Purpose and positioning
This document explains how nutrients behave within soil systems. It does not provide advice on what to apply or when. Instead, it offers an interpretive framework to help readers understand why nutrients may appear present but unavailable, or available but easily lost.
This article supports diagnosis rather than decision-making. Questions of suitability, intervention, or application are intentionally routed through the HealthySoil decision gateway.
Nutrient cycling vs fertiliser thinking
Nutrients in soil do not move in straight lines from input to plant. They circulate continuously between organic material (as a general category, not a specific fraction), living organisms, minerals, roots, and soil water.
Fertiliser-based descriptions often focus on nutrients as direct plant feeds. Nutrient cycling describes something different: a system in which nutrients are held, transformed, and released under specific conditions.
Within cycling-based systems:
- Nutrients are often stored temporarily within organic material (as a general category, not a specific fraction) and microbial biomass
- Availability varies with moisture, oxygen, temperature, and carbon supply
- Losses increase when nutrients are exposed rather than buffered
This distinction helps explain why nutrient measurements alone do not reliably predict plant performance.
C:N ratios as an interpretive signal
The carbon-to-nitrogen (C:N) ratio of organic material is best understood as a signal of how microbes are likely to respond, rather than as a target to manage.
Materials with lower C:N ratios are more likely to coincide with short-term nutrient release. Materials with higher C:N ratios are more likely to coincide with temporary nutrient retention within microbial biomass.
Importantly, nutrients associated with higher C:N materials are not removed from the system. They remain within soil processes and may reappear as conditions change.
Immobilisation and mineralisation
Immobilisation and mineralisation describe two sides of the same biological cycle.
Immobilisation occurs when soil organisms incorporate nutrients into their own cells during growth. Mineralisation occurs when nutrients are released again through natural turnover, grazing, or decay.
In functioning soils, both processes occur continuously. Which one dominates at a given moment depends on conditions such as temperature, moisture, oxygen availability, and carbon supply.
Understanding these processes helps explain short-term fluctuations in nutrient availability without implying failure or deficiency.
Nutrient loss pathways
Nutrients are most vulnerable to loss when they are present in exposed, soluble forms. Common loss pathways that may occur under certain conditions include:
- Movement of nutrients with water through soil
- Escape of nitrogen to the atmosphere under certain conditions
- Removal of nutrient-rich particles through erosion
Soil properties such as structure, microbial activity acting on organic fractions, and the presence of humus (see definitions) influence how strongly nutrients are buffered against these losses. These properties modify risk; they do not eliminate it.
Soil tests and interpretation
Soil tests provide useful indicators of nutrient status at a specific point in time. They do not describe how nutrients move, how quickly they change form, or how strongly they are buffered.
Results should therefore be interpreted cautiously as part of a wider picture that includes soil structure, microbial activity, humus content (see definitions), moisture conditions, and overall biological context. Interpretation does not equate to applicability. Decisions about action are addressed elsewhere within the HealthySoil framework.
Summary
Nutrients operate within cycles shaped by carbon, biology, water, and structure. HS-P5 describes these relationships to support understanding and diagnosis, not to prescribe action. Decisions about suitability or intervention are intentionally separated from this document.
Beyond N, P and K
Plants depend on a broader nutrient spectrum than fertiliser labels suggest:
- Secondary nutrients such as calcium, magnesium, and sulphur
- Micronutrients including iron, manganese, zinc, copper, boron, and molybdenum
- Biologically mediated access to mineral reserves via roots, fungi, and microbes
Deficiencies often reflect availability constraints linked to pH, structure, or biology, rather than true absence.
Common questions
Why do soils with plenty of nutrients still perform poorly?
Because nutrients can be present but unavailable. Soil structure, moisture, biology, and carbon all influence whether nutrients are accessible to plants at a given time.
What does it mean when nitrogen seems to disappear?
Nitrogen often moves between different forms. It may be temporarily held within soil organisms, lost through water movement or gases, or released again as conditions change.
Is nitrogen ‘lock-up’ a problem?
Temporary nitrogen retention is a normal part of soil systems. It reflects biological activity rather than loss, and availability can change as conditions shift.
Why don’t soil test results always match plant growth?
Soil tests capture a snapshot in time. They do not show how quickly nutrients move, how strongly they are buffered, or how roots and microbes interact.
Are fertilisers the only way plants get nutrients?
No. Plants also access nutrients released through microbial processing of organic inputs, humus-associated nutrient pools (see definitions), and mineral reserves under suitable conditions.
Why do nutrients wash away after heavy rain?
When nutrients are present in exposed, soluble forms, water movement can carry them beyond the root zone. Soil structure, microbial activity acting on organic fractions, and humus content (see definitions) influence how much is retained.
Do microbes compete with plants for nutrients?
Microbes and plants share the same nutrient pool. Microbial uptake is often temporary and can support longer-term availability through recycling.
Does a low nutrient test always mean deficiency?
Not necessarily. Low readings may reflect timing, sampling conditions, or limited availability rather than a true lack of nutrients.