by their semi-arid climate and characteristically low precipitation levels, which favor the accumulation of salts due to high evaporation rates and limited leaching.
Impacts and Correction Strategies
The impact of soil salinity and sodicity on agricultural output in Burundi is substantial. The average yield of paddy rice in the lower Rusizi region is approximately 4.2 tonnes per hectare. This indicates that significant agricultural losses can occur when land is affected by soil degradation, as productive land is rendered unusable or less productive. For every ten hectares of land abandoned due to salinity or sodicity, losses of approximately 42 tonnes of rice per season can result, according to agricultural data. These figures highlight the economic detriment faced by farmers and the broader agricultural sector.
Correcting saline soils primarily involves removing excess salts through flushing with good quality water. This process necessitates effective drainage conditions and a water table that is not close to the surface, allowing salts to be leached away from the root zone. However, correcting such soils can be challenging in certain areas due to specific soil characteristics. Vertisols, which are common in Gihanga, contain high percentages of smectite clay. These soils are inherently difficult to work, possess a strong capacity to retain water and salts, which significantly complicates efforts to correct salinity and sodicity, making remediation a long and resource-intensive process.
The correction of sodic soils is particularly difficult and requires the introduction of another cation, typically calcium. This is often applied in the form of gypsum. Gypsum, a common amendment for sodic soils due to its calcium content, is not locally available in Burundi and would need to be imported, adding to the cost and logistical challenges of remediation. While calcium chloride is even more effective for sodic soil correction due to its higher solubility and calcium content, its high cost makes it a less viable option for widespread use in agricultural settings, especially for smallholder farmers.
Prevention and Future Outlook
Promoting resistant crops such as cotton, tomato, sorghum, date palm, and various fodder types is recommended as a strategic approach to manage soil salinity and sodicity. These crops are better adapted to high salt concentrations, allowing for continued agricultural production in affected areas. Further prevention measures involve isolating unaffected agricultural plots to prevent the spread of contamination from degraded areas to healthy ones. Agricultural practices should also actively avoid irrigation using water sourced from already affected areas to limit further soil degradation, as such water often carries dissolved salts that can exacerbate the problem. Implementing careful water management strategies and selecting appropriate irrigation sources are critical steps in mitigating the expansion of saline and sodic soils, ensuring the long-term sustainability of agricultural land in Burundi.