The savanna regions of Africa and Latin America are very old land surfaces that are at an advanced stage in the weathering process. This means that the soils are acidic and have low levels of plant available nutrients, especially phosphorus (P) and nutrient cations, and high levels of potentially exchangeable aluminum (Al). When these soils are strongly acidic, aluminum becomes soluble and interacts with phosphorus and calcium to render them unavailable. Since soluble aluminum is toxic to the roots of most plants, these soils are considered to be highly marginal for the production of most food crops, unless special management strategies are employed. Strongly acid soils can be rehabilitated by liming. Increasing soil pH to 5.5 eliminates the aluminum toxicity problem and increases the range of crop types that can be grown, although nutrient additions are still required to achieve good crop productivity. Liming, however, does not correct the acidity of the subsoil below the plow layer (0-20 cm) where root growth is restricted in susceptible cultivars causing susceptibility to drought and restricted nutrient uptake. Also, in many poorer areas of the world where soil acidity is a significant constraint, agricultural lime and fertilizer inputs are not readily available or affordable.
Where lime is not a viable solution to the soil acidity problem and where acid subsoil persist, farmers must rely on Al-tolerant plants. A number of important tropical food crops, such as cassava and sweetpotato, are able to grow in soils with a pH as low as 4. Special Al-tolerant genotypes have also been identified within otherwise Al-intolerant crop species, such as maize and sorghum. These Al-tolerant genotypes modify the rhizosphere environment by secreting organic acids that detoxify Al as well as allow the plant to access P more efficiently. Over time, Al-tolerant crop genotypes have been selected by farmers and breeders working in areas with acid soil problems. Today's plant breeders hope to accelerate the creation of new Al-tolerant and P-efficient food crop varieties by using marker-assisted breeding and genetic transformation.
The second phase of the project “New Approach for Improving Phosphorous Acquisition and Aluminum Tolerance for Plants in Marginal Soils” is a joint activity between Embrapa, Brazil, Moi University, Kenya, Purdue University, USA and USDA-Cornell University, USA undertake both basic and applied research modules aimed at improving the productivity of maize and sorghum in the Brazilian and East African acid soils.
The team's research activities seek to advance the understanding of key plant and soil factors that influence Al-tolerance and P-acquisition and use efficiency in maize and sorghum. The plant studies focus primarily on the molecular, genetics, physiological and breeding aspects of Al-tolerance and P efficiency, while the soil studies are concerned with understanding how various soil components influence and/or interact with the plant responses. This information will provide a better understanding of the soils components and on the plant mechanisms of Al tolerance and P efficiency acquisition in order to generate maize and sorghum cultivars with better adaptability and improved yields under stressful acid soils.