Plants are mostly water, but the “dry matter” that builds stems, leaves and roots is made largely of carbon-based compounds. For decades, research and gardening advice have leaned heavily on nitrogen. Nitrogen matters, but soil health is often defined by its carbon content, especially the organic matter that fuels microbes, holds water and helps plants access nutrients.

Organic matter is where soil stores carbon and energy. In many Western soils, organic matter averages about 1 to 1.5%. A commonly cited target for a strong soil profile is closer to 5%.

That difference has real consequences in a dry climate. Research from the University of Florida has shown that for every 1% increase in organic matter, soil’s water-holding capacity can increase five-fold. More water stored in the soil means stronger plants and less irrigation.

Organic matter also supports nutrient availability. When soil organic matter reaches roughly 3 to 5%, many plant nutrients become more available. Colorado State University has also found that when growers use diverse cover crops and minimize soil disturbance, they can build measurable nitrogen in the soil. Add managed grazing animals, and that benefit can increase even more. 

Building organic matter has economic value because it can reduce water and fertilizer needs while improving plant growth and the nutritional quality of what you harvest.

THREE KINDS OF ORGANIC MATTER

Organic matter comes in three broad forms: the living, the dead and the very dead. A healthy garden uses them together.

1. Living organic matter is most often living plants in the landscape. As plants photosynthesize, they release sugary compounds through their roots. These “root exudates” help feed soil microbes and can also help soil particles stick together. That improves pore space for oxygen exchange and creates pathways for water to infiltrate.

Plants also build organic matter as they grow. Roots naturally shed older tissue while pushing into new soil in search of water and nutrients. Those old roots break down and become part of the soil’s organic matter.

The other living component is microbes: bacteria, protozoa and fungi. In a healthy garden, microbes gather around plant roots as long as chemicals and aggressive tillage aren’t wiping them out. As they feed on exudates and decaying material, they produce substances that help bind soil, reducing erosion while lowering the pH around the root zone of the plants.  

It’s not too late to incorporate living organic matter into the garden this year.

• Option 1: Plant a cover crop mix now, such as annual wheat, rye or barley paired with peas, beans and mustards. Let it germinate and grow to establish roots through the soil profile. In late April, terminate the cover crop by crimping, hoeing it down or spraying vinegar so the tops die back as mulch. Let it rest two to three weeks, then plant your desired crops into that spot.

• Option 2: Plant vining peas or beans between your main crops in May. Their vines shade the soil, and their roots help build organic matter as plants grow and later die back. If you’re worried about vines climbing your peppers or tomatoes, plant annual legumes on the edges of beds so they can trail outward. You still get the soil-building benefits and you may get a bonus harvest of young pods.

2. Dead organic matter includes the remains of the organisms that live in our landscapes: insects, microbes and small critters. Insects contain chitin in their exoskeletons, which is rich in nitrogen and helps plants to grow new leaves.

For home gardens, dead organic matter also includes fallen leaves, spent flowers, feathers and seed husks left over from germination. Naturally derived soil amendments such as kelp meal, greensand and blood meal fit here, too. These materials add nutrients and trace minerals that can support growth, plant vigor and overall resilience.

3. Very dead organic matter is the finished product of decomposition. Compost and vermicompost look nothing like the leaves and food scraps they started from because many compounds have already been broken down into forms plants can use.

Compost is also packed with microbes, which makes it a useful inoculant for garden beds. Compost made on-site tends to carry more living biology. Store-bought compost can still provide nutrients, but if it’s old and dry, it may have fewer viable microbes.

Aged manure also falls into this category. Given time, manure loses excess urine and reduces pathogen concerns that can come with fresh material. If you’re hauling in manure, pay attention to source and quality. Manure can carry weed seeds, salts and residues depending on the animal’s diet and management. Many gardeners avoid cow manure for those reasons. Horse manure can be good if animals are fed clean hay, while sheep and goat manure is small and breaks down quickly. Rabbit and llama manure are often considered lower-risk in terms of pathogens, which can be a consideration for gardeners with compromised immune systems.