Some anions are also soil nutrients — and essential ones at that. These include the various forms of phosphate [PO43-, HPO42- and H2PO4- — sources of phosphorus (P)], sulfate [SO42- — source of sulfur (S)], nitrate [NO3- — source of nitrogen (N)], molybdate [MoO42- — source of molybdenum (Mo)], and bicarbonate (HCO3-).
Other commonly present anions include flouride [F- (the correct word for the ionic form of flourine)] and chloride [Cl- (the correct word for the ionic form of chlorine)]. Chloride is a micronutrient (needed in small quantities). Soil flouride is not generally available to plants, though it can enter plants via diffusion and lead to toxicity if levels build up too much.
Boric acid (B(OH)3), the main source of boron in soil, is not an anion but does behave like one.
While cations are attracted to, and adhere to, the negatively-charged surfaces of clays and humus, anions are attracted to positively-charged surfaces. However, the number of positively-charged sites in soil is often less than 1% of the number of negatively-charged sites, and rarely above 5%.
This means anions remain in the soil water or combine with minerals such as iron and aluminium oxides. This combination is strongest for the phosphates, then molybdate, followed by boric acid and sulfate equally, then chloride, and finally nitrate.
The anions held by the minerals are released slowly into the soil water to replace the ones already in the soil water as they are uptaken by plants.
The strength at which anions bind to minerals, as well as their rate of release into soil water, has important consequences.
Nitrates, being the weakest bound, are the most likely to be found in soil water. This makes them susceptible to leaching following heavy rains or watering. Deep-rooted plants may still be able to access nitrates further down the soil profile, but shallow-rooted and potted plants will especially succumb to deficiencies. All will benefit from a regular replenishment.
Anions take cations with them when leaching from soil — the excess water will draw cations into solution alongside the anions, where they attract each other and combine to form salts (though these salts remain in solution). A ’salt’ in chemistry is a compound made up of cations and anions, and which has a net neutral charge. Ordinary table salt, sodium chloride (NaCl), is but one salt of many.
The cation most leached is usually ionic calcium (Ca2+). Sodium ions (Na+) are the next most leached, if it is present, followed by potassium (K+) and magnesium (Mg2+) ions. Phosphate ions, held the tightest of all, are the least likely to leach.
We’ll be revisting both cations and anions some more in an upcoming section on bore water and water infiltration.
Having zoomed in on soil over the past few weeks right to the ionic level, next week we’ll zoom out a tiny bit and look at soil pore spaces and their role in soil structure!
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