Plant Growth on Serpentine Soil
Summary:Calcium is the most important nutritional element for the development of self-sustaining plant populations. It is critical to cell division and cell elongation; i.e., plant growth. And large healthy plants are needed to produce large numbers of seed (or other reproductive structures) to establish and maintain populations. Magnesium is only needed by plants in very small quantities, but it can contribute to very poor growth if in large amounts when calcium levels are low. Good plant growth occurs in soils with a Mg:Ca ratio of 1:1; gardeners lime their soils to achieve this ratio. At Soldiers Delight, the Mg:Ca ratio in topsoil (A horizon) is over 3:1, and it can be over 27:1 in subsoil (B horizon). This excessive Mg:Ca ratio is the main reason why serpentine areas around the globe develop distinctive vegetation, very different from surrounding vegetation. Heavy metal toxicity was once hypothesized to contribute to the distinctive composition of serpentine vegetation, because nickel (Ni), chromium (Cr), and cobalt (Co) occur naturally in serpentinite, and Ni and Cr are at high levels. However, none of these elements has been found to be toxic on serpentine in eastern North America and elsewhere because of unsuitable pH levels. That is, these elements do not occur in a toxic chemical state in the circumneutral pH range typical of serpentine. Low to very low levels of these elements have been found in plants at Soldiers Delight. Other essential elements which are naturally low in serpentinite, and therefore low in serpentine soils, are phosphorus and potassium, but their effects to plant growth are masked by excessive Mg:Ca. Nitrogen is not particularly low in serpentine soils.
Discussion: Calcium is the most important nutritional element for the development of self-sustaining plant populations as it is essential to cell division and cell elongation; i.e., plant growth. And large healthy plants are needed to produce large numbers of seed (or other reproductive structures) to establish and maintain populations. When parent ultramafic rock is converted to serpentine rock, calcium is lost because it cannot fit in the molecular structure of the new minerals (Alexander et al. 2007). As a result, this essential element for the growth of plants is in short supply in serpentine soils (Rabenhorst et al. 1982). In contrast, the high level of magnesium in ultramafic rock remains unchanged during the formation of serpentinite. And this combination of high magnesium and low calcium is deadly for most species of plants. Good growth occurs in soils with a Mg:Ca ratio of 1:1. Gardeners lime their soils to achieve this ratio. At Soldiers Delight, the Mg:Ca ratio in topsoil (A horizon) is over 3:1 in both silt loam and gravelly sandy loam soils (Tyndall 2012), and it can be over 27:1 in subsoil (B horizon) (Rabenhorst et al. 1982). This Mg:Ca factor is the main reason why serpentine areas around the globe develop distinctive vegetation. That is, the vegetation on serpentine is very different from surrounding vegetation, worldwide. In order for plants to grow and develop self-sustaining populations on serpentine, they must be able to compensate for this excessive ratio of high Mg to very low Ca. The physiological mechanisms for Mg:Ca tolerance can be found in Baker (1981).
Heavy metal toxicity was once hypothesized to contribute to the distinctive composition of serpentine vegetation, because nickel (Ni), chromium (Cr), and cobalt (Co) occur naturally in serpentinite, and Ni and Cr are at high levels. But Cr and Co fell out of scientific discussions when it was understood that they are not in biologically-available chemical forms in serpentine soil (Alexander et al. 2007). That is, they are not present in chemical forms which are useable by plants and, therefore, cannot become toxic to plants.
Ni continues to be part of the metal toxicity discussion as a number of plants on serpentine, including eastern North America, can have higher levels of Ni in their tissues than plants on adjacent non-serpentine (Briscoe et al. 2009, Milton and Purdy 1988, Pollard 2016). In addition, laboratory grown seedlings of a common plant on serpentine in Maryland and Pennsylvania (lyreleaf rockcress, Arabidopsis lyrata) have shown genetic signs of local adaptation to Ni (Veatch-Blohm et al. 2017). However, Ni phytotoxicity has not been verified for any plant species in eastern North America (Pollard 2016, Rajarakuna et al. 2009). Hochman (2001) measured low levels of Ni in the tissues of plants growing in serpentine soil at Soldiers Delight, with some plants having higher levels in nearby non-serpentine. Wood (1984) found no differences in available Ni between Soldiers Delight serpentine and non-serpentine soils. These results indicate that only a small proportion of total Ni in the soil is in a chemical form biologically available to plants. Ni experts have shown that biologically available Ni decreases exponentially with increasing pH (Kukier and Chaney 2004). Areas at Soldiers Delight with indigenous oak savanna or grassland vegetation do not have phytotoxic levels of Ni due to pH being above 5.8 (Kukier and Chaney 2004, Siebielec et al. 2007, Tyndall 2012). Areas which have been lost to pine-greenbrier woodland were hypothesized to have higher potential for Ni phytotoxicity as their soils become acidic, but a study by Cumming and Kelly (2007) found very low levels of Ni in leaves of herbaceous plants and lower total Ni levels in the pine woodland soil.
In the tropics, especially in Cuba and New Caledonia (Reeves 2003), over 300 species hyperaccumulate Ni to levels which would be toxic to most plants, including many plants which grow on serpentine. But no hyperaccumulator of Ni has been verified in eastern North America, and only two plant species in western North America (Rajakaruna et al. 2009).
Other essential elements which are naturally low in serpentinite, and therefore low in serpentine soils, are phosphorus and potassium (Alexander et al. 2007, Alexander 2009). But their effects to plant growth are masked by excessive Mg:Ca. Nitrogen is not particularly low in serpentine soils.
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