On the road…again!
Afghanistan to Zambia
Chronicles of a Footloose Forester
By Dick Pellek
Sand, Silt, and Clay
At one time or another in our everyday lives, everyone is guilty of using the improper or imprecise word or words to describe things around us. Some of us do it thoughtlessly and often, others of us do it because we haven’t come up with better words to describe things. The more precise the words, the greater the clarity in the description. And there are consequences when we miss the mark badly.
When it comes to describing soil it is reasonable to assume that most people have a clear enough understanding of the basic idea of sand. For example; the soil type commonly described as sand is readily perceived as loose, gritty, and grainy particles that are large enough to be isolated and set aside as individual specks. Sand as a noun is a legitimate description as a soil type. The word sand as an adjective also has technical applications and implications. There are various kinds of sand: quartz sand, silica sand, coral sand, etc. In soil science, various sands are further described in size ranges such as very coarse sands, coarse sands, medium, fine, and very fine sands. Thus, knowing the parent materials and size ranges of various sands helps to define and describe the overall soil matrix in which they occur. Precise words as descriptors of parent materials is one thing, and knowing particle sizes as complementary descriptors of sands helps us to understand why some agricultural crops can be grown in sand and others cannot; and why some kinds of sand are better able to meet certain objectives than others.
The precise size category of a sand as part of its suite of descriptors can be deduced by sifting a bucket of sand through a series of nested sieves and adding up the percentages that pass through each sieve. Perhaps it does not seem important to distinguish a medium sand from a fine sand, but if virtually everything found in the bucket stayed in the uppermost sieve, it is likely that the soil type is not sand, at all. Particles that are too large to classify as sand must be classified as gravels. On the other end of the size range, if virtually all of the properly dried samples in the bucket pass through the lowest, finest sieve; then the soil type might be a silt or a clay.
soil texture triangle
Why is it important to know the difference between a sand and a silt, or between a silt and a clay? Because most crops cannot be grown in true sands, especially when rainfall is limiting. Yet, sand soils can support vegetation if the key environmental factors are factored in; and if the proper kinds of vegetation are chosen for the task.
Dune stabilization has been very successful using Casuarina equisetifolia, a tropical tree that is known to thrive in deep sands. The Footloose Forester was pleased to see over 200,000 Casuarina trees transform large patches of barren sand at Cam Ranh Bay in Viet Nam into stabilized landscapes that also developed with aesthetic appeal into sources of useful building materials, on sites where they did not previously exist. Close to the sea, such species as seashore saltgrass (Distichlis spicata) are capable of holding back dunes under the added stresses of constant wind and salt spray. In desert areas, there are numerous Acacia tree species that can thrive there.
On a much larger scale, millions of hectares of arid and semi-arid lands around the world that are primarily sandy in nature can and do benefit through afforestation campaigns, using tree and shrub species that can survive in sand. Afforestation refers to the planting of trees and shrubs in places where they did not naturally exist. Vivid examples of successful afforestation campaigns can be seen in Mali, Niger, China, and elsewhere.
Silt as a soil descriptor is also generally and widely understood. The process of silting of delta regions and the formation of silt banks along rivers are apt examples as relatable circumstances germane to silt and siltation, but not exclusively so. They derive from similar parent materials like sands, and they are also defined by their particle size. Indeed, the fine particle size of silt facilitates displacing associated silts into groundwater and dispersing them in the process of lateral transport. Silt is also dispersed into the wind, particularly at the site of active volcanoes.
At 50 microns in diameter as the lower limit of particle size, individual grains of silt are barely visible with the naked eye, and only then with the aid of strong light. Silt as a soil type in relatively pure form is not often found in nature because the narrow size range of silts is transitory and progressive. Breakdown of larger particles of gravel and sand, through weathering, grinding, and erosion is continuous until it reaches the endpoint which is clay. Not too much can be said about silt to distinguish it from very fine sand at one end of the size measurement range and clay at the other end, if only because individual particles of silt are so small. As a personal observation, however; the Footloose Forester is confident in stating that a person with good eyesight can distinguish individual particles of dry silt under good light conditions. And that means seeing something as small as 50 microns in diameter.
In a few areas of the world, relatively pure silt soils can be found. The upland slopes in the Volcanoes region of El Salvador are rich in silt that is a direct result and by-product of the emission of volcanic ash that has been dispersed over a period of centuries. Deep profiles of silt that are more commonly thought of as ash blankets ring the upper slopes of Volcan Santa Ana and Volcan Izalco in Santa Ana Department of El Salvador. The area is also an important coffee growing region that has benefitted from the natural mineralization and easily workable silt soils. One can only speculate whether the production of high-quality coffee is a direct result of the fertilizing benefits of the silt soils.
This chronicle entry is meant to be more than a dry, semi-technical discussion of soil particles and their properties. The Footloose Forester learned valuable lessons on the ground about how fundamental natural resources such as soil, water, forests, and grasslands are intertwined and interactive in the varied landscapes we humans occupy and depend on for our very existence. In the case of silt as a natural resource, the ash blankets in the remote uplands of the Volcanoes Region of El Salvador constitute one of the best forest fire fighting tools available. A person is able to dig it up and use it to easily snuff out ground fires by smothering the flames. At one time, the Footloose Forester and a Salvadoran Ministry of Agriculture employee were able to extinguish 100 meters of ground fire by simply digging into the ground with our hands and scooping up the silty soil to snuff out the flames.
In the nomograph above, commonly referred to as a soil texture triangle, the precise description of a soil type can be determined if you have data on the particle size distribution to back it up. As deduced, most arable soils contain admixtures of sand, silt, and clay.
As important as soil texture is in the agricultural milieu, there are differences of opinion regarding the percentages of sand, silt, and clay that should be used as boundary descriptors. As a result, there are a half dozen variations of the soil texture triangle used around the world, although the one used in the United States is considered to be the standard model.
At the opposite end of the size range of soil particles, we find clay. Paradoxically, when we handle a clod of wet, heavy soil that we may readily identify as clay, we might be precisely correct. The paradox in play has to do with the fact that clay particles are so small that they cannot be inspected individually. They are smaller than silt particles but can, nonetheless, form into clods because of the binding properties of organic matter that is normally contained in native soils. Water tension within the soil matrix also helps to keep particles coalesced.
In a soils laboratory, clay is extracted by washing the contents of the whole, natural sample through the entire set of nested sieves and then concentrating the clay found in suspension by using a centrifuge. Individual soil bodies may be classified as clays in a general way, but similar to soil particles of sand and silt, the clays may also be classified into mapping units to include their various other properties. Perhaps more than named sandy soil types or silt soils, it is the panoply of chemical and physical properties of various clays that sets them apart and makes those characteristics so important in agriculture. Microscopic clay particles may be small but they are the most chemically reactive and thus are vitally important in every aspect of soil fertility.
Even clays are different enough that good descriptors are useful in distinguishing one from another. In the most basic description, one group of clays can be described as sticky in structure; and another group can be described as non-sticky. It also has to do with a shrink-swell capacity of their cell structures. Those that exhibit shrinking when dry and swelling when wet are known as 2:1 ratio clays. Those that don’t exhibit shrinking and swelling are generally referred to as 1:1 ratio clays. Needless to say, the working qualities are very different, thus knowing what kind of clay a farmer has makes for fundamental differences in their management. As the final step in the breakdown of larger soil particles, various clays around the world present the most challenges when it comes to issues of flooding and extreme drought. Both circumstances have to be dealt with because both circumstances limit the practical value of any parcel of land.
There is a quite fuzzy recollection of anecdotal stories about clays about which the Footloose Forester sometimes daydreams. One of them is about the dark-colored 2:1 shrink-swell clay found at Brickfield, Trinidad. When he was working the dry land there with its distinctly wide open cracks in the soil, it began to rain. He hoped to see the soil profile swell up with moisture, so he stuck a pencil into the crack and watched as the surface of the soil began to close shut. In just a few minutes the surface was sealed and only the eraser at the end of the pencil was visible.
One other remarkable memory is about a pale yellow clay that he was analyzing at a depth of 1.5 meters in a tropical forest in Indonesia. With its high rainfall in the region, soil weathering was extreme and extended deep into soil profiles. But even at 1.5 meters, it was almost all pure clay, including adjacent to some stone parent material that formed the spine of the highest hill in Ujung Kulon National Park. The stage of weathering was so obvious that the Footloose Forester was able to break off chunks of the sunken stone and proceed to gouge out the soft exterior with his fingernails. In a matter of minutes, he had reduced the stone to a pile of clay. The stone itself could rightly be labeled as claystone: one minute it was stone, and the next minute it was clay.
As a final anecdote about clay, the Footloose Forester allowed the contents of a dream to seep out into his waking consciousness to provide another observation about clay. The sizable accumulation of decomposed leaves shed from an American Elm tree in Newton, New Jersey convinced him that decomposed litter from trees and shrubs becomes incorporated into the ground as clay; clay as a size descriptor and indistinguishable in appearance from any other dark-colored clay.
The reason he believes that is because, at a height of about five feet above the ground, he excavated several hundred grams of decomposed leaves from a deep fissure in a crook of the parent tree. The decomposed material, by now readily recognized as organic matter looked like and had the texture of clay. No sand, no silt. The real-world implication is clear...when archeological remains are unearthed, they are usually discovered as buried artifacts. The cumulative depth of "soil" that covers them, for the most part, is a result of hundreds or thousands of years of shed plant materials that eventually develop into a soil profile. Of course, shifting sands in desert ecosystems, buried under volcanic eruptions, and rapid depositions due to river and stream bed shifting also contribute to the development of soil profiles that mask what is underneath. And that is why archeologists acknowledge that there are tens of thousands of artifacts under the prominent mounds you can still see in the jungles around Tikal, Guatemala. Archeologists have been busy unearthing them for decades but there are thousands more still waiting to be "discovered."