Soils of Homoxi

The soil observations and samples listed in Table 7 were made at selected sites whose locations are indicated in Figs 58 and 59. Data on soil profiles and chemical analyses are provided in Appendices 5 and 6.

Natural soil types

Red-yellow dystrophic latosols (medium texture) dominate the upper slopes and tops of the hills in the Homoxi region. They present deep profiles (greater than 1.5m) and are probably based on basic intrusive rocks from the Complexo Guianense. They are old, acidic to strongly acidic, and well drained. The profiles show the following sequence of horizons: A, AB, BA and B. Colours range from brown (7.5YR 3/3, 5/8) to red-yellow (5YR 5/8) with little differentiation between the A and B horizons. The sand fraction makes up over 60% of the granulometric composition. The textural relationship B/A of 1.8 shows characteristics of argisols.

The red-yellow latosols at Homoxi are of low to medium natural fertility and dystrophic (base saturation <50% across the profile). CEC (Cation Exchange Capacity) is highest towards the surface on account of the presence of organic matter there. According to Schaeffer (1996) these types of soils are developed from reworked material, normally resulting in chemically poor soils. These characteristics are reinforced by the climatic conditions in which they have developed, which intensify the processes of leaching.

Yellow dystrophic latosols (medium texture) are found on the lower slopes of the hills at Homoxi and in the valleys (where they had not been removed by garimpeiros). They are generally deep and are probably overlying granites and gneisses of the Complexo Guianense and sandstones of the Paleolithic formations. These soils are old, acidic and well drained, and in some cases are substantially clayey. Profiles show A, AB, BA and B horizons and the colour is predominantly brown (10YR 4/2 and 10YR 6/8) both in the A and B horizons, although the latter may be yellowish (10YR 7/8). In general there is little differentiation between the horizons.

Like the red-yellow latosols, these soils are dominated by their sand fractions and show low natural fertility. They are dystrophic and the highest CTC values are found at the surface. Again, the textural relationship between the B and A horizons suggests an argisol.

Hydromorphic sandy soils are found in the baixadas (damp depressions and streamside areas) in the forest. However, in most of these areas the soils have been disturbed by garimpeiros.

Soils and gravels on areas disturbed by mining

How the soils were altered

The garimpeiros generally started at the edge of the river and gradually worked their way away from it. The forest was cut down in the area to be mined and the topsoil was removed with high-pressure water hoses. The liquefied soil was washed into the hole left by the previous workings. The gravels were then worked for their gold, and transported to the same hole. When the layer of greenish or bluish clay underlying the gravels was reached, the equipment was moved on and a new site was begun (passing the waste materials into the new hole). Thus, as the garimpeiros moved away from the river, they left a swathe of gravel behind them. The topsoil that had originally covered it was now either underneath the gravel (effectively an inversion of the original profile) or else washed downstream.

The most significant anthropogenic substrates in the Homoxi region are the (predominantly quartzite) gravels. These are most obvious in the raised banks and mounds that have been formed along the course of the Mucajaí and some of its tributaries. There is considerable variation in these gravels and in the quantities of sand present among them, but in general the finer fractions are most abundant in the areas of lowest topography. In some of these places sand and silt have accumulated on top of the gravels to form a distinct layer. This may contain some organic matter resulting from decomposing vegetation, which is generally more abundant and exuberant (shrubs and occasionally trees) in the depressions than on the higher gravels (which mainly support a sparse cover of grasses and lichen).

Figure 1: Profile across gravels beside the Rio Mucajaí

The finer organic material in the depressions was almost certainly washed out of the gravels above it by the rain. In some areas, however, these materials have probably been deposited by flooding of the river. There is substantial chemical variation in these substrates, but in general they are poor in disposable nutrients.

Table 1: Soil samples/observations taken at Homoxi

*Sample*	*Substrate*	*Brief description*
1	Gravel	Bottom of a baixada in the forest. Site of small mine working. Some secondary species (e.g. Cecropia sciadophylla) present in the canopy. Sandy substrate with gravel. Some shrubby regeneration.
2	Soil	Yellow latosol in dense terra firme forest towards the bottom of a hill.
3	Gravel and sand	Open mining site (relatively flat) covered in herbaceous vegetation (many Pityrogramma calomelanos ferns) with some shrubs. Sandy substrate. This site has probably been mined manually.
4	Gravel and sand	Small open area at edge of secondary forest, probably seasonally flooded by nearby stream. Possibly former course of diverted stream. Shrubby vegetation. Top 5cm with organic material. Grey sand from 5-20cm.
5	Gravel	Dry raised area at edge of secondary forest with large quantities of Inga edulis, to a height of about 15m. Dense leaf litter and root system present on surface.
6	Soil	Red-yellow latosol on the top of a hill. Dense terra firme forest.
7	Gravel	Area of gravels with small patches of shrubby regeneration to a height of 2.5-3m (with small accumulations of leaf litter) in the depressions.
8	Gravels	Alternating hillocks of gravels with depressions with finer substrate. The hillocks are colonised by grasses and occasional small bushes. Shrubby regeneration to 2.5-5m on the finer (sandy) substrates in the depressions.
9	Sand	Hydromorphic substrate with water table 60cm below ground level. Alternating sand and fine gravel under secondary forest.
10	Soil	Yellow latosol in an abandoned garden (with some old banana trees) dominated by the tall grass Setaria sulcata.
11	Soil	Yellow latosol under secondary forest to 15m dominated by Cecropia sciadophylla. Abandoned Yanomami garden.
12	Soil	Yellow latosol under a dense cover of Pteridium arachnoideum in an area that was accidentally burned by the Yanomami.
13	Soil	Yellow latosol on the Baiano Formiga airstrip under a dense cover of tall Paspalum grass and other herbs.
13a	Soil	Yellow latosol under secondary forest dominated by Vismia guianensis at the edge of the Baiano Formiga airstrip. The soil is more clayey than the sample from the strip itself (13).
14	Soil	Yellow latosol in abandoned Yanomami garden (at least 15 years old) dominated by Cecropia sciadophylla trees to 30-35m to 50cm dbh.
15	Gravels	Secondary vegetation dominated by dense, slender-stemmed Vismia guianensis on gravel substrate near the end of the Macarrão airstrip.
16	Soil	Sandy yellow latosol on the Macarrão airstrip under a dense cover of tall Paspalum grass and other herbs.
16a	Soil	Sandy yellow latosol under secondary forest dominated by Vismia guianensis at the edge of the Macarrão airstrip.
17	Soil	Sandy yellow latosol on the Chimarrão airstrip under a dense cover of tall Paspalum grass and other herbs.
17a	Soil	Sandy yellow latosol under secondary forest dominated by Vismia guianensis at the edge of the Chimarrão airstrip.
17aa	Soil	Crater formed by explosion on Chimarrão airstrip, surrounded by shrubs on loosened earth. Sandy yellow latosol.
18	Gravels	Coarse gravels underlain by friable sandy substrate mixed with gravel. Gravels supporting sparse grassy vegetation, with small islands of shrub regeneration in lower areas.
*19-19b. Profile across gravels (Fig. 52). Coarse gravels alternating with patches of white or yellow sand [evidence of oxidisation; the samples were collected after a night of rain.*
19	Gravels	Gravel bank approximately 1m high. Top covered by grassy vegetation; slopes with grasses and shrubs. Material damp to 40cm.
19a	Gravel and sand	Flat surface; substrate slightly damp. White sand mixed with yellow (hydromorphic) sand. Vegetation cover of grasses, ferns and lichens.
19b	Gravel and sand	Flat surface with friable sandy substrate.
20	Soil	Yelow latosol at edge of abandoned Julio de Blefe airstrip. Secondary forest.
*21-21k. Profile across gravels upstream of Pista Chimarrão (see Fig. 53).*
21	Gravel and sand	Small island in river. Sand dark grey to approx. 50cm; below that bluish-grey (apparently hydromorphic). Secondary vegetation 5-10m.
21a	Gravel and sand	Right bank of the river; soils and vegetation similar to 21.
21b	Gravels	Upper part of river bank with friable sand/gravel mixture. Below 40cm gravels too coarse to be sampled with augur. Secondary shrubby vegetation.
21c	Gravel and sand	Flat area with coarse gravels and some sand, varying from white to yellow. Herbaceous vegetation with occasional shrubs.
21d	Gravels	Elevated area of coarse gravels with grasses and lichens.
21e	Gravels	Depression in gravels with some finer material and island of shrubby regeneration, situated at base of a gravel pile 1m high.
21f	Gravels	Elevated area of coarse gravels with grasses and lichens.
21g	Gravels	Gravel slope; finer material more abundant towards bottom. Sparse grassy vegetation with shrubs becoming denser towards bottom.
21h	Gravel and sand	Damp low area with sandy substrate; presence of organic material in the top 5cm. Shrubby regeneration dominated by Piper aduncum.
21i	Sand	Sandy substrate close to lake, becoming hydromorphic. Herbaceous vegetation with Pityrogramma ferns and Lycopodium cernuum.
21j	Soil	Shallow yellow latosol on slope clothed in secondary forest to 15-20m.
21k	Soil	Deep red-yellow latosol towards top of small hill with secondary forest.
22	Soil	Red-yellow latosol beneath dense terra firme forest on top of a ridge SW of Homoxi on the trail to Wiramapiu.
22a	Soil	Red-yellow latosol beneath dense terra firme forest half-way up a ridge SW of Homoxi on the trail to Wiramapiu.
22b	Soil	Yellow latossol near the bottom of a ridge SW of Homoxi on the trail to Wiramapiu.
23	Sand	Hydromorphic grey sandy substrate in a baixada close to a stream in an area apparently not disturbed by gold mining.
24	Soil	Recently burned Yanomami garden.
25	Soil	Soil under secondary forest on slope of ridge alongside the Homoxi airstrip; possibly a transition to yellow latosol from red-yellow latosol.
26	Soil	Red-yellow latosol under terra firme forest on top of the ridge alongside the Homoxi airstrip.
27	Soil	Yanomami garden behind the Tirei maloca; the top 15cm is black and hard, passing to yellow (latosol) at greater depth. Low productivity of bananas in this garden but high manioc productivity.
28	Soil	Yanomami banana plantation on slope SE of the Post. Dense cover of herbaceous material with ferns.

Yanomami gardens

In general the soils underlying the Yanomami gardens that were visited in the Homoxi region are yellow latosols. This is partly a result of the position chosen for these gardens (i.e. in the on the lower slopes and valley bottoms, where the predominant soils are yellow latosols) but also a result of positive choice of locations on account of their soil properties.

The soils at each of the three sites sampled (24, 27 and 28) were similar, showing low natural fertility and a sandy texture (more then 60% sand fraction). The fertility, however, had been boosted at the surface by burning of the vegetation cover, resulting in darkening of the top 15cm. Below this level there was little difference from the yellow latosols found elsewhere in the area. Considerably more biological activity (termites and ants) was observed at 28 than at 27, and there was a denser ground layer of herbaceous vegetation. In addition, the soil was more compacted at 27 and the bananas appeared to be growing less successfully. All of these differences may be due to the fact that the garden at 28 had only been burned once whereas that at 27 was on its third cycle.

Additional information on soils in the Homoxi region, based on interview data, is given in the section Spatial use of resources at Homoxi.

Substrate analyses

Granulometry

Forest soils: Analysis of granulometry of soils in terra firme forest sampled with mini-profiles demonstrated considerable textural diversity. The soils from samples 2, 6, 24 and 26 were in general more silty. The soil at 21j showed a sandy texture with little granulomatric differentiation with depth. On the other hand, 21k (on the same transect) was better distributed with no predominance of any one fraction along the profile. Soils at 22 and 22b showed more sand and clay in their constitution than the silt fraction (the presence of a silt fraction suggests immature soils). The textural diversity of the soils is probably a function of their topographic position, although parent material (mother rock) has doubtless influenced the granulometry as well.

Secondary forest and grassy areas (including airstrips and abandoned gardens): In the secondary forest areas the soil texture was very diverse. There was generally an association of fractions present of which the most common was the sandy-silty fraction. However, in some samples (e.g. 12 and 13a) there was predominance of silt, or of clay (point 17). This diversity in textures may be connected to the position of the samples in the landscape and topography.

New gardens (including one recently felled and burned): In general the soils in the gardens were clayey in texture, with the exception of the soil at point 27, which contained a higher proportion of sand. The highest levels of clay found at point 28 may be responsible for the higher availability of nutrients favouring the production of the banana plantation on the site.

Substrates altered by mining: The gravel substrate is made of a mixture of stones, pebbles and sand. This is mainly coarse in texture and its proportions vary. The mineralogy is predominantly quartz, but some mica (muscovite) was present. A few samples showed a finer granulometry, e.g. sites 1 and 4, in which silt was present in significant proportions. At some points across the profiles at 19 and 21 clay was also a significant component. These finer fractions were generally found in the topographically lower areas.

Chemical properties

Forest soils: In general these soils are highly acidic with medium to low fertility. They show low levels of exchangeable minerals (Ca, Mg, N e K) and low to medium levels of C, N e P. The high acidity and base poverty are reflected in the raised levels of exchangeable aluminium. In terms of organic material the carbon, nitrogen and phosphorus are highest at the surface on account of the contribution from decomposing vegetation. These decrease rapidly with depth - particularly the carbon.

Secondary forest and grassy areas (including airstrips and abandoned gardens): In general the soil surface in secondary vegetation showed a decrease in levels of carbon, nitrogen and phosphorus when compared with forest soils, and little incorporation of organic matter in the 10-20cm depth. The low fertility and high acidity of the soils remain unchanged.

New gardens (including one recently felled and burned): In general these soils, like those of forest and secondary forest, were highly acid and low in exchangeable elements (Ca, Mg, N and K). In terms of organic matter they showed low levels of carbon, nitrogen and (principally) phosphorus at sites 27 and 28, in contrast with those at site 24 (recently burned). In comparing the samples it can be seen that at site 27 (a garden where the bananas had not grown well) the levels of organic matter were slightly higher at the surface. The levels of nitrogen were similar, and those of phosphorus and CEC (cation exchange capacity) were slightly higher at the surface of the soil at point 28 (a garden where the bananas were growing well). The better chemical conditions allied with the clayey texture appears to have resulted in a greater availability of nutrients at this site and hence a better production of bananas.

Substrates altered by mining: The gravel-dominated substrates in the main mining areas were in general highly acidic and with extremely low levels of nutrients. Those without vegetation cover showed particularly low levels of carbon, nitrogen and phosphorus. In contrast, areas supporting some regeneration showed slightly higher traces of these elements. Some samples were found to contain higher levels of potassium, sodium and calcium than the forest soils, however. This may be the result of the breakdown of the mica that was present in the mining waste. This may in turn have contributed to the regeneration of vegetation at these sites.

Microbial content

Preliminary data on microbial analyses of the substrates sampled at Homoxi are presented in Appendix 8. These show that although the gravels are being recolonised by microorganisms, their density and diversity remain lower than those of undisturbed forest.

Soils and the regeneration of vegetation

There is considerable variation in the regeneration of vegetation in the altered areas at Homoxi. In some areas, for example, secondary forest to a height of several metres has established itself on gravels, with accumulation of leaf litter and development of a root mat on the surface. The level of regeneration is determined by a number of factors including topography and the distance to undisturbed forest,[1] but one of the most significant factors is the nature of the substrate on which it is growing. In general it is the areas with greater humidity and/or finer granulometry (accumulations of sand and silt) that have regenerated most successfully. In the isolated, raised patches of gravels where there has been least regeneration, the effects of small accumulations of finer materials in depressions in the terrain can clearly be seen by the survival of shrubs and in some cases small trees. Small amounts of leaf litter accumulate underneath these shrubs and nutrient levels are raised.[2]

At the abandoned airstrips the yellow latosols on the strip itself have been compacted, allowing little regeneration beyond a dense cover of grasses. The effect of the loosening of these soils can be seen around the bomb holes made by the Federal Police, where regeneration is considerably more advanced (shrubs and small trees). In the areas surrounding the airstrips that were felled by the garimpeiros but where the soil has not been compacted, secondary forest has developed.

[1] Proximity to undisturbed forest results in a higher seed rain and greater accumulation of litter.

[2] It was observed on the gravel banks that the roots of the grasses and herbs were penetrating deeply into the gravel/sand substrate (possibly on account of its porosity, which may facilitate water movement and root penetration). Some development of biotic activity was observable in these substrates, including termite activity and fungal growth.