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The residual value of phosphorus

03 Aug, 2017
The 2017 growing season has proved to be a very challenging one for growers in the central to northern wheat belt with decile 1 growing season rainfall in most areas.
By Wongan Hills Area Manager, Mitchell Hutton and Senior Agronomist, Luke Dawson.

The 2017 growing season has proved to be a very challenging one for growers in the central to northern wheat belt with decile 1 growing season rainfall in most areas.

Kalannie, 19 July

Moving into August we are seeing a below average yield potential in these areas. Due to the fact that the crops most limiting factor in 2017 will be rainfall, most of the crops nutrient requirements will have been met at seeding, with very few needing any top up nitrogen (N) at all. Many farmers are asking the question about residual nutrients from fertiliser applied in the 2017 season and how this may impact their nutrition decisions in 2018. It is important to understand the factors that drive residual nutrient availability from one year to the next. This article will focus on phosphorus (P) and the implications of a dry 2017 on next year’s P inputs.


P is essential for plant growth. No other nutrient can be substituted for it as it's required by the plant to complete its normal production cycle. P plays a role in photosynthesis, energy storage and transfer, cell division, cell enlargement and several other processes. It is important for annual crops to have access to readily available phosphorus throughout the whole season, however it is especially important in the early stages of development (first 2-3 weeks of growth). 

Phosphorus in soil

Soil interactions are very complex and can change depending on a number of factors which will be discussed in this article.

Elemental P is very reactive chemically, so it is not present in its pure state in nature. Soil P can be categorised into three groups:

  • Phosphates readily available in soil solution
  • Labile P (Slowly available phosphate)
  • Non-labile P (Very slowly available phosphorus)

80-90% of soil phosphorus is in the non-labile state and most of the remainder is in the labile form. 1% of soil phosphorus is expected to be readily available in soil solution. The rest of the P required by the plant needs to be applied as fertiliser P at seeding time. Only up to 30% of this freshly applied P will be available to the plant in year 1 with the rest binding to soil particles (fixation). Over a period of years this P will become plant available again, as more fresh P is applied the soil will fix freshly applied P whilst releasing previously fixed P.

Soil phosphorus is either in the organic or inorganic form and just like nitrogen it needs to be in the inorganic form to be absorbed by plants. Soil P can move from the inorganic and organic state through mineralisation and immobilisation. Organic matter contains about 1-3% P and represents about 50% of total soil P. The amount of organic P is closely correlated with the organic carbon (OC) level. Mineralisation depends on soil temperature, moisture and pH. Once soil P is mineralised soluble P compounds are released and are subject to plant uptake or fixation by the soil. Similar to the Carbon:Nitrogen (C:N) ratio, if soil organic matter is low in P and high in other nutrients available P will temporarily disappear.

Soil P is very slow moving throughout the soil profile. Soil P moves by diffusion which is a slow process and requires moisture. In a loamy soil P needs to be within 1cm from the surface of the roots to be taken up by the plant. Considering roots only intercept 1-3% of the soil in the top 15-20cm it is important fertiliser P is applied close to the roots.

Factors affecting phosphorus availability

  • % of clay
  • Type of clay
  • Phosphorus buffering capacity (PBI)
  • Time of application
  • Phosphorus status of soil (Fertiliser history, P bank)
  • Soil temperature
  • Soil pH
  • Other nutrients

Figure 1 highlights the various factors affecting P availability and uptake each year. 

figure 1
Figure 1 - The P cycle explained.

What does this mean for the 2018 season?

Many farmers are thinking about fertiliser decisions for 2018 and are asking the question about P rates following a drought year. We have dug up some trial data from 2003 following 2002 which was a below average year, much the same as this year is shaping up. In these trials we looked at the effect of freshly applied P following a nil yield year.

Panizza 2003- Southern Cross

Soil results

Trial results

Wenlock 2003 - Lake Grace 

Soil results

Trial results


Both trials demonstrate the value of fresh P. in each trial there is a significant yield response to 5 kg/ha of freshly applied P and another significant increase in yield when 10kg/ha of P is applied, bearing in mind that 2002 was a complete blow out for these areas and very little yield if any at all was harvested off these paddocks. These trials highlight that there is no escaping the fact that you will need fresh P in 2018. The fresh P applied in 2017 will have very quickly become fixed to the soil and move to the labile and non-labile state.

If you are looking to reduce P rates ensure you fully understand your soils to make an informed decision. Soil testing is the best way to aid in the decision making process and it is recommended to discuss your options with your local CSBP area manager. There are a number of solutions CSBP can provide with a wide range of products that can meet your needs.

An aspect that has not been mentioned yet is distribution of P. If the rate is going to be reduced it is pivotal that the distribution of P is sufficient so the plant can have early access to applied P. The removal rate of P is 3 kg/ha per ton of wheat so for yield potentials between 1t/h-3t/ha you should be targeting anywhere from 4.5-9 kg/ha of P. Table 1 shows the rates needed of different products to achieve those targets:

Table 1- Rates of CSBP Fertilisers required to supply varying P requirements. 

Below 40 kg/ha P distribution can become an issue as some plants may become P deficient which can limit yield. Row spacing will affect the distribution of P and the closer your row spacing’s are the less concentrated the fertiliser is along the drill row which may increase the requirement for a higher rate to ensure adequate P distribution. 

Take home message

2017 will most likely be a below average yield and input P will be higher than output P however increasing and decreasing soil levels is a long term process. It takes 3 – 4 kg/ha of un-utilised fertiliser P to change Soil P by 1ppm due to fixation and immobilisation, so any P applied this year will have little impact on overall soil levels.

It is important to remember that we should look beyond one season and it is important to match P rates with target yield using soil results regardless of the year so we are not wasting dollars on unnecessary P or losing dollars limiting yield due to low P rates.

As a general rule the best practice for 2018 will be to apply maintenance rates of all nutrients for target yield at seeding time, there is no replacement for freshly applied P and there is no instance where we can escape that. Previous data shows us that we cannot place much value on residual P from the previous year regardless of the application rate. The advice to farmers is to maintain their P, K, S and trace elements application at seeding as per normal and look for savings in N applications if needed. If you see the opportunity to chase cost savings with reducing P rates, the advice is to speak to your local CSBP area manager and soil test to see if it is an option. The phosphorus cycle is a very complex system and there are a huge amount of factors affecting it. To add to the complexity, every farm is different and every paddock within a farm is different. There is definitely no one size fits all, so it is very difficult to make a general recommendation that will work for most farms.

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