Composting toilet systems conserve water and retain nutrients and organic matter in a usable loop. Their cured output—the Conditioner—acts as a soil conditioner, supporting soil microbiology, buffering pH, improving water-holding capacity, increasing soil organic carbon, and reducing nutrient losses and downstream pollution. This article outlines the core environmental mechanisms behind these benefits and explains how composting toilets operate to deliver them.

Deep Reads

As founder of WCTNZ® and a thought leader in sustainable sanitation, I, Dylan Timney, have been advocating for composting toilets for years. These systems represent a unique and valuable waste management technology that reintroduces human excreta into the environment safely as a resource, yielding munificent environmental benefits. Composting toilets offer a practical, eco-friendly approach by conserving water, recycling nutrients, and reducing pollution. They provide ecological advantages while transforming what we consider "waste" into a valuable resource. This knowledge base explores how composting toilets work, the nature of their output—often called humanure—and why that output acts as a soil conditioner rather than just a fertiliser. We'll detail how the conditioner enriches soil life, balances pH, sequesters carbon, and restores nutrients, ultimately closing the nutrient loop. Additionally, we'll highlight the cultural shifts and social empowerment that composting toilets can foster around the world.

What Are Composting Toilets and How Do They Work?

A composting toilet is a type of dry toilet that treats human excreta through biological decomposition. In simple terms, it is a collection and treatment system that turns our bodily outputs into safe, stable humus—organic compost—via controlled aerobic composting. A composting toilet is a recycling device that contains and transforms organic material on-site. How it works: Waste is deposited into a compost chamber or receptacle, often along with a bulking agent such as carbon-rich materials like sawdust, wood shavings, straw, or coconut coir. This mix absorbs liquids, provides aeration, and creates the right carbon-to-nitrogen ratio for composting. Beneficial bacteria and fungi then decompose the material aerobically, generating a modest amount of heat in the process—though high heat is not a requirement for effective composting. Most true composting systems handle both urine and fecal matter together in an all-in-one setup. Many composting toilet designs include vent pipes and fans to ensure airflow for oxygen supply and to vent away moisture and odours. Some advanced units have multiple chambers or rotating drums to separate fresh deposits from curing compost, maintaining optimal conditions for decomposition. Over time—typically several months to over a year—the microbes break down the waste, reducing pathogens through sustained decomposition. The end result is a sanitised, earthy-smelling compost that resembles dark soil and is rich in organic matter. This material can then cure further until it is safe and ready to use as a soil amendment. 

What they produce: Composting toilets produce a nutrient-rich compost often termed "humanure." When done properly, this compost is a dry, crumbly humus that is stable, pathogen-free, and odour-free. It contains decomposed feces, urine, and any bulking materials added. Importantly, this product is not raw excreta—it has been fully processed by microbes. Proper composting eliminates harmful pathogens and foul odours, yielding a material that can be safely handled and utilised. In fact, the conditioner (compost) from humanure looks and smells like rich soil. It can be used to condition soil for non-food plants or fruit trees, with regulations often guiding use on edible crops for safety. Many communities safely use the conditioner on gardens, landscapes, or to reforest and rebuild soils without chemical inputs. 

To put it simply, a composting toilet allows us to recover the nutrients and organic matter in our bodily outputs and return them to the soil. By mixing excreta with dry carbon materials and fostering aerobic decay, composting toilets mimic nature’s recycling system, turning our personal by-products into a beneficial resource. 

Disclaimer on Safe Use: Safe burial of the conditioner is an important step in ensuring human safety. It creates a natural behavioural barrier to reduce risk, allowing time for any remaining pathogens to die off naturally before broader application.

Humanure: Turning Waste Into a Soil Conditioner, Not a Fertiliser

As I've taught for years as a thought leader in this field, when we talk about the output from composting toilets—humanure—it's essential to understand its role in soil. Rather than viewing it as a powerful chemical fertiliser, it is better seen as a soil conditioner or amendment. A conventional fertiliser is primarily valued for its immediate nutrient content to feed plants. The conditioner (humanure compost), by contrast, improves the overall health and structure of soil, providing steady nourishment alongside other ecological benefits.

Why call it a soil conditioner?

For one, the conditioner is relatively modest in N-P-K (nitrogen-phosphorus-potassium) concentration compared to chemical fertilisers. The N-P-K of well-finished compost might be on the order of only a few percent, making it a slow-release source of nutrients. What it lacks in concentrated N-P-K, it makes up for in organic matter and biological richness. The conditioner introduces humus, microscopic life, and trace minerals to the soil. These attributes enhance soil quality in ways fertilisers do not—by improving soil texture, moisture retention, and microbial activity. As gardening experts note, compost is more of a "soil amendment" that yields multiple benefits: it aerates compacted soil, boosts water retention, and introduces beneficial microorganisms that aid plant growth and disease resistance. 

Slow, Steady Nourishment: The nutrients in the conditioner are released slowly as soil organisms continue to break down organic matter. This means plants receive a more balanced diet over time, and there is less risk of nutrient leaching compared to soluble fertilisers. For instance, nitrogen in the conditioner is largely in organic forms that become available gradually; this prevents sudden nutrient flushes and potential runoff. Studies have found that soils amended with the conditioner tend to retain nutrients better, reducing contamination of waterways by nitrates or phosphates. 

Improving, Not Just Feeding, the Soil: Perhaps the greatest distinction is that the conditioner builds fertile topsoil rather than bypassing it. It increases soil organic carbon, fosters a crumbly soil structure, and boosts the soil food web. By adding the conditioner year after year, depleted soils can be restored to productivity much faster than natural processes alone would allow. This is why I emphasise conditioning—the conditioner conditions the soil to be healthy, which in turn supports robust plant growth. The conditioner takes the opposite approach to chemical fertilisers: it regenerates the soil ecosystem first, and the soil will take care of the plants. 

Another reason to treat humanure as a soil conditioner is safety and public perception. Humanure is the product of careful composting; calling it a fertiliser outright might raise unwarranted fears. In practice, well-finished conditioner is used similarly to how one would use compost from food scraps or leaves—worked into garden beds, spread around fruit trees, or used to enrich landscaping soil. Its greatest value is in the total improvement of soil health rather than a spike in fertility. In short, the conditioner transforms outputs into an organic soil-builder, helping soils hold water, nutrients, and life. This makes it a cornerstone of sustainable soil management, not just a source of plant food. 

Ecological Benefits of Humanure Compost

Composting toilets deliver a suite of ecological benefits through the material they produce. Below, we examine how the conditioner (humanure compost) supports soil microbiology, helps balance soil pH, enhances the soil’s carbon sequestration ability, restores nutrients to depleted lands, and ultimately closes the nutrient cycle between our bodies and the earth.

Boosting Soil Life and Microbiological Activity

What is soil microbiological activity?

It refers to the dynamic processes carried out by microorganisms in the soil, such as bacteria, fungi, protozoa, and nematodes, which are essential for nutrient cycling, organic matter decomposition, and plant health. Healthy soil teems with these organisms, forming a complex food web that sustains ecosystem functions. The conditioner is teeming with these beneficial microbes and the organic matter that sustains them. By adding the conditioner to soil, we effectively inoculate the soil with beneficial organisms and provide food for the native soil biota. The result is an increase in microbial activity in the soil, which is highly desirable for ecological balance. Research shows that compost additions, including those from humanure, can significantly enhance microbial diversity and activity, leading to improved soil fertility and plant growth. 

These microbes decompose organic matter into humus, fix nitrogen, synthesise natural plant hormones, and outcompete many plant pathogens in the soil. In practical terms, soils enriched with the conditioner tend to show higher microbial biomass and respiration, indicating a vibrant soil food web. The conditioner introduces not just microbes, but also larger decomposers like fungi and even earthworms (often, worm activity increases once the conditioner is applied). All these organisms contribute to creating a porous, aerated soil structure. 

Composting supports abundant microbial populations that add life to the soil. When that humus is returned to land, those microbes continue their work in the soil ecosystem. Additionally, any remaining dormant microbes or spores in cured conditioner can colonise the soil. This microbial inoculation can help suppress plant diseases and pests naturally, as a diverse microbiota keeps harmful organisms in check. 

Crucially, the conditioner feeds soil life. It contains carbon compounds that serve as energy sources for microbes, and it buffers moisture and temperature to create a hospitable environment for them. Soil organisms flourish in the presence of the conditioner’s organic matter, breaking it down further and in the process releasing nutrients in plant-available forms. This synergy is why agronomists often say we must feed the soil, not just the plant. The conditioner is a key soil food, and by nourishing the microbial workforce, it indirectly nourishes plants better over the long term. In summary, the conditioner greatly enhances soil microbiological activity, fostering a living soil that is resilient, fertile, and self-regulating. 

Balancing Soil pH and Buffering Extremes: How does the compost balance soil pH?

Compost acts as a natural buffer due to its organic matter and humic substances, which can neutralise extremes by releasing ions that adjust acidity or alkalinity over time. Mature compost typically has a near-neutral pH and helps stabilise soil pH fluctuations. Soil pH—the measure of acidity or alkalinity—is a key factor in nutrient availability for plants. If the soil is too acidic or too alkaline, plants struggle to absorb certain nutrients, and soil life can be inhibited. The conditioner has a buffering effect on soil pH, helping to neutralise overly acidic or basic conditions. Mature composts, including humanure compost, often end up near neutral pH (around 7) regardless of the initial inputs, because the composting process tends to produce humic substances that are weakly acidic and alkaline components that even out over time. 

When such a conditioner is added to soil, it can help moderate pH swings, pushing the soil pH toward the middle range ideal for most plants. The conditioner helps balance the soil pH, preventing extremes that could harm plant growth. For example, in acidic soils (low pH), the conditioner’s humus and ash content can raise the pH by providing a gentle liming effect (it contains calcium, magnesium, and potassium in organic forms). In alkaline soils (high pH), the organic acids in the conditioner can slightly acidify the soil microsites as they decompose, bringing pH down a bit. More importantly, the buffering capacity of the conditioner means the soil resists pH changes—an abundance of organic matter can neutralise acids or bases, protecting plant roots from sudden shifts. 

This buffering is one reason gardeners often apply the conditioner to gardens annually: it gradually coaxes soil pH toward a healthy range and maintains it there. Beyond chemistry, a balanced pH fostered by the conditioner correlates with optimal conditions for nutrient availability. Most nutrients are bioavailable in the soil pH range of ~6 to 7.5. By keeping soils in this sweet spot, the conditioner indirectly ensures that plants can access the full spectrum of nutrients already present in soil or added via fertilisers. In contrast, in un-amended soils that are too acidic, elements like phosphorus, calcium, or magnesium may get locked up; in overly alkaline soils, iron and manganese may become insoluble. Conditioner addition mitigates these issues, often reducing the need for separate pH amendments (like lime or sulphur). 

Furthermore, the conditioner’s pH-balancing act ties back to microbiology—many soil microbes prefer near-neutral pH, so by buffering the soil, the conditioner also supports a more diverse and active microbial community. The bottom line is that the conditioner acts as a natural pH stabiliser, creating a soil environment where plants and microorganisms can thrive together without the stress of pH extremes. 

Sequestering Carbon in the Soil

Did you know soil has CO₂ sequestration potential? Soils can act as carbon sinks, storing atmospheric CO₂ in stable organic forms, and compost enhances this by providing organic matter that microbes convert into long-lasting humus. One of the benefits of using the conditioner on soil is carbon sequestration. Soils are a major reservoir of carbon, containing more carbon globally than the atmosphere and biomass combined. When we add stable organic matter like the conditioner to soils, we are effectively transferring CO₂ from the atmosphere into the ground in the form of soil carbon. The conditioner contributes to this by adding a rich mix of partially decomposed organic compounds that can further stabilise into long-term humus. Research has shown that applying the conditioner can increase soil organic carbon content over time, even in agricultural settings. In a long-term field study, adding compost increased soil carbon significantly—far exceeding global targets for soil carbon gain. The key is that the conditioner provides not just raw carbon, but also the nutrients (like nitrogen and phosphorus) that soil microbes need to incorporate that carbon into their biomass and soil humus. 

The conditioner is particularly valuable because it closes a loop—instead of carbon in our excreta ending up oxidised in treatment or landfills (where it might emit methane or CO₂), it gets reintegrated into soil organic matter. This not only reduces greenhouse gas emissions from waste but also actively stores carbon in soils. Even semi-arid and degraded lands show promise: conditioner additions have been observed to build up carbon in deeper soil layers where it can remain for decades. If scaled up, widespread use of the conditioner could make a meaningful contribution to climate change mitigation by turning soils into carbon sinks. 

Moreover, carbon-rich soils have multiple co-benefits: they are more fertile, hold more water, and erode less. So the carbon sequestration aspect of the conditioner is intertwined with climate resilience. For example, adding the conditioner to rangelands not only sequesters carbon but also improves soil moisture and vegetation cover, creating a positive feedback for the ecosystem. In summary, by composting our outputs and returning them to the earth, we harness a natural carbon cycle, offsetting some CO₂ emissions while rejuvenating soils. Each handful of the conditioner applied is a small step toward drawing down atmospheric carbon and storing it in the life-giving bank of the soil. 

Restoring Nutrients and Soil Fertility: Agricultural Erosion

Intensive agriculture and erosion have depleted soils in many regions, stripping away nutrients and organic matter. The conditioner is a tool for nutrient restoration in such depleted soils. It contains all the essential plant nutrients—not just the big three (N, P, K), but also secondary nutrients (calcium, magnesium, sulfur) and a suite of micronutrients (like boron, zinc, iron, etc.) that are often missing in synthetic fertilisers. These nutrients in the conditioner come in organic or mineral forms that become available slowly, which means they tend to stay in the soil and get taken up by plants as needed. Over time, regular additions of the conditioner can remineralise impoverished soils, correcting micronutrient deficiencies and bringing soil nutrient levels back to balance. 

Retaining the Nutrients

Beyond adding nutrients, the conditioner improves nutrient cycling efficiency. In a living soil with ample organic matter, nutrients are continually exchanged between organic and inorganic states by soil organisms. The conditioner increases the cation-exchange capacity (CEC) of soil—essentially the soil’s ability to hold onto nutrient cations like potassium, calcium, and ammonium rather than letting them leach away. Humus particles in the conditioner have many charged sites that bind nutrients and release them slowly to plant roots. This means soils amended with the conditioner are less prone to nutrient losses during rain or irrigation. For farmers and gardeners, that translates into more of the nutrients staying where the crops can use them, and less fertiliser needed overall. 

Water Retention

The conditioner also helps restore the soil’s physical fertility. By improving structure (aggregation) and preventing compaction, the conditioner creates a soil environment where roots can penetrate deeply and access nutrients and water more freely. It also helps soils retain water, which further means nutrients dissolved in soil moisture are more consistently available to plants instead of washing out. Compost can hold approximately 9 times its weight in water, enhancing drought resilience. 

Soil Fertility for the Community

Soils we deplete in a matter of decades could take nature centuries to replenish without intervention. Composting our organic discards and returning them to soil allows us to accelerate the rebuilding of topsoil that would otherwise be lost. This is the essence of sustainable fertility. A vivid example is provided by the people of the Hunza region in South Asia, who historically composted all human, animal, and plant refuse and returned it to their terraced fields; observers noted their soils remained fertile and their health robust, compared to regions that did not recycle organic matter. In modern contexts, the conditioner can play a similar role in regenerating soil fertility, especially in communities where chemical fertilisers are unaffordable or in ecological farming systems aiming to eliminate synthetic inputs. By restoring nutrients naturally, the conditioner supports long-term productivity of the land while avoiding the pollution associated with overuse of chemical fertilisers. 

Closing the Loop on the Nutrient Cycle

Arguably, the most profound benefit of composting toilets is that they close the loop on the human nutrient cycle. In nature, nutrients cycle continuously: what one organism excretes becomes food for others, and eventually those nutrients return to the soil to be taken up by plants again. Modern linear systems of sanitation broke this cycle—we remove nutrients, essentially wasting what should be a resource. This contributes to pollution but also means farmers must manufacture new fertilisers to replenish soil nutrients that could have come from recycled outputs.

Composting humanure is a direct answer to this problem: it reframes outputs as resources and keeps the nutrient cycle local and sustainable. Urine and feces are rich in the same nutrients (nitrogen, phosphorus, potassium, etc.) that crops require. By composting them, we safely convert those nutrients into forms that plants can reuse, thus completing the loop from food → human → conditioner → soil → food again. This concept is at the heart of what’s called “ecological sanitation.” Ecological sanitation aims to return nutrients to soil instead of contaminating water, thereby closing the gap between sanitation and agriculture. 

What is Your Relationship with Your Waste

Closing the nutrient loop has multiple ripple effects. Environmentally, it reduces the dependency on chemical fertiliser production (a process which is energy-intensive and often relies on finite resources like phosphate rock). It also prevents water pollution—nutrients from human outputs, when properly composted, do not end up causing algal blooms in rivers or dead zones in oceans. Culturally, closing the loop alters our relationship with our own outputs. Instead of something to banish immediately, it becomes something to transform and give back. This shift can foster a sense of responsibility and connection to the land: we realise that what we take from the soil in the form of food, we can and should return to maintain the balance. 

Closing the loop via composting toilets means we stop the cycle of discarding valuable nutrients. Instead of treating human outputs as garbage, we sanitise and recycle them, just as nature has always done. In a world facing resource limits, this circular approach is environmentally sound and necessary. Composting toilets are the practical tool to achieve it, turning a household practice into an act of ecological regeneration. 

Copyright © 2025 Waterless Composting Toilets NZ Limited (WCTNZ®). All rights reserved.

This content has been reviewed and approved by Dylan Timney, Managing Director of WCTNZ®, who brings over 17 years of composting toilet expertise and 16 years of experience in building and eco-construction in New Zealand.

Last reviewed: August 29, 2025