SANS, L. M. A.

SANTANA, D. P.

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.~d~ll~s!O.1Proceedings of a Workshop onImproving Phosphorus AcquisitionEfficiency in Marginal SoilsOctober, 17-22, 1999Robert E. SchaffertEditorMilho e SorgoTABLE OF CONTENTSWorkshop Objectives - Robert E. Schaffert 7Nature And Distribution of Acid Soils in The World - V.C.Baligar and J.L. Ahlrichs 9The Distribution and Nature of Soils With Low Phosphorus inthe Brazilian Savannas - Derli Santana and Luis Marcelo A.Sans 27Mineralogy of Highly Weathered Soils and Its Role inPhosphorous and Trace Element Availability - Darrell Schulze,João José Marques and Nilton Curi 35Surface Chemistry Of Phosphorus In Highly Weathered SoilsCliff Johnston and Nicholas B. Comerford 47Role of Organic Acid Release in Phosphate Mobiliz~tion andAcquisition by Plants - Soil and Plant Factors - Richard H.Loppert and Sarah E. Johnson 61Mechanisms of Phosphorus Efficiency in Maize - EmbrapaMaize and Sorghum Experience - Vera M. C. Alves, Antônio F.C.Bahia Filho, Carlos A. Vasconcellos, Gilson V.E. Pitta, Gonçalo E. França,Hélio T. Prates and Sidney N. Parentoni 71Can We Gain Insights into Mechanisms of P.Efficiency byStudying Aluminum Tolerance in Crop Plants? - Leon V.Kochian 77Genetic and Molecular Dissection of Aluminum Tolerance inGrain Crops- David F. Garvin.....•...•.•.•.•.....•.............•...•.•...•.•.•.•.. 89Prospects for using conventlonal techniques and molecularbiological tools to enhance efficiency of crop plants in low-nutrient environments - Running title: QTL mapping andMAS for improved phosphorus uptake - C. Tom Hash, Robert ESchaffert and John M. Peacock 99Maize Genetic Resources with Contrasting PhosphorusEfficiency - Sidney N. Parentoni, Vera M. C. Alves, Carlos A.Vasconcelos, Elto E. G. Gama, Manoel X. Santos, Cleso A. P. Pacheco,Walter F. Meirelles, Robert E. Schaffert, Antonio F. C. 8ahia Filho.... 127Sorghum Genetic Resources with Contrasting PhosphorusEfficiency - Robert E. Schaffert, Vera M. C. Alves, Antônio F. C. 8ahiaFilho, Gilson V. E. Pitta, Fredolino G. Santos and C. A. Oliveira 139Engineering Citrate Overproduction in Transgenic Plants toIncrease Aluminum Tolerance and Improve PhosphorusUptake in Marginal Soils - José López-8ucio, María Fernanda Nieto-Jacobo, Vereníce Ramírez-Rodríguez and Luis Herrera-Estrella 151Molecular Determinants of Phosphate Acquisition and TheirUtility in Crop Improvement - K.G. Raghothama 171Aluminum tolerance in maize - Marcelo Menossi, Maron L.G, JorgeR. A, Felix J. M, Fukada M. K, Stukart G. C, 8oscolo P. R. S, Mussi L.,Kalvan H. C., 8ergamo R. F., Ottoboni L. M. M and Paulo Arruda ..... 177Comparison of Phosphorus Recommendation Based onAverage Soil Test and .on Spatial Variability Maps - Gonçalo E.Franca, Evandro Mantováni Daniel Marçal Queiroz, Roberto CarlosOrlando and Antônio F. C. 8ahia Filho 185A National Canipaign of Maize and Bean Seed Production inSmall Rural 'Communities - José Heitor Vasconcellos João CarlosGarcia, Luiz Gomes de Souza, José Carlos Cruz, Antônio F. C. 8ahiaFilho 189Summary: Considering Options in Biotechnology for AfricanCrops - Exploration Summary - Joe Devries (RockefellerFoundation) 193State of the Science and Art -Developing Grain Cultivars forAcid Savannas of Brazil - Antono F.C. 8ahia Filho and Mauricio A.Lopes 205Workshop Participants 239THE DISTRIBUTION ANO NATURE OF SOILS WITH LowPHOSPHORUS IN THE BRAZILlAN SAVANNASDerli P. Santana & Luis Marcelo A. SansEmbrapa Milho e Sorgo, Cx. Postal 15135701-970 Sete Lagoas, MGderli@cnpms.embrapa.brINTRODUCTIONWith a surface area of approximately 8,500,000 squarekilometers of continuous land of which a large part arable,Brazil enjoys abundant water resources; both tropical andsubtropical climates and a vast vegetative cover possessinggreat biological diversity. This diversity of species is attributableto edaphoclimatic characteristics that result in vegetationpatterns which can be fitted into six principal ecological regions:the Amazonian Forest, Cerrados of Central Brazil, Caatinga ofthe Northeast, Mata Atlantic, Southern Forests and Grasslandsand the Pantanalof Mato Grosso. In spite of the large extensionof arable lands, Brazil has only about 40% of its territoryoccupied by agro-silvo-pastoral activities. Today, Brazil is theworld leader in the production of coffee, sugar cane, citrusjuices, the second largest producer of soybeans, and the thirdlargest world exporter of grains.Brazilian agricultural activities are, traditionally,concentrated in the regions of the Southern Forests andGrasslands and the Mata Atlantic. However, during the lasttwenty-five years, the country has gone through a phase ofsignificant expansion of the agricultural frontier, principally inthe direction of the Cerrados region, the Brazilian savanna,situated in the central part of Brazil. Prior to the 1970s, theCerrado region was thinly populated with extensive cattle27grazing as its main land use. The soils in this area are highlyweathered Oxisols with serious limitations for crop productionin terms of low natural soil fertility. In addition to these chemicalproblems, there are other production limitations: a) typically a5 to 6 months dry season (April to September); b) dry spellsduring the rainy season generally associated with highevapotranspiration rates; c) low water holding capacity evenin clayey soils; and d) limiting rooting depth of many crops asa function of AI toxicity and/or Ca deficiency in subsurfacesoil layers .In spite of ali these problems, a burst in agriculturaldevelopment has taken place as a result of governmentalincentive programs and the use of new technology, with greatemphasis placed on both plant breeding and soil management.Nevertheless, as Nobel Peace Prize winner Norman Borlaugpointed out (Borlaug and Oowswell, 1997), Brazil has failed toemphasize the fantastic current development of the Cerrado,which is converting tens of millions of hectares of formerlyworthless land into productive farmland. This is nowcontributing greatly to food production while at the same timeindirectly slowing destruction of the Amazon rain forest.Moreover, it should be emphasized that the vast Cerradoecosystem, over untold millions of years, was leached ofnutrients and made unproductive by Mother Nature, whereasover the last three decades, it is being converted into a hugeand highly productive, "bread basket" through the collectiveefforts of science, effective government and private sectorprograms, plus aggressive, creative farmers and ranchers. The"Cerrado Revolution" can be described as the Second GreenRevolution, concluded Dr. Borlaug. In ali probability, much ofthe technology being developed for the "Cerrados" will be of2829considerable value in the future in opening areas with similarsoil problems to agricultural production, such as the vast plains(lIanos) of the Orinoco River watershed in Colombia andVenezuela and large tracts in Central and Southern Africa.THE CERRADOREGIONCerrado is a name given to different types of savannalike vegetation in which a continuous herbaceous stratum isassociated with an arboreal stratum of woody species of varieddensities. It can be defined as a subhumid wooded savanna,although there is much variation, from pure grassland to aclosed tree canopy (Eiten 1972; Ferri, 1977). The Cerradoregion is situated in the central part of Brazil, and small areasfringing other regions such as the Northeast and the North,between the latitudes 6 and 21?S, occupying approximately25% of the country (Figure 1). Its total area is around 205million hectares, an area approximately equivalent to thecombined areas of Spain, France, Italy, Germany, Portugal andGreat Britain.The average monthly temperature of 22°C is almostconstant throughout the year, and the annual rainfall in theregion varies from 900 to 2000 mm. The most importantclimatic characteristic is the seasonal distribution of theprecipitation with two well-defined seasons, one rainy, with 5to 7 months of duration (between November and April), andthe other dry (Figure 4). Short-term droughts commonly occurduring the rainy season (mainly ·in January and February) andmay be detrimental to crop growth because of highevapotranspiration rates, low soil water retention capacity andadverse chemical conditions for root growth. Dry spells in therainy season are locally known as "veranicos".Figure 1. Localization of Zone 61 "Cerrado of Central Brazil-in the Brazilian "Cerrado" EcosystemThe present landscape is best visualized as a broadthree-Ievel terrain resulting from different types of surfaceerosion: the upper tableland -"Chapadões" if there are big riversdivides or "Chapadas" if there are small rivers divides, theslightly rolling intermediate levei regions (a long pediment) andthe river valleys. The upper tableland is at altitudes higherthan 850m and slopes vary from O to 3% being almost flat inmany stretches and are formed of clayey Red Yellow and OarkRed Latosols (Oxisols). The origin of the clayey parent materialstill remains questionable. The cerrado (savanna), in the broadsense, is the main phytoqeoqraphic unit. The upper tableland"breaks" in the second surface is usually formed by abruptescarpment where shallow soils (Lithosols) or outcropping rocksare common. In some places the transition to this intermediatelevei is subtle and laterite is common. The unit is typically3031more rolling than the Chapadão (3 to 8%), but there are flatstretches. The fine-Ioamy Oxisols developed from sandstoneare widespread in this intermediate levei, but clayey Oxisolsstill occurs and the vegetation is still cerrado. The contactwith the lowest levei occurs in the sameway. If there is basalticrock or limestone it will have fertile soils and the vegetation isbasically semideciduous forest but cerrado occurs on the flatterstretches.Oxisols cover more than 50% of the region, theremainder consisting mostly of Entisols, Inceptisols and Ultisolsand sandy soils may cover as much as 15% of the area. Mostof the soils are deep and well drained which, together withvery good microaggregate stability, makes them easy tocultivate at almost any moisture status. A combination of thesefactors with the gentle slopes (commonly less than 3%) favorsintensive agricultural mechanization. Red Yellow Latosols andDark Red Latosols, most in the suborder Ustox, have beenpreferred for intensive agriculture. Their texture is variable,but most are high in clay (over 35% of clay) with kaolinite,gibbsite and iron oxides (goethite and hematite) dominate theclay fraction. The predominance of these low activity clays isresponsible for most of the soil behavior, especially in termsof ion exchange, water holding capacity and phosphateadsorption characteristics. The organic matter content isgreater than in most tropical ustic conditions (to 3%) and isimportant as a source of nutrients.CONSTRAINTS TO SOIL QUALlTYDeclining and more erratic yields due to soil degradationand biotic pressures (weeds, insects, diseasesand nematodes),are widely reported by farmers in the area. At the same time,they report higher costs of production due to increased use offertilizers and pesticides in an attempt to maintain yields. Onthe other hand, the increased use of chemicals pose seriousthreats to the environment. lhe main constraints identifiedare:Organic Matter - lhe average values are around 2%.Following land clearing, liming and fertilization, the organicmatter content declines. lhe consequences of organic matterloss are: soil structure destruction, compaction, erosion, CECreduction, nutrient loss, higher crop vulnerability to droughtand reduction in water infiltration.Soil Acidity - lhe soils are acid and high in AI saturation(Lopes, 1996). A marked yield response to lime is observedfor ali crops.Phosphorus - lhe soils are poor in P and have high Pfixation. lhe performance of ali crops, without addition of P,is poor, and large responses are observed with addition of thisnutrient.Nutrient Balance - lhe soils have low nutrientavailability. Nitrogen fertilization is recommended for most non-legume crops; the rate being a function of expected yield,commonly varying between 30 to 120 kg N/ ha. Splitapplications usually minimize N losses. Usuallv K is not aproblem, except at continuous planting of soybeanmonocropping in soils of low K supply. Excessive dolomitelimestone applications in some locations have led to narrowCa:Mg relationship, inducing Mn deficiency.Compaction - Intensive use of disc harrowing hasresulted in the formation of dense plow pans ("plow layer"). inmost soils under continuous cultivation. As a consequence,porosity decreases, infiltration rates reduce and runoff anderosion increase .32Erosion - Erosivity in the "Cerrado" region can be veryhigh because 70% of the total precipitation occur betweenNovember and March. This coincides with early stages ofannual crop growth, which, in most cases have not fullycovered the ground. On the other hand, repetitive diskharrowing for every crop, common in the "Cerrado" region,creates one loose superficial layer and a compact subsurfacelayer, which facilitate erosion.Monocropping - Continuous planting of soybeans andinappropriate soil management practices has resulted indeclining productivity and increased costs of production,particularly in older areas. Sole cultivation of soybeans hasbeen shown to reduce the soil organic matter because of thelow C/N ratio and high rates of decomposition.PREDOMINANT FARMING SYSTEMSSoybeans have been the main driving force for increasedgrain production and the expansion of the areaplanted to annualcrops in the region (Spehar, 1996). However, continuousplanting of soybeans and inappropriate soil managementpractices in many farms, have resulted in declining yield andincreased costs of production.Maize has only recently become a major crop in theregion. From a share of 17%, it evolved to about one third ofthe national production Maize has been employed in rotationwith soybeans, when it yields as high as in the best areaswhere it is traditionally cultivated in the country. In the areaswhere the rainy season is prolonged, maize is also cultivatedin double cropping, after the soybeans.A system which is becoming common is the annualcrop-pasture rotation. The grass-Iegume association in rotation33with annual crops offers the following advantages: soil fertilityimprovement by incorporating nutrients through crop residues;increased biological activity in the subsoil due to the deep rootgrowth of perennial acid tolerant plants and efficient nutrientrecycling by plants and animais; improvement of soil physicalconditions by better organic matter management; and,consequently, reduced erosion.Another system which is becoming very successful isno-tillage or Zero Tillage. ZT is based on permanent soil coverwith crop residues, crop rotations including cover crops,specializedplanters/drills, and maximization of biological activityand enhanced management capabilities of the farmer, leadingto environmental responsibility. Besides reducing soil erosionlosses by up to 90%, and substantially improving rainfallinfiltration rates, ZT generates a series of direct and indirectbenefits to the farmer as rapid operation, suitable for doublecropping and crop rotation, time and money saving, and moredurable machinery.In a substantial area of the region crops irrigated bycenter pivot are becoming common. Besides making thegrowing of a winter crop possible, the irrigation guaranteessummer production in the case of "veranicos". .34

The distribution and nature of soils with low phosphorus in the Brazilian savannas.

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