Sixteen elements or nutrients are essential for plant growth and reproduction. 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), and chlorine (Cl). Nutrients required for plants to complete their life cycle are considered essential nutrients. 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 nutrient availability, hence the use of stinging nettle and horsetail (both silica-rich) macerations in biodynamic agriculture. With the exception of carbon, hydrogen and oxygen, which are supplied by carbon dioxide and water, and nitrogen, provided through nitrogen fixation, the nutrients derive originally from the mineral component of the soil. The Liebig's law of the minimum expresses that when the available form of a nutrient is not in enough proportion in the soil solution, then other nutrients cannot be taken up at an optimum rate by a plant because of stoichiometry constraints. A particular nutrient ratio of the soil solution is thus mandatory for optimizing plant growth, a value which might differ from nutrient ratios calculated from plant composition.
Plant uptake of nutrients can only proceed when they are present in a plant-available form. In most situations, nutrients are absorbed from the soil water in an ionic form by diffusion, mass transport, and root interception. Although minerals are the origin of most nutrients, and the bulk of most nutrient elements in the soil is held in crystalline form within primary and secondary minerals, they weather too slowly to support rapid plant growth. 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. However, the association of plant roots with mycorrhizal fungal partners increases the dissolution rate of phosphorus-containing apatite.
The nutrients adsorbed in an exchangeable form onto the surfaces of clay colloids and soil organic matter (the soil matrix) provide a more accessible reservoir of many plant nutrients (e.g. K, Ca, Mg, P, Zn). As plants absorb the nutrients from the soil water, the soluble pool is replenished (exchanged) from the surface-bound pool. The decomposition of soil organic matter by microorganisms is another mechanism whereby the soluble pool of nutrients is replenished – this is important for the supply of plant-available N, S, P, and B from soil.
Gram for gram, the capacity of humus to hold nutrients and water is far greater than that of clay minerals, most of the soil cation exchange capacity arising from charged carboxylic groups on organic matter. However, despite the great capacity of humus to retain water once water-soaked, its high hydrophobicity decreases its wettability. All in all, small amounts of humus may remarkably increase the soil's capacity to promote plant growth.
+ Plant nutrients, their chemical symbols, and the ionic forms common in soils and available for plant uptake Element Symbol Ion or molecule Carbon C CO2 (mostly through leaf and root litter) Hydrogen H H+, HOH (water) Oxygen O O2−, OH −, CO32−, SO42−, CO2 Phosphorus P H2PO4 −, HPO42− (phosphates) Potassium K K+ Nitrogen N NH4+, NO3 − (ammonium, nitrate) Sulfur S SO42− Calcium Ca Ca2+ Iron Fe Fe2+, Fe3+ (ferrous, ferric) Magnesium Mg Mg2+ Boron B H3BO3, H2BO3 −, B(OH)4 − Manganese Mn Mn2+ Copper Cu Cu2+ Zinc Zn Zn2+ Molybdenum Mo MoO42− (molybdate) Chlorine Cl Cl − (chloride)
Uptake processes
Nutrients in the soil are taken up by the plant through its roots, and in particular its root hairs. To be taken up by a plant, a nutrient element must be located near the root surface; however, the supply of nutrients in contact with the root is rapidly depleted within a distance of ca. 2 mm. There are three basic mechanisms whereby nutrient ions dissolved in the soil solution are brought into contact with plant roots:
Mass flow of water Diffusion within water Interception by root growth
All three mechanisms operate simultaneously, but one mechanism or another may be most important for a particular nutrient. 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), mass flow alone can usually bring sufficient amounts to the root surface. However, in the case of phosphorus, diffusion is needed to supplement mass flow. For the most part, nutrient ions must travel some distance in the soil solution to reach the root surface. This movement can take place by mass flow, as when dissolved nutrients are carried along with the soil water flowing toward a root that is actively drawing water from the soil. In this type of movement, the nutrient ions are somewhat analogous to leaves floating down a stream. In addition, nutrient ions continually move by diffusion from areas of greater concentration toward the nutrient-depleted areas of lower concentration around the root surface. That process is due to random motion, also called Brownian motion, of molecules within a gradient of decreasing concentration. By the mean of diffusion, plants can continue to take up nutrients even when transpiration stops following stomatal closure and thus mass flow is interrupted. Finally, root interception comes into play as roots continually grow into new, undepleted soil. By this way roots are also able to absorb nanomaterials such as nanoparticulate organic matter.
+ Estimated relative importance of mass flow, diffusion and root interception as mechanisms in supplying plant nutrients to corn plant roots in soils Nutrient Approximate percentage supplied by: Mass flow Root interception Diffusion Nitrogen 98.8 1.2 0 Phosphorus 6.3 2.8 90.9 Potassium 20.0 2.3 77.7 Calcium 71.4 28.6 0 Sulfur 95.0 5.0 0 Molybdenum 95.2 4.8 0
In the above table, phosphorus and potassium nutrients move more by diffusion than they do by mass flow in the soil water solution, as they are rapidly taken up by the roots creating a concentration of almost zero near the roots (the plants cannot transpire enough water to draw more of those nutrients near the roots). The very steep concentration gradient is of greater influence in the movement of those ions than is the movement of those by mass flow. The movement by mass flow requires the transpiration of water from the plant causing water and solution ions to also move toward the roots. Movement by root interception is slowest, being at the rate plants extend their roots.
Plants move ions out of their roots in an effort to move nutrients in from the soil, an exchange process which occurs in the root apoplast. Hydrogen H+ is exchanged for other cations, and carbonate (HCO3−) and hydroxide (OH−) anions are exchanged for nutrient anions. As plant roots remove nutrients from the soil water solution, they are replenished as other ions move off of clay and humus (by ion exchange or desorption), are added from the weathering of soil minerals, and are released by the decomposition of soil organic matter. However, the rate at which plant roots remove nutrients may not cope with the rate at which they are replenished in the soil solution, stemming in nutrient limitation to plant growth. Plants derive a large proportion of their anion nutrients from decomposing organic matter, which typically holds 90–95% of the soil nitrogen and sulfur and 20–75% of the soil phosphorus. Where crops are produced, the replenishment of nutrients in the soil must usually be augmented by the addition of fertilizer or organic matter.
Because nutrient uptake is an active metabolic process, conditions that inhibit root metabolism may also inhibit nutrient uptake. Examples of such conditions include waterlogging or soil compaction resulting in poor soil aeration, excessively high or low soil temperatures, and all above-ground conditions that result in low translocation of sugars to plant roots.
Carbon
Plants obtain their carbon from atmospheric carbon dioxide through photosynthetic carboxylation, to which must be added the uptake of dissolved carbon from the soil solution and carbon transfer through mycorrhizal networks. About 45% of a plant's dry mass is carbon; plant residues typically have a carbon to nitrogen ratio (C/N) of between 13:1 and 100:1. As the soil organic material is digested by micro-organisms and saprophagous soil fauna, the C/N decreases as the carbonaceous material is metabolized and carbon dioxide (CO2) is released as a byproduct which then finds its way out of the soil and into the atmosphere. Nitrogen turnover (mostly involved in protein turnover) is lesser than that of carbon (mostly involved in respiration) in the living, then dead matter of decomposers, which are always richer in nitrogen than plant litter, and so it builds up in the soil. Normal CO2 concentration in the atmosphere is 0.03%, this can be the factor limiting plant growth when no other factor like water or nutrient scarcity is involved. The respiration of CO2 by soil microorganisms decomposing soil organic matter and the CO2 respired by roots contribute an important amount of CO2 to the photosynthesising plants, to which must be added the CO2 respired by aboveground plant tissues. Root-respired CO2 can be accumulated overnight within hollow stems of plants, to be further used for photosynthesis during the day. Within the soil, CO2 concentration is 10 to 100 times that of atmospheric levels but may rise to toxic levels if the soil porosity is low or if diffusion is impeded by flooding.
Nitrogen
Nitrogen is the most critical element obtained by plants from the soil, to the exception of moist tropical forests (tropical rainforests) where phosphorus is the limiting soil nutrient, and nitrogen deficiency often limits plant growth through decreased photosynthetic rate. Plants can use nitrogen as either the ammonium cation (NH4+) or the anion nitrate (NO3−). Plants are commonly classified as ammonium or nitrate plants according to their preferential nitrogen nutrition. Usually, most of the nitrogen in soil is bound within organic compounds that make up the soil organic matter, and must be mineralized to the ammonium or nitrate form before it can be taken up by most plants. However, symbiosis with mycorrhizal fungi allow plants to get access to the organic nitrogen pool where and when mineral forms of nitrogen are poorly available. The total nitrogen content depends largely on the soil organic matter content, which in turn depends on texture, climate, vegetation, topography, age and soil management. Soil nitrogen typically decreases by 0.2 to 0.3% for every temperature increase by 10 °C. Usually, grassland soils contain more soil nitrogen than forest soils, because of a higher turnover rate of grassland organic matter. Cultivation decreases soil nitrogen by exposing soil organic matter to decomposition by microorganisms, most losses being caused by denitrification, and soils under no-tillage maintain more soil nitrogen than tilled soils.