Lush Grass

Chasing Optimal Phosphorus Levels in Pastures

While soil sampling is a fairly tedious annual job on the farm, receiving the soil analysis results is one of the most exciting and possibly my favourite annual event!

After a decade of tough seasonal conditions and ordinary cattle prices, in 2013 we decided to make a concerted effort to lift our phosphorus levels to an optimal level.  Conventional thinking suggested Colwell P of 34 (Olsen P of 15) was our optimal target.  From a limited soil tests I had at hand, I could see our Colwell P levels at the time were down at around 20.


  1. Soil test every paddock
  2. Set an annual total budget
  3. Calculate annual stocking rates for each paddock to establish maintenance P requirements.
  4. Apply capital applications of Phosphorus, where required, staying within my total budget, prioritising the paddocks that were likely to give the best response.

The strategy was simple.  Soil testing every paddock seemed extravagant at the time and I still get the odd raised eyebrow when I tell someone about this annual policy.  However those initial tests cost around 5% of my total fertiliser budget.  They are cheaper now – I’ll talk more about that later.  Importantly, a soil test for each paddock showed me which paddocks were already on target (e.g. Jns and SbH in the below graphs).

Chasing Optimal P, paddock by paddock.

What do these graphs show?

  • The horizontal axis shows which year.
  • The red bar and right axis show how many kgs of single super (or equiv) I applied in the Autumn.  The most I’ve put out is 500kgs and the minimum I have put out is 75kgs.
  • The blue line and left axis shows what effect that had on Colwell P later in the Spring.
  • The green line is my target of 34.

It’s very clear that there is no precision when chasing that target!

Taking Action

There’s plenty of margin for error in managing P levels in our paddocks. Soil Test results can be volatile so getting a couple of years results gives the best picture of what is happening in a given paddock.  This is demonstrated in the result above for Jns where the Colwell P scores make extreme movements.

Calculating our annual stocking rate is essential to predicting the maintenance P requirements.  This is a fairly inaccurate science with different seasonal conditions affecting the consumption of P.  If you make a lot of rotations then it takes good record keeping to keep track of each paddocks stocking rate. If you use your paddocks  the same way (class and quantity of livestock) each year, the test results give good insights into whether you are applying the correct rate for maintenance.

Top Tips for Soil Testing

There’s a misconception that soil testing is expensive. Testing can cost as little as $15 per sample. In our region, phosphorus is generally the most limiting factor to pasture production which is why Superphosphate is the most commonly applied fertiliser. At $15 for a single Colwell Phosphorus and Potassium test, producers can do multiple tests across their farm and vary their rate of fertiliser application accordingly.

  • Test at the same time of year each year.  I like Spring because the ground is not too hard!
  • Test in the same spot in the paddock each year.  I open the maps app on my phone with the satellite layer turned onto find the centre of the paddock and do a lap of the ute collecting 20 plugs per sample.
  • Buy or borrow a proper soil sampler for ease of use and depth accuracy
  • Use the same lab each year so that your results are more comparable.  I use CSBP in W.A.
  • If changing labs, choose a NATA accredited laboratory
  • Double bag soil samples in zip lock bags and use a prepaid satchel for postage
  • Consider using different types of tests. E.g. Single tests for Colwell P to make informed decisions about Superphosphate applications and comprehensive tests to make informed decisions ahead of pasture renovations.
  • If you don’t know your Phosphorus Buffering Index (PBI) it’s a cheap add-on to any test. You’ll only have to do it once as it won’t change from year to year.


There’s lots of useful resources online such as those at MLA and Agriculture Victoria.  The one I keep going back to is the Phosphorus Budget Tool developed for the Evergraze project.  This tool is great for helping you decide which paddocks to apply a maintenance rate of P and which to apply both maintenance and capital P applications.