How Loess Enriches the Soil

It was made clear that hills mantled with loess can be biologically special in Eugene Odum's classic college textbook called Fundamentals of Ecology. Here Odum says that "Soils which have developed on material transported by glaciers, water, and wind are often extremely fertile (witness the deep loess soils of Iowa and the rich soils of the deltas of large rivers)."

Why?

One answer relates to the manner by which glaciers advance over the landscape, grinding the many kinds of rocks below them into small, silt-size particles. When the glacier melts, these particles are sent downstream in the melt water. Since the particles are derived from rocks of various mineral compositions, the mud deposited downstream is composed of a spectrum of elements. This same spectrum of varied elements is distributed over the land as loess when the mud dries and wind carries away the dried mud's loose particles.

Thus, a handful of loess is almost like a multivitamin containing "all the essential elements." If the original soil below the loess happens to be a little deficient in magnesium, say, well, there's surely a bit of magnesium in the loess, since the loess-producing glacier during part of its journey in the north surely rolled over a dolomite outcrop, and dolomite contains magnesium.

There are less direct ways by which loess enriches soil, too.

For example, loess, we see elsewhere in this site, is composed of mineral particles of a very narrow range of sizes. Loess is a nearly pure form of silt, with almost no sand (particles larger than those of silt), and very little clay (particles smaller than those of silt). Thus, if loess is added to a typical soil containing a normal amount of sand, that soil's average particle-size is reduced. Causing a soil to become "less sandy" and "more silty" produces a variety of effects.

For one thing, it so happens that the addition of silty loess to a typical rather sandy soil improves that soil's capacity to hold and retain organic matter. Organic matter is the remains of once-living plants and animals in the process of being broken down to their basic mineral and chemical constituents. In general, the more organic matter in a soil, the more water and nutrients it can hold, so therefore the more diverse the ecology it can support.

A study by C.B. Williams et al of a very sandy topsoil in North Carolina revealed that it contained only 0.80% organic matter. A clayey North Carolina topsoil with a history similar to the sandy topsoil, however, composed of much smaller particles, on the average consisted of 1.46% organic matter.

Organic matter is important in soil not only because it acts like a sponge holding water, but also, since it is the remains of once-living organisms, because it contains much the same rainbow of minerals as once the organisms did, and therefore is very nutrient rich. Organic matter also improves a soil by loosening it up, making it easier for plant roots, earthworms, and other living things to penetrate it.

An addition of loess to a sandy soil also increases the sandy soil's available water. Available water is the water readily absorbed by plant roots. A handful of soil that's been dug up and deposited on a table will have water in it, even if that water does not drain onto the table. The water is held as a thin layer on each soil particle. If the water layer is thick enough, plant roots coming into contact with the particle will be able to absorb that water, even if the water is held tightly enough by the particle that it doesn't drain onto the table. Water thus available is... available water. However, if the layer of water on the particle is so thin that the particle's ability to hold onto it is greater than the root's ability to pry it loose, then that's.., unavailable water. The point is that loess-size particles typically have absorbed to them more available water than sand grains or tiny particles of clay.

Some faculty and students at Millsaps College in Jackson (C.D. Caplenor, et al) studied the effects of Mississippi loess on the amount of available water in Mississippi. They measured available water in various locations where there was "thick loess," "thin loess," and "upland non-loess." Here are the results, expressed in percent water by dry weight:

THICK LOESS
19.8%

THIN LOESS
12.5%

UPLAND NON-LOESS
9.0%

Clearly, the addition of loess to our soils has been a blessing for all plants needing water from the soil, and all animals depending on those plants.

Loess can also make a soil's pH more congenial for living things. Soil pH is a measure of the soil's acidity or alkalinity. Other things being equal, and dealing with average soils, the higher the soil's pH (the less acid it is) the more nutrients become available for plants and animals in and on the soil. Technically, a soil's ability to hold important nutrients such as calcium, magnesium, and potassium is referred to as its cation-exchange capacity. A typical acid soil with a pH of 4 may have a cation-exchange capacity of only 85 me./100 g; an alkaline soil with similar structure but with a pH of 8 may have a capacity of around 210 me./100 g.

In the same Millsaps College study mentioned above, the effects of Mississippi loess on soil pH was clearly demonstrated. The researchers measured the soil's pH in various locations, again where there was "thick loess," "thin loess," and "upland non-loess." Remembering that a pH lower than 7 is acidic, while a pH higher than 7 is alkaline, here are the results:

THICK LOESS
pH = ±6.85

THIN LOESS
pH = ±5.08

UPLAND NON-LOESS
pH = ±4.75

Thus, all the soils tended toward the acidic, though soils developed on thick loess were close to being neutral (pH =7). And the less loess there was, the more acidic the soil. Since the pH scale is logarithmic (a pH of 4 is ten times more acidic than a pH of 5), we can say that an upland soil not overlain with loess in the area of study was over a hundred times more acidic than soil developed on deep loess!

Why has loess made our soils less acidic? Elsewhere in this site we see that Mississippi loess is about 20% carbonate, mainly in the dolomite form, CaMg(CO3)2. The carbonate known as lime is used by farmers to "sweeten" their fields, or make them less acid, and dolomite is often a constituent of agricultural lime. Once again the Millsaps College researchers shed light on how loess increases the carbonate content of the soil it is deposited on. Again measuring soils where there was "thick loess," "thin loess," and "upland non-loess," they measured for the main carbonate in agricultural lime, CaO. Here's what they found:

THICK LOESS
CaO = 2.00%

THIN LOESS
CaO = 0.51%

UPLAND NON-LOESS
CaO = 0.43%

Finally, the same Millsaps College researchers gathered some data strongly supporting the assertion that by enriching the soil, the addition of loess in our area has enabled local ecology to increase in diversity. In the same "heavy loess," "thin loess," and "upland non-loess" locations, they determined the approximate numbers of species (not individuals, but different kinds of plants) found in each of the tree, shrub, and herb layers. Here are their very convincing results:

THICK LOESS
±40 trees
±51 shrubs
>66 herbs

THIN LOESS
15 trees
±34 shrubs
>49 herbs

UPLAND NON-LOESS
13 trees
±38 shrubs
>44 herbs

The "±" and ">" ("greater than") signs must be used because sometimes identifications were difficult or impossible.

Therefore, during the millennia when the wind laid down our vast layer of loess, it was analogous to a farmer going into his field and "liming" it, according to the best agricultural traditions. Moreover, just as with lime on a field, the loess that fell onto the lower Mississippi Valley enriched the soil in many ways, bestowing biological communities there with remarkable vigor and diversity.


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