Materials used with soil

Compost and other organic inputs are frequently added to land with the expectation that they will improve soil. This article explains how these materials behave once introduced to soil systems, how they interact with existing soil components, and the limits within which they influence soil function. It maintains a strict distinction between soil and materials applied to soil, and between short‑lived biological effects and longer‑term soil change.

What is meant by organic inputs

Organic inputs are materials introduced from outside the soil profile that interact with soil but are not themselves soil or topsoil. They do not replace mineral soil fractions and do not constitute soil structure by definition. Their influence arises only through interaction with existing soil minerals, water, air, and organisms.

Organic inputs considered here include:

Compost derived from plant‑based, food, or mixed organic feedstocks
Animal manures, whether composted or applied following storage
Surface mulches such as straw, wood chip, leaf mould, or crop residues
Digestate solids produced through anaerobic digestion

Although these materials are often grouped together in practice, they differ materially in composition, biological activity, nutrient form, and stability. Treating them as interchangeable obscures their mechanisms and exaggerates expected outcomes.

Compost as a processed organic input

Compost is organic material that has undergone managed biological decomposition prior to contact with soil. During composting, readily decomposable compounds are metabolised by microbial communities, heat is generated, and the most reactive fractions are reduced. This processing changes how the material behaves once added to soil.

Compared with fresh residues, compost is typically less phytotoxic and more predictable. However, compost is not a single material class. Its behaviour depends on feedstock composition, oxygen management, temperature history, moisture, and degree of stabilisation achieved during processing.

Compost remains an external input. Its presence alone does not confer soil structure, persistence, or resilience.

Compost types and maturity

Different composting pathways produce materials with distinct soil interactions:

On‑site and garden composts often contain diverse feedstocks and active microbial populations, but show high variability in stability and nutrient form.
Green‑waste composts are typically more uniform in particle size and bulk composition, though extended processing can reduce biological activity.
Food‑derived composts may contain higher concentrations of soluble nutrients and salts and can remain biologically reactive if insufficiently stabilised.

Maturity describes the extent to which easily degradable compounds have been processed prior to soil application. Less mature composts continue to respire rapidly after addition, consuming oxygen and releasing reduced nitrogen compounds or organic acids. More mature composts exhibit lower respiration rates and fewer acute soil disturbances, but still function primarily as transient biological inputs.

Manures, mulches, and digestate

Other organic inputs influence soil through different dominant pathways:

Manures introduce readily mineralisable nutrients alongside organic material. Their effects are strongly time‑dependent and influenced by moisture, temperature, and handling history.
Mulches primarily affect the soil surface environment, moderating temperature, evaporation, and erosion while decomposing gradually.
Digestate solids supply nutrients in soluble or weakly bound forms but contribute little stabilised organic material unless further biological or mineral interactions occur within the soil.

These materials influence nutrient availability and biological activity, but none can be assumed to deliver durable changes to soil structure or persistence in isolation.

Biochar as a material used with soil

Biochar refers to a broad class of carbon‑based materials produced by heating biomass in low‑oxygen conditions (pyrolysis). In soil systems, biochar behaves very differently from compost, manures, or plant residues. Rather than acting as a nutrient source or short‑term biological input, certain soil‑fit biochars may influence soil behaviour through physical and chemical interactions, depending on their production conditions and the soil context in which they are used. Other biochars may be unsuitable for soil use altogether.

For a more comprehensive review of biochar in soil, including suitability, boundaries, and definitions, follow the link to our dedicated Biochar FAQ page.

Short‑term biological stimulation and conditional soil change

Organic inputs influence soil systems through two distinct, but often conflated, pathways:

Short‑term biological stimulation — readily available compounds increase microbial activity and nutrient turnover, with most added material rapidly respired.
Conditional contribution to longer‑lived soil fractions — a smaller proportion of organic inputs may contribute indirectly to longer‑term soil change through microbial processing and subsequent association with soil minerals, depending on soil texture, mineralogy, moisture regime, and time horizon.

Most material introduced via composts, manures, or digestates is not retained within the soil system. Repeated inputs are therefore required to sustain biological effects, and expectations of permanence must remain constrained.

Risks, constraints, and quality limits

Organic inputs also impose constraints that arise from their composition and processing history:

Salinity from manures or digestate can impair plant and microbial function.
Physical contamination such as plastics, glass, or grit may persist within soil systems.
Biological instability from incompletely processed materials can reduce oxygen availability and disrupt root–microbe interactions.
Surviving weed seeds or pathogens reflect insufficient temperature control during processing.

These risks originate from the input materials themselves rather than from soil processes.

Organic inputs within soil systems

Organic inputs exert influence only through interaction with existing soil conditions. Outcomes depend on:

Soil mineral composition and aggregation state
Continuity of plant roots and biological activity
Moisture and aeration regime
Time horizon and frequency of input

Soil behaviour emerges from the interaction between materials, organisms, structure, and environment. Inputs do not act independently and do not override soil constraints.

Boundaries and limits

Organic inputs can stimulate biological activity, influence nutrient availability, and modify surface conditions. They cannot, by themselves:

Create soil where mineral structure is absent
Substitute for mineral soil fractions
Guarantee formation of persistent soil fractions
Override constraints imposed by texture, climate, or hydrology

Their effects are conditional, time‑limited, and mediated by soil properties rather than inherent to the materials alone.

How HealthySoil interprets this within the Soil Transition Model (STM)

Within the Soil Transition Model, organic inputs are treated as external drivers that influence soil condition indirectly through interaction with existing soil structure, biology, and mineral capacity. The model uses these interactions diagnostically rather than as guarantees of outcome.e Soil Transition Model (STM)

Within the Soil Transition Model, organic inputs are treated as external drivers that influence soil condition indirectly through interaction with existing soil structure, biology, and mineral capacity. The model uses these interactions diagnostically rather than as guarantees of outcome.