Soil Solution Phosphate and Plant Root Interactions

After reading this article you will learn about soil solution phosphate and plant root interactions.

Phosphates are absorbed by plants mostly as primary (H₂PO₄^–) and secondary (HPO₄^2-) orthophosphate ions which are present in the soil solution. The amount of each species of orthophosphate ions are controlled by the pH of the soil solution. At pH 7.2 there are almost equal amounts of H₂PO₄^– and HPO₄^2-.

Below this pH value, HPO₄^2- is the dominant form whereas the secondary orthophosphate ion (HPO₄^2-) becomes most important at pH values > 7.20. Plant uptake of HPO₄^2- is much slower than that of H₂PO₄^–. Distribution of orthophosphate ions in soil solution as influenced by pH is shown in Fig. 21.5.

Concentrations or orthophosphate ions in the soil solution are of immense importance in plant growth and nutrition. The amount of phosphate present in the soil solution is very low as compared to adsorbed phosphate. Adsorbed phosphate exceeds the phosphate of the soil solution by a factor of 10²-10³.

As plant roots find their way through the soil with which plant roots come in contact with the soil solution phosphorus. Provided that the roots have a high demand for P and for growing plants, phosphate is absorbed by the roots at a faster rate and the soil solution in the direct vicinity of root is depleted of phosphate.

Such depletion creates a gradient between the phosphate concentration near the root surface and that concentration in the bulk soil and hence the rate of phosphate diffusion towards the plant root (rhizosphere zone) is regulated by the concentration gradient. Besides, mass flow can also, play a part in the transport of phosphate towards plant roots.

However, the phosphate uptake due to mass flow is minimum as the concentration of phosphorus in the soil solution is very low. It has been found that root infection by endotrophic mycorrhizal fungi can stimulate plant growth by enhancing the phosphate uptake.

Mycorrhizal hyphae are also able to grow well even under conditions of low soil water potential and this may be of particular importance for the mobilisation of soil phosphate under dry soil conditions.

High amounts of available soil phosphate depress the development of mycorrhiz and this effect is related to the carbohydrate content of roots which is usually high in phosphorus deficient plants. The release of organic carbon is related to the colonisation of roots with vesicular-arbuscular mycorrhiza (VAM).

Root Exudates:

The influence of root exudates on the solubility of phosphate in the root vicinity has been found and it is now established that relatively large amounts of C assimilated in photosynthesis are transferred from the roots into the soil.

Organic chelating agents can exchange with surface bonded phosphate releasing phosphate for plant uptake. 2-Keto gluconate (isolated from rhizosphere of wheat roots) solubilize considerable amounts of phosphate from hydroxyapatite and such a chelate release from roots and root hairs can provide an efficient means of solubilising phosphate.

Rhizosphere pH:

Another important effect of roots on phosphorus availability is the change of pH in the rhizosphere. The pH at the root surface may be as such as one unit different from that of the bulk soil. Such differences in pH result differential rates of uptake of both cations and anions. Plants supplied with NH₄—N or molecular N₂ (symbiotic N₂ fixation) take up more cations than anions releasing more H⁺ ions into the soil solution resulting rhizosphere soil more acidic.

Such change in pH in the root zone may influence the uptake of phosphorus by the plants. In soils where adsorbed phosphate is the main source of P, and increase in the rhizosphere pH leads to a desorption of soil phosphate and therefore an increase in the availability of phosphate.

Many micro-organisms play a very important role in enhancing the P uptake by the plant through producing various acids and chelating agents. Such micro-organisms are: Aspergillus niger, some Penicillium sp. and pseudomonas sp. etc.

Soil Solution to Root Surfaces:

Movement of phosphorus:

Movement of phosphorus from the soil solution to the surfaces of roots is an important factor for the phosphorus need of plants. Such movement is usually carried out by mass flow and diffusion. These mechanisms have been discussed earlier.

However, the amount of nutrients, (phosphorus) moving towards the root surfaces by the mass flow are determined by the rate of water flow or water used by plants and the average phosphorus concentrations in the soil water. The amount of phosphorus around the root will vary depending upon the balance between the rate at which it moves to the root surfaces and the rate of P uptake by the root.

Mass Flow:

The flow of P through this mechanism usually provides only a small amount of phosphorus requirement to crops growing in soils containing low phosphorus. Mass flow of phosphorus can be calculated by using the transpiration ratio or the weight of water transpired per unit weight of plant produced.

Assuming a value of 400 for the transpiration ratio and a phosphorus concentration of 0.2% in the crop, an average phosphorus concentration of 5 ppm in the soil solution will be required to meet P supply to the plant.

The most important mechanism involved in the movement of phosphorus from soil solution to the plant roots is diffusion. With the exception of soils extremely high in phosphorus, diffusion process is responsible for carrying phosphorus to the plant roots. It depends on the concentration gradient of a particular ion (phosphorus).

Maintenance of a suitable concentration of phosphorus in the soil solution (phosphorus intensity) depends on the solid phase phosphorus releasing into the soil solution to replace the amounts taken up by the plants. The capacity or quantity factor (reserve-solid phase phosphorus) is generally used to describe gradients that relate quantity to intensity.

The presence of high amounts of rapidly absorbable cations (e.g. NH₄⁺) increased the P uptake. Example, the amount of P absorption has been found to be depressed in the presence of rapidly absorbable NO₃—N.

Molybdenum absorption has been found to be increased with an increase in the phosphate concentration. However, the phosphorus absorption from the soil solution is partly dependent upon the amount of interfering ions.