The Loam Ranger – Shotgun pellets and wet soils
Dear Loam Ranger,
The local ants have been making piles of big round pebbles. Where did the pebbles come from? My soil’s sandy.
These pebbles are a characteristic feature of soils that experience cycles of wetting and drying.
For a change, we’ve been having a lot of wet weather on the east coast. So instead of talking about dry soils, let’s look at some peculiarities of wet soils.
Wet and dry: La Niña and El Niño
The Australian climate, particularly on the east coast, is subject to extremes of dry and wet cycles, known respectively as El Niño (“the boy child”) and La Niña (“the girl child”). These cause the “droughts and flooding rains” of Dorothea McKellar’s Sunburnt Country. We now understand that these are due to ocean temperature oscillations across the Pacific Ocean (as measured by the Southern Oscillation Index).
Interestingly, this wet-and-dry cycling has a profound effect upon the morphology (physical structure) of our soils. Soils subject to such wetting and drying cycles tend to form a strong “texture-contrast” soil known as a podzol, podzolic or kurosol. These soils have a sandy or loamy topsoil (A horizon) changing abruptly into a much heavier (usually clay) subsoil (B horizon). This often seems to happen regardless of the underlying rock or the vegetation, and is common in the high-rainfall or humid zone. In fact, these are the dominant soil types along the whole east coast of Australia.
Very often in such soils we can find a distinct layering in the A horizon, with the surface layer (A1) being darker, but the underlying layer (A2), just on top of the B horizon, being white or bleached. Also, very often, there are various hard “shotgun pellets” (sometimes flattened) of a black, red or iron colour. Sometimes these are so abundant as to form most of the soil. This is not partly weathered rock: these stony inclusions are formed in situ. Why?
Chemistry is the answer
When it rains like mad and the surface soil gets saturated (forming a “perched” or temporary water table), chemical changes occur:
- The clay minerals are weathered and broken down.
- The clay particles are leached downwards by the water, leaving the A horizon sandier and the B horizon clayier.
- Certain minerals, particularly iron and manganese, are chemically “reduced”.
The more this happens (or the longer the time over which it happens), the stronger the texture contrast becomes.
The third mechanism, reduction, is a fundamentally important piece of soil chemistry. The metals iron (Fe) and manganese (Mn) occur as oxides and hydroxides with the following general formulae:
- Goethite – Fe(OH)3 – rusty red
- Ferrihydrite – FeOOH – orangey yellow
- Manganese dioxide – MnO2 – black
- Manganite – MnOOH – black
In these forms, both Fe and Mn have had all loose electrons (2 or 3 of them) stripped off and replaced with those attached to oxygen (O). Think of them as fully “rusted” or oxidised. These forms are extremely insoluble, which is why even after the silica has been washed away (such as in some rainforests), there can be nothing left but iron minerals.
When a soil is waterlogged, however, conditions become anaerobic (that is, without air). Without the oxygen in the air, the conditions are now called “reducing” (the opposite of oxidising). In chemical reduction, some electrons are added back. The resulting forms of Fe and Mn are much more soluble, dissolving in the soil water. As the soil dries out again, they oxidise back to the insoluble forms, forming hailstone-like balls. If you crack these balls open you can often see the roughly concentric growth rings resembling an archery target.
Shotgun pellets
Different forms of pellets can be found: hard, soft, round, slightly angular and even platelike. Where manganese predominates they are black. Where iron predominates they are red. Collectively they are called ferromanganiferous nodules, but “shotgun pellets” will do!
The presence of shotgun pellets at the A/B boundary or in the A2 horizon is a sure sign that the soil waterlogs occasionally. There are two consequences of this:
- Manganese and iron toxicity in plants. Although Mn and Fe are essential trace elements, too much in a soluble form can induce toxicity. Susceptibility to Mn toxicity varies. Generally legumes (peas, beans, clovers, lucerne etc.) are susceptible, whereas other species, like many eucalypts (which of course are adapted to these soils) are highly tolerant.
- Death due to waterlogging. A good example is olives. Olive trees are often planted in podzolic soils with no attention to drainage, as they are considered tough plants. Yet good initial growth can be followed by sudden death (usually in low-lying areas) after sustained heavy rainfall.
In the lab we can recognise soils that are periodically or permanently waterlogged because of elevated Fe and Mn levels, the characteristic colour and texture contrasts, and the presence of the shotgun pellets. These clues tell us whether the soil is suffering anaerobic conditions and whether plants are likely to suffer from Fe and Mn toxicity and low-oxygen stress.
When the current floods pass, have a look at a low-lying area of your soil and see whether you can spot any nodules or any stark colour or texture contrasts. If you can, you now know that that soil is subject to periodic waterlogging and possible Fe and Mn toxicity.
Further information
Wikipedia: El Niño-Southern Oscillation.
Wikipedia: Podsol.