Micronutrients include iron, manganese, zinc, copper, boron, chlorine and molybdenum. The term refers to plants’ needs, not to their abundance in soil. They are required in very small amounts but are essential to plant health in that most are required parts of some enzyme system which speeds up plants’ metabolisms. They are generally available in the mineral component of the soil, but the heavy application of phosphates can cause a deficiency in zinc and iron by the formation of insoluble phosphates. Iron deficiency may also result from excessive amounts of heavy metals or calcium minerals (lime) in the soil. Excess amounts of soluble boron, molybdenum and chloride are toxic.

Most sulfur is made available to plants, like phosphorus, by its release from decomposing organic matter. Deficiencies may exist in some soils and if cropped, sulfur needs to be added. A 15-ton crop of onions uses up to 19 lb of sulfur and 4 tons of alfalfa uses 15 lb per acre. Sulfur abundance varies with depth. In a sample of soils in Ohio, United States, the sulfur abundance varied with depths, 0-6 inches, 6-12 inches, 12-18 inches, 18-24 inches in the amounts: 1056, 830, 686, 528 lb per acre respectively.

Magnesium is central to chlorophyll and aids in the uptake of phosphorus. The minimum amount of magnesium required for plant health is not sufficient for the health of forage animals. Magnesium is generally available, but is missing from some soils along the Gulf and Atlantic coasts of the United States due to leaching by heavy precipitation.

Calcium is 1 percent by weight of soils and is generally available but may be low as it is soluble and can be leached. It is thus low in sandy and heavily leached soil or strongly acidic mineral soil. Calcium is supplied to the plant in the form of exchangeable ions and moderately soluble minerals. Calcium is more available on the soil colloids than is potassium because the common mineral calcite, CaCO3, is more soluble than potassium-bearing minerals

The amount of potassium in a soil may be as much as 80,000 lb per acre, of which only 150 lb or 2 percent is available for plant growth. When solubilised, half will be held as exchangeable cations on clay while the other half is in the soil water solution. Potassium fixation occurs when soils dry and the potassium is bonded between layers of clay. 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. Potassium may be leached from soils low in clay.

Phosphorus is the second most critical plant nutrient. The soil mineral apatite is the most common mineral source of phosphorus. While there is on average 1000 lb of phosphorus per acre in the soil, it is generally in unavailable forms. The available portion of phosphorus is low as it is in the form of phosphates of low solubility. Total phosphorus is about 0.1 percent by weight of the soil, but only one percent of that is available. Of the part available, more than half comes from the mineralisation of organic matter. Agricultural fields may need to be fertilised to make up for the phosphorus that has been removed in the crop. When phosphorus does form solubilised ions of H2PO4–, they rapidly form insoluble phosphates of calcium or hydrous oxides of iron and aluminum. Phosphorus is largely immobile in the soil and is not leached but actually builds up in the surface layer if not cropped. The application of soluble fertilisers to soils may result in zinc deficiencies as zinc phosphates form. Conversely, the application of zinc to soils may immobilise phosphorus as zinc phosphate. Lack of phosphorus may interfere with the normal opening of the plant leaf stomata, resulting in plant temperatures 10 percent higher than normal. Phosphorus is most available when soil pH is 6.5 in mineral soils and 5.5 in organic soils.

Usable nitrogen may be lost from soils when it is in the form of nitrate, as it is easily leached. Further losses of nitrogen occur by denitrification, the process whereby soil bacteria convert nitrate (NO3-) to nitrogen gas, N2 or N2O. This occurs when poor soil aeration limits free oxygen, forcing bacteria to use the oxygen in nitrate for their respiratory process. Denitrification increases when oxidisable organic material is available and when soils are warm and slightly acidic. Denitrification may vary throughout a soil as the aeration varies from place to place. The conversion of nitrate to gases causes nitrogen to be lost from the soil to the atmosphere. Denitrification may cause the loss of 10 to 20 percent of the available nitrates within a day and when conditions are favourable to that process, losses of up to 60 percent of nitrate applied as fertiliser may occur.[103] Ammonium volatilisation occurs when ammonium reacts chemically with an alkaline soil, converting NH4+ to NH3. The application of ammonium fertiliser to such a field can result in volatilisation losses of as much as 30 percent.

When bacteria feed on soluble forms of nitrogen (ammonium and nitrite), they temporarily sequester that nitrogen in their bodies in a process called immobilisation. At a later time when those bacteria die, their nitrogen may be released as ammonium by the processes of mineralisation. Protein material is easily broken down, but the rate of its decomposition is slowed by its attachment to the crystalline structure of clay and trapped between the clay layers. The layers are small enough that bacteria cannot enter. Some organisms can exude extracellular enzymes that can act on the sequestered proteins. However, those enzymes too may be trapped on the clay crystals. Ammonium fixation occurs when ammonium replaces the potassium ions that normally exist between the layers of clay such as illite or montmorillonite. Only a small fraction of nitrogen is held this way.

In a process called mineralisation, certain bacteria feed on organic matter, releasing ammonia (NH3) (which may be reduced to ammonium NH4+) and other nutrients. As long as the carbon to nitrogen ratio (C/N) in the soil is above 30:1, nitrogen will be in short supply and other bacteria will feed on the ammonium and incorporate its nitrogen into their cells. In that form the nitrogen is said to be immobilised. Later, when such bacteria die, they too are mineralised and some of the nitrogen is released as ammonium and nitrate. If the C/N is less than 15, ammonia is freed to the soil, where it may be used by bacteria which oxidise it to nitrate in a process called nitrification. Bacteria may on average add 25 pounds nitrogen per acre, and in an unfertilised field, this is the most important source of usable nitrogen. In a soil with 5 percent organic matter perhaps 2 to 5 percent of that is released to the soil by such decomposition. It occurs fastest in warm, moist, well aerated soil. The mineralisation of 3 percent of a soil that is 4 percent organic matter would release 120 pounds of nitrogen as ammonium per acre. In symbiotic fixation, Rhizobium bacteria convert N2 to nitrate by way of nitrogen fixation. They have a symbiotic relationship with host plants, wherein they supply the host with nitrogen and the host provides the bacteria with nutrients and a safe environment. 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. Other, free-living nitrogen-fixing bacteria and blue-green algae live independently in the soil and release nitrate when their dead bodies are converted by way of mineralisation. Some amount of …

Nitrogen is the most critical element obtained by plants from the soil and is a bottleneck in plant growth. Plants can use the nitrogen as either the ammonium cation ammonium (NH4+) or the anion nitrate (NO3–). Nitrogen is seldom missing in the soil, but is often in the form of raw organic material which cannot be used directly. Carbon/Nitrogen Ratio of Various Organic Materials                              Organic Material C:N Ratio Alfalfa 13 Bacteria 4 Clover, green sweet 16 Clover, mature sweet 23 Fungi 9 Forest litter 30 Humus in warm cultivated soils 11 Legume-grass hay 25 Legumes (alfalfa or clover), mature 20 Oat straw 80 Straw, cornstalks 90 Sawdust 250 Some micro-organisms are able to metabolise the organic matter and release ammonium in a process called mineralisation. Others take free ammonium and oxidise it to nitrate. Particular bacteria are capable of metabolising N2 into the form of nitrate in a process called nitrogen fixation. Both ammonium and nitrate can be lost from the soil by incorporation into the microbes’ living cells, where it is temporarily immobilised or sequestered. Nitrate may also be lost from the soil when bacteria metabolise it to the gases N2 and N2O. In that gaseous form, nitrogen escapes to the atmosphere in a process called denitrification. Nitrogen may also be leached from the soil if it is in the form of nitrate or lost to the atmosphere as ammonia due to a chemical reaction of ammonium with alkaline soil by way of a process called volatilisation. Ammonium may also be sequestered in clay by fixation. Nitrogen is added to soil by rainfall.