A Practical Guide To Planting With Compost in Fire Impacted Landscapes

Understanding compost, mulch, and soil amendments that rebuild healthy soils and fire-impacted urban landscapes

Preparing Healthy Soil Is Like Making Your Bed

Healthy soil begins with a simple, familiar analogy: selecting and making your bed. Just as a bed has layers that work together to create comfort and support, soil has layers that work together to create a thriving environment for plants.

The ground, the sand, silt, and clay beneath us, acts like the floor your bed frame rests on. It is the base layer, inorganic and structural, but not ready to receive a living body. The bed frame and mattress, the soil bed, represents the structure and housing of healthy soil: stable, supportive, and able to hold the living beings in place.

The sheets are the compost. This biologically active layer is filled with microbes that come into direct contact with plant roots just as the sheets come in contact with our skin. These organisms drive nutrient cycling, nutrient delivery, and nutrient uptake. Compost is where the living part of soil begins.

Finally, the comforter or blanket is the mulch layer, whether wood chips, chipped yard/greenwaste, or leaf litter that protect the newly constructed soil bed surface. Mulch keeps moisture in, shields roots from heat, reduces erosion, and creates the conditions that allow compost and soil biology to thrive. A rock or hardscape layer cannot serve this role. Rocks do not breathe, they are not alive, and do not support biological life. Plants need a living blanket their roots can move through, not a stone one.

The Language of Soil, Organic Matter and Amendments

Understanding soil care begins with understanding the terms we use to describe it.

Mulch is a broad category of organic materials placed on top of the ground.  It includes fresh wood chips, leaf litter, chipped tree trimmings, chipped yard waste or greenwaste, compost, and dried grass clippings. Mulch protects the underlying living soil, conserves water, moderates temperature, and feeds the soil biology as it decomposes or breaks down.

The ground is simply the surface we walk and build on, sand, silt, clay, and rock fragments. It is not alive (inorganic) and it supports both our structures and landscapes, but it is not soil until biology and organic matter enters the picture.

Dirt is a mix of sand, silt, and clay. It is inorganic and lifeless. Dirt becomes soil only when organic matter, water and microbial life are added.

Organic matter refers to any living, dead, or decaying organism containing carbon and hydrogen bonds. This includes plants, trees, animals, fungi, insects, soil microorganisms, and even people. Organic matter is the fuel for soil life.

A soil amendment is any material that improves soil quality, its nutrient content, structure, texture, biodiversity, and organic matter levels. Compost is one of the most effective soil amendments available because of its soil biodiversity over nutrient content. It is not a fertilizer.

NOTE:  Synthetic products like Miracle‑Gro, Terra‑Sorb, Stockosorb, and Profile 2SC give plants quick nutrients or water, but they don’t help soil rebuild its living community of microbes. A peer‑reviewed study in Arboriculture & Urban Forestry, tested these additives (excluding Miracle-Gro) in tree‑planting holes and found they did not benefit the trees or improve soil health; and sometimes even reduced oxygen in the root zone, which makes it harder for microbes to survive. Compost works completely differently as a soil amendment: it’s alive, and it houses the microbes that not only repair soil over time but work in concert with plants. Its a partnership.  Different composts support different parts of the soil food web: bacterial compost for fast nutrient cycling, fungal compost for long‑term structure, and biodiverse compost for full ecosystem recovery.  So, choosing the right compost matters. In short, synthetic additives or soil amendments give quick results, but compost brings soil back to life and optimizes plant growth for resiliency and longevity. (Day, S.D., Wiseman, P.E., Dickinson, S.B., & Harris, J.R. (2004). Effectiveness of Hydrogels in Amending Tree Planting Backfill. Arboriculture & Urban Forestry, 30(5), 301–310.)

Fertilizer, by contrast, is generally chemical-based, engineered in a laboratory, salt-based, and soluble. Fertilizers deliver measured nutrients but do not build soil biology or soil structure. Compost builds both.

Compost: What It Is and Why It Matters

Compost is decomposed organic matter transformed by microorganisms into a stable, biologically rich material. It is the most powerful soil amendment available because it adds organic matter, increases microbial diversity, improves soil structure, enhances water retention, supports nutrient cycling, and builds resilience in plants and landscapes.

Compost does not “feed” plants directly. Instead, it supplies the microbes that make up the soil food web, the interconnected community of microbes, fungi, and organisms.  The microbes in compost are responsible for making nutrients available to plants naturally. Healthy soil is not a chemical system; it is a biological one.

Compost as Mulch

Compost can also be used as mulch, and when applied correctly, it becomes one of the most effective ways to restore soil biology and protect the root zone.

When using compost as mulch, it should be spread beneath the drip line of the tree, where feeder roots actively take up water and nutrients. This placement ensures that the compost’s biology comes into direct contact with the area of greatest root activity.

Once the compost layer is in place, it should be capped with a second layer of dry mulch, a different material such as fresh wood chips, chipped greenwaste or yard waste, or leaf litter. This top layer protects the compost from drying out, shields soil from heat, reduces evaporation, and creates the moisture‑stable environment microbes need to thrive.

Used this way, compost acts like the sheet layer in the bed analogy, touching the soil and roots directly, while the dry mulch acts like the blanket, offering protection and comfort to the living soil beneath.

Types of Compost

Different composts offer different biological and nutrient profiles.

Greenwaste compost is made from chipped yard trimmings, leaves, and plant material. It is rich in bacteria and moderate in fungi. 

Food‑scrap compost comes from kitchen scraps, and is typically nutrient‑dense with excellent microbial diversity.

Manure‑based compost is made from livestock manure. It is bacterial dominated and high in nitrogen and must be fully aged to avoid burning plants.

Leaf‑litter compost is created from decomposed leaves. It is fungal‑rich and ideal for trees and perennial landscapes.

Blended or mixed-compost combines greenwaste, food scraps, manure, or leaf litter to create a balanced nutrient profile and strong biological diversity.

Why Aged or Mature Compost Matters

Compost must be fully mature before use. Mature compost has completed the thermophilic (hot) phase, no longer heats up when piled, and smells earthy rather than sour or ammonia‑like. It is safe for plant roots, supports fungal and microbial communities, and does not rob nitrogen from the soil.

Immature compost can burn roots, tie up nitrogen, introduce pathogens, attract pests, and damage seedlings. Mature compost is like clean sheets, ready to support healthy biological activity without causing stress or disease.

How to Use Compost in the Landscape

1 cup = 1 square foot

Compost can be used in several ways.  As a soil amendment, biologically diverse compost is recommended and just 1 cup is sufficient to amend 1 square foot of existing soil either by placing it directly on top of the ground or pre-mixed with the existing soil for transplanting and garden beds. 

For degraded, compacted and/or fire impacted soils, blanketing the ground with up to 1” is recommended without tilling or further disturbing the soil.  This improves soil structure, water infiltration, nutrient availability, and root development of newly germinated seeds.  Biologically diverse compost is powerful. You don’t need a thick layer to restore microbial life. If kept moist, microbes multiply rapidly once they’re in contact with the ground, so a thin layer is enough to "populate" the ground with bacteria, protozoa, nematodes and fungi.

For transplanting of trees, root contact with biologically diverse and mature compost is the goal.  It serves as the living interface layer between the tree roots and the minerals in the ground where the tree will be planted. The microbes in the compost will go to work and begin mining the minerals in the ground and deliver them to the tree in the amount and form it needs. 

NOTE: Peer‑reviewed research shows that soil microbes dissolve and transform minerals, making nutrients like phosphorus, iron, and sulfur available for plants. A 2025 review in Frontiers in Microbiology (Pradhan et al.) explains how these microbes break down mineral particles and release nutrients in forms that roots can absorb, highlighting their essential role in natural nutrient delivery. (Pradhan, N., Singh, S., Saxena, G. G., Pradhan, N., Koul, M., Kharkwal, A. C., & Sayyed, R. (2025). A review on microbe–mineral transformations and their impact on plant growth. Frontiers in Microbiology, 16, Article 1549022. https://doi.org/10.3389/fmicb.2025.1549022)

Creating a liquid compost, or compost extract, also referred to as a biological inoculant, made from high quality and biologically diverse compost can be used to coat the root ball as means of reintroducing these microbial partners to support the tree's growth.

As mulch, compost is applied as a thin top layer not only beneath the drip line of trees, shrubs, perennials, but also extending several feet beyond in anticipation of lateral roots and feeder roots growing outward to expand the trees reach for moisture and nutrients.  After spreading compost, it should be capped with a protective layer of dry mulch such as wood chips, chipped yard waste/greenwaste, or leaf litter. This two‑layer system keeps biology thriving and maintains moisture stability.

Mulch + Compost: The Perfect Partnership

Healthy soil needs both compost and mulch. Compost provides biology, nutrient cycling, and soil structure. Mulch provides protection, moisture retention, a food source for fungi and temperature regulation. Together, they create a living system that supports resilient plants, thriving landscapes, and long‑term soil health.

A New Way to See Garden Beauty

A garden carpeted with compost and mulch, instead of raked bare soil, is not messy, it is alive. Golden leaf litter, soft wood chips, and rich compost are signs of a functioning ecosystem. These materials are what plants evolved with, and they are what keep soil healthy today.

When we embrace compost and mulch, we embrace the natural processes that make landscapes and trees thrive.