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How healthy is your soil?

Research & Extension
The Tasmanian Institute of Agriculture is leading PIPS3 Program research to learn more about how orchard floor management practices influence soil and tree health, fruit yield and quality, with demonstration sites in five growing regions. There are simple methods you can use to do the same!

The PIPS3 team for Improved Australian apple and pear orchards soil health and plant nutrition (AP19006) have been busy measuring many aspects of soil health in their test orchards. Last year we all had a lot of fun burying undies in the orchard looking for differences in soil health. But what is a healthy soil, how is one soil healthier than another and how do you measure this (other than with undies)?

PIPS3 researcher Dr Sally Bound from the Tasmanian Institute of Agriculture (TIA) has reviewed the scientific literature to capture the current academic view on what is a healthy soil.

“A healthy soil for apple production is best summed up as being fit for this purpose, so its characteristics might be completely different to say a healthy soil for growing proteas. A healthy soil, by definition, also continues to function effectively and efficiently in the long term – that is, it continues to sustain apple tree productivity while also maintaining healthy humans and the environment,” Sally said.

If you could paint a picture of soil health, it would be a complex life-support system for the apple tree, which interacts positively with the surrounding environment.

More and more, soils are being defined by both their health and their resilience, or how well they return to normal functioning after a stress or disturbance such as drought, compaction – yes, that heavy tractor – or cultivation.

Drilling down further, you get to the nitty gritty ingredients of soil health: the physical structure, biology and chemistry, in relation to the functions it performs.

Let’s get physical

Soil structure, it’s that great feeling you get as you rumble a handful of soil and it crumbles satisfyingly into clumps or aggregates.

The physical measures we are using in the PIPS trial all relate in some way to the size and stability of soil aggregates. Sally says these are critical as they create the pores necessary for movement and storage of air and water that are essential for healthy, functioning roots. One physical measure of a healthy soil structure is the size and stability of soil aggregates when wet.

Strong, stable aggregates resist breaking down and are vital for maintaining good conditions for root growth. When aggregates are unstable, soil pores can clog, and the soil surface can crust and become impenetrable. Aggregate stability is affected by soil texture, the type and amount of organic matter present, and the nature and size of the microbial population. For example, long strands of fungal mycelia bind soil particles together more effectively than smaller organisms such as bacteria.

In the PIPS study, we are measuring:

  • aggregate stability
  • soil compaction using a soil penetrometer and bulk density measurement
  • how much water the soil holds, for how long and how much is available to the plant.

What we are looking for is whether our orchard floor treatments are having a positive impact on soil structure.

Pure chemistry (to a soil scientist!)

This is a measure we are very familiar with from annual soil nutrient tests. The test results indicate how soil nutrient levels are changing from year to year. Other measures such as soil organic carbon and mineralisable nitrogen indicate the energy available for microbial activity. Photosynthesis is a key driver of soil health as this is how carbon is captured by the plants in the system, the apple tree and the plants on the orchard floor or amendments added to the orchard system as mulches and composts. Soil micro-organisms are dependent on these plants for a source of fixed carbon.

Measures of salinity, pH and cation exchange capacity indicate potential constraints to apple productivity.

Optimum ranges for chemical measures are mostly well documented. However, there is now more emphasis being placed on measures of potentially available nutrients due to microbial activity such as potentially mineralisable nitrogen. Measuring this could prevent over-fertilising by knowing how much nitrogen could be released over a season through microbial activity. Researchers from the Australian Soil Cooperative Research Centres (CRC) have taken up the challenge to develop faster, meaningful soil test methods for growers.

This PIPS study is conducting a complete nutritional analysis of soil from each plot to see how the chemical indicators of soil health are modified by orchard floor management practices.

Soil biology: the big, the little and the microscopic

The living organisms, the “soil biology”, are the munchers and builders in the soil system, transforming plant material into smaller and smaller parts. Earthworms are a favourite and particularly useful indicator of soil health, but smaller creatures like nematodes can be powerful indicators of changes to soil function.

Microbiological indicators are now recognised as the most sensitive indicators of soil health. Soil microorganisms are incredibly dynamic as their populations respond rapidly to changes in conditions, whether they are man-made or environmental such as temperature and rainfall.

The dynamic nature of microbial communities is a huge challenge to developing meaningful test methods for soil health.

PhD candidate Phil Kay (TIA) testing soil at Rookwood Orchard, Ranelagh, Tasmania.

In the PIPS study, TIA PhD candidate Phil Kay is conducting a range of microbial tests to answer questions around what microbes are in the soil and how well the microbial communities are functioning and how resilient they are to change.

These will be related to the physical and chemical changes in the soil and different orchard floor management practices.

Many tests are now available for testing the size, activity and identity of the microbial population in the soil. The Australian Soil CRC is developing practical soil tests specifically to help growers make decisions based on soil health indicators.

There are many simple ways of keeping track of your soil health, from commercially available soil test kits to simple tests using everyday equipment. PIPS3 team member Jessica Fearnley (NSW Department of Primary Industries and Future Orchards Front Line Advisor) and local apple growers compared the biological activity of different orchard floor treatments at the PIPS Orange demonstration site using simple field kits, such as Solvita, that measure soil respiration or carbon dioxide production (Figure 1).

Soil test kit

Figure 1: Jessica Fearnley (NSW DPI) and Orange district apple growers used commercially available field microbial respiration kits to look at differences between orchard floor management treatments.

If you want to test your own orchard soil you can:

  • dig a representative sample of soil and store it in an ice cream container – do the same in subsequent seasons to see how the physical character of your soil changes
  • count earthworms in a set volume of soil
  • have some fun while testing your soil – just bury some cotton undies and see how quickly they degrade!

When deciding which soil test to use, remember soil health testing is not so much about absolute values but showing how a soil is responding to change, whether that change is time or a new management practice. Watch for changes over time but make sure you are consistent in the time of year you take a sample, the way you take a soil sample and the lab you use. And most importantly, remember that a spade is one of your most valuable tools to see what is happening in your soil.

Find out more about the research Australian Soil CRC is doing.

This article was written by Michele Buntain, Horticulturalist, Extension & Communication, Tasmanian Institute of Agriculture, and was first published in the Winter 2022 edition of AFG.









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