What are soil nutrients?

Nutrients can be defined as any substance used by an organism as food. Macronutrients are found in concentrations in excess of 1000mg/Kg and micronutrients have concentrations below 1000mg/Kg. Carbon, hydrogen and oxygen are obtained predominantly from water and atmospheric sources.

How is the availability of nitrogen, potassium and phosphorus affected by the soil colloids?

Nitrogen (N)

The majority of the nitrogen contained in the soil is held in organic matter. This must be mineralized by soil micro-organisms before the nitrogen it contains is released for plant use. Nitrate, because it is anion, can be susceptible to leaching.

Potassium (K)

Potassium is an essential element in many enzymes; it also regulates the opening and closing of the plant stomata, as well as being essential for photosynthesis and disease resistance. Exchangeable potassium can readily exchange with the soil solution to buffer changes. Plants use a lot of potassium and this means that it can be in short supply.

Phosphorus (P)

Phosphorus is essential for photosynthesis, nitrogen fixation, growth and root development. Phosphorus occurs as the phosphate anion, but, unlike nitrate, potential leaching losses are reduced because it forms insoluble compounds when the pH is outside the narrow band of 6-7 and ligand bonds with other soil chemicals that modify its anionic character.

As roots remove nutrients from the soil solution, losses are compensated for by buffering, as nutrients held on exchange sites are exchanged with the soil solution. In well-buffered soils, despite plant absorption and leaching losses, the concentration of nutrients in the soil solution is maintained.