Plant nutrients in soil

They are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), iron (Fe), boron (B), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), nickel (Ni) and chlorine (Cl).

Nutrients that enhance the growth of plants but are not necessary to complete the plant's life cycle are considered non-essential, although some of them, such as silicon (Si), have been shown to improve nutrent availability,[4] hence the use of stinging nettle and horsetail (both silica-rich) macerations in Biodynamic agriculture.

For example, the application of finely ground minerals, feldspar and apatite, to soil seldom provides the necessary amounts of potassium and phosphorus at a rate sufficient for good plant growth, as most of the nutrients remain bound in the crystals of those minerals.

[10] However, despite the great capacity of humus to retain water once water-soaked, its high hydrophobicity decreases its wettability.

[11] All in all, small amounts of humus may remarkably increase the soil's capacity to promote plant growth.

[15] For example, in the case of calcium, which is generally plentiful in the soil solution, except when aluminium over competes calcium on cation exchange sites in very acid soils (pH less than 4),[16] mass flow alone can usually bring sufficient amounts to the root surface.

[17] For the most part, nutrient ions must travel some distance in the soil solution to reach the root surface.

[18] By this means, plants can continue to take up nutrients even at night, when water is only slowly absorbed into the roots as transpiration has almost stopped following stomatal closure.

Where crops are produced, the replenishment of nutrients in the soil must usually be augmented by the addition of fertilizer or organic matter.

Others, called nitrifiers, take free ammonium or nitrite as an intermediary step in the process of nitrification, and oxidise it to nitrate.

Both ammonium and nitrate can be immobilized by their incorporation into microbial living cells, where it is temporarily sequestered in the form of amino acids and proteins.

Nitrate may be lost from the soil to the atmosphere when bacteria metabolise it to the gases NH3, N2 and N2O, a process called denitrification.

Nitrogen may also be leached from the vadose zone if in the form of nitrate, acting as a pollutant if it reaches the water table or flows over land, more especially in agricultural soils under high use of nutrient fertilizers.

[47][48][9][49][50][51] In the process of mineralisation, microbes feed on organic matter, releasing ammonia (NH3), ammonium (NH4+), nitrate (NO3−) and other nutrients.

Predation of bacteria by soil fauna, in particular protozoa and nematodes, play a decisive role in the return of immobilized nitrogen to mineral forms.

It is estimated that such symbiotic bacteria in the root nodules of legumes add 45 to 250 pounds of nitrogen per acre per year, which may be sufficient for the crop.

Ammonia, NH3, previously emitted from the soil, may fall with precipitation as nitric acid at a rate of about five pounds nitrogen per acre per year.

At a later time when those bacteria die, their nitrogen may be released as ammonium by the process of mineralization, sped up by predatory fauna.

[66] Ammonium fixation occurs mainly between the layers of 2:1 type clay minerals such as illite, vermiculite or montmorillonite, together with ions of similar ionic radius and low hydration energy such as potassium, but a small proportion of ammonium is also fixed in the silt fraction.

[74] All kinds of nitrogen losses, whether by leaching or volatilization, are responsible for a large part of aquifer pollution[75] and air pollution, with concomitant effects on soil acidification and eutrophication,[76] a novel combination of environmental threats (acidity and excess nitrogen) to which extant organisms are badly adapted, causing severe biodiversity losses in natural ecosystems.

[77] After nitrogen, phosphorus is probably the element most likely to be deficient in soils, although it often turns to be the most deficient in tropical soils where the mineral pool is depleted under intense leaching and mineral weathering while, contrary to nitrogen, phosphorus reserves cannot be replenished from other sources.

[88] Under certain conditions, dependent on the soil texture, intensity of drying, and initial amount of exchangeable potassium, the fixed percentage may be as much as 90 percent within ten minutes.

[91] Calcium is supplied to the plant in the form of exchangeable ions and moderately soluble minerals.

[93] Calcium uptake by roots is essential for plant nutrition, contrary to an old tenet that it was luxury consumption.

[94] Calcium is considered as an essential component of plant cell membranes, a counterion for inorganic and organic anions in the vacuole, and an intracellular messenger in the cytosol, playing a role in cellular learning and memory.

Magnesium is an essential element for plants, microbes and animals, being involved in many catalytic reactions and in the synthesis of chlorophyll.

[98] The micronutrients essential in plant life, in their order of importance, include iron,[99] manganese,[100] zinc,[101] copper,[102] boron,[103] chlorine[104] and molybdenum.

[107] Iron deficiency, stemming in plant chlorosis and rhizosphere acidification, may also result from excessive amounts of heavy metals or calcium minerals (lime) in the soil.

[110][111] Nutrients which enhance the health but whose deficiency does not stop the life cycle of plants include: cobalt, strontium, vanadium, silicon and nickel.

[112] As their importance is evaluated they may be added to the list of essential plant nutrients, as is the case for silicon.