Improving soil structure is important to garden health

The task of improving soil structure requires more than just digging in a bit of compost. Actually, it would be more accurate to speak in terms of the "Friability and Aggregate Stability" of soil, but to keep thing simple here, lets just say "Soil Structure". However, let's just start by describing what these two soil characteristics are:

What is Friability & Aggregate Stability

Friability refers to the ease with which a soil crumbles. The moisture range in which soils are most friable is also the range in which conditions are optimal for tillage. This means there is sufficient moisture between the particles to minimize any cementation, but not enough to cause the soil structure aggregates to fall apart. The greater the moisture range over which the soil remains friable, the better. Therefore, friability is a function of aggregate stability, or the ability of aggregates to remain stable as the moisture content goes up.

In fact, it is the aggregation of soil particles that creates soil structure - with small spaces between the particles making up the aggregates and larger spaces between the aggregates themselves.

The very fine colloidal material in the soil (clay and humus) is a key component in the development the aggregates. For a more detailed description of 'colloids' you should read our "What is Soil" page.

The formation of strong soil structure cannot take place in sandy or silty soils, lacking these colloids.

A soil that is strongly friable with well developed, stable aggregates and will be much more resistant to damage caused by sudden wetting (after having been dried), than a less friable soil.

Soil aggregation is affected by many physical, chemical and biological factors. The most important of these are the balance proportions of exchangeable cations adsorbed onto soil colloid particles, the type of clay minerals, the nature of organic and inorganic colloids and climate.

The size distribution of soil aggregates, or alternatively, the size distribution of the pore spaces between them, determines soil permeability (how well roots and moisture can get in). Strongly structured soil is generally more permeable to water and is well aerated. In such "well-structured" soils these aggregates are stable to rain and the soil can be worked easily over a wide range of moisture levels.

Factors affecting Soil Structure

As mentioned earlier, there are many factors contributing to this, but the main ones are the relative proportions (or balance) of exchangeable cations in the soil, the percentage of organic matter present and biological activity. The effect of each of these factors is also modified by interactions between them. These interactions are defined by the Mikhail System, which focuses strongly on improving soil structure.

Since the exchangeable cations have the strongest influence on soil structure (and are also the most manageable), let's look at them in more detail.

Exchangeable Sodium

It is generally accepted that a high exchangeable Sodium percentage contributes to a weakly aggregated soil. Exchangeable Sodium measurements are frequently used as an index of the physical properties of a soil. The effect will differ depending on the part of the profile in which the Sodium is found. Soils with high exchangeable Sodium percentages in the surface layer are liable to have unfavorable physical characteristics if the layer is subjected to cultivation and wetting.

As the Sodium content of the soil increases, there is a tendency towards increased dispersion of the colloid fraction of the soil, destroying friability and filling pore spaces. Even in non-sodic soils, the degree of friability is affected by both the soluble and exchangeable Sodium. The properties of different sodic soils also vary depending on the flocculating effects that are produced by salinity. The presence of excess salts causes a depression of ionization, which results in a decrease in the potential hydration, swelling and dispersive tendency of the soil. It also increases colloid flocculation.

Soil exhibits adverse physical properties at exchangeable Sodium percentages in excess of 15%, however, in Australia 5% is considered a more appropriate as a defining level for sodic vs non-sodic soils. The reason for this apparent disparity is due to the interaction between the effects of exchangeable Sodium and Magnesium, along with the generally higher exchangeable Magnesium percentages found in Australian soils.

Exchangeable Magnesium

The effect of exchangeable Magnesium depends on the nature of the complementary cations in the exchange complex. For example, soils with low Ca:Mg ratios behaved similarly to soils with high exchangeable Sodium.

At high Ca:Mg ratios, higher exchangeable Sodium percentages can be tolerated. While soils with 60% Calcium and more than 25% Magnesium on the exchange complex have poor permeability, irrespective of the level of exchangeable Sodium. Similarly soils with Ca:Mg ratios below 2.0, or with exchangeable Magnesium percentages greater than 40% would have similar properties to sodic soils – even when the exchangeable Sodium percentage is low.

Exchangeable Calcium

It is generally accepted that exchangeable Calcium contributes to strong soil structure. It has been conclusively demonstrated that the poor structural qualities of sodic soils can be significantly improved if exchangeable Sodium is replaced by Calcium to flocculate soil colloids.

Both soluble and exchangeable Calcium appear to contribute to strongly aggregated soils.

Exchangeable Potassium

It is often believed that Potassium has a similar effect on soil aggregation to Sodium and that application of Potassium fertilizer will cause structural dispersion. However, the application of Potassium fertilizer generally does not cause any structural dispersion under field conditions and the effect of exchangeable Potassium on soil structure has been shown to depend in the nature of the predominant clay mineral present in the soil.

Exchangeable Hydrogen

It has been generally accepted that the flocculating effect of Calcium is the main contributing factor for stable aggregation. However, experimental observations indicate that the direct effect of Calcium on aggregation of acid soils is not as important as was originally thought.

Early experiments on the effect of exchangeable cations on the physical properties of soils indicated that Hydrogen and Calcium saturated soils were somewhat similar. In some cases, Hydrogen soils were more flocculated than Calcium soils. In another instance, the reverse was true.

Organic Matter

It is recognized that organic matter serves as an aggregating agent in soil. Organic colloids affect not only the degree of aggregation, but also aggregate size. The application of organic matter such as straw, crop residues and manures increases the bonding of mineral particles into water-stable aggregates.

Synthetic long-chain organic molecules can also act as aggregating agents. They appear to combine chemically with mineral particles and form bridges between them. Organic matter also reduces the slaking of dry aggregates in water.

The effects of organic matter are partly short-term and partly long-term in nature. Stabilization of mineral particles by microbial filaments and mucilaginous polysaccharides may occur within a few days, in the presence of an ample supply of decomposable organic material. Long-term, continued stabilization by these materials probably requires continuous production because they are not highly resistant to decay.

The major part of the organic matter in soils, however, has been accumulated over many years and has a long half-life. Formation and stabilization of soil by this portion of the organic matter is thus a long-term process.

Interaction between Organic Matter and Soil Particles

Organic matter is conducive to the formation of relatively large stable aggregates. If various soils are grouped according to their clay contents, it is seen that the effect of organic matter in improving soil function and productive capacity is more pronounced in those soils containing smaller amounts of clay. A very high correlation exists between organic matter and the formation of aggregates in soils containing less than 25% clay. However, in soils with more clay, organic matter appears to be effective in supporting the development of larger and more stable aggregates.