By Tony Hartshorn, Lesego Khomo, Kevin Rogers, Arjun Heimsath, and Oliver ChadwickKruger National Park is justly famous for its Big Five and the enlightened management practices that ensure the sustainability of these populations of buffalo, elephants, leopards, lions, and rhino. Less well known is that Kruger represents a world-class laboratory for understanding how landscapes are sculpted into ridges and valleys.
Krugers Hidden Secrets
Yes, this is a premier place to study the dirt underfoot and over which your bakkie and millions of animals—from elephants to dung beetles—travel. Kruger straddles three important gradients that shape the soils across which people and animals travel. These are well documented in the tourist map at any shop inside the park, and they include the changes imposed by rainfall, rock type, and hilliness.
The impact of these gradients can be seen in the soils as you drive through the park, especially in the spectacular river cuts, so take particular note of those. Other factors influence soils too: plants and animals as well as time. Because Kruger happens to be where humans evolved nearly two million years ago, one could argue that all of Kruger's soils are likely to contain the fingerprints of our ancestors, making humans another important soil-forming factor.
Since the northern part of the park is approximately twice as dry as the south, this leads to large differences in the soils. For example, granite soils are generally rockier and shallower in the north. These differences cause changes in the plant community.This is especially evident as one passes into the endless sea of mopane at Olifants from the knobthorn-dominated south. The geological gradient shows its importance in the west-east drive from Orpen gate to Satara. The flat eastern half of the park is underlain by soft basaltic rock which produces rich soil that can support large amounts of grass.
The burning of this grass in the dry season limits the growth of trees and that is why the Satara area is such a postcard savanna. As you drive through the eastern half of the park, by contrast, you will notice a more rolling landscape. Here, the soil landscapes are derived from harder granitic rock that makes poor soils and less grass, and this gives trees the opportunity to escape fire leading to a much more closed savanna. Interestingly, granitic soils need not be poor throughout their extent—they can actually be quite rich due to the action of another soil forming factor, hilliness. It has long been known, for example, that some topographic sequences show repeatable patterns as one drives from the crest, or the highest parts of the landscape between rivers or channels, down to those rivers or channels.
A typical sequence is shown in Figure 1 and can be summarised as red/coarse, white/coarse, black/fine. In many hillslope sequences, the lowest parts of the landscape are often sodic sites, a technical term for the very salt-enriched, and relatively sparsely vegetated areas, that are commonly found next to rivers across the park (Figure 2). What accounts for this great variability in granitic soils? These special sequences of soils may actually represent Kruger's hidden secret—the differences along slopes occur because material is moving downslope over very long periods Chadwickof time. Our measurements suggest that Kruger is eroding at a rate of three centimetres per 10,000 years.
This means that since sea levels were 100 m lower than they are today (surfing at Durban was not as good back then), Kruger has lost less than the thickness of a fat novel! This is a very slow rate of erosion compared with the fastest eroding parts of the planet in the Himalayas, which have lost almost 50 metres of relief in the last 10,000 years. Kruger's very slow erosion rates mean that the soils from crests to channels have been stable for very long periods of time—perhaps 1,000,000 years. Thus even very slow movements of material downslope have left their distinctive imprint in the landscape. What kinds of materials move over time?
The smallest materials may represent individual molecules of water, but in the dry climate of Kruger, many of these molecules fail to move very far in the soil because much of the water that rains down on Kruger is evaporated or transpired by trees. We have estimated that these losses of water range from three to four times the amount of rain falling across Kruger.This imbalance between losses and additions of water is a defining characteristic of savannas and helps explain the great variability in the soil landscape. It is neither too dry for things to wash slowly downslope as in the Kalahari, nor too wet, when water just washes everything away. Kruger, and the lowveld in general, are in just the right place to form well-differentiated soil landscapes.
If some of the smallest materials slowly moving downslope are water molecules, what other materials are slowly being washed downhill toward the rivers crossing the park, rivers such as the Shingwedzi, Olifants, and Sabie? These large rivers are ultimately responsible for carrying small pieces of Kruger through the Lebombo Mountains to the Indian Ocean. The materials we have measured include salts such as sodium ions and clay-sized particles. Remember that the reason the oceans are salty is that, over time, the breakdown of rock frees salts that were once trapped in the rock. In wetter parts of the world with metres of rain (versus Kruger's half metre), the release of salt from rock, and the transport of that salt to the ocean, occurs extremely quickly.
Here in Kruger, that two-step sequence of breaking away and transport occurs slowly enough to lead to great soil variability from crest to channel. How exactly is sodium liberated from rock? One way to think about this is to imagine a rock sitting in a bath of acid.
That acid dissolves the rock, freeing elements that make up the rock, including sodium. Rock under soil is bathed in acid. Most of the acid is produced by soil organisms and plant roots, although Kruger is receiving more and more acid via its rainfall because of the large concentration of power plants upwind. The characteristic termite mounds that punctuate the horizon are important producers of acidity.
The next time you see a termite mound, think about it as an acidity pump, injecting acid deep into the soil, where it meets the granite rock and begins a sodium ion on its long odyssey to the Indian Ocean. Just to appreciate the scales involved, if a sodium ion is the size of a rugby ball with its average short-axis radius of ~10 cm, a water molecule would occupy about 10 times the volume of that ball.Clay-size particles, by contrast, are 1200 times larger than the rugby ball and sand-size particles are more than a trillion (1,000,000,000,000) times the size of the rugby ball (Figure 3). If you took a ball and turned it into a cube, you could fit about 360,000 of them into an Olympic size swimming pool.
Thus, if a sodium particle is thought of as a rugby ball, moving a sand-size particle downhill would require a much greater amount of energy—because it would be like moving an object three million times larger than a swimming pool! As you might imagine, it is much easier to move a sodium ion downhill than it is to move a sand particle.And this difference helps us understand why salts move further in the landscape than even clay-size particles. What happens to a water drop that falls on the crest of a slope? It slowly trickles downhill toward the nearest river. Just as walking across Kruger represents an extremely risky proposition unless you have made arrangements for a wilderness walk, with an armed escort, that journey from crest to river is a hazardous one.
In savannas, there is a very high probability that a water molecule will be sucked up a plant root and transpired or evaporated directly from the soil. If it is not lost from the system, it may be able to transport material like salts and clays downslope.
However, because of the large size difference in these materials, we might expect more salts to be carried further downhill than clays. In fact, this is exactly what Kruger's granitic hillslopes end up looking like: sandy, leached crests grade to clayey, dark soils partway downslope and these grade next to channels into salt-rich sodic zones.
So on your next game drive, be sure to take time to reflect on how the much slower movements of things like sodium and clays lead to the soil differences that lead to the vegetation patterns that ultimately influence the movements of the Big Five.