Rainforest soils are catagorized as oxisols (the red regions of the map). The A horizon, comprised of humus and nutrients, is shallow, thus easily depleted by deforestation. These soils lack a substantial nutrient layer due to extensive rainfall and high temperatures. As a result, the flora has adapted to absorb nutrients directly from decaying plant matter. Rapid bacterial decay makes this possible, as nutrients are released quickly once something begins to decompose. Beneath the A horizon, the thick B horizon consists mostly of clay, sand, and iron and aluminum oxides (giving the soil its red color) that do not retain water well.
Like most soils, tropical soils have a low pH. This causes high concentrations of aluminum and iron. Tropical species can tolerate these high levels of alumunium (whereas it would be toxic to other plants), however, if acidity is increased, the soils can become infertile.
Oxisols also have low ion exchange capacity. That is, ions are not readily available to the plant life. To accommodate this, certain fungi have developed a symbiotic relationship with higher order plants. The fungi, called myccorhizae, dissolves P, Ca, and Mg for the infected plants, while the latter provides the fungus with sugars. If the soil becomes overexposed to sunlight, however, the mycorrhizae die in the baked soil, decreasing the amount of ion absorption by plants, and thus their health and resilience.
Soil Locations:
Monitoring:
We have identified pH and ion concentrations to be important factors in monitoring.
Here is one lab method commonly used:
1. Drying
- soils should be dried as rapidly as possible to minimize microbial activity
- accomplished by exposing as much as surface of the soil to circulate air as possible and by evaluating the drying temperature*
*not to exceed 38 C to preserve the physiochemical properties of the sample
2. Crushing
- By hand or mechanical (stainless steel)
- Filter through a 2mm screen to remove rocks and crate a uniform sample
3. Divide samples by volume into equal portions
Testing for pH
- use pH meter with two separate electrodes
- prepare a “soil slurry” of 0.01M CaCl2 2H20* in ratio 1:2
*Recommended for sandy soils or soils with low cation exchange. Usually causes the pH to appear 0.3 to 0.5 lower therefore, take this into account
Testing for ions
- Use CaCl2 to indicate the presence of Na, Mg, K ions.
- Add 0.01M CaCl2 to soil sample and shake it for two hours (mechanically).
- Centrifuge and collect the supernatant and analyze for elemental composition.
- Use plasma emission spectrometry (with emission lines, etc.) to determine percent of Na, Mg, and K ions.
References:
General Soil Characteristics:
“Brazil.” Microsoft Encarta Encyclopedia 2002. (1993-2001). Microsoft Corporation.
