Why soil test?

There are a number of benefits to soil testing:

• Soil testing provides information on other possible constraints to crop growth, such as soil acidity and salinity
• It can be used to monitor changes in organic matter, which will influence the soil's resilience and mineralisation of nitrogen
• Testing is essential to making accurate fertiliser decisions.

Growers will have the opportunity to hear the results from a GRDC investment looking at the value of using soil and plant-testing data to inform soil management and fertiliser investments in the southern region through a series of workshops in early 2020.

Approximately 100 growers in the southern region have implemented tailored fertiliser recommendations based on the soil and plant testing undertaken throughout the 2019 season. In addition, a survey has been conducted of a further 200 growers and agronomists to better understand their attitudes and decision-making around soil testing.

The project aims to provide grain growers with the confidence, knowledge and ability to make more effective and profitable nutrient management decisions through effective soil and plant testing.

One of the project's leaders, Agronomy Solutions research agronomist and director Dr Sean Mason, says the value of soil testing cannot be understated, with the results from the in-field testing and the survey to be extended to growers across the southern region.

Initially, Dr Mason says, factors such as the time needed to soil sample, the cost of the soil testing and the turnaround time at the laboratory were all considerations for growers in deciding whether or not to soil test. He says that while soil testing every cropping paddock each year is desirable, this can be often be difficult due to the amount of cropping land sown by most farmers. He says many growers opt to test each paddock every four or five years on a rotational basis, or when a paddock is going to be planted to wheat.

"We've found during the initial stages of the project that the cost of soil sampling and analysis can be seen as a hinderance," he says.

"However, the cost of soil testing is often only a small percentage of the actual fertiliser cost.

"It is a worthwhile investment to make sure you're applying the right amounts of the right nutrients."

Soil testing also provides information on other possible constraints to crop growth such as soil acidity and salinity and can be used to monitor changes in organic matter, which will influence the soil's resilience and mineralisation of nitrogen.

When considered in combination with information about target yield, available soil moisture, last year's crop and nutrient removal and soil type, soil tests are essential to making accurate fertiliser decisions.

Dr Mason says topsoil testing to a depth of 10 centimetres is important in understanding the level of immobile nutrients available, particularly phosphorus, while deeper sampling is key for identifying nitrogen, potassium and sulphur levels which, due to their mobility and inherent soil properties, are prevelant throughout the rooting depth.

Deeper sampling is also important for monitoring subsoil acidity.

Developing a testing strategy

Before diving into soil testing, it is important for growers and agronomists to have a soil sampling strategy in place.

Dr Mason says it is necessary to first understand the variability within a paddock and identify different production zones.

Most growers will have a good sense of the high and low-yielding parts of the paddock and changes in soil types from their previous experience, but tools such as grain yied maps, in-seasonn Normalised Difference Vegetation Index maps or mapping technologies such as EM38 can provide a more accurate picture.

Testing in different production zones can help a grower or advisor identify constraints to root growth and the causes of variable crop growth and yield.

"There are often significant changes in soil types across a paddock which will drive nutrient availability and crop performance," he says.

"Constraints can vary enormously within a paddock, so there is a shift to applying fertilisers and soil amendments based upon the type of constraint within each of the production zones, soil types and yield maps."

Recommended soil-sampling methods are described in the Fertilizer Australia soil-sampling guide.

Key nutrients levels

Dr Mason says most advisors and agronomists use critical soil and plant nutrient levels to make fertiliser recommendations, which are determined by interpreting data bases that contain data from replicated field trials conducted on similar soils and production areas.

The bulk of the data regarding both soil and plant testing is for wheat, although recent research efforts have improved, the database for barley, canola and other crop types.

"The Better Fertiliser Decisions fro Crops database is a powerful tool which allows for advisors to view thousands of field trials to identify critical soil nitrogen, phosphorus, potassium and sulphur levels," Dr Mason says.

Economic analysis

Another major component of the three-year investment is the development of an economic framework which will quantify the likely returns from improved nutrient-management techniques and the opportunity to boost farm profit while managing risk.

The framework will assess soil and plant-testing approaches including frequency, phase in the rotation, sampling intensity, soil depths and sampling strategies in controlled traffic paddocks.

The framework will be developed from survey and workshop analyses, farmscale assessment via fertiliser test strips and existing literature. It is designed to be used together with large scale on-farm paddock trials to demonstrate how fertiliser decisions informed by test results lead to increased profitability.

Results from this paddock trials program will be included in the ongoing development of the economic framework.

One of the main aims of this project and an overarching GRDC objective, is to improve nutrient management best practice through the increased use of soil and plant testing to provide grain growers with the confidence, knowledge and ability to make more effective and profitable nutrient decisions.

'Using soil and plant testing data to better inform nutrient management and optimise fertiliser investments for grain growers in the southern region' is being led by Dr Mason and Harm van Rees in conjunction with Australian Precision Ag Laboratory (APAL), CSIRO, Landmark, Hart Field-Site Group and AgCommunicators with support from Mallee Sustainable Farming, Southern Farming Systems and Eyre Peninsula Agricultural Research Foundation.

More information: Sean Mason, 0422 066 635, sean@agronomysolutions.com.au.