Blog by Suzanne Jones 


Protecting and preserving our environment is an important topic to most of us today. Environmental concerns are often in the news and are always on the political agenda. But so are economic stability and food security. The Peel region is a popular holiday destination known for its beaches, restaurants and beautiful waterways. It is also home to many orchards, wineries, and produces high quality beef and dairy. How do we find a balance between our growing agriculture industry and our environment?


Peel-Harvey Catchment Council is working with the Regional Estuaries Initiative to run a program that hopes to do just that: to better our understanding of fertilising practices and producing from our land, as well as reducing the nutrient run-off into our waterways.

Each year 140 tonnes of phosphate enters the Peel-Harvey Estuary. Just as on land, phosphate can be a very effective fertiliser. But it has the unwanted effect of prompting algal blooms, lowering the oxygen content and contributing to fish deaths. In order to maintain a healthy ecosystem in our waterways, we needed to know where this phosphate is coming from and if it is possible to reduce the inflow.

The Peel-Harvey catchment covers 1.17 million hectares with three major river systems draining into the estuary; the Harvey, Murray and Serpentine Rivers. However, 90% of the phosphorus that enters the estuary comes from the narrow sandy coastal strip of the catchment that is part of the Swan Coastal Plain.

Studies of the waterways of south west WA found that three quarters of the phosphate was coming from grazing livestock; that is beef, sheep and dairy. The irony is that these land uses apply some of the least amount of phosphorus per unit area. It makes sense that this would be the first place to look to see if there is any way to reduce that run-off because it is such a large area. Urban land management is also being addressed through regulations on bagged garden fertilisers and through changes to septic systems.

Previous and ongoing programs of soil testing in the region showed that adding more fertiliser to the soil does not necessarily lead to higher production. Between 2009 and 2019, 934 farms were tested (that’s over 220,000 hectares, 19,600 samples and around half a million soil cores). The results were surprising. They found that due to soil conditions, 78% of paddocks would not have responded to the application of more phosphorus, and 63% continued to add more than was needed. Adding more fertiliser in these paddocks poses an unnecessary economic and environmental risk.

The tests also revealed that of the paddocks tested 65-80% had enough or too much phosphorus and just 20-30% had too little. Almost 100% had acidic soil and a significant number were deficient in both sulphur and potassium.


So what does this all mean?

To a trained eye, those numbers make a lot of sense.

When the acidity increases (i.e. the pH decreases) those essential elements for plant growth such as phosphorus, sulphur and potassium become tied up with other elements and are less available to the crop. At the same time, elements such as aluminium increase, which can poison the plants.

The pH scale goes from 1 to 14 with 1 being the most acidic, 7 being neutral and 14 being highly alkaline. For example: if a soil goes from a pH5 to pH4 it has become more acidic.

The ideal soil pH is 5.5 however many paddocks were showing readings as low as 4.2, and a few below 4.0. This is well into the unhealthy range, reducing the availability of phosphorus to plants.

The tests revealed that a combination of acidic soil and low phosphorus could lead to deficiencies, however, most soils have accumulated large stores of phosphorus and this can overcome the acidity effect. It requires a close assessment of both pH and phosphorus to make a decision on whether to apply phosphorus. What is clear is that applying lime will enable reserves of phosphorus to be more available for plants as well as managing the soil health issues.

Adding lime to a soil will increase the pH. The main constituent of lime is calcium carbonate (CaCo2) which has a neutralising affect.

By testing the soil for these key elements, a farmer might choose not to add any fertiliser but apply lime instead to reduce the acidity and release the phosphorus that is already present. By doing this they may decrease their costs but increase the productivity of the land, and in doing so increase the profit margin. A positive by-product of this switch is that less fertiliser is then flushed into our waterways.


Long-term testing program

As part of the Regional Estuaries Initiative’s Sustainable Agriculture program, farmers from the around Peel and the South West were invited to take part in a soil testing scheme. Running for four years between 2016 and 2020, the program aims to assess the condition of the agricultural land and guide farmers through best fertiliser practices. The results could be a potential win-win situation with farmers getting the best output from their land with minimal fertilising as well as reduced nutrient input into our waterways.

For each farm that participated, soil samples were collected from each paddock, tested for phosphate, sulphur and potassium, among other things, and the results presented to the farmers at a soil testing workshop.

I attended one of these workshops in Pinjarra in February 2019 along with several dozen farmers from the Peel-Harvey catchment (pictured below).

The morning started with a brief introduction to Regional Estuaries Initiative and followed with presentations by experts in soil testing techniques, testing methods and how to interpret the results. After lunch the farmers were presented with a folder containing detailed results from their farm.


The Mapping Report


Each personalised mapping report is a comprehensive booklet presenting results in both map and table form. For ease of interpretation a simple traffic light system was used:

  • Green – high values. There are sufficient nutrients in the soil for the plants to ‘mine’ and no fertiliser should be added
  • Yellow – medium and ideal values
  • Red – low values – add more nutrients

Exact values are also displayed in each paddock on the maps.

Phosphorus Buffer Index

The Phosphorus Buffer Index (PBI) is a simple way to describe how well the soil can hold onto the nutrients and is determined by the soil type. Clay soils hold more nutrients and for longer, whereas sandy soils tend to ‘wash out’ faster. The PBI is used to weight the rest of the results and give an idea of how often to retest.

pH map

Identifying the pH of each paddock helps us to understand the relationship between nutrients already in the ground and plant growth. It may be possible to have all the nutrients in place, but if the pH is too low (too acidic) the plants may be unable to access them resulting in lower production than expected given the fertilisers applied.

Phosphorus Status Maps at a range of production values

Four maps showing the phosphorus status at 80%, 85%, 90% and 95% production were produced. Not every paddock will be farmed at 100% of its production capability. If an intensive crop is being grown that field may want 95% or 100% of the soil’s capability to support that crop. However, if growth is grass to feed grazing cattle it may need only 80% of the soil’s growing capacity. By the same token, the soil may not need 100% saturation of phosphorus.

For this reason, phosphorus maps were created at the four different production percentages. A paddock may be green (too much) phosphorus for 80% production, but at 95% it may be red (not enough). This makes it easy for anyone reading the report to see which fields are suited to different intensities of plant growth, and which may need more or less nutrient added.

Sulphur and Potassium Status (2 separate maps)

These two show the levels of respective nutrients in the soil for each paddock using the traffic light system.

Fertility Index

This map shows how each paddock compares to a chosen ‘critical value’ – it will show which paddocks to back off fertilising, and which to increase.

Phosphorus Environmental Risk Index

This clearly identifies which areas are likely to be contributing to the high nutrient input to our waterways. This value is closely linked to the Phosphorus Buffering Index number. Sandy soils are more prone to losing nutrients than clay soils. This doesn’t let clay soils off because erosion can carry the clay carrying the phosphorus off the paddocks and into the waterways.


Trends in the results

As I am not a farmer I had no results of my own to read through. I could sense a little trepidation in the room as the folders were being handed out, almost like waiting for exam results. I watched as people flicked through the pages and found many were amazed with the results.

It came as a surprise to many that a lot of their phosphorus status maps were mostly green meaning they had sufficient or too much phosphorus in the soil already. An interesting conversation arose around the room with some farmers commenting that they routinely added fertiliser to their land as ‘that’s what has always been done’. Around a hundred kilograms per hectare appears to be the norm. The soil testing showed that this may not be the best course of action. 

Another result common to many farms was having acidic soil. If you recall from earlier, having a low pH, or acidic soil means the plants are less able to utilise the phosphorus, sulphur and potassium in the soil.


So is there a more efficient way of treating the soil to yield better production?


In these cases it may be more cost effective to apply lime treat the acid soil, and potash for potassium where need be. With the high acidity of the soil treated, phosphorus already present in the ground will be more available for the crop, meaning we wouldn’t need to add any more fertiliser.

It is estimated that over $400 million of excess fertiliser is used in WA each year. I heard anecdotes of fertiliser costs around $40,000 a year where none was necessary – we joked that you could have a wonderful holiday for that amount of money!


Tissue Testing

A second test that is available for farmers free of charge as part of this program is tissue testing. In this test a small sample of leaf is taken at several locations through the paddock. The leaf tissue is then tested to see what nutrients it contains and in what amounts.

This is the plant is telling us what is has, and what it is deficient in. It is a good way to check that we have the right balance of fertiliser and lime.


A win for farmers and waterways

Without the soil testing, making any changes would have been trial and error, often having to wait a year or so before knowing if you were successful or not. By subsidising the soil testing and collecting the results together to a central database, we can get an overall idea of the health of our agricultural land, and assist farmers to get the most out of their land and potentially increase their profits.

The aim of the program is not to test farmers, it is to educate both them and the community in the most economic and sustainable farming practices. It is an ever-evolving science and all this testing helps develop knowledge of the land in the region, and how agriculture and environment are tied together. The plus side for all of us is that we should be able to protect our estuaries for many years to come while continuing to support our agriculture industry.


About the author

Suzanne is a geoscientist and outdoor enthusiast. She is passionate about sharing her love for science and nature with younger generations. You can read about her adventures with her young family on her blog Keeping Up With Little Joneses.