How to examine your soil structure

A simple way to examine your soil structure

BY VO THE TRUYEN | REGIONAL DEVELOPMENT OFFICER, VEGETABLESWA

TOOL EXAMINING SOIL FERTILITY

WHAT DOES IT DO EDUCATING GROWERS TO TEST SOIL STRUCTURE

WHO WOULD USE IT ALL GROWERS

THE COST

FREE

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TRUYEN.VO@VEGETABLESWA. COM.AU SOIL FERTILITY IS examined by three main things — soil structural, chemical and biological properties. With increasing cropping intensity, soils of vegetable farms are becoming more vulnerable to the threat of deterioration which make soils become unable to sustain good productivity. The problems are usually unseen below ground and difficult to assess properly, many fail to put them right.

This article outlines vegetable growers with basic knowledge of soil structure and a simple way to examine it before providing guidance on how a better soil structure can be achieved and then maintained in the next articles. Soil structure is defined as the way the primary soil particles (sand, silt and clay) are combined and arranged with other solid soil components (organic matters, soil fauna) to form clumps or aggregates. The size, shape and arrangement of these soil solids and the spaces between them influences the soil’s capacity to retain and transmit air, water, organic and inorganic substances, and its ability to support root growth and development (Figure 1). Good soil structure is vital for sustainable agriculture. It limits gaseous exchange rates, for instance the gaseous form of soil fumigants in the treated soil profile. In the field trial on tomatoes and capsicum, the measurement of the biocidal breakdown product of Metham sodium in the case of applications via strickle irrigation was higher on the upper 15cm than at 25cm, and 15–20 times lower at the edge than in the middle of the growing bed 1 . These greatly reduce the efficacy of soil fumigation practice to kill soil borne pests and weeds. The soil structure influences the movement and storage of water, soil temperature, respiration and development, nutrient cycling, resistance to structural degradation, and supports biological activity. For example, cracks between aggregates that are larger than 0.2mm can be seen. These pores will allow air movement, rooting and drainage. Cracks smaller than this only hold water or fertigation liquid due to capillary force, they don’t drain 2 .

FIGURE 1 SOIL PARTICLES TO FORM SOIL AGGREGATES AND THE ARRANGEMENT OF SOIL AGGREGATES TO FORM SOIL STRUCTURE Soil particles (sand, silt, clay, organic matter)

Soil aggregate Soil structure

A simple way to examine your soil structure

All that is needed is a spade, a penknife and careful observation. 25–30cm topsoil structure can be examined by simply digging out a ‘spit’ of soil then carefully observe against a pre-defined index. It is useful to compare different soils and land uses. In vegetables farms the examination should be done within the growing beds. Topsoils should be examined when moist, preferably in the spring or early summer. This is when bright white roots are most obvious and probably at their greatest depth. Simply push the spade into the surface soil around three sides of a square, then, on the final dig, remove the spit of soil and, keeping it on the spade gently lay it on the ground. Examine the soil looking at the roots and the structures. Carefully probe the spit apart using a knife or your fingers and notice their shape, strength and development. It is also useful to bang the spit of heavy loamy clay soil on the ground and see how it breaks. stopping emergence of seedlings and infiltration of water

3 WELL-STRUCTURED soil with numerous and well branched roots.

TABLE 1 A STRUCTURAL INDEX

Poor

Clay and loamy soils

Plough layer is dense and consists of large clods; roots only in cracks; top 6cm has angular aggregates; dense below 6cm

Moderate Plough layer has large, but porous, aggregates; top 7 to 8cm small porous

Sandy soils

Almost single-grain structures; little cohesion; collapsed top layer; compacted lower layer; surface capping very thin but very effective at blocks, denser below

Slight cohesion of particles; moderate aggregation; some collapse

Example

Good Plough layer is mainly porous crumbs with few dense aggregates Entire plough layer is stable crumbs and few dense aggregates

Source: www.agricology.co.uk/resources/guide-better-soil-structure. Retrieved by 09/05/2020

1. Note compacted zones: During digging and by probing the pit sides with a knife look out for any hard or compacted areas. 2. Look at the cracks: Cracks larger than 0.2mm can be seen. These pores will allow air movement, rooting and drainage. Cracks smaller than this only hold water or fertigation liquid due to capillary force, they don’t drain. 3. Look at the roots: Examine the root depth and root forms. In a wellstructured soil, the roots will be numerous and well branched with lots of fine root hairs. If the root growth restricted or they are balling up, then there may be soil structural problem. 4. Look for earthworm holes: Earthworm will only live in well aerated soils.

Their channels help drainage, aeration and rooting. 5. Look at the structures: Note the size and shape of the structural units.

Are they too fine and collapse on each other or are they too large and separated leaving spaces without root or any organic matter? 6. Look for evidence of cultivation: Look to identify the depth where previous cultivations/rotary hoeing have taken place and examine what effects these have had on the soil such as plough pans, crop residues.

Management for enhancing soil structure

Soil structure holds a vital, but often overlooked role in sustainable vegetable farming and the well-being of the environment. It can impact the vigour and yield of individual plants and also nutrient and water cycles. Basic knowledge and guidance on improving soil structure of the three components of soil fertility: the structural chemical, biological properties will be outlines in our next publication.

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Contact Truyen Vo, Vietnamese Regional Development Officer, (08) 9486 7515 or email truyen.vo@vegetableswa.com.au

This project has been funded by Hort Innovation using the vegetable research and development levy and funds from the Australian Government. For more information on the fund and strategic levy investment visit horticulture.com.au

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