Phosphorus – Queensland

Key Points

Phosphorus (P) is one of the most critical and limiting nutrients in agriculture in Western Australia.

Phosphorus cycling in soils is particularly complex, and agronomic advice is recommended when interpreting soil test results.

Only 5 – 30 % of phosphorus applied as fertiliser is taken up by the plant in the year of application.

Phosphorus does not move readily is soils except very light sandy soils in high rainfall areas.

Background

Ancient and highly weathered soils with very low levels of natural phosphorus dominate much of Australia, particularly Western Australia. Many of our agricultural soils are among the most acutely phosphorus deficient in the world, and profitable crop production has only been possible through significant applications of phosphorus-fertilisers.

Phosphorus is an essential element for plant and animal growth and important during cell division and growth. Complex soil process influence the availability of phosphorus applied to the soil, with many soils able to ‘tie up’ phosphorus, making it unavailable to plants. Your soil’s ability to do this must be measured when determining requirements for crops and pastures.

Deficiency

Phosphorus deficiency is difficult to detect visually in many field crops, as the whole plant tends to be affected. Stunted growth, leaf distortion, chlorotic areas and delayed maturity are all indicators of phosphorus deficiency (figure 1). Phosphorus is concentrated at the growth tip, resulting in deficient areas visible first on lower parts of the plant.

Figure 1: Wheat plant showing phosphorous deficiency (source: Grundon, 1987).

A purple or reddish colour associated with accumulation of sugars, is often seen in deficient plants, especially when temperatures are low. Deficient cereal crops are often poorly tillered. Visual symptoms, other than stunted growth and reduced yield are not as clear as are those for nitrogen and potassium. At some growth stages, phosphorus deficiency may cause the crop to look darker green.

The role of phosphorus in cell division and expansion means crop establishment and early growth is highly dependent on sufficient sources of the nutrient. Trials have shown significant agronomic penalties from applying phosphorus more than 10 days after germination. Most phosphorus timing trials indicate the optimum time for phosphorus fertiliser application is before or during seeding.

It should also be noted that recent Queensland research has indicated that around 50 % of phosphorus removed by crops comes from below 10 cm, and that phosphorus fertiliser applied with seed may only supply approximately 10 % of the crop’s total needs. While phosphorous at seeding is necessary to ensure seedling vigour and establishment, the plant also needs a continuous supply of phosphorous to produce dry matter through the rest of the season. This means that growers need to consider phosphorus reserves in both the top 10 cm, which help the crop’s early growth, and in the subsoil (10–30 cm), to understand that soil’s ability to supply phosphorus later in the growing season.

Fate of applied fertiliser

Phosphorus fertiliser is mostly applied in a water soluble form which can be taken up by plants, retained by soil and lost through erosion and leaching (figure 2). In the water soluble form phosphorus is not stable, and rapidly reacts in the soil (principally with iron, aluminium and calcium) to form insoluble, more stable compounds. Therefore, competition between the soil and plant roots for water soluble phosphorus arises, with only 5 % to 30 % of the phosphorus applied taken up by the crop in the year following application. Furthermore, at low pH (< 5.0), the soil’s ability to fix phosphorus rises dramatically, thereby decreasing plant availability.

Measuring a soil’s ability to fix phosphorus

Knowing the soil’s ability to fix phosphorus is vital in determining the rates of fertiliser application. A high fixing soil will require significantly more phosphorus -fertiliser, and commercial tests have been developed to determine this. These are used in conjunction with other soil and crop traits to optimise fertiliser phosphorus requirements:

Reactive Iron Test measures the amount of iron extracted from soil by ammonium oxalate. This indirect measure of a soil’s ability to fix phosphorus is only accurate when soil is adjusted for pH.

Phosphorus Retention Index (PRI) is a direct measure of phosphorus -sorption and involves mixing a quantity of soil in solution with a single amount of phosphorus for a set period of time. The amount of P remaining in solution measures the soil’s ability to fix phosphorus.

Phosphorus Buffering Index (PBI) is similar to PRI except that a range of phosphorus rates are mixed with the soil, and the index is adjusted for pH. This is becoming the Australian standard for measuring soil phosphorus -sorption.

Phosphorus retention and removal

Phosphorus that is not removed from the soil system remains as (i) undissolved in fertiliser granules, (ii) adsorbed by the soil, or (iii) present in organic matter. These sources all supply some phosphorus for plant uptake and thus maintain a residual fertiliser value (figure 2). A long term regime of applying phosphorus fertiliser decreases the capacity of the soil to adsorb phosphorus, giving increased effectiveness of subsequent applications.

Placement of phosphorus

Phosphorus movement in soil varies depending on soil type, although it generally stays very close to where it is placed. With the exception of deep sandy soils, very little phosphorus is lost to leaching.

Starter phosphorous fertilisers are most effective when applied at planting in direct contact with or just below the seed. Recent Queensland research has also indicated that where subsoil reserves of phosphorus are low, growth and yield increases as high as 20 % can be achieved in wheat and sorghum if crops are supplied with phosphorus in addition to starter fertiliser. The best responses have occurred when the additional fertiliser is applied in multiple bands as deep as 20 cm. Research is currently being conducted to determine the best methods for applying phosphorus deeper into the profile, and what fertiliser rates should be used.

Figure 2: The phosphorus cycle in a typical cropping system is particularly complex, where movement through the soil is minimal and availability to crops is severely limited (from Fertiliser Industry Federation of Australia Inc., 2000).

Fact Sheets Phosphorus - Qld