Forms / stores of nutrients in soil

• Inorganic - original rocks & minerals, dissolved or adsorbed ions.
• Biomass - living tissue of plants & soil organisms (especially microbes)
• Organic - dead tissue in various stages of decay, including soil humus.

Nutrients are not locked up in one store - natural processes constantly change them from one form to another. It is this continual process of biological-geological-chemical transformations, that we call the Soil Nutrient Cycle, as depicted in the diagram, below:

Plant roots and soil organisms extract nutrients as simple inorganic ions released by decay of rocks & minerals and soil organic matter. Plants, in particular can only take up nutrients as simple inorganic ions.

When organisms die their tissue is added to the soil organic matter store. In addition, some material is excreted as waste organic matter and some is released accidentally by damaged cells. All of this material immediately starts to decay. Some of the more resistant material forms soil "humus" which decays very slowly.

Organic residues provide food for other soil organisms. This carbohydrate material is utilized by decomposing organisms (especially microbes) in the process known as "mineralization", which releases simple inorganic ions from decaying organic matter.

• erosion - water & wind can carry away nutrients in eroding soil particles
• leaching - percolating water can carry dissolved nutrients downward beyond the reach of plant roots.
• gaseous losses - volatile forms of nutrients such as nitrogen & sulfur can escape into the air.

• Natural precipitation carrying dust particles and dissolved gases.
• Acid rain depositing nitrogen and sulfur from industrial pollution.
• Nitrogen fixation by certain microbes (notably the legume root-nodule bacteria) converting N₂ gas in the air to ammonium (NH₄⁺) ions which can be used by plants and other soil organisms.

Most natural soil ecosystems tend toward an equilibrium state wherein the losses are balanced by natural inputs. The diagram below shows data from nutrient cycling studies in an unharvested forest soil ecosystem. Tally up the losses and inputs of calcium to confirm that the cycle is in equilibrium.

Harvested vs Unharvested Soils:

Farms soils differ from unharvested soils in that nutrients are removed with the harvested crops. In addition, some farm management practices increase the amount of nutrients lost by other mechanisms, such as erosion, leaching and gaseous loss.

The additional losses from harvested soils require special management in order to maintain soil fertility for crop production. Farmers seek to reduce or replace the losses by good soil husbandry, by recycling crop residues in the form of unharvested plant material and livestock manure, and by taking advantage of natural inputs.

1. Reduce losses from the system.
2. Take advantage of nitrogen fixation by legumes.
3. Return harvest residues (plant residues & livestock manure) to the soil.
4. Manage soil pH for optimum nutrient recovery.
5. Use supplemental (off-farm) nutrients to balance deficits (as determined by soil test).

In North American agriculture, continuing harvests require that some additional supplemental nutrients inputs be applied to maintain high crop yields. Such nutrient inputs include fertilizers and lime, as well as off-farm livestock manure, sewage and food-processing wastes.

The traditional method of farming in this region involves production of forages which are fed to livestock on-farm. Most of the nutrients in the feed are excreted by the animals. Mixed farms have an important supply of nutrients for the next year's crop stored in the manure. Combined with the nitrogen fixed by legume forages, manure nutrients may provide most of the crop requirements.

In many areas crops are raised to be sold. There is no manure nutrient supply. This type of production relies on off-farm supplies of nutrients for next year's crops. Most often these nutrients are purchased in the form of commercial fertilizers, which are highly concentrated and highly soluble forms of nutrients.

Other agricultural production systems raise animals exclusively. Their feed is provided by purchased crops. These systems have little or no land base and end up with a surplus of nutrients in the manure from the livestock. In this type of system, manure nutrients are a disposal problem, rather than a resource for soil fertlity replenishment.

Separating the production of crops from the production of livestock has produced twin problems - nutrient deficiencies for crop production and nutrient surplusses from animal production. There are also serious environmental issues associated with heavy use of fertizers on the one hand, and disposal of manure on the other.

The solution would appear to be to use the manure from livestock production to supply the nutrients for cash cropping. However, manure is a bulky commodity and presents many transportation problems. Often the different production systems are distant from each other.