Phosphate is essentially a rock that is mined throughout the world, which contains a macro-nutrient used by plants – Phosphorus.

Phosphorus in plants

Phosphorus plays a role in many of the complex functions of a plant. When adequate supplies of Phosphorus are available within the plant it can promote or enhance the following benefits:

  • Early root formation and growth
  • Greater flowering and seed production
  • Fruit, vegetable, and grain quality
  • Better growth in cold temperatures
  • Water use efficiency
  • Early maturation of fruit and grain

Phosphorus in the Soil

Phosphorous is a highly reactive element. The majority of Phosphorous in most soil is in essentially insoluble forms, and unavailable to plants.

Phosphorus is held within the soil in 3 different “Pools”:

The Solution Pool

This is the primary pool in which phosphorus is available to the plant, and is stored in the soil solution

The Active Pool

This is the pool that feeds the Solution Pool, or conversely may withdraw phosphate from the Solution Pool if the quantities of phosphorus in the Solution Pool are extremely concentrated. In this pool the phosphorus is adsorbed to particles in the soil, ready to feed the Solution Pool. It can also provide phosphorus in small amounts to the plant roots.

The Fixed Pool

The Fixed Pool contains very insoluble phosphate that may remain unavailable to plants for many years. Conversion into the the Active Pool occurs very slowly.

Within the Solution Pool plants will take up nearly all Phosphorus as either:

  1. Primary orthophosphate anion
  2. Secondary orthophosphate anion

Primary orthophosphate is taken up about 10 times as readily as the secondary orthophosphate form. All Phosphorus sources applied to the soil must be converted to the orthophosphate forms before a plant can utilize them. However, applying these forms of Phosphorus to the soil does not guarantee that they will remain in that form for very long. Because phosphorous is highly reactive, it is readily converted to other, less soluble forms. The particular forms that are created depend on other soil factors such as the soil pH, temperature, moisture, other elements, and others. This is one reason all aspects of the soil must be optimized before plants will perform at their best.

In most situations there is very little soluble Phosphorus in the soil at any point in time. It has been estimated that at any point in time, the solution/available forms of Phosphorus in many soils may only amount to from 0.01 to 0.06 ppm (0.02-0.12 lb P/acre). This Phosphorus will typically move no more than about one tenth of an inch in the soil. Roots quickly deplete the 0.10 inch cylinder of soil around each root and must continually grow into new areas of the soil to maintain adequate Phosphorus intake.

Increasing Phosphorus availability for your plant

Apart from increasing the total amount of Phosphorus, there are several important steps you should take to ensure the Phosphorus you apply is readily available to your plants:

1 – Maintain a soil pH between 6 – 7

As the soil pH increases above about pH 7.0, soil P is increasingly “fixed” into less available forms by excess calcium. As the soil pH decreases below about pH 6.0 soil P is increasingly “fixed” into less available forms by excess soluble aluminium and iron.

2 – Manage soil compaction

Phosphorous moves very little in the soil. Because of this, plant roots must be healthy and actively explore new areas of the soil daily in order to obtain adequate P nutrition. Anything that inhibits aggressive root growth is likely to reduce P uptake, even in high P soils.

3 -Increase soil aeration

Inadequate soil aeration is often related to soil clay content, soil drainage, and soil compaction. Most cultivated plants require adequate oxygen (O2) in the soil atmosphere. A lack of adequate soil O2 can reduce P uptake by as much as 50%.

4 – Increase soil moisture

As moisture stress increases, P availability and uptake decrease. Higher levels of soil P result in higher P uptake at all moisture levels. However, as soil moisture begins to exceed field capacity, the excess water excludes the needed oxygen from the soil and P uptake begins to suffer due to the lack of O2 in the soil.

5 – Keep in mind soil temperature

Cold soil reduces P uptake, as well as most other chemical and biological activity in the soil. As the seasons begin to warm up, uptake efficiency of both the plants and the soil improve. However, permanent yield losses can occur from early season P shortages.

6 – Increase Soil Organic Matter

The organic matter (OM) in soil may account for anywhere from 3% to 75% of the total P in a soil (not necessarily the same as “available P”). Generally, increased OM results in greater fixation of Iron and Aluminium, resulting in less P fixation by these elements, and more available P. Such reactions also tend to reduce the fixation of applied P as well. Typically, in soils developed in temperate climates, the contribution of P by OM is relatively small and the main source of P for plants is the inorganic forms.

7 – Choose plants with suitable root systems

As mentioned earlier, soil P is essentially immobile, and the portion of soil P that is soluble and immediately available to plants is exceptionally small. Therefore, plant roots must constantly explore large volumes of soil to satisfy their need for P. There are significant differences between species in the relative size and effectiveness of their root systems. There can also be significant differences of this type between hybrids and varieties within the same species. This is one factor in explaining why some plants or crops require different soil P levels for a given level of performance.

8 – Increase beneficial soil fungi

Mycorrhizae are soil fungi that form a symbiotic association with plant roots. The hyphae of the fungus connect with plant roots and extend into the soil, acting like extensions of the plants root system by absorbing nutrients and transporting them back to the plant roots. A major benefit in this respect is an increase in P uptake. In exchange, the mycorrhizae receive sugars manufactured by the plant.

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