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Does orchard floor management improve soil health?

Research & Extension

“Soils are the basis of life. Ninetyfive percent of our food comes from the soil.Maria Helena Semedo, Deputy Director-General Natural Resources, Food and Agriculture Organization 

Key messages 

  • Soil improvement is a long-term process and investment. 
  • There was a general trend for improved soil health indicators under living grass/clover mulches, composts and organic mulches.  
  • There was limited influence on fruit quality outcomes; however, fruit firmness improved over time with orchard floor treatments compared to the herbicide control. 
  • Native species proved unsuitable in the orchard environment due to cost, poor establishment and lack of persistence.  
  • Herbaceous meadow plants would require careful management to allow self-seeding to ensure persistence. 
  • Future research will be directed at improving soil health in the tree line which showed the greatest potential for economic and ecosystem benefits.

The PIPS3 Program’s Improved Australian apple and pear orchards soil health and plant nutrition (AP19006) project set out to test some key principles of sustainable production systems with different orchard floor management strategies. Specifically, did these strategies improve soil health and contribute to long-term orchard resilience. We tested and demonstrated a range of orchard floor management strategies in the Huon Valley (Tasmania), Adelaide Hills (SA), Manjimup (WA) and Orange (NSW). 

The key question

“If we grow different cover crop species (living mulches) or add different organic mulches to the orchard floor, what are the impacts on key soil health properties and does this improve fruit quality and orchard productivity?” 

With a timeline of three years, just two growing seasons were used to test soil processes that normally take many years to impact soil properties and ultimately plant health. Despite the short timeframe, the research plots demonstrated progress and positive trends that provide some initial evidence-based recommendations for incorporating living mulches, composts or organic mulches into the orchard design. 

 

Bare earth

 

Compacted wheel rut

 

Cover crop

 

Field testing – what worked, what didn’t

The project was an opportunity to test unconventional practices alongside more accepted ways of managing the orchard floor including the inter row and tree-line strip (Table 1). Native species and meadow plants alongside tree lines thick with vegetation raised a few eyebrows. Their selection stemmed from an understanding that plant diversity encourages beneficial organisms and improves soil health with multiple ecosystem benefits.  


Table 1:
Orchard floor treatments: species and composts/mulches varied at different sites to reflect regional priorities and local soils, climate and management systems. 

Tree line  Inter row 
Herbicide bare earth  Grass/clover mix 
Compost  Mixed flowering meadow plants 
Grass/clover living mulch  Native plant mix 
Mow and throw  Grower sward 


What actually grew and persisted was a different story.
 

Native plants had the odds stacked against them. Despite dedicated support, these failed to establish or persist. Their inability to compete with weeds and pasture species was their death knell.  

The meadow plants fared better initially, with good stands flourishing in the first season. However, few persisted and the inter row generally reverted to a few dominant species. In the Tasmanian plots, this was mostly plantains, grasses and clover. Potentially, their persistence could be improved with more controlled mowing strategies.  

For native grasses and herbaceous species to have any chance of success in orchard inter rows, a long lead-in time of at least two years with intensive herbicide application would be needed. With the need for intensive labour requirements and the impractical disruptions this would have upon ‘typical’ orchard management systems, natives alone are uneconomic for most orchards. These plots reverted to a naturalised state, with grasses and clover dominant. 

 

Figure 1: Meadow plants in a NSW orchard. Photo: Jess Fearnley

 

Did soil health improve?

Even with the relatively brief time frame of this trial, the project observed encouraging changes in soil health indicators, specifically in the tree line, while the inter row was relatively unaffected by treatment. 

Changes to tree-line soil health indicators

Soil life was enhanced in both compost and grass/clover plots. Compost-treated plots with consistently higher soil moisture had increased levels of soil microbial carbon and bacterial species richness. The number of earthworms found in the living grass/clover plots was 440 per cent higher than in the compost, mulch or herbicide plots. 

Soil physical characteristics such as soil bulk density, hydraulic conductivity (the way in which water moves through the soil) and aggregate stability improved over time in most tree-line treatments compared to the bare earth herbicide strip.  

One way of interpreting soil physical health is to look at the amount of water available in the soil for tree use. This integrates many different physical soil measures and interprets them in a way that provides insight into the soil-water-tree relationship.   

In the 15+ year old orchard, the available water for tree use was unsurprisingly lowest in the tractor ruts, reflecting the compaction which occurs. The inter row, with its 15+ year history of ground cover, shows a trend for higher water availability for tree growth compared to the more recently established tree-line treatments. In a longer-term study, we would expect the tree-line treatments of compost or grass/clover to ‘catch up’ to the inter row and show significantly improved levels of water availability compared to bare earth. The extra organic matter supplied by compost or living mulch treatments should be reflected as increased soil carbon, biota and structural improvements, and water available for tree growth. 

Nutrient level changes were not consistent for treatments over the two seasons. However, the trend was for increased levels of some useful nutrients in the tree-line treatments that provided ground cover compared to bare earth (herbicide treatment). In the tree-line treatments, standouts were significantly elevated levels of some major nutrients in compost and grass/clover plots compared to bare earth (herbicide treatment), along with elevated pH (Table 2). 


Table 2:
Effect of tree-line treatments on key nutrients in a 15+ year old orchard. Levels in herbicide plots shown in brackets. 

Nutrient   Treatment  Change from herbicide plots 
pH   Grass/clover 6.27 (6.08)  Less acidic in grass/clover 
Nitrate nitrogen (mg/kg)  Compost 33.5 (11.5)  Compost 3 times higher 
Ammonium nitrogen (mg/kg)  Grass/clover 10 (4.5)  Grass/clover 2.2 times higher 
Phosphorus (mg/kg) (Colwell)  Compost 215 (98)  Compost 2.2 times higher 
Potassium (mg/kg) (Colwell)  Compost 590 (164)  Compost over 3 times higher 

 

Building up good soil health is a gradual process and while some benefits to any management changes aimed at improving soil health and resilience may be observed within the first 12 months, it usually takes several years before noticeable improvements can be observed. Healthy soil attributes to work towards include: 

  • good levels of organic matter 
  • abundant populations of earthworms, macro- and meso-arthropods 
  • thriving populations of micro-organisms: bacteria, fungi, protozoa and nematodes 
  • nutrient cycling to provide plant-available nutrients 
  • 100 per cent ground cover with a diverse mix of species 
  • small and large pore spaces for air and water (physical measures) 
  • good soil aggregation 
  • good water infiltration rates (>100 mm/hr) 
  • absence of a compaction or crusting layer. 

The different components of this project are all in agreement that soil life can be increased by increasing organic matter in soil. Practices such as reducing herbicide use and adding organic matter to the soil will assist in increasing soil life. While addition of compost and/or mulches such as straw or mower clippings plays an important role, actively growing plants are critical to healthy microbial communities/populations that support nutrient cycling.   

Did the orchard floor management treatments impact fruit quality? 

It is challenging to link changes in fruit quality to orchard floor management with just two seasons of data; however, the results did reveal some noteworthy influences.  

In the 2023 season fruit in the grass/clover living mulch tree-line plots was 11g lighter than in the herbicide plots. One explanation for this difference is that the well-established and vigorously growing grass in this season actively competed with trees for water and nutrients. Potentially if these plots had been mown regularly there may have been no effect. This is worth exploring in future studies.  

Fruit firmness was higher in the grass/clover plots compared with the herbicide plots in both seasons, while fruit in the compost plots showed the lowest firmness in the 2022 season and the highest firmness in the 2023 season.  

These initial results point to the need for longer-term studies to draw the link between improved soil properties and tree performance. 

 

Table 3: Impact of compost and grass/clover tree-line treatments on crop load, return bloom and fruit quality parameters in an established orchard.  

  Compost*  Grass/clover* 
  2022  2023  2022  2023 
Crop load  nil  nil  nil  nil 
Return bloom  nil  nil  nil  nil 
Fruit weight  nil  nil  nil  -11.0g 
Fruit shape  nil  nil  nil  nil 
Total soluble solids  nil  nil  +0.25 Brix  -0.2 Brix 
Dry matter content  nil  nil  +0.34%  nil 
Firmness  -0.13kg  +0.14kg  +0.25kg  +0.12kg 
Maturity (SPI)  -0.21  nil  -0.42  nil 
DA Index (green colour)  +0.12  -0.05  -0.09  +0.19 
Colour  nil  nil  nil  nil 
Pest damage  nil  nil  nil  nil 

*compared to herbicide treatment. 

 

What would we do differently?

The project team accept that native species were an ambitious inclusion in the trial. Apart from the intensive labour requirements, they would be impractical for most orchards other than in exceptional circumstances. 

The inter-row treatments revealed very little change in the soil health indicators we measured, suggesting that any ground cover will provide benefits in the inter row. This leads to the next phase of research to focus on the tree line, where we believe greater economic and ecological impact can be achieved.  

Where to from here?

Three years is a short time frame when working with soils as changes can be slow, often taking many years. However, the positive trends observed have provided valuable information that can be used as a framework for longer-term studies. In the PIPS 4 Profit Program’s Building Sustainable Soils (AP22003) project, we will continue monitoring the existing PIPS3 intensive trial sites in the Huon Valley (Tas), with a focus on tree-line treatments. Additional studies in Tasmania and across growing regions – with the support of team members from Pomewest (WA), NSW DPI, Fruit Producers SA and APAL (economics) – will examine the following: 

1. Tree-line living mulches: Measuring how these and their vigour affect tree growth, crop yield and fruit quality.

2. Soil microbial populations: how do we shift the orchard from a bacteria-dominated environment (changes to bacterial populations) to a mycorrhiza-dominated environment (what species, rates of colonisation?).

3. Fruit quality: what influence do changes to soil health have on fruit quality?  

  • What are the influences beyond simple and traditional fruit quality measures?
  • Apply research tools such as e-tongue and e-nose to fruit quality evaluation.

4. Improving water availability and tree water use efficiency  

  • Measuring how living and organic mulches and compost addition improve tree water availability over time with increased soil carbon/organic matter and benefits to soil structure and water infiltration/retention.  
  • Potential effects of orchard floor treatments on fruit and leaf tissue carbon isotope ratios may indicate improved resilience of the growing system to tree water deficit. 

5. Soil health tool kit: develop a soil health tool kit for growers.  

6. Economic analysis: cost benefits of living mulches, modelling of soil carbon changes and benefits, determining resilience and economic benefit of a more resilient orchard. 

 

Figure 3: Bringing it all together.

 

Acknowledgements  

The PIPS 4 Profit Program’s Building Sustainable Soils (AP22003) project has been funded by Hort Innovation, using the apple and pear research and development levy, contributions from the Australian Government and co-investment from the Tasmanian Institute of Agriculture, with delivery support from Pomewest, NSW DPI, Fruit Producers SA and APAL. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture.  

About the authors: 

Sally Bound, Michele Buntain, Nigel Swarts 

Tasmanian Institute of Agriculture (TIA)  

Project supported in regions by:  

Jessica Fearnley (NSW DPI), Susie Murphy-White (Pomewest) and Paul James (Fruit Producers SA)

This article was first published in the Summer 2023/4 edition of AFG.

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