Mapping Zn Availability in Brazilian Soils.

Brazilian Cerrado soils cover the principal and most challenging agricultural area in the country, accounting for ~70% of Brazil’s farm output. Its geographical and agricultural importance justifies any efforts concerning a better soil fertility survey of yield-limiting nutrients in agroecosystems - Zn included -, which is the aim of this study. Previous studies comprised a first (n=909, ≈12.5 million ha) and second (n=18,636, ≈40 million ha) tentative survey for assessing the Zn status in Brazilian soils. In this survey, we have included data from additional 22,754 (≈55.1 million ha) geo-referenced soil samples from the Cerrado to improve our map of Zn availability in this important area. In order to better evaluate Zn phytoavailability, we also evaluated soil attributes that have a direct effect upon this micronutrient (Cu, Mn, pH, P and Clay). Soil Zn levels (Mehlich-1) ranged from 0.1 to 92.1 mg Zn dm^-3 (average=4.73 mg dm^-3). Nearly one out of every four samples (n=11,293) contained <1.6 mg dm^-3, which is the lower limit for adequate Zn levels for crop production in Cerrado soils. Values for pH varied from 3.8 to 8.0 (average =5.89). This value is in accordance with those described as ideal in the region (5.5-6.3). Phosphorous levels ranged from 0.1 to 225.2 mg P dm^-3 (average=11.3 mg dm^-3). Such high P levels can promote the reduction of Zn uptake by plants. In addition to high P, high clay contents can also lead to increasing Zn deficiency. In our study, clay values varied from 2.0 to 89% (average=51%). Nearly 59% of the samples were classified as very clayey (clay>60%) and 21% as clayey soils (36-60% clay). Considering the area covered by the database (≈55.1 million ha) and cross-checkings of soil information, there are indications that 58% of the area studied (≈32 million ha) is under risk of Zn deficiency, either due to low soil Zn levels or because of a combination of factors (high P levels, pH or clay content) that could induce Zn deficiency to plants. 

Acknowledgment

We thank the Associação de Mineradores de Pains, Arcos e Região (AMPAR), International Zinc Association (IZA), CNPq, CAPES and FAPEMIG for financial support and scholarships. We also thank CAMPO, SIAP Agribusiness, AMPAR and COMIGO for kindly providing the soil analysis results.