What Mushrooms Eat Matters More Than Most People Think

This article was entirely generated by Chat GPT but edited and reviewed by myself for accuracy

A customer once asked me a question that seemed simple on the surface but ended up changing how I thought about my entire operation. He was standing at my market table looking over the mushrooms and asked me whether we sprayed glyphosate on our crops. I answered immediately and honestly: no, we do not spray anything on the mushrooms. At the time, the conversation moved on, and I thought nothing of it.

It was not until later that day, after the market had ended and I was home thinking through the interaction, that the full implication of that question hit me. While it was true that I did not spray glyphosate directly on the mushrooms, I had been using Master’s Mix as my primary substrate. Master’s Mix contains soy hulls. And I was not using organic soy hulls. At that moment, I realized that although I was not applying herbicide to the mushrooms themselves, I was almost certainly introducing trace amounts of glyphosate indirectly through the very material the mushrooms were consuming.

Soy hulls are the outer skin of the soybean. They are exactly the part of the plant most exposed to herbicides during cultivation and harvest. It is similar to how nutritionists warn people about pesticide residues accumulating in the skins of apples or berries. If glyphosate is present anywhere on a soybean, it is most likely concentrated in the hull. That realization made my earlier answer feel technically true but philosophically incomplete. I was focused on what I was not doing at the surface level, while ignoring what I was feeding the organism itself.

Glyphosate itself is not the focus of this article, but it matters as context. It is the most widely used herbicide in the world. It is heavily applied to soy, corn, and grain crops. Its persistence in soil and water, and its potential links to endocrine disruption and chronic disease, have made it one of the most controversial agricultural chemicals of the modern era. Whether someone views the risks as severe or minimal, the deeper lesson remains unchanged: what mushrooms grow on becomes part of what people ultimately consume.

Mushrooms are often treated like plants in public imagination, but biologically they behave much more like animals. They do not photosynthesize. They digest externally. They secrete enzymes into their environment, break down complex materials, and absorb nutrients into their tissues. In that sense, mushrooms are far closer to cows, chickens, and pigs than they are to lettuce or tomatoes. And just like animals, their health, resilience, texture, and internal composition are downstream of what they eat.

In conventional livestock farming, everyone understands that feed quality matters. A grass-fed cow is not nutritionally equivalent to a corn-finished cow. A free-range chicken fed diversified grains is not metabolically equivalent to one raised on industrial feed pellets. Yet in mushroom farming, substrate is often treated as a purely mechanical input. Something that exists only to support growth and yield. This view ignores the fact that substrate is not just a structural medium. It is the primary nutritional source of the organism.

Modern mushroom farming has become increasingly focused on biological efficiency. The industry standard metric is how many pounds of mushrooms can be produced per pound of dry substrate. This makes economic sense. Growers operate on tight margins. Substrate is one of the largest costs. If another ingredient can dramatically boost yield, it quickly becomes dominant. Master’s Mix rose to prominence precisely because it does this so well. By combining hardwood sawdust with soy hulls, it delivers an aggressive nitrogen charge that drives rapid mycelial expansion and large fruiting bodies.

From a yield perspective, it is extremely effective. From a biological and nutritional perspective, it raises deeper questions. Soy hulls are not a natural food for most forest fungi. They are an industrial byproduct of large-scale soybean agriculture. Unless explicitly sourced as organic, they are deeply embedded in a chemical-intensive farming system. When yield becomes the dominant goal, the origin and chemical history of the substrate quickly becomes secondary.

This pattern is not unique to mushrooms. It is the central pattern of industrial agriculture as a whole. Corn is optimized for tonnage, not micronutrient density. Cattle are optimized for weight gain, not metabolic resilience. The same logic is now being applied to fungi. Substrates are selected for speed and mass rather than for long-term biological similarity to natural growth environments.

This does not mean that all high-yield substrates are inherently harmful. It means that the optimization process itself tends to push quality into the background unless there is a strong cultural or economic reason to protect it.

Different substrates serve different functions, and each comes with trade-offs. Straw is one of the oldest and most widely used substrates in mushroom farming. It is cheap, abundant, and works well for fast-fruiting species like oysters. Straw produces mushrooms quickly with minimal input cost. However, it is nutritionally thin and highly variable depending on where it is grown. It also carries elevated contamination risk unless properly pasteurized. Straw-grown mushrooms are efficient from a volume standpoint, but they do not necessarily represent the highest nutritional ceiling.

Master’s Mix occupies the industrial end of the spectrum. The soy hull component delivers rapid growth and heavy fruiting. For commercial farms aiming to maximize output per square foot, it is extremely attractive. It is also where the philosophical compromise becomes most visible. Soy hulls are not consumed by humans directly in most cases. They are primarily used as animal feed. Their safety threshold is set for livestock, not for nutraceutical food production. When used in mushroom cultivation, they bridge two different regulatory worlds without much public scrutiny.

In contrast, the hardwood sawdust and organic bran formula reflects a more traditional approach. Oak sawdust provides a lignin-rich, structurally stable carbon source. Bran contributes nitrogen, vitamins, and micronutrients in a controlled percentage. Growth is slower. Yields are lower. The mushrooms are often denser and take longer to mature. Biologically, this more closely resembles how many of these species evolved in nature. A reishi growing on an oak stump over many months is being metabolically shaped by slow carbon breakdown, not nitrogen acceleration.

If organic soy hulls are used, many of the chemical concerns disappear. But the optimization logic remains. Acceleration in biological systems almost always trades something for speed. In plants, it often trades flavor and micronutrient complexity for bulk. In animals, it often trades metabolic resilience for rapid weight gain. My intuition, supported by repeated agricultural history, is that fungi will not be immune to that same pattern.

The question of what “organic” actually means in mushroom farming adds another layer of complexity. For most cultivated species, between 50 and 90 percent of the diet is wood. Very few hardwood logs or sawdust sources are formally certified organic. At the same time, untreated hardwood sourced from unmanaged forests may be exposed to fewer synthetic chemicals than many certified agricultural crops. The industry operates in a gray space where practical cleanliness often substitutes for formal certification.

A grower might use organic bran but non-certified sawdust. Or untreated sawdust but conventional soy hulls. Consumers see the word “organic” on a label without realizing that substrate itself may be a mosaic of certified and uncertified inputs. Organic frameworks were designed for plants and animals. Fungi fall awkwardly between those regulatory categories.

For consumers, substrate choice rarely enters the purchasing decision. People ask whether mushrooms are local. They ask whether they are wild or cultivated. They ask whether they are fresh or dried. Almost no one asks what they were grown on. Yet substrate determines mineral uptake, contamination resistance, texture, flavor nuance, and potentially even medicinal compound expression. The substrate is the invisible half of every mushroom sold.

A mushroom grown on straw is not chemically identical to one grown on oak. A mushroom grown on soy hulls is not metabolically identical to one grown on wheat bran. These differences may be subtle on a plate, but they matter at the cellular level. In medicinal species especially, substrate likely influences secondary metabolite profiles in ways the industry has barely begun to quantify.

That customer who asked about glyphosate was not thinking about substrates when he asked the question. He was thinking about safety at the surface level. What that moment revealed, however, was how shallow most safety conversations in agriculture really are. Surface sprays are easy to see. Substrate chemistry is invisible. Yet it is the deeper and more consequential variable.

Switching from Master’s Mix to an oak-and-organic-bran substrate reduced my yields. It lengthened growth cycles. It complicated sourcing. It also gave me far greater confidence in what was entering the food system under my name. That trade-off is not attractive to large industrial farms chasing maximum output. It is, however, the same trade-off that has always distinguished commodity agriculture from craft agriculture.

Substrate choice is where mushroom farming quietly decides what kind of system it wants to be. A machine optimized for biological efficiency, or a food system optimized for nutritional integrity. These two goals are not always aligned. When speed and volume dominate, input quality almost always drifts downward. When biological alignment and chemical cleanliness dominate, yield predictably falls.

For consumers, the application is straightforward. Ask growers what their mushrooms are grown on. Ask whether soy hulls are organic. Ask whether the wood is treated. Ask whether additives are used to drive yield. These questions are not fringe or ideological. They go to the biological core of how food quality is shaped.

The mushroom industry is still young enough that these questions can influence its trajectory. It does not yet have to replicate the full industrial arc of corn, soy, and livestock production. It can still choose transparency over silence. But that choice depends on whether growers and consumers are willing to look beneath the fruiting body and into the substrate that built it.

That single question from a customer forced me to do exactly that. It did not lead to panic. It led to recalibration. Mushrooms are what they eat. And what they eat is something most people never see, even though it determines nearly everything that follows.