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Agrochemistry

Mechanisms for the delivery of mineral nutrients to the plant root system

For students

3 min read

AGROCHEMISTRY A

For the applied mineral fertilizers to be effective, nutrients must physically reach the root surface. The transport of elements to the root system occurs in three ways: through root interception, mass flow of water, or diffusion. The contribution of each mechanism determines how quickly and completely a crop absorbs nutrients from the soil.

  • Root system volume — no more than 1–2% of the soil volume
  • Water flow intensity — from 6·10-7 to 1 cm3 per 1 cm2 per second

If the content of elements in the soil is lower than the plants' requirements, root interception cannot provide the crop with nutrients. In this case, mass flow and diffusion begin to play the primary role.

How nutrient transport pathways are distributed

In root interception, the root absorbs nutrients that it directly contacts while growing and constantly regenerating its root hairs. In essence, the plant seeks out nutrients itself by pushing through the soil. However, this pathway is inefficient for most elements, as the root volume makes up a negligible fraction of the total soil volume.

Mass flow transports water-dissolved nutrients under the influence of root suction force. The amount of substances reaching 1 cm2 of the root surface is calculated by multiplying the rate of water absorption by the concentration of elements in the equilibrium soil solution. This mechanism works over longer distances than diffusion and is highly dependent on weather conditions and crop species.

Diffusion is activated when roots absorb nutrients faster than they arrive with the water flow. A deficit arises in the zone around the root, and ions begin to move from more concentrated soil layers to the root system. The ratio of these transport mechanisms can be clearly observed using maize nutrition as an example.

Nutrient Requirement for life activity, kg/ha Supplied by interception, kg/ha Supplied by mass flow, kg/ha Supplied by diffusion, kg/ha
N (Nitrogen) 190 2 150 38
P (Phosphorus) 40 1 2 37
K (Potassium) 195 4 35 156
Ca (Calcium) 40 60 150 0
Mg (Magnesium) 45 15 100 0
S (Sulfur) 22 1 65 0

As the test results show, root interception makes a minimal contribution to maize nutrition for all elements except calcium, whose reserve in the soil is excessive for plants. Mass flow fully covers the crop's requirement for all elements, with the exception of nitrogen, phosphorus, and potassium. Plants are forced to compensate for the deficit of these vital nutrients through diffusion.

Above all, diffusion is critically important for supplying plants with the following elements:

  • phosphorus;
  • potassium;
  • partially nitrogen.

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