With increasing awareness of the environmental impacts of synthetic fertilisers, herbicides and pesticides, there is a growing understanding that an ecologically balanced soil system is essential for maintaining healthy crops, Dr Sally Bound* writes.

As such, there is a growing interest in farming systems using alternate nutrient sources such as humates and supplementary microbes. Although these amendments have been around for some time, there is little scientific evidence as to their effectiveness, and in commercial fruit production, there is uncertainty as to whether these alternate systems can adequately support the production of quality fruit.

The aim of the recently completed Hort Innovation-funded project 'Improving fruit quality and consistency in cherries through maximised nutrient availability' was to take a critical look at how alternative nutrient management influences soil health and, ultimately, fruit quality.

The idea is that healthy soil includes a diverse micro and macro biological population that assists in maintaining a balanced nutrient supply to the plant with smaller peaks and troughs. The desired impact of this biological diversity and balanced soil nutrition is a healthy tree with consistent fruit quality.

This project, which ran for five years, looked at the longer-term impacts of alternative nutrient regimes on soil physical structure and biology and cherry fruit quality.

Treatment application

Test sites were commercial cherry orchards in Tasmania's Derwent and Huon Valleys and included the cultivars 'Sweetheart', 'Staccato' and 'Lapin'. Four treatments were applied at each site:

1. conventional nutrition and herbicide program

2. alternate nutrient regime (based on humates)

3. conventional plus effective microbes (EM)

4. alternate plus EM.

The alternate management amendments included Ferbon, a lignite-based soil conditioner (Interstate Energy Group), and humified compost (Foundation Aerobic Compost, Pure Living Soils). These amendments were applied in spring and autumn, along with targeted minerals based on annual soil tests.

Effective microbes (EM1, VRM Pty Ltd) are a mix of ~80 species of co-existing beneficial micro-organisms, predominantly lactic acid bacteria and yeasts, plus photosynthetic bacteria and actinomycetes, which are brewed to activate. The activated EM was applied monthly as a soil drench throughout the five-year study period.

Impact on soil health

At the end of the study, key soil health indicators had improved under alternate treatments compared with the conventional. The alternate plots showed higher soil organic matter and improved water infiltration. Soil moisture content (Figure two) was higher in the alternate plots, and soil compaction (Figure three) was reduced. Figures three and four also show the high level of compaction in the wheel ruts in the inter-row.

There was an increase in soil biology in the alternate plots, with worm numbers more than doubling at the Derwent Valley site and quadrupling at the Huon Valley site. Mycorrhizal colonisation of roots increased by 30 per cent in the alternate plots, and the addition of EM to both the alternate and conventional plots further increased colonisation by 10 per cent. Mycorrhiza increases the effective root area, allowing for greater access to and uptake of nutrients in the surrounding soil.

Fruit set and pack-out improved

The alternate regime resulted in a higher fruit set than the conventional in most years, but EM had no effect. There was a general trend for increase in percentage of A-grade fruit in the alternate regime compared with the conventional in most years. A significant increase in A-grade fruit in years two, three and four was associated with EM application. Overall, the alternate regime showed increased fruit set and pack-out and a reduction in percentage reject fruit in most years.

Fruit cracking reduced

There was significantly less fruit cracking in the alternate regime in years three and four which both had significant rain events just prior to the harvest period. EM application reduced the incidence of cracking under both alternate and conventional regimes in every season.

Other quality parameters

In all treatments, firmness, total soluble solids content (TSS) and stem retention force met Australian 'export finest' standards in all years of the study. TSS content increased with increasing mycorrhizal colonisation. Both firmness and TSS content were higher in the alternate treatment in some years, but not others.

Field Day – key points

As part of the project, we held an on-site field day with soil scientist Dr Bill Cotching. Not having been to the trial site before and not knowing what the treatments were, Bill dug holes in the various treatments and discussed what he found.

The key points arising from the discussions with Dr Cotching were:

• Soil structure is as much about growing roots as the physical structure

- Clods indicate a lack of biology and/or root material

- To reconstitute soil needs actively growing roots (plant roots exert pressure of 2000 kilopascals, helping to break up clods and improve structure

• Worms are a strong indicator of soil health

• Soil aeration is important for good soil structure

- Grey and yellow soil colours indicate anaerobic soils

- Hydrogen sulphide is an indication of a lack of air

• Good soils can be in poor condition. Conversely, poor soils can be in good condition

• Organic matter (OM) levels are influenced by rainfall and clay content. OM is 58per cent carbon, and the rest is other nutrients.

- Compost can increase OM but doesn't rejuvenate the soil

• Moss on surface is a sign that the soil is not very active

• In perennial situations where the soil is left undisturbed, fungi are predominant over bacteria (compared with tilled soils that are bacterial dominated, as fungi don't like being disturbed)

• Improving the soil's physical structure will increase the number of native bacteria and fungi.

How do I determine the condition of my orchard soil?

There are some relatively easy ways to assess the condition of your soils without resorting to expensive laboratory tests, although depending on what you find, it may still be useful to send some samples for a full analysis.

1. Physically inspect your soil.

A spade is your best friend. The first step is to dig a hole so you can see the subsoil:

• Are old inactive roots decomposing (evidence of bacteria and fungi)

• Does the soil smell earthy (actinomycetes)

• Is the soil dark in colour (soil organic carbon)

• Is the soil well structured (soil aggregation)

• Is there evidence of bioturbation (macrofauna – earthworms and beetles)

2. Look for evidence of organisms

• Count earthworms – 10–12 per spadeful indicates good soil health

• Insert calico (cotton) strips into the soil (vertically) to monitor decomposition – the rate of decomposition gives an indication of fungal activity as fungi break down the cellulose in the cotton. Wooden stakes can also be used

• Set pitfall traps for macro and mesofauna

• Examine nodules on legume roots – a bright pink/red colour indicates active nitrogen-fixing bacteria

Key Points

Humate-based nutrition programs are capable of yielding high-quality cherry fruit with good pack-outs. The alternate regime showed increased fruit set and pack-out and a reduction in percentage reject fruit in most years

Fruit cracking incidence was reduced in alternate regimes in most years, while effective microbe application reduced cracking every season

Key soil health indicators improved under alternate treatments, including reduced soil compaction, improved water infiltration, increased soil organic matter and a higher abundance of mycorrhizal fungi.

Acknowledgements

This study is a strategic levy investment under the Hort Innovation Cherry Fund. It is funded by Hort Innovation using the cherry levy and funds from the Australian Government.

*Dr Sally Bound is senior research fellow, Horticulture Centre, Tasmanian Institute of Agriculture. Contact: sally.bound@utas.edu.au