Root foraging response to gradients of calcium and magnesium, essential bivalent cations with low mobility in the soil

Plants forage for nutrients by root proliferation in nutrient-rich patches. While foraging for nitrogen and phosphorus has been repeatedly confirmed, foraging for calcium and magnesium, which are essential for plant growth and form much more stable patches in the soil, has never been examined. We examined preferential root placement into dolomite-limestone-rich patches in a pot experiment with 17 species, and compared it with foraging for a nitrogen, phosphorus and potassium mixture (NPK). About one half of the species showed root proliferation in dolomite-rich patches. It was less pronounced than foraging for NPK and did not show any relationship to species field preferences to soil reaction, or dicots-grass difference, but it showed clear negative relationship to species-specific Ca+Mg tissue concentrations. While foraging for NPK shows the potential of species to change their root systems by proliferation, only some species use this potential to respond to the Ca+Mg gradient. The negative correlation of this response to Ca+Mg tissue concentrations implies that nonresponding species compensate for it by physiological mechanisms. The response to Ca+Mg also implies that in contrast to nitrogen, which never shows stable patches in the soil, Ca+Mg-rich patches, which are much more stable, can be exploited by root proliferation.