CALIDAD DE VIDA Y MEDICINA

The work reported by Cochrane (1989) was carried out as a corollary to a comprehensive land systems soil survey of the Geo-Economic Region of Brasilia, covering some 22 million hectares surrounding and including the 2 million hectares of the Federal District (Cochrane et al. 1989). The study of the Geo-economic Region of Brasilia has been summarized in Chapter 9.

Past work.

Improved crop production in the savannas (cerrados) of central Brazil was correctly associated with increasing rooting depth. This ameliorates water stress during the “veranicos” or dry spells that are common towards the latter part of the wet season (Goedert et al.1980). Deeper rooting of crops was achieved by deep liming (Gonzalez-Erico et al. 1979), attributed to overcoming Al toxicity. Ritchey et al. (1980) also attributed deep rooting in corn (Zea mays L.) to the “correction” of Al toxicity by the downward movement of calcium incorporated in the plow layer as lime. However, following additional work, Ritchey et al. (1982) hypothesized that rooting depth in the cerrados oxisolic soils was more likely to by limited by Ca deficiency than Al toxicity; they established a “critical level” for soil Ca of less 0.02 cmolckg-1. Although not observed by those latter

authors, the acrustox soils of their study had Ca/Mg ratios varying from 0.4 to 1.8, with an average of 0.9 to a depth of 90 cm. These low ratios suggested to the senior author a possible imbalance between Ca and Mg.

The study of the chemistry of the soils of the Geo-Economic Region of Brasilia.

Cochrane (1989, and refer to Appendix 7) analyzed soil samples from the A1 and B2 horizons of 48 profiles of well-drained soils representing the major landscapes of the Geo-Economic Region of Brasilia, on the basis of their occurrence under native vegetation. Exchangeable cations including K, Ca, Mg, Na, Mn and Al were determined in an initial extraction with 0.5M NH4NO3, and CEC

and AEC by equilibrating the soil with NH₄NO₃ solution strengths reflecting measured field conditions (Cochrane and de Souza, 1985). Thirty eight of the profiles analyzed were taken from sites found in the predominant cerrados ecosystems of the region and 10 from the smaller inclusions of native forest covered lands. The savanna vegetation of the cerrados sites had been modified by extensive cattle grazing and annual burning. The native forests had been periodically cut over for timber, but were distinguishable from heavily wooded cerrados (locally called “cerradão”), by the paucity of grass species in their shrub layers. In terms of US Soil Taxonomy (Soil Survey Staff, 1975), the cerrados soils included 21 acrustox, 12 haplustox, 2 eutrustox, 2 rhodustalf, 1 eutropept and 1 ustropept. The chemical data from the soils under cerrados were compared with those under forests with special reference to their Al, Ca and Mn levels, cation ratios, CEC, AEC and AEC/CEC ratios. Mean, standard deviation (SD) and minimum and maximum values were calculated for their A1 and B2 horizons. Student’s t-test was used to compare the data. -The mean, SD, and minimum and maximum soil

analytical values for the A1 and B2 horizons of the 10 forest and 38 cerrados soils representing the Geo-Economic Region of Brasilia are recorded in Table 14-1, together with the comparisons of the mean values of the cerrados and forest soils by the t-test. From these data, the following observations may be made:

Al Toxicity: The relative Al saturation of the CEC and the relative Al contribution to the sum of the Ca, Mg, K, and Al values, which would approximate the effective CEC (ECEC) in these low pH soils, were highly correlated: % Al(CEC) = 10.772 + 1.617 % Al (sum of cations); r = 0.88, for the 48 B2 horizon samples. From this regression equation, it was calculated that 30% Al saturation of the CEC approximates 60% Al saturation of the ECEC, a level considered toxic for many crops (Sanchez et al. 1982). On this basis, the original data indicated that only one forest soil had toxic levels of Al in its A1 horizon, and none in their B2 horizons. In contrast, 24 cerrados soils had toxic levels of Al in A1 horizons and 13 in their B2 horizons. Clearly, Al toxic conditions were more common in the cerrados soils. The condition appears not so severe in the B2 horizons of either soil group. In fact, extractable Al was not detectable in seven of the B2 horizons of the cerrados soils, which would confirm that Al toxicity is not a ubiquitous condition in those soils.

Ca Deficiency: Calcium deficient conditions defined by Ritchey et al. (1982) as < 0.02 cmolckg-1 soil, were found only in the cerrados soils,

although one forest soil had a marginal value of 0.03 cmolckg-1 in

its B2 horizon. The SDs of the mean Ca values were large in both the forest and cerrados soils.

Eighteen of the 38 cerrados soil profiles have Ca deficient levels in their B horizons. Six of these soils would also be considered Al toxic; hence, 12 of the 38 profiles or about 32 % appear to have Ca deficient conditions in the absence of Al toxicity. On the other hand, 11 of the remaining soils, or 29%, had neither Ca deficient nor Al toxic conditions in their B2 horizons.

Calcium deficient conditions were found only in the cerrados soils. However, cerrados also occurred on soils with high levels of Ca. A little over 30% of the cerrados subsoils had Ca deficient conditions in the absence of Al toxicity, indicating that the former condition is often independent of the latter.

Mn Toxicity: The original data showed that the proportion of Mn in the total extractable bases was just over 20% in the B2 horizons of two of the forest soils and eight of the cerrados soils. In three of the cerrados soils, this proportion exceeded 40% suggesting a greater potential for Mn toxicity.

Table 14-I. A comparison of chemical analyses of the Al and B2 horizons of soils under forest and cerrados vegetation in the Geo-economic Region of Brasilia.

pH K + Ca2+ Mg2+ Mn2+ Al3+ CEC AEC %AI AEC/CEC Ca/Mg Ca/K Mg/K

cmolc kg-1

Horizon A1

Forest soils (10 profiles)

Mean 5.4 0.23 4.08 1.06 0.11 0.14 4.33 0.11 10 0.09 3 .1* 7.1 4.5

SD 0.5 0.20 4.92 0.95 0.12 0.16 3.53 0.19 17 0.20 1.4 6.2 3.0

Min. 4.9 0.05 0.05 0.02 0.01 0.02 0.96 0.00 0 0.00 1.5 1.0 0.5

Max. 6.3 0.64 15.5 2.68 0.39 0.50 10.1 0.62 48 0.65 5.8 21.7 8.7

Cerrados soils (38 profiles)

Mean 5.2 0.14 0.27 0.38 0.04 0.52 1.60 0.22 33 0.16 0.9* 1.9 2.3

SD 0.3 0.08 0.43 0.93 0.04 0.32 0.68 0.19 16 0.14 0.5 2.1 3.0

Min. 4.6 0.03 0.01 0.03 0.00 0.00 0.50 0.00 0 0.00 0.1 0.2 0.3

Max. 6.0 0.45 2.43 5.83 0.18 1.50 3.40 1.07 69 0.56 2.5 10.6 16.7 Horizon B2

Forest soils (10 profiles)

Mean 5.5 0.13 1.92 0.62 0.08 0.10 2.91 0.39 4 0.18 3.0** 16.9 5.5

SD 0.4 0.14 2.27 0.61 0.07 0.14 2.11 0.41 6 0.21 1.5 17.4 5.3

Min. 4.8 0.01 0.03 0.01 0.01 0.00 0.89 0.00 0 0.00 1.2 0.9 0.4

Max. 6.2 0.38 7.40 1.76 0.21 0.45 7.60 1.15 15 0.55 5.8 53.7 17.0

Cerrados soils (38 profiles)

Mean 5.4 0.06 0.07 0.20 0.03 0.33 1.40 0.50 20 0.41 0.7** 1.2 2.2

SD 0.3 0.05 0.17 0.67 0.03 0.38 0.70 0.35 17 0.28 0.5 1.5 5.1

Min. 4.5 0.01 0.01 0.02 0.00 0.00 0.33 0.00 0 0.00 0.1 0.1 0.1

Max. 6.1 0.36 1.00 4.20 0.13 1.33 3.27 1.60 57 1.00 1.7 9.2 32.0

* Mean values significantly different by t-test (P<0.01)

** The only mean values significantly different by t-test (P < 0.001). All other comparisons of the means of the chemical data were not significantly different at P<0.05

Plate 14-7. A forest remnant near the city of Goias Velho in central Brazil, with “bottle trees” typical of soils with high exchangeable calcium levels and high Ca/Mg ratios.

It is not suggested that a low Ca/Mg ratio is a ubiquitous feature of all well-drained savannas lands. Soil analysis previously carried out on a series of soil profile samples taken throughout the well- drained eastern savannas (“Llanos”) of Colombia suggested that adverse Mg/K ratios may well be a limiting factor for plant growth (Cochrane et al., 1985). However in retrospect, it is also possible that the analyses carried out to determine Ca and Mg levels by the CIAT laboratory (a versenate titration method) were faulty or at least insensitive. (The senior author later detected some serious flaws in that laboratory on being asked to review an analyses related paper by a colleague. -After an acrimonious denial, the senior author was eventually proven right, and the problems corrected.).