L. Tropical Soils

Soils are the basis for all terrestrial life, and the soil on which a forest grows is a critical determinant of forest type and vegetation. Soils are the transformers, regulators, buffers, and water and nutrient filtration systems of the forest. They act as links between the nutrient and mineral cycles and the atmosphere. They provide physical support for plants; they absorb, retain, and release water; they provide essential minerals and other chemical compounds for plant growth and maintenance; and they provide “waste disposal” services and nutrient cycling services through their microorganisms and soil fauna.

1) Soil formation

Soil is created by an immensely slow process involving the weathering of bedrock. Bedrock is dissociated by water and heat, and gradually forms particles at a rate of a few millimeters per millennium, although in the tropics weathering is more rapid than in temperate climates because of heavy rainfall and high temperatures. Because weathering is such a slow process, only minuscule amounts of minerals are generated by it. Some of these minerals remain dissolved or suspended in the moisture surrounding soil particles, and represent nutrients for future plant growth. In addition, the roots of plants fracture rocks, while soil organisms produce acids and CO2 which also help break the rocks down. While soil is being formed, the minerals associated with the particles are continuously lost by leaching as water passes over the soil or rock. Generally mineral losses are balanced by continual weathering of the rock. The minerals must be in a form which plants can absorb, but generally no more than 10% of soil minerals are, so that plant growth is often limited by their availability. As plants become established on the soil particles, and they (and animals) die and are decomposed by bacteria and fungi, their organic material mixes with the particles and accumulates near the surface to form topsoil. In this way, organic matter (humus) is added to the soil, making it able to support life. Since humus is resistant to further decomposition, it prevents compaction of soil and also plays a role in soil chemistry. Humus in soil also absorbs a great deal of water and prevents runoff.

2) Soils of tropical rainforests

Rainforests are very fragile habitats. In many places they are “wet deserts,” which grow on soils poor in nutrients. In many tropical regions, the bedrock is very old and weathered, and, consequently, depleted in minerals and nutrients. Mineral release is also inhibited by the acidic nature of many tropical soils. The soil types derived from the bedrock underlying tropical forests are mainly soils called oxisols and ultisols. (There are many kinds of tropical soils, each with its own characteristic array of minerals; see Richter and Babbar, 1991, for a detailed discussion). Oxisols have a high aluminum and iron oxide content and a low silica content. Ultisols are highly-weathered, acidic soils and are less frequently found than oxisols. These two types of soils, generally of low fertility, comprise about 43% of the soils under tropical rainforests (Hoffman and Carroll, 1995). Another 40% consists of variably fertile soils, some of which are suitable for agriculture, but many of which have low pH, poor physical structure, low phosphorus and other nutrient deficiencies, or high salt or aluminum levels. Interestingly, tropical soils can vary a great deal within a relatively small area, which leads to a variety of vegetation types because of differences in nutrient concentrations and availability, variations in the ability of the soil types to retain water, and the like. (Many other factors are also involved in determining the vegetation which grows in any particular area.)

Oxisols are acidic soils and contain considerable quantities of iron and aluminum. These minerals form insoluble compounds with phosphorus, which decreases the availability of the latter to plants. Also, under dry conditions and, particularly in soils with high iron contents and low silicate content, the oxides in oxisols form impermeable layers, known as laterite, below the surface. Thus, when the forests overlying such oxisols are cut down, the logged area becomes much drier and eroded, and this often leads to laterization. This will not happen if the surface is covered with trees and vegetation. Because laterite is impermeable, rain will run off quickly, leading to erosion and flooding. Laterization is not reversible.

Many tropical soils are acidic and depleted in weatherable minerals such as calcium, potassium and magnesium, essential for plants. Many lowland forests are limited by a lack of phosphorus, or sometimes calcium and magnesium; others, on spodosols (periodically-flooded sands) seem to be limited by low nitrogen levels. But plant growth is dependent upon the presence and interactions of many nutrients. To add to the intricacy of the situation, the presence – or limitation – of one mineral may affect the uptake and metabolism of others. For instance, the ability of leguminous trees to “fix” atmospheric nitrogen and convert it to nitrates and nitrites may be compromised by deficiencies in iron, molybdenum and/or calcium. Because there are so many types of tropical soils, and their mineral profiles are so complex, not a great deal is known about them.

Many essential elements such as calcium and potassium are easily leached out by the heavy tropical rainfall, further reducing soil nutrient levels. There are few nutrients more than 5cm (2 inches) below the surface of the soil in tropical rainforests. This poverty of soils (which is common but not universal in rainforests) has the consequence that the forest is dependent on the recycling of nutrients, most of which are contained within the vegetation and not in the soil, unlike temperate forests. Because many rainforest trees are evergreen and drop their leaves infrequently, there is relatively little “litterfall” in comparison with temperate forests. Leaves and dead plants and animals which fall on the forest floor are rapidly decomposed by fungi and bacteria, and the resulting chemical compounds are quickly reabsorbed by the living plants. Plants on tropical soils typically recycle 60% to 80% of nutrients, and in the case of calcium and phosphorus, more than 99% of these minerals appear to be recaptured from the soil by the roots of forest trees. The remainder of necessary nutrients must come from soil or from rainfall. [See also Section G5.]

How do plants retain nutrients under such stringent conditions? Many have adaptations which allow them to exploit the limited quantities of nutrients in tropical soils. Root biomass is very high where soils are infertile, so that plants can “locate” whatever nutrients might be available. Because so much energy must be invested in root systems, energy expenditures for leaves are minimized by retaining them for a considerable time, although this risks attack by pests. As an adaptation to this situation, many tropical plants form tough leaves containing noxious tannins and reinforced with woody fibers. Such leaves are unfortunately resistant to decomposition and inhibit the cycling of nutrients. Rainforest plants also produce vast quantities of leaves to capture as efficiently as possible the abundant CO2 in the air. Additionally the canopies as well as “crooks” in branches capture organic matter and provide a medium for its decomposition before it can reach the ground. Essential ingredients in nutrient acquisition and transfer are the associations tropical forest plants form with many species of flora and fauna in the soil; these organisms promote nutrient recycling and soil aeration. Forest trees grow on a mat of fungi [mycorrhizae, see above in Section G10e], which absorb nutrients, phosphorus and other minerals and transfer them to roots. The many soil fungi, bacteria and other detritivores rapidly decompose organic material on the forest floor, and these compounds become part of the soil’s nutrient supply. High-quality soil may have as much as 4000 kilograms of fungi per square meter, and 3000 kilograms of bacteria in the same volume. Trees form associations with nitrogen-fixing bacteria and fungi, which can extract gaseous nitrogen from the air and convert it to compounds usable by plants. In addition, forest soils contain large numbers of arthropods (perhaps 200,000), earthworms (which aerate the soil and increase water infiltration), nematodes, and many other organisms. Forests, by means of their roots, stabilize the soil and thereby reduce runoff into rivers and lakes, and, eventually, into the oceans. Where soils are stable, balanced nutrient relationships between freshwater bodies and land are maintained.

Since many tropical soils are already heavily weathered, they are highly vulnerable to nutrient loss and this is why many tropical soils are difficult arenas for the establishment of agriculture. Only about 20% of tropical soils are suitable for conventional agriculture, and many of these are found in alluvial plains and volcanic highlands. Disruptions of the nutrient cycle by clearing or burning (usually for agriculture or pasture) can be catastrophic for the soil, as nutrients will be rapidly lost and often the soil cannot support the same species as before, only an impoverished flora. A deforested experimental plot in Peru gave only moderate yields even with substantial inputs of fertilizer, but without them, yields dropped to zero. Even one crop depleted the soil too much for a good yield (Buol, 1995). Then, too, the organic materials contained in the nutrient-rich food crops are removed from the fields, not recycled as in a forest, and so are lost to the soil. Only the “refuse” or nonconsumable parts of the plants remain after harvesting to be decomposed and return to the soil as nutrients