Soil Health

Is soil structure important?

The physical condition of a soil influences farm economics and environment and because some Southland soils are very susceptible to wind erosion it pays to know your soil.

Soil structure refers to the arrangement of the aggregates and pores within the soil. The structure of soil determines the porosity, strength and stability of a soil which influences:

Water movement and storage
The roots ability to penetrate, grow and withdraw water and nutrients
The ability of a soil to resist erosion

Good soil structure

A well-structured soil has many stable aggregates in a wide range of sizes. Aggregate size of 2-5 mm diameter is considered to be the best and most wind erosion resistant. Good soils also have a large number of pores within and between these aggregates. These pores maintain the correct balance of air and water in the soil and allow easy seedling emergence and plant root growth.

Poor soil structure

Poorly structured soil either does not have aggregates of many different sizes or the aggregates are packed tightly together with few pores. In light textured soils the soil appears fine and powdery and has unstable aggregates. Poor soil structure can markedly increase the risk of wind erosion.

Main forms of soil damage

Aggregate breakdown - is caused by natural processes such as wet/dry and freeze/thaw cycles. Soils comprised mostly of silt and sand, with a small proportion of clay, are most susceptible as they are loosely bound. As organic matter is a major soil binding constituent, those soils with low organic matter are particularly susceptible. Cultivation significantly reduces soil organic matter content and can rapidly result in serious aggregate breakdown with the risk of erosion, crusting, and poor infiltration of rain and irrigation water. Recent soils and many pallic soils are susceptible to aggregate breakdown.

Soil compaction - is the compression or squeezing of a soil which makes it denser and reduces the number and volume of large soil pores. The wetter a soil is, the more susceptible to damage, because water reduces soil cohesion and lubricates the movement of aggregates. With compaction, the proportion of fine pores is increased and decreases the soils water drainage and air exchange capability. Plant root growth is also restricted, causing a decline in the plant's ability to absorb water and nutrients. Farm machinery and animal treading causes compaction especially in wet conditions. Heavy loads can compact soil to over 50 cm in depth.

Assessing soil structure

Assess your soil structure by carefully examining a spadeful of topsoil at 5-10 locations in each paddock. Loosen and compare the condition of the soil against the photographs, or with pasture soil from an adjacent area. Assign a score to each sample and average for the paddock. A soil in good condition will have a high score (7-10) and a poor soil a low score (1-4). Soil should have a condition score "above 5" to ensure high crop yields and a low of erosion.

The condition score given is based on the size, shape, and apparent porosity of the soil aggregates. Also taken into consideration is the cohesion of the soil, the amount of dispersion or crusting on the surface, and the extent of root development.

The most erosion prone soils are poorly structured light textured soils. Poorly structured medium and heavy textured soils consist of dense clods with few roots, smooth faces and occasional rusty orange spots. There will often be a distinct grey colouration, particularly for poorly drained and/or compacted soils. Although these are less prone to erosion, this condition will reduce crop yield.

Score 1-2: Single grain structure, no cohesion of particles. Low in humus, very loose or big dense clods with smooth faces. Roots only in cracks.

Score 3-4: A few aggregates of low stability. Low cohesion. Loose or big dense aggregates with few pores.

Score 5-6: Entire topsoil of large porous aggregates.

Score 7-8: Plough layer almost entirely porous, stable crumbstructure, occasional dense aggregates.

Score 9-10: Entire plough layer consists of stable crumbs and few dense aggregates.

Best cultivation practices

Retain a grass phase in the crop rotation and return crop residues to the soil.
Keep cultivation to a minimum and use minimum tillage or direct drilling methods to reduce soil damage and to retain organic matter levels.
Leave the seedbed as rough for sowing to reduce the surface windspeed and work at right angles to the prevailing wind. Use light harrows to leave the sown bed surface rough.
Plant shelter belts at right angles to the direction of the prevailing/damaging wind.

Facts about soil porosity and water holding capacity

In a well-structured soil, over half the soil volume consists of pore and about 40% of these pores are very small and nearly always filled with tightly held water which can’t be withdrawn by plant roots.
The volume of the small pores is determined by the proportions of sand, silt, clay and organic matter.
Medium sized pores retain water, which can be taken up by the roots, after rain or irrigation. The volume of these pores determines the water holding capacity of a soil and its susceptibility to drought.
Large pores (macropores, >0.03mm diameter) are the structural soil pores, and are important for water infiltration into the soil, drainage of excess water, aeration of the root zone, and act as passages for root growth in the soil. In pasture, there are few macropores >5mm, other than earthworm channels.

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Taking Care of Topsoil