Compost - Why it's important and how to do it

Once you start to learn about organic gardening you can easily fall into the rabbit hole of organic principles and practices. Everything is interconnected to everything else in some way, which in reality makes things pretty simple, but the effect can often be confusing (at least for my brain, which I’ve found prefers to work in straight(ish) lines).

Today we’ll dive a little deeper into one of the main building blocks of organic gardening – compost- and hopefully stay out of its web-like rabbit hole. Compost is an integral part of organics  because it naturally builds and fertilizes soil, and in turn improves pest management.

[You might be wondering what the heck do compost and pests have to do with each other. At the pest management workshop led by Farmers Dave and Greg at River Road, we learned that a good offense is the best defense against pests; in other words, prevention is key. The fundamentals of pest prevention are: building soil (which includes adding compost), growing healthy plants, rotating crops, and promoting crop diversity. ..... But we are dangerously close to entering the rabbit hole, so more on those later!]

What is Compost?


Back to the basics. What is compost? It is the end product of the composting process, which breaks down large organic materials (as in materials derived from living things, such as plants and animal manures) into a uniform, soil-like substance that is a great source of nutrients for plants and soil organisms.

Compost improves soil structure by helping to bind soil particles together and by adding more organic matter into the soil. Better soil structure results in better water absorption, drainage, and retention. Compost also slowly releases nutrients and increases the availability of minerals in the soil, which helps plants grow stronger and healthier (another fundamental of pest prevention, remember?!)


A bit on compost biology

In order to better understand how to manage the composting process, it helps to understand a bit of the biology of what is happening in there.

A compost pile only works because of the microbes it contains. Basically, all organic matter contains some population of microbes, so when you throw all of your organic waste (i.e. veggie scraps, garden waste, etc) into a pile, the microbes have a little party, in three phases.

Phase 1: The microbes (mainly bacteria and fungi) multiply rapidly and feed on the plant material in the pile by secreting enzymes and acids that break down plant materials. They then absorb the nutritious sugars and simple proteins that are the byproduct of the decomposing plant material.

Phase 2: As the first phase gets rolling, things start to heat up, which attracts heat-loving organisms (still some bacteria, but more fungi). Fungi then chemically decompose the more complex carbon compounds in the compost pile.

Phase 3: Once the fun-guys finish their job, the pile cools and begins its curing process. A new type of bacteria joins the party to degrade tough, resistant-to-rot woody stems and bark.

After the pile has cooled and cured macro-organisms such as centipedes, millipedes, sowbugs, ants, and earthworms move in to physically degrade the pile even further by using their “mouths” to chew and shred resistant materials, as well as feed on the dead bacteria and fungi who partied themselves out.

Building a compost pile

Throwing a great microbial party (a.k.a. building a compost pile) involves these major components:

  • Scale. A small compost pile won’t ever get hot enough to move to phase two of the kitchen compostparty. In order to really get things rolling, a pile should be at least a few feet in width and length. So be sure to really unload your vegetable scraps, coffee grounds, and eggshells in your garden’s compost pile. [But be sure not to unload any oils, dairy, meats, or pet waste. These materials attract pests and rodents.]
  • Smaller particle sizes. A smaller particle size has greater surface area, and therefore more microbes. The more microbes the merrier, and faster decomposition ensues. This is why we have a separate pile for stalks at our community gardens – they are the party poopers of the compost pile.
  • Oxygen. Oxygen fuels the decomposition process, so a pile needs regular injections of oxygen. This is typically called aerating the pile. The pile can be aerated by layering coarse materials (such as straw or shredded newspaper) over finer materials (such as veggie scraps). This layering action naturally leaves plenty of pore/air space within the pile. The pile should also be aerated, ideally a little while after the decomposition process has been underway, when the temperature of the pile drops and materials start to settle. This is a critical time to add some oxygen to keep the party going (sounds like a Las Vegas casino…..). Turning the pile too often will cool it down and make it harder to move on to phase 2 of the process.
  • Water. Microbes need water in addition to oxygen in order to keep chugging along as they work to decompose the matter in the compost pile. A compost pile should be wet enough that it feels similar to a wrung-out sponge. Too little water and the microbes will get sluggish, too much water and they’ll suffocate – so it’s a balancing act. In short: a compost pile needs to be watered periodically.
  • Carbon and Nitrogen. The materials you throw in the compost pile are a mix of carbon and nitrogen – so these are the stars of the pile. Ideally, a compost pile is made up of 30-40 times more carbon-rich material* than nitrogen-rich* materials. The ideal carbon:nitrogen ratio (30-40:1) represents the ideal “diet” of the microbes that are decomposing the pile.  Note this is the ideal ratio. The fact is that decomposition will still happen in the compost pile even if the carbon:nitrogen ratio is off. The closer the ratio is to the ideal, the faster the decomposition. In aiming to reach this ideal ratio, it is best to combine comparative volumes of carbon-rich and nitrogen-rich ingredients* in layers.
  • The decomp on nitrogen-rich vs carbon-rich materials: Nitrogen-rich materials are “green” materials, such as plant material and vegetable scraps. Carbon-rich material are “brown” materials, such as straw and leaves.

Compost pile – illustrated

Former community garden manager Linda Sliter created this wonderful compost illustration. Thanks Linda!

How to apply compost

Finished compost can be used both to fertilize plants and to improve soil structure.

  • Improving soil structure. At our community gardens, we bring truckloads of compost to each garden in the fall. This compost is used for soil building and fertility maintenance. It should be spread onto the surface of your plot, then dug down into the soil. If you have an established plot and your soil is pretty healthy, the compost should be dug down about 4-8 inches. If you are in a newer garden and are still working on developing your soil, dig the compost down 12-24 inches into the soil. Typical garden application rates are ½–2 pounds of compost/square foot (depending on your present soil development and fertility).
  • Fertilizing plants. To fertilize your plants mid-season, “side dress” them with some of your garden’s house compost. Work a bit of compost about 1-4 inches into the soil around established plantings. [Don’t have much house compost? Encourage your fellow gardeners to bring in their kitchen scraps and lend a hand in maintaining the compost by periodically turning and watering it when needed]

IMG_0340Don’t fret

The above information is all about creating the ideal atmosphere for fastest decomposition in a compost pile. But microbes like to party – so even if the pile isn’t turned quite enough, or it’s not wet enough, or it doesn’t contain the ideal ratio of carbon: nitrogen, it will still decompose…..................eventually. At the very least, composting is a useful way to get rid of garden waste and vegetable scraps and it may even relieve any guilt you might have about not getting to all the veggies sitting in your refrigerator. If they can’t nourish your body, they’ll at least nourish the soil!

Learn more about compost and other useful gardening topics at the University of California Santa Cruz Center for Agroecology and Sustainable Food Systems