Understanding soil biology in

commercial orchards

Belinda Rawnsley

South Australian Research and Development Institute

(SARDI)

Introduction

• Why soil biology is important

• Tests to measure soil biology

• How and when to sample

• Improving soil biological health

Soil biology

• Break down organic matter and release nutrients into plant available forms

BAM!

Soil biology

• Break down organic matter and release nutrients into plant available forms

• Improve and maintain soil structure

Soil biology

• Break down organic matter and release nutrients into plant available forms

• Improve and maintain soil structure

• Suppress soil borne diseases

• Degrade chemicals

• Lock up greenhouse gases in soil

Soil biology is essential for the soil to function properly

Soil biological indicators

• Tests designed to measure activity or populations of soil organisms

• Soil biology quick response to soil changes

• Routine soil tests do not measure soil biology

• Wide range of biological indicators to measure effect of soil management

Amount (e.g. microbial biomass, plate counts)

Activity (e.g. soil respiration, enzyme activity)

Diversity (e.g. community composition)

Microbial biomass

• Total population of active microbes in the

soil at the time of sampling

• Influenced by soil properties

• Measured by amount of carbon, nitrogen,

phosphorus and sulphur

• Responds quickly to soil management

• 1 to 5 % of total organic carbon (source DPIVic).

Microbial biomass and soil carbon

Lower than expected microbial biomass may indicate

a constraint to biological fertility Source:Soilhealthknowledge.com.au

Seasonal variability of microbial

biomass under perennial crop

0

5

10

15

20

25

30

35

40

Cultivated Biodynamic Permanent

sward

Mulch Grazing

land

To

tal m

icro

bia

l b

iom

ass (

ug

C/g

)

2 Nov 2009

1 Feb 2010

25 March 2010

HIGH

MODERATE

(Barossa Valley, 2009/2010)

Fungi and bacteria

• Measured as a proportion of fungi and

bacteria in soil

• Fungi associated with decomposition and

soil structure stability

High carbon requirement

• Bacteria regulate nutrient supply

transform nitrogen into a useable

form

How to use soil biology tests

• No target values for a healthy soil in

regards to soil biology

• Use tests to monitor changes in soil biology

over time at the same time of year

• Measure soil biology in relation to

management and/or problem areas

• Consider soil type, land use, topography

and plant growth - cannot compare regions

Soil Sampling

• Majority of organisms live

in top rootzone

• Macadamia shallow

fibrous root system

• Sample from within 1 m of

the trunk

• Random sample of 8 - 20

cores bulked to give

representative sample

20 cm

40 cm

Microbial activity at depth

0

20

40

60

80

100

120

140

0-5 5-10 10-20 20-30 30-40

Soil depth (cm)

B-g

luco

sid

ase e

nzym

e a

cti

vit

y

(m

g p

-Nit

rop

hen

ol/

kg

so

il/h

)

Rhizosphere soil

(Rawnsley, 2008)

When to sample

• Coincide sampling

with periods of peak

root growth in

autumn

• Avoid after

application of

fertilisers

• Monitor at same time

of the year

Chemical use and soil biology

• Soil biology decompose chemicals and

pesticides – use them as a food source

• Impact of a pesticide depends upon the

rate of degradation

• Minimise use of heavy metals, e.g. copper

How to improve soil biology

• Feed the microbes!

• Organic matter must be continually supplied

How to improve soil biology

• Avoid bare soil

• Maintain grassed interrow

• Use groundcovers under trees

• Apply compost – apply early to firm up

• Soil additives/amendments

• Minimise compaction

What about microbial inoculants?

• If current practices have low microbial

populations, additions of new microbes

or organisms are unlikely to thrive and

persist

• Products stimulate organisms already in soil

• Manage existing soil biology by feeding the microbes

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

Conventional

cultivation

Biodynamic Permanent sward Undervine mulch

no

. fr

ee

-liv

ing

ne

ma

tod

es

/20

0 g

so

il

Nov 2009

Feb 2010

Mar 2010

Take home message

• Use soil biological tests in conjunction with chemical and physical analysis

• Monitor soil health over time

– will reflect changes in soil management practices

• Monitor changes within a single orchard

• Low microbial biomass restrains nutrient

availability…build up soil biology

• Healthy soils are resilient and more productive

More web information

• Soil health knowledge bank

Soilhealthknowledge.com.au

• Victorian Resources online >soil

health vro.dpi.vic.gov.au

Soil health factsheets

• soilquality.org.au