Soil acidity poses a critical challenge to agricultural productivity in Tanzania, impacting farmers’ ability to grow healthy crops and hindering the nation’s food security. In a compelling and enlightening conversation, Anthony Muchoki and Beda Msimbe sat down with esteemed soil scientist, Dr. Joel L. Meliyo, to delve into the depths of this pressing issue. Dr. Meliyo, the National Project Coordinator of Guiding Acid Soil Management Investment in Africa (GAIA), an International Project Funded by the Bill and Melinda Gates Foundation (BMGF) through CIMMYT Harare, holds a wealth of knowledge and expertise on the subject. As the local Principal Investigator for GAIA, he collaborates closely with TARI Mlingano, TARI Uyole, TARI Ukiriguru, and SAGCOT Centre Ltd, making him a leading authority on soil health in the region.
Question 1: What is soil acidity and its implication?
Ans: Soil acidity is measured in pH units. Soil pH is a measure of the concentration of hydrogen ions in the soil solution. The lower the pH of soil, the greater the acidity. pH is measured on a logarithmic scale from 1 to 14, with 7 being neutral. Therefore, below pH 7 there are classes of acidity. The pH levels range from 0 to 14. The optimal pH range for most plants is between 5.5 and 7.0.
The United States Department of Agriculture Natural Resources Conservation Service provides a comprehensive classification of soil pH ranges, offering valuable insights for researchers, farmers, and land managers. The classifications are divided into distinct denominations, each corresponding to specific pH ranges. At the lower end of the spectrum, soils are categorized as ultra acidic with a pH below 3.5, and as extremely acidic within the range of 3.5–4.4. Moving up the scale, soils are classified as very strongly acidic (pH 4.5–5.0) and strongly acidic (pH 5.1–5.5). The moderately acidic category covers soils with a pH range of 5.6–6.0, followed by slightly acidic soils falling between 6.1–6.5. The neutral classification encompasses soils with a pH range of 6.6–7.3, while slightly alkaline soils are within 7.4–7.8. For those on the higher end of the pH scale, soils are categorized as moderately alkaline (pH 7.9–8.4), strongly alkaline (pH 8.5–9.0), and very strongly alkaline with a pH exceeding 9.0.
These classifications help researchers, farmers, and land managers understand soil acidity or alkalinity levels, which is crucial in making informed decisions about agricultural practices and soil management strategies.
Implication: The negative effects of acidity include reducing the availability of plant nutrients, such as phosphorus and molybdenum, and increasing the availability of some elements to toxic levels, particularly aluminum and manganese. Essential plant nutrients such as calcium, potassium, and magnesium are leached below the rooting zone, becoming unavailable for plant uptake.
Qn 2: What regions of Tanzania are most affected by soil acidity?
Ans: It’s important to note that most of the acidity in Tanzania is natural and attributed to parent materials, climatic conditions, and landform. The regions most affected are those with high rainfall, including areas around Lake Victoria and Tanganyika (Geita, Kagera, Mwanza, Mara, Tabora, and Kigoma), the Southern Highlands, and some areas along the coastal regions of the Indian Ocean (Njombe, Makete, and others).
Qn 3: How does soil acidity impact agriculture in Tanzania?
Ans: Soil acidity is measured by soil pH, which affects the availability of plant nutrients from the soil solution. In Tanzania, soil pH ranges between 4 and 10, with extremes that restrict plant nutrient uptake. Extreme soil pH levels release substances from soils in amounts that can be toxic to plants, such as aluminum and manganese. Acid soils may dissolve toxic amounts of metals and result in subsoil nutrient depletion and stunted root growth. Soil microbe activity, particularly nitrogen-fixing bacteria, may also be reduced in acidic soil. Soil acidity influences plant nutrient uptake, leading to poor crop stand and reduced yields, as well as a decrease in fertilizer use.
Qn 4: Which crops are most affected by soil acidity in Tanzania?
Different crops have different pH preferences. Most varieties of wheat and barley grow best in soils with a pH ranging from 6 to 7, with 6.5 often mentioned as the target level. Maize, in general, does not tolerate pH conditions of less than 5.5, preferring an optimal range of 6-7.2. In acidic soils, the roots of maize crops suffer from aluminum toxicity, limiting nutrient uptake. Besides root damage, acidic soils also negatively affect nutrient availability. Other crops affected by soil acidity include cabbage (pH: 5.20–6.80), mushrooms (pH: 6.00–6.70), broccoli (pH: 6.30-6.85), and collard greens (pH: 6.50–7.50).
Qn 5: How much economic loss does Tanzania face annually due to reduced crop output and inefficient use of fertilizers?
Ans: This has not been well computed, but it is assumed to be a significant loss. Simple calculations show that in areas where subsidies are provided, the government estimates that crops do not utilize 50% of the applied fertilizer. Additionally, in agricultural land where beans are grown, around 80% of the yield is not realized due to strong acidity, resulting in substantial losses. These losses need attention and reversal because they are potentially avoidable.
Qn 6: What are the natural factors contributing to soil acidification in Tanzania?
Ans: The pH of newly formed soils is determined by the minerals in the parent material. Temperature and rainfall affect the intensity of leaching and the weathering of parent materials and soil minerals. In warm, humid environments, soil pH decreases over time through acidification due to leaching of bases from high amounts of rainfall.
Under natural farming practices, soil acidification is influenced by four factors or causes: i) Removal of crops and residues from the farm or paddock. ii) Leaching of nitrogen (applied as fertilizers) below the plant root zone. iii) Inappropriate use of nitrogenous fertilizers.
Qn 7: How do human activities, such as farming practices, contribute to soil acidity in Tanzania?
Ans: Soils under cultivation in Tanzania are becoming more acidic due to several factors:
- Rainfall and leaching leave soils bare, and when rains leach, acidity increases.
- Soil erosion exposes acidic parent materials.
- Harvesting high-yielding crops involves the removal of crops and residues.
- Nitrification of ammonium fertilizers can contribute to soil acidity, although fertilizer use in Tanzania is limited.
Qn 8: Please provide an overview of the GAIA project and its objectives in addressing soil acidity in Tanzania.
The Guiding Acid Soil Investment in Africa (GAIA) project aims to help stakeholders understand the importance of soil health and agronomy in boosting the productivity of smallholder enterprises. The project seeks to address key knowledge gaps by providing reliable, timely, and actionable data and insights on soil health and crop performance at the farm and regional levels. This includes diagnostic approaches, data assets, decision aids, and better farm management practices. These services can be integrated with solution-focused, bundled services that support timely management and operational decisions by farmers, including digitally enabled agricultural advisory systems.
Key expected results of the project include the integration of national Soil Information Services (SIS) with functioning agronomy research pipelines in at least seven focus countries in SSA and SA. SIS solutions are also integrated with Agricultural Advisory Services (AAS) into overall decision agriculture platforms at the national level. Innovative diagnostic tools and decision aids are expected to be increasingly used at the farm and regional levels. Additionally, the project promotes the application of FAIR (findable, accessible, interoperable, reusable) data principles and practices in all investments.
Qn 9: Can you discuss the gains, challenges, and obstacles encountered in implementing the GAIA project?
Ans 1 Gains: The project is addressing pressing problems, and initial data and results have impressed stakeholders. Farmers are showing interest in obtaining lime, and experimental data indicates significant yield increases, up to four times in some cases. Lime application neutralizes soil acidity and adds essential calcium for plant growth. It also improves the environment for soil microorganisms, promoting organic matter breakdown and nutrient release.
Ans 2 Challenges: Lime is expensive, and its availability is limited by distance. Agri-lime is only available at Dodoma Cement and Tanga, which are far from areas like Mbozi and Geita. The cost and logistics of transporting lime can make its use difficult or impossible in some regions.
Qn 10: What strategies are currently in place in Tanzania to manage and mitigate soil acidity?
Ans: The most practical management option for acid soils in Tanzania is applying agricultural lime (ag-lime) to reduce acidity and increase crop productivity. The government is advocating for this strategy and emphasizes soil testing to determine the need for lime or fertilizer application.
The government has also reduced lime royalties from 3% to 1% to encourage limestone factory investment. Tax reductions for the lime industry have been implemented to attract more investment. There is strong political will to promote lime and fertilizer use, and further implementation is expected.
Qn 11: What policies are currently in place in Tanzania to regulate soil acidity?
Tanzania has several policies related to agriculture and food security that indirectly address soil acidity, including:
- Tanzania Agricultural Sector Policy 2003: This national policy aims to promote sustainable development of the agricultural sector for economic, social, and environmental benefits, including addressing food insecurity and nutrition.
- The National Agriculture Policy 2013 (NAP 2013): This policy focuses on developing an efficient, competitive, and profitable agricultural industry to improve livelihoods and economic growth.
- Kilimo Kwanza: This initiative aims to transform agriculture through various pillars, including political will, enhanced financing, land management, and more.
Qn 12: How effective are these policies in combating the problem of soil acidity?
Ans: While these policies address various aspects of agriculture, they do not specifically target lime application or soil acidity. To be effective, a specific policy for lime application should be developed, considering its importance and the need for significant quantities, which may require substantial mobilization for transportation.
Qn 13: How can Tanzania’s current policies related to soil acidity be improved?
Ans: Tanzania’s current agricultural policies are promising, with substantial budget allocations for agriculture, emphasis on soil testing, and investments in irrigation and improved seeds. To further address soil acidity, a dedicated policy for lime application should be developed. This can be coupled with soil analysis, which has been shown to be a cost-effective investment for farmers.
Qn 14: What successful policies from other countries could Tanzania emulate to address soil acidity?
Ans: Rwanda provides an example of a successful policy for lime application. The government ensures the availability and quality of lime to farmers, and this policy has been well-received by farmers. In the United States, government subsidies for lime are offered to farmers, making it more accessible.
Qn 15: What role can International Organizations play in helping Tanzania develop and implement policies to combat soil acidity?
Ans: International organizations, such as FAO, can play a crucial role in several ways:
- Conducting research to determine the correct amount of lime needed per district or zone in Tanzania.
- Creating awareness among farmers about the benefits of lime application, thereby creating demand and encouraging private investment in lime production.
- Supporting soil testing and data collection efforts to inform policy decisions.
- Offering expertise and resources to help implement lime-related policies effectively.