Geochemical properties and migration patterns of strontium in soil
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How strontium behaves in soil and why it is important for plants
In soil processes, strontium acts as a direct analogue of calcium due to the similarity of their ionic radii. Plants easily absorb it from the soil solution, where the element exists in the form of Sr²⁺ ions or mobile chelate complexes. Since strontium is actively involved in the nutrient cycle, its excess directly affects the quality and safety of the harvested crop.
Upon excessive accumulation in the environment, strontium begins to replace calcium in plant tissues. The maximum permissible concentration of the element in soil is 600–1000 mg/kg, while its actual content in surface horizons can range from 18 to 3500 mg/kg.
During the weathering of minerals, strontium turns into soluble bicarbonates, chlorides, and sulfates. Sulfates and carbonates of this element are less soluble than calcium salts, but its bicarbonate is significantly more soluble. Because of this, in regions with high humidity, strontium is leached from the root zone more intensively than calcium. Free strontium is partially bound by clay minerals and organic matter, but the bulk of it is carried away by water runoff into the ocean.
In geochemical processes, strontium behaves specifically. It concentrates in sedimentary rocks during the evaporation of moisture along with gypsum, and also accumulates in endorheic chloride waters. Compared to calcium, this element is less biophilic and more prone to accumulation in seawater. In the biosphere, the Sr:Ca ratio remains relatively constant, which is why its deviation is used to identify dangerous concentrations of the element.
- Clarke in the Earth's crust — 4·10–2 %
- Clarke in soil — 3·10–2 %
- Content in plants — 1·10–4 %
- Content in livestock animals — 1·10–3 %
- Content in seawater — 1·10–3 %
Geography of distribution: zones of excess and deficiency
The concentration of strontium in the arable layer depends on the composition of parent rocks and the climate. In soil-forming rocks, the element content ranges from 72 to 1380 mg/kg, with heavy clay fractions retaining it significantly better than light sandy ones. In loess-like rocks, strontium is distributed evenly, while in alluvial deposits, limestones, and marls, it is extremely patchy due to the heterogeneity of the particle-size distribution.
| Soil-forming rock | Strontium content, mg/kg |
|---|---|
| Loess-like heavy loams | 167–279 |
| Loess-like clays | 129–209 |
| Clayey alluvium | 150–410 |
| Loamy alluvium | 316–414 |
| Clayey deluvium | 118–233 |
| Shale eluvium | 111–154 |
| Marls, chalk, limestones | 143–544 |
Increased strontium content is characteristic of peat-bog soils of the tundra on alkaline weathering rocks, as well as for the chernozems of the Trans-Urals and light chestnut soils. This element also actively accumulates in sierozems, floodplain and meadow-bog complexes of taiga-forest regions.
A strontium deficiency with an average total level of no more than 120 mg/kg is usually noted in podzolic and sod-podzolic soils of the forest zone. Soils of the steppe and forest-steppe chernozem zone, sandy massifs of deserts, as well as chernozems of mountainous regions, contain few of the element.
In the soils of the southern regions, the average amount of strontium is 2–3 times lower than the national average. The deficiency is especially pronounced in mountain non-carbonate soils of humid zones and in light alluvial-meadow soils.
The behavior of strontium in soil directly depends on its type, acidity, and water regime. In acidic soils of humid zones, the element is actively leached into lower horizons and the soil-forming rock. At the same time, carbonate and leached low-humus chernozems retain strontium in the upper humus-accumulative layer.
In the soil solution, the element is present in the form of soluble salts — carbonates, bicarbonates, chlorides, and sulfates, and is also part of the soil adsorption complex in the form of exchangeable cations. It is from these mobile forms that strontium is easily absorbed by the root system of plants and migrates through the profile. The constant presence of the element in soil solutions ensures its continuous cycle in the "soil — water — plants — living organisms" system.
| Soil type | Strontium content, mg/kg |
|---|---|
| Low-humus carbonate chernozems | 271–259 |
| Low-humus leached chernozems | 167–187 |
| Vertic chernozems | 129–155 |
| Low-humus chernozems | 167–279 |
| Medium-humus leached chernozems | 209 |
| Meadow-chernozem soils | 298–410 |
| Alluvial-meadow soils | 316–414 |
| Meadow-bog soils | 150–170 |
| Grey forest and forest-steppe soils | 118–233 |
| Brown mountain-forest soils | 111–154 |
| Humus-carbonate soils | 278–544 |
| Mountain-meadow soils | 143–208 |
The displacement of strontium by solutions containing calcium has key practical significance. This technique is used for land reclamation and the restoration of soils contaminated with radioactive isotopes.
Behavior of the radioactive isotope strontium-90 in the soil profile
The 90Sr isotope poses a serious danger, as it mimics calcium in biological processes. Entering the human body through the "soil — plants — livestock" chain, radioactive strontium accumulates in bone tissues and can have a carcinogenic effect.
The fixation strength of strontium-90 depends on the particle-size and mineralogical composition of the soil, its pH, cation exchange capacity, and organic matter content. Soils with a heavy particle-size composition possess the highest sorption capacity and retain this nuclide most firmly. Further vertical movement of the element deeper into the profile occurs due to diffusion, convective mass transfer, and plant root systems.
Depending on the strength of the bond with the soil adsorption complex, strontium-90 is divided into three forms: water-soluble, exchangeable, and non-exchangeable. Their ratio determines the bioavailability of the isotope for agricultural crops. In arable lands, the proportion of the most firmly bound, non-exchangeable form differs significantly from similar indicators in virgin sites.
- Variation of content in mountain-forest soils — 16.6–18.1 %
- Fixation of 90Sr in the top layer (0–5 cm) — 70–90 %
- Proportion of non-exchangeable 90Sr in the arable layer — 20–30 %
- Proportion of non-exchangeable 90Sr in virgin soil — 6–40 %
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