Soil amendments

What are soil amendments?

Soil amendments are organic, or inorganic materials added to soil with the aim of altering chemical, physical or biological soil properties. The addition of amendments to soil generally has the goal of overcoming a soil constraint or improving soil function. Amendments can aim to improve specific soil properties such as water infiltration, structure, and nutrient retention; or target improved function in soil.

Incorporating soil amendments:

• Surface (mulch; can include plant residues)
• Broadcast and/or mixed into topsoil
• Targeted delivery and potential mixing into subsoil

Types of soil amendments

Mulch

Any organic, or inorganic material that physically protects and shades the surface of soil can act as a mulch. The most likely source of mulch in Australian farming systems is the stubble and surface residues of past crops and pastures, though there is interest in gravel also acting as a mulch.

Surface mulches can have multiple benefits:

• Minimising soil water evaporation
• Decreasing movement of salt to soil surface
• Decreasing the risk of hardsetting and crusting
• Acts as a weed suppressant

Factors to consider when choosing a soil amendment

Soil amendments are not perfect solutions, and different amendments have different considerations for best practice soil management. The most important factor is determining what soil constraints are most limiting and targeting solutions to address these, with a combination of management strategies that can be implemented both in a practical and economic sense.

Gypsum

Gypsum is a common soil amendment applied to sodic (dispersive) soil, and also to alkaline soil.

Incorporating gypsum can:

• Decrease the sodium exchange percentage
• Reduce dispersion
• Increase stable soil structure

How does gypsum impact soil?

Gypsum causes an ‘electrolyte effect’. As it dissolves, the electrolyte concentration in soil increases. This causes clay flocculation, and improved soil structure.

As well as the electrolyte effect, gypsum also decreases soil dispersion. Sodium ions in soil are replaced with calcium ions in the diffuse double layer of clay platelets. The replacement of sodium ions with calcium ions is a long-term process, resulting in sustained improvement in structural stability only with repeated applications.

Not all soil is gypsum responsive. Responses are affected by pH, mineralogy, electrical conductivity, co-cations, organic carbon, and the depth of the dispersive layer in the soil profile.

Gypsum quality

The quality of gypsum is primarily determined by its purity.

Purity is determined by the sulfur content as a percentage of the gypsum sample.

Most gypsum mines in southern Western Australia contain 14-18% sulfur, equating to gypsum purity of 75-97%.

Claying

Claying involves incorporating clay-rich subsoil into water repellent topsoil to try and overcome the water repellency soil constraint.

Incorporating clay can:

• Reduce soil water repellence, including long-term impact (30 years or more)
• Increase soil water holding capacity
• Reduce wind erosion risk
• Reduce frost risk
• Help increase soil organic carbon
• Improve soil structure
• Improve plant establishment

Claying is best-suited to medium to high rainfall environments, in areas with higher yield production.

Claying can benefit sandy textured topsoils with less than 5% clay content, where water repellence is a constraint limiting crop and pasture growth.

Effective claying can increase yields by 20-100% and in some cases even more. These yield benefits can last for more than 30 years. Production increases are more likely when:

• Sands are deep (>60 cm), but have reasonable plant-available water holding capacity
• Soils are highly water repellent
• Cation exchange capacity is low (<3)
• Potassium is marginal or deficient (<50 ppm)

How does claying impact soil?

Claying increases the soil particle surface area (clay soil particles are smaller than sandy soil particles).

Claying also coats organic matter, which is hydrophobic, helping to repel less water.

Clay application

Clay is often sourced on-farm.

Clay spreading

Spreading is often done using carry graders, purpose-built broadcast spreaders, or heavy-duty multi-spreaders.

The typical spread of clay is at rates of 100-450 tonnes per hectare. This increase the topsoil clay content to 3-6%, which is required to overcome water repellence.

Once clay is spread, a range of different deep cultivation approaches are used to incorporate the clay into the top 100-200 mm of the soil profile.

Increasingly, rotary spreaders have also become a common method of incorporating clay following spreading.

Clay delving

Clay delving uses deep working tines (600 mm or deeper) to penetrate the clay in the subsurface soil layer of texture contrast or duplex soil, while at the same time breaking up compacted or cemented layers above. Following delving, soil is typically worked with offset disks or a rotary spader to mix and incorporate the clay.

Delving of clay is generally cheaper than clay spreading. Clay delving also has the added benefit of distributing clay-rich subsoil through the soil profile, rather than mixing it from the surface.

Risks of claying

• Using the wrong clay
• Using clay with deficiencies or toxicities
• Poor incorporation or too much clay
• Increased subsoil compaction
• Claying when other constraints limit benefits

Compost

Compost is one of the the most used organic soil amendments.

What is compost?

A composted product is defined as having undergone a controlled aerobic and higher temperature biological transformation to produce a stable and mature product.

Composts are made from recycled nutrients, such as:

• Household food and garden waste
• Farm waste
• Sewage sludge

The nutrient content of compost can be highly variable and depends on the source material(s) and compost processing. The inconsistency of nutrient levels throughout a compost sample can make it difficult to reliably predict the effect on plant production. Because of this, composts are often supplemented with other applied nutrients to achieve a desired balance.

What can the effects of compost be?

Composts can influence many soil properties and processes, depending upon the context of the environment they are being added to, and the properties of the compost itself.

These properties include:

• pH
• Nutrient availability
• Electrical conductivity
• Sodicity, disersibility, and/or slaking
• Water repellence
• Biological activity

Composting organic wastes produces a more stable product but organic matter, nitrogen and other nutrient losses can occur during composting. This can be avoided by adding material with a high carbon to nitrogen ratio (e.g. straw, sawdust) which will help stabilise nutrients in the compost, but will also influence the amount and timing of nutrient release. Composting can be useful for disposal of nutrient-rich water wastes such as piggery effluent, which increases the efficiency of the composting process and captures nutrients

Compost application

Compost application often takes place before planting.

Compost can be applied at large paddock scales, or to smaller targeted areas.

Risk considerations for compost

• Human or plant pathogens – composts need to be correctly processed and treated before application
• Impurities – poor compost manufacturing can lead to compost products with highly variable nutrient levels, or high levels of contaminants
• Inappropriate context – consider longevity and soil properties before applying compost to ensure effects align with management aims

Manure

The simplest approach is to apply manure as a phosphorus fertiliser and add additional nitrogen to meet demand. One tonne of feedlot and poultry manure contains approximately 7 kg of phosphorus (equivalent to 35 kg MAP), 24 kg of nitrogen (equivalent of 35 kg MAP + 43 kg urea) and 25 kg of potassium. 

Where animal manures are used frequently at very high rates there is some potential to increase soil salt levels, but nutrient build up in this case would be more of a concern.

Application

Even application can be achieved by aging manure and screening for size distribution (larger lumps travel further from a spreader). Check swath width and ensure sufficiently even distribution to maintain adequate nutrition. Larger swath widths (eg. 9m) will reduce potential compaction from smaller width wheel tracks (eg. 3m).

Fresh manures should be incorporated into soil within an hour of distribution to minimise nutrient loss. This reduces the potential for nutrient runoff into waterways and ammonia volatilisation.

Leaving manure on the soil surface will reduce nutrient availability, so manage different soils appropriately. For example, on a cracking clay distribute prior to rainfall to allow manure to wash down cracks, or incorporate using cultivation to distribute nutrients.

Timing

Timing of application must be considered as mineralisation from an organic to an inorganic form may take several weeks, during which time plant-available nutrients may be limited. Alternatively, early application on soils prone to leaching may be risky if warm, moist conditions result in rapid mineralisation and subsequent loss of nitrogen from the system.

Time the cultivation to match other management strategies such as cotton pest pupae busting to reduce tillage operations where possible. Even in zero tillage programs, cultivation to incorporate once every 5-10 years may be desirable to mix nutrients and address other soil constraints.

Biosolids

Biosolids contain much higher nitrogen and phosphorus concentrations and are mineralised more rapidly. Slower rates of nutrient release are attainable if straw or sawdust is composted with the biosolid product.

Sewage sludge has in the past contained potentially harmful contaminants such as heavy metals and pesticide residues. Some sewage sludges (biosolids) may contain a significant amount of cadmium or other heavy metal impurities. However, phosphorus, nitrogen, copper and zinc concentrations are generally the rate limiting factors in the application of biosolids to soils for beneficial use. Control strategies governing the disposal of hazardous wastes and the reduction in use of residual pesticides have decreased the risk of contaminants.