A snapshot of some of the more significant regional trends in soil health

By Summit Fertilizers

Each year Summit Fertilizers processes thousands of soil samples from across WA. Samples are analysed by an independent laboratory, and the results help guide our clients to make more informed fertilizer decisions. They also provide us with valuable long-term information on soil health and nutrition trends. As our farming systems keep evolving, our soils too will continue to change in their condition and nutrient status.

In this spring newsletter we report on some of the noteworthy regional trends and nutrition updates, with a focus on:

• pH;
• phosphorus (P);
• and, potassium (K) content.

North Midlands region pH

2023-24 topsoil and subsoil pH readings from the North Midlands shires of Mingenew, Three Springs, Carnamah, Coorow, Morawa and Perenjori are a good news story indeed! When compared to Summit samples taken back in 2014-15, both topsoil and subsoil pH have shown healthy improvement.

Most WA crop and pasture species have proven to perform best when topsoil pH is above 5.5 (in CaCl2) and subsoils above 4.8. pH values below these can reduce the availability of many soil nutrients and reduce root exploration via an increase in availability of the toxic form of Aluminium (Al).

Figure 1 clearly shows the benefit from investing in lime and soil amelioration in the North Midlands. Summit testing in 2023-24 showed 72% of North Midlands region topsoils sampled had pH above 5.5. That’s a significant improvement on the 43% from samples taken in 2014-15.

Midland region subsoils have likewise benefited with a similar trend.70% of the regions subsoils were measured with pH of above 5 in 2023-24, compared with 41% in 2014-15.Extensive amelioration programs would most likely have helped with this shift, by moving applied lime into the subsoil. Not only can improved topsoil and subsoil pH increase root exploration and the availability of most nutrients, it can enhance soil conditions to increase the survival of soil organisms such as bacteria, legume inoculum, and earthworms.

The data indicates many paddocks in the North Midlands region are now better placed than they were a decade ago to take full advantage of applied fertilizers and favourable seasonal conditions.

P & K

Critical soil test values for P and K are harder to determine as definitively as pH. Soil type, rainfall, crop or pasture type and other soil test parameters need to be considered along with pH, to build a more complete picture of P and K status.

North Midlands region Colwell P test results for 2023-24 are shown in Figure 2. Summit inSITE gives a “P status” indication that takes all soil test parameters into account to advise the likelihood of a response (or lack of)to P application.

Topsoil samples taken in the North Midlands region in 2023-24 showed close to 60% had Colwell P readings below 20mg/kg. It’s a surprising result given 2023 applied P was followed by such a poor growing season in the area. But, perhaps not surprising given the huge crops and nutrient removal in the previous years. Summit trials are continuing to evaluate the new DGT-P test as a potentially better method than the Colwell P test for identifying responses on soils with a PBI > 30.

Figure 3 shows topsoil and subsoil K results. We know how difficult it is to increase soil K on very sandy soils. The best strategy on these soil types is to apply K until you get to a stage where responses are less visible/common. In-season Summit K gauges can help assess your K status. Plant testing can also provide valuable insight.

Key messages

North Midlands region growers should continue to keep an eye on nutrient removal. Make sure areas that test low in plant available P are addressed and rates are in balance with yield potential.

Maintain a replacement strategy on sites that have adequate P. Nutrient removal calculations area good way to get an overview of what you are exporting. Post-harvest calculations will help fine tune your rates so you can adjust your fertilizer order accordingly.

Lastly, have a long term-nutrition vision. You would want to be in the position in five or ten years time whereby soils still have healthy pH and, P and K levels are not limiting farm production.

Read the rest of the Summit Fertilizers Broadacre Spring 2024 Newsletter here.