All the nutrients with the exception of carbon are taken up by the plant through its roots. All those brought through the roots, with the exception of hydrogen, which is derived from water, are taken up in the form of ions. Carbon, in the form of carbon dioxide, enters primarily through the leaf stomata. All the hydrogen utilised by the plant originates from soil water and participates with the carbon dioxide in the photosynthetic production of sugars and release of oxygen as a byproduct. Plants may have their nutrient needs supplemented by spraying a water solution of nutrients on their leaves, but nutrients are typically received through the roots by:
- Mass flow
- Root interception
The nutrient needs of a plant may be carried to the plant by the movement of the soil water solution in what is called mass flow. The absorption of nutrients from the soil solution with which the roots are in contact causes the concentration of nutrients in that area to be reduced. Diffusion of nutrients from areas with high concentration to those of lower concentration moves nutrients near the roots as they take up those nutrients. Plants constantly send out roots to seek new sources of nutrients in a process called root interception. Meanwhile older, less effective roots die back. Water is lifted to the leaves, where it is lost by transpiration and in the process it brings soil nutrients with it. A maize plant, for example, will use one quart of water per day at the height of its growing season.
In the above table, phosphorus and potassium nutrients move more by diffusion than they do by mass flow in water solution, as they are rapidly taken up by the roots creating a concentration near zero 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 as the plants must extend their roots. Plants move ions out of their roots in proportion to the amount of nutrients they move in. Hydrogen H+ is exchanged for cations, and carbonate (HCO3–) and hydroxide (OH–) anions are exchanged for nutrient anions. Plants derive most of their anion nutrients from decomposing organic matter, which holds 95 percent of the nitrogen, 5 to 60 percent of the phosphorus and 80 percent of the sulfur. As plant roots remove nutrients from the soil water solution, nutrients are added to the soil water as other ions move off of clay and humus, are added from the decomposition of soil minerals, and are released by the decomposition of organic matter. Where crops are produced, the replenishment of nutrients in the soil must be augmented by the addition of fertiliser or organic matter