soil your undies

This spring, we decided to get our hands dirty — with a quirky experiment in some of our citrus orchards. The method? Burying cotton underwear in the soil to measure biological activity. The name? “Soil Your Undies”.

Words Bronwyn Walsh, Industry Development Manager, WA Citrus

IT sparked a few laughs — especially when one young helper asked if we’d be wearing the undies again after digging them up. (Spoiler: “No.”) But behind the humour was some insight about the living world beneath our trees and a good conversation starter on soils.

What we did

1. Chose 5 sites with different soils, temperature and rainfall.

2. Buried 100% cotton undies at a depth of about 10–20cm

3. Marked the spot extensively, so we could find them again

4. Dug them up after 7 weeks

When we returned to dig them up, the results were interesting.

What we found

The undies from the 5 sites had different levels of decomposition (see Figure 1). The most decomposed was from the Carmel orchard, this site had gravel loam. The least decomposed was at the West Gingin orchard, which had sandy soils. At some sites, as we dug, we spotted earthworms, slaters, beetle larvae, and fine roots — clear signs of biological activity.

FIGURE 1. Control and decomposed undies buried at 5 sites: a. Control b. West Gingin c. Tutunup d. Capel e. Gingin f. Carmel

The decomposition of the undies is due to biological activity in the soil. The different soils are shown in Figure 2.

Many thanks to the growers for volunteering their soils and time for this activity.

Why it matters

Soil Science Australia says that a healthy soil will:

• Retain more water and resist erosion

• Enhance nutrient cycling and retention

• Support strong, productive plants, and

• Store carbon and help to mitigate climate change

The role of organisms in the soil to achieving these outcomes is by affecting the physical, chemical and biological quality of soil.

The soil biological properties include living soil organisms and organic matter on and in the soil.

The organisms are responsible for biological activity and range in size and role. The smallest, some microscopic, include bacteria, fungi, and protozoa (micro organisms), nematodes to larger animals (mega organisms) like earthworms, earwigs, beetle larvae and ants.

Nutrient cycling by many of the organisms will affect the chemical quality of the soil. This occurs when the organisms breakdown the organic matter in soil, making nutrients available in a suitable format to plants. In addition, organisms such as nematodes feed on fungi, bacteria and roots, also contributing to nutrients in the soil. The physical quality of the soil can be affected by the movement of earthworms and ants (macro-organisms) through the soil.

Types of soil biology tests

A combination of measures may be used to assess the impact of new management practices — including yield, quality, nutrition density and Indicators from biology tests, chemical and physical tests. An excellent article from Neil Lantzke provides an overview of soil quality tests for citrus, including biology tests.

Soil biological tests mostly measure the amount, activity and diversity of soil organisms and their biochemical processes. However, they are difficult to quantify, and benchmark values are less available than for physical and chemical tests. Where data exists, it often comes from broadacre agriculture rather than horticulture. Tracking trends from test results over several seasons may be a practical alternative.

There are at least four companies in Australia offering soil tests, ranging in cost from $50 to over $500.

For more information on tests, follow the links for the factsheets found at the end of this article.

Management

Soil biological activity is affected by many factors including temperature, soil moisture, organic matter content, soil pH, salinity, cultivation and chemical contamination. Therefore, Management practices like cultivation, mulching, irrigation, aeration and ground cover can affect soil biology (there is limited information on how orchard plant protection and nutrition practices affect soil biology properties).

Some orchards in WA have tried composts, mulching, and adding molasses and fish emulsion to encourage soil biological activity. Tom and Emma Mitchell from Worrolong Produce in West Gingin have a wealth of experience in tackling their sandy soils.

Currently the pome industry is exploring under-tree treatments as alternatives to bare herbicide strips, focusing on adding organic matter. Yield and economics will be part of the evaluation.

An article by SouthWest NRM says, “A lot of information about how to manage soil biology is coming from the marketplace, which now offers a wide range of biological amendments aimed at improving biological function. However with many amendments not widely tested or used, it is difficult for farmers to decide which product, if any, will help in their situation”.

Three groups of amendments discussed are:

• Biostimulants (e.g. humic substances, hydrolysates, amino acids, seaweed products)

• Organic amendments (e.g. manures, composts, teas, biochars)

• Microbe inoculation (there were mixed results possibly due to soil type, crop, disease, moisture and side effects)

The article recommends comparing treated areas to untreated areas.

This burying undies exercise revealed how complex understanding soil and its management can be. Some useful sources used for this article are listed below. Or if you just want something lighter to do — why not soil your own undies?

Introduction

• Assessing and understanding soil health — Overview, Rise 360 eBook, Free, AgVic and North Central Catchment Management Authority. 16 Chapters, including:

• Soil Health Score Card and videos

• Evidence of soil biological activity

• Grower converted to Organic and uses farm-produced organic compost. Podcast

• Video of the function of earthworms, nematodes, fungi and bacteria in the decomposition of plant residues in the soil (2.5min)

• Evidence of soil biological activity

• Evidence of soil biological activity

Tests

• Soil biology testing factsheet, NSWDPI, 2005. Greg Reid and Justine Cox.

• 2025 Factsheet Soil microbiology and pathogen testing: what’s out there? AUSVEG

Management

• Pome orchard research

– Does orchard floor management improve soil health? Apple and Pear Australia Limited (APAL), Hort Innovation (AP19006)

– Video interview Exploring sustainable soil practices in WA orchards. Pomewest, Hort Innovation project (AP22003)

• Fumigation, inoculation and disease suppression, 2005, NSDPI Greg Reid

• Compost production and use in horticulture, Dept of Agriculture WA

• Case-Study_-Biological-Amendments_reduced.pdf, 2021, South West NRM’s Regional Agriculture Landcare Facilitator