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Composting 101, Part II
October 20, 2011

A steaming compost pile at St. Michael’s College in Colchester, VT. Proof that composting is not merely a gardener or farmer activity. This college and others like it are making dedicated efforts to divert cafeteria food wastes away from the landfills, and instead channel those wastes into productive new substances. Check out St. Michael’s composting page for more information.


Our last blog entry introduced the process of composting, along with some preliminary information about composting ingredients, the C:N ratio, particle size, moisture, process time/temperature, and common pile shapes/sizes. That could all be considered the first half of composting. But what about when the finished product is put onto the land? How does it work as a soil amendment? Can it be called a fertilizer? What other benefits does it provide? How is it different from inorganic fertilizers? These application questions will be today’s topic.

So, let’s say you have created a three-bin format composting setup. You have taken reasonable steps to ensure the desired 30:1 carbon:nitrogen ratio, appropriate moisture content and particle size for your feedstock, you’ve monitored the temperatures over time, and you now have a bin full of finished compost ready for application. What are the current recommendations for how to use, and how NOT to use, that black gold?

As hinted at above, soil is much more than a three dimensional matrix to support and feed roots. It is one part the shattered pieces of whatever parent bedrock lay underneath. It is one part the shattered pieces of whatever soils, stones or large rocks have washed down from other locations, or been deposited there by previous flooding and/or glacial activity. It is a continuously changing reservoir of water, which ebbs and flows according to the seasons, the underlying water table, the movement of soil and surface waters in the area, and precipitation. And soil is, at the end of the day, a repository of all the life that has gone before - the plants and animals which have lived and died in that location over time. A graveyard, if you will, for all those previous generations. And here is where our population of microorganisms pay their way. All those geologic, hydrologic and biologic materials contain a wealth of nutrients.  But how to get at those nutrients? Sadly, some of those nutrients (many soil scientists would argue that MOST of those nutrients) are chemically and/or physically locked to the inert substances in the soil, due to a variety of chemical and physical bonds. We won’t get into that level of detail here. Suffice to say that without microorganisms to break down those bonds and free up those nutrients, our plants would never have access to them. Yet introduce those hungry little microorganisms and their various digestive processes, and voila! Their waste products (you could think of it as microscopic poo) become the water-soluble nutrient solution that our plants take up in their roots. Let’s be very specific here.  If compost did not contain any nutrients at all, but merely provided a goodly volume of beneficial microorganism populations, it would still help feed our plants.  Why? Because the microorganisms would digest large soil particles and free up soil nutrients, which would otherwise be unavailable to the plant roots. Many would argue that process by itself is what makes compost so valuable.

First, let’s consider what compost actually does for soil. Strictly speaking, compost is not considered a fertilizer for the soil. Yes, it does contain nutrients, but that is not its primary function. Instead, compost is a soil amendment, or what some would call a soil conditioner. That’s because its primary function is to improve the overall biological, chemical and physical conditions of the soil, rather than merely adding more of this-or-that specific nutrient. And how does compost make these fundamental changes? As it turns out, in several ways. First, it inoculates the soil with a tremendous volume of living microorganisms. Soil should never be thought of as an inert physical substance, with only some chemical qualities to be considered. Much of conventional writing about soil will freely discuss the need for this-or-that nutrient, treating soil as little more than a matrix for delivering that nutrient. That mentality borders on treating soil-based agriculture like hydroponics. I have a tremendous respect for hydroponics, and we use hydroponics here for various crops. The basic premise of hydroponics is to work with a soil-less, inert growing medium which merely provides a three dimensional substrate to support roots and deliver nutrient in measured amounts. But if we are working with soil, my goodness let’s treat it with the respect it deserves. Soil is so much more than a three dimensional matrix for supporting roots and delivering nutrients. It is a microscopic urban metroplex teeming with life of various sizes and purposes. That life can work for us, or against us, in our quest to raise and harvest profitable crops, depending on how we treat the soil. One of compost’s primary purposes is to add to and build a beneficial population of soil microorganisms. In fact, a number of growers I’ve known will deliberately use compost for that very purpose as their primary goal. Any nutrient content delivered with the compost is, for their purposes, secondary.

OK, so soil is a living breathing community. So what? How does that help plants? At this point we have already begun to diverge from conventional ag methods which simply provide nutrients, and get into the sustainable farming portion of the conversation. Compost introduces microorganisms, and those microorganisms in turn begin to work on digesting various materials present in the soil. That digestive process does two things - it liberates nutrients for plant use, and gives the soil better water-holding and aeration characteristics. Let’s take a closer look at those processes.

But wait, there’s more. Compost DOES contain nutrients, sometimes a great deal of nutrients. And those nutrients go into the soil and become available to the plant roots at various rates throughout the growing season. First and foremost in most people’s minds is nitrogen. It is a cruel irony that our atmosphere contains a preponderance of nitrogen, yet nitrogen is the nutrient needed the most, yet it the priciest nutrient to provide to our plants. Many consider compost an excellent source of natural nitrogen, and for good reason. Adding to the cruel irony is that nitrogen is abundantly available in both plant and animal wastes, but it is so easily mobilized in the air, in water and in soil that it can be lost before we’ve had a chance to use it. When we talked last time about the ideal carbon:nitrogen ratios in compost, there were two main reasons for really focusing on that particular ratio. First, most microorganisms need a certain mix of carbonaceous and nitrogenous materials in their diet to be happy. If one ingredient is either lacking or in surplus, the microorganisms can’t metabolize their meals as efficiently and the whole process slows down. But if we tweak that ratio just a little to either side, we can preserve most of that efficiency, and more closely start to match up what the compost can provide versus what our soil actually needs. For instance, if our soils are already rich in nitrogen for whatever reason and we merely want the benefit of boosting the microorganism populations, we would want to go a little heavy on the carbon fraction of the C:N ratio. That way, the soil’s excess nitrogen would be bound up with the incoming compost and then brought into that ongoing digestion. That situation presents itself when soils are at the receiving end of upstream nitrogen-heavy discharge, for instance from feed lots or fertilizer-heavy irrigation sources. If the nitrogen isn’t captured and sequestered, it will simply continue to move downstream where it can cause things like damaging algal blooms in either fresh or salt water bodies. But a carbon-rich compost application can tie up that nitrogen, keep it from washing downstream, and make it available to subsequent crops. On the other hand, if nitrogen is lacking in the soils, a slightly nitrogen-heavy C:N ratio in the compost will ensure that some nitrogen is immediately available to plants after a compost application, while the bulk of nitrogen is bound up and released more slowly.

Other nutrients are also released by compost, depending on what ingredients went into the compost. Macronutrients such as phosphorus, potassium and calcium can all be ingredients in compost depending on how plentiful these nutrients were in the starting materials. Happily, the same general principles apply for these other nutrients as with nitrogen - most of the nutrients will be available for long-term release into the soil as the microorganisms digest those parent materials. But a bumper crop of one or more nutrients may be available right away if present in sufficient quantities in the parent material. The same holds true for trace elements as well, and here’s where making your own compost can start to really pay dividends. Once growers have the basic composting process figured out and established, they can start to customize their composting activities to provide extra ingredients as needed by their particular soils. For instance, selenium is deficient in many areas with plentiful precipitation. Those growers who also raise feed crops either for their own livestock or for sale, may want to introduce selenium to the soils since selenium is required by all classes of livestock for healthy musculature growth and performance. Or perhaps growers want to introduce a group of elements, via materials such as rock dust, to tired soils. Those ingredients can be applied separately, per a soil analysis recommendation. But when farmers and ranchers start looking at multiple passes over the same field, applying different specific chemicals, at some point they start to ask if they can combine those applications into one trip. Yes, they can, if those ingredients are in the compost they’re already applying.

So, compost introduces microorganisms to the soil, and nutrients to the soil. What else does it do? Remember when we mentioned above that compost will help soils either hold water or drain water? How does that work? Let’s go back to looking at what soil is actually made of. Most soil scientists would start with the parent geologic material - the sands, gravels, silts and clays that have weathered over time into that location’s soil. But we also know that soils are made up of all the countless generations of plants and animals that have lived and died in that location over time. That introduces a wide variety of additional materials, in various states of decomposition. Add to that the current generation of life at that particular location and you have a soil that has some measurable ability to hold water. Add to that the locale’s climate, and what the grower is trying to grow. It’s a rare thing that a grower would coincidentally have a soil that happens to store or drain water at exactly the right pace for the desired crops, in that particular climate. Most of us want to tweak that a little, by either speeding up or slowing down the drainage rate. For instance, our first property was on the high plains of the American West, where soils are typically very sandy, rainfall is sparse, and the biomass of previous generations is relatively lacking. We added compost to that soil to increase the water holding ability and boost the organic content of the soils. Water wouldn’t drain through quite as fast, and we had better water retention for our thirsty plants in those hot, dry conditions. But when we moved out to the Pacific Northwest, we were in much different conditions. Our soils are a mix of silt and clay, with a tremendous amount of forest litter deposited over countless generations. At our rental land, we have all that plus the deposits from countless previous floods. Those ingredients make for a very heavy soil that doesn’t drain nearly fast enough, and is very prone to compaction. Instead of craving water, our plants are at risk of drowning and/or suffocating. Plus, our soils are relatively cold. Here, compost introduces enough undigested biological material that the actual air spaces are opened up between each microscopic plate of clay, and each bit of silt. The compost’s microorganisms re-inoculate the soil each season when the previous year’s microorganism populations may have slowed down or drowned out over the winter. Our soils come awake faster, we have better nutrient availability, and better digestion of the muck from all those countless generations. In that regard then, compost is the great equalizer of soils, helping to heal whatever imbalances there may be so that productive growth may be optimized.

This has been another long tour through the realm of compost, but we’re not quite done yet. Next time, we’ll look at when to apply compost, how much to apply and how to ensure your particular compost is providing what your particular soil really needs. Thanks for continuing to read and hang in there - compost is worth this long hard look.