For nutrient uptake to occur, the individual nutrient ion must be positioned adjacent to the root. There are three ways this positioning occurs – the root can “bump” into the nutrient ion as it grows (interception), nutrients in the soil solution flow to the root as water is taken up (Mass Flow) or by diffusion. Nutrients such as phosphate (P) and potassium (K) are held strongly by soils and as their concentration around a root reduces through absorption, a gradient is created for the nutrient to diffuse from an area of higher concentration to a depleted area.
Mass flow supplies the bulk of the nitrogen (N) requirement, as well as calcium and sulphur as these elements are more soluble and taken in as the plant takes in water. Diffusion is responsible for the majority of the P, K and trace elements moving to the root zone. They are taken into the root by an active process therefore it requires energy to do so. A protein carrier binds with a nutrient ion and carries it across a protective membrane and into the xylem system for delivery to the growth areas.
In normal patterns of uptake, nutrient uptake mirrors plant growth. The plant will take up most of its nutrients during periods of vegetative growth and then move stored nutrients to the developing grain during the reproductive stage. Nutrient uptake increases rapidly after the 4 leaf stage and remains high until after flowering. After pollination, nutrient uptake slows, and nutrient loss from the plant will occur after the dent stage.
Growers have a responsibility to ensure they only supply the nutrients that are essential for crop development – anything above that has the potential to remain in the soil and leach in groundwater (especially nitrogenous fertilisers).
The timing of the availability of nitrogen (N) is one of the key building blocks of a successful crop – too little or too late will compromise crop potential. A soil test before cultivation will determine the levels of available N in the soil and provide a baseline for determining the crops needs.
A maize plant will use very little N in the first month. This increases to reach a peak prior to flowering – and with a need for phosphate (P) and potassium (K) as well as water in this key period, any stress or deficiency at this stage has the potential to cause major yield penalties for the crop. With N, split applications present the fertiliser in a more natural way – therefore a drip feed approach (or the use of slow release products) will gain the best response in the majority of situations. With a nitrogen efficiency coefficient of 60% before 4 leaves to 80% thereafter, the plant has a large need for nitrogen from 6 leaves onwards.N efficiency improves with yield. With a good root system and good soil structure, roots are able to access a greater soil area. This means increased root mass, along with good soil moisture results in decreased stress. Stress inhibits growth and causes a downward spiral to begin.