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Composting - a primer

Phil Reilly

April, 2003

Compost piles are a gardener's best friend. They are easy to build and there are few rules to making a functional compost pile. The ideal result is sterilized, nutrient-rich mulch for your garden.

Compost piles are typically built from lawn clippings and pruned garden vegetation. They often contain unwanted seeds and root pieces. To prevent the re-introduction of weeds to your garden, the compost pile must kill all seeds and root fragments added to it.

To do this, the entire compost pile has to reach a temperature of about 130 to 160 degrees Farenheit (definitely hot to touch!) and maintain this temperature for 1 - 3 weeks. This can only be done if the pile is large enough (about 3' x 3' x 3') and has a good supply of moisture, air spaces and lots of nitrogen-rich material. A compost pile is literally a bacteria factory – it is the bacteria that produce the heat as they grow, multiply and feed on the plant material in the pile.

If sterilization of weed seeds and root pieces is not of concern, then the attainment of the high temperatures is not a must. Simple decomposition (rotting) can take place at lower temperature. But beware of this limitation. Also be aware that the typical backyard ‘Black Box Composter’ is too small to build up sterilization temperatures –especially if you are constantly adding material to it. Rotting, not composting, is the common process taking place in those contraptions!

How much area will a compost pile occupy? A garden 50’ x 10’ (15 m x 3 m.) will generate enough organic matter in the fall to occupy an area about 4’ x 6’ x 3’ high. It takes about 3-4 months for a compost pile to finish work if the pile is turned once a month. If it is turned every week, it will finish in about three weeks. You should not add new organic material to a pile that is in the active sterilization process. Depending on your plans for turning, you should leave enough space for at least a second pile to be created while the sterilizing pile is at work.

Building a Compost Pile

To build a compost pile that reaches the 160 degrees F. sterilizing temperature, it is best to build the pile like a layered cake. Also, it is best to construct it in a shaded area to reduce the drying effects of sun and wind.

Here are the basic steps to constructing a compost pile.

Build your compost pile in an area which is at least 3 feet square at the base. This area can be walled or fenced to help create vertical sides. Solid sides also help keep the exteriors of the pile moist.

Create 8" thick layers of garden trimmings (plant stems help create air pockets), followed by 2" layers of fresh grass clippings (rich in nitrogen) followed by 1" layer of soil. The soil contains innoculants of bacteria and fungi to get the composting underway. If you have access to farm-yard manure, adding 1" layers will enrich your compost and help drive the temperature up to the desired 160 degree F. mark. If you add sawdust or straw to your pile (these don’t contain much nitrogen to feed the population of bacteria and fungi), add extra grass clippings and extra nitrogen-rich fertilizer. The finer the size of material added to the pile, the faster bacterial breakdown occurs. Mechanical shredders are a great help if you plan to do a lot of composting.

Sprinkle a dusting of bone meal over each soil layer: This adds phosphorus required by the bacteria to multiply quickly.

Spray these layers with water to the point of being obviously moist but not waterlogged. Repeat this sequence until the pile is at least 3 feet high.

Don't add diseased plant material, oily kitchen waste, meat or dog/cat feces. Also, don't add wood ashes to any piles to be used as mulch in woodland gardens. This would create compost whose pH is too alkaline for acid-loving woodland plants.

There is no need to add commercial compost starters if you add thin layers of soil for every 8” –12” height of your pile. Soil naturally contains the decomposer bacteria needed to inoculate any compost pile.

Composting success is often limited by too little nitrogen-rich material needed to feed the decomposition bacteria.

The following table gives examples of materials with high carbon and nitrogen contents.

Compostable Materials

High Carbon	High Nitrogen	Other
Bark	Bloodmeal	Rock phosphate
Corn cobs	Bonemeal	Seaweed
Dry leaves (all are slightly acidic)	Coffee grounds	Soil
Dry weeds	Cottonseed meal	Wood ashes(sprinkle periodically, rather than in big doses)
Hay	Fruit waste
Pits	Green weeds
Sawdust	Hair
Shredded newspaper	Manure
Stalks	Nut shells
Straw	Tea bags
Wood chips	Vegetable waste

Maintaining a Functioning Compost Pile

Once a compost pile is built (or even in between creating successive layers), place a plastic sheet or other covering over the pile to help maintain the moisture level in the pile. Keep an eye on whether it needs added moisture. Simply plunge your hand into the pile and pull out a sample of the compost. If it is even slightly dry, spray the pile to re-moisten it. Add just enough water for it to soak into the inner areas of the pile! Too much water creates a soggy, oxygen-starved pile.

Now is the time to keep an eye on the temperature of the compost pile. Make sure that the pile heats up and that it stays heated for at least a week. You should be able to see steam rising from the pile on a cool fall evening. If you plunge your hand to a depth of 12 inches and you can feel the heat, this will be a good indication that the pile is working properly.

At the end of a week at 160 degrees F., the compost pile will normally start to cool down. It is time to turn the pile inside out. Turning puts the outside materials on the inside of the rebuilt pile. This chore can be made easier if you have a second close-by place to re-establish the pile. You don't need to worry about layers at this point. But, do make sure that the pile is very moist. Sprinkle bone meal and perhaps some high-nitrogen pelletized fertilizer at regular intervals to stimulate the bacteria's activity. The pile must reheat to 130 degrees F (somewhat lower than the first temperature) for a second weeklong period.

After the second heating has been achieved and maintained for a week, the compost is ready to apply to the garden. It will still have much of its stringy texture, but it should look blackened from the decomposition and heating process. Composting reduces the volume and weight of the original material by about 50%.

It takes 3-4 months for a compost pile to finish its work if the pile is turned once a month. If a pile is turned every week, the time can be reduced to about three weeks.

If the compost pile is to be left exposed to the elements over the winter, it is advisable to place a plastic covering over the pile to keep fall and spring rains from washing away all of the nutrients.

Applying Composts and Manures.

When preparing a perennial bed for the first time, add about 6 inches of compost or well-aged manure and work it into the soil to a depth of 10 inches.

To maintain soil fertility, a one-inch layer of compost added annually to most gardens is sufficient. This can be done at any time during the growing season, but is most effective if applied before the heat of summer begins. This may mean that you are applying year-old compost that has been stored over the winter. A one inch layer is enough to act as a mulch to keep new weed seed from germinating and will help keep moisture in the soil.

Manures of many types are beneficial additions to garden soils. Fresh manure is very rich in nitrogen and can burn tender plant feeder roots. If you use fresh manure, allow 8 weeks between application and planting for the nitrogen to be diluted by rains. Keep the manured soil uniformly moist during this 8-week period to ensure that the manure is decomposing in the soil. In a drought period, the manure will simply stop decomposing, only to resume when moisture becomes plentiful.

Manure application to established perennial beds is not recommended. Manures contain too much nitrogen for perennial plants to produce flowers. Manures cause perennials to produce heavy leaf production and no flowers. Backyard compost, richer in phosphorus than nitrogen, is the ideal soil enrichener. One-inch annual topdressings to perennial beds provide the nutrient balance that favours perennial flower production.

Growing Cover Crops or Green Manures.

Good gardens are usually found on good soils. If you don't already have a good garden soil on your property, and in the city this is a real possibility, a gardener's main chore is to create one. Often soils are too compacted and lack organic material, or humus, required to supply nutrients and retain moisture for growing plants.

It is possible to import bought soil with the right composition for gardening ... but this can be an expensive proposition.

It is also possible to 'grow' a good quality soil right on the spot where the garden is to be located. The idea is to grow a cover crop whose sole purpose is to be rototilled into your existing soil. This will build up the humus content of the soil, which in turn, helps make the soil lighter in texture, able to hold more moisture and which is more weed-free.

A cover crop is simply a crop of a legume (member of the bean family) or a grass chosen for its ability to grow quickly. When rototilled into the soil, it quickly decomposes and adds humus to the compacted soils. Buckwheat, peas and beans, alfalfa, clovers, wheat and annual rye grass are typical crops grown as cover, or green manure, crops. Buckwheat has a special appeal because it grows so quickly and densely (shading out other weed varieties) that it can be resown up to three times in a growing season.

If you are planning a perennial flower bed, keep in mind that many perennial flowers are easily planted in the early fall. Grow a green manure crop or two until the end of August and then plant your perennials. No time is actually lost. In fact, on the plus side, soils on which cover crops have been grown contain more humus, have been relatively weed-free for a growing season, and have been enriched by natural fertilizers released by the decomposition of the cover crop plant material.

In vegetable gardens, wherever there is a bare spot during any season, why not plant an edible 'green manure' such as beans or peas. In the fall, after the last cultivation of your soil, plant a crop of annual ryegrass, winter rye or kale. In the spring this crop will be rototilled into the soil to add extra nutrients to the soil.

Because growing green manures are such a good fertilizer investment (many can be cut or mown to add to the compost pile), why not design your vegetable garden to be 1/3 larger than you plan to use? Sound silly? Not really, because now you can plan for crop rotation whereby each year you can plant a cover crop on 1/3 the area where you are just improving your soil (but not mowing lawn!). In the second and third years you plant your garden on this area while growing cover crops on one of the other 1/3 of the garden area.

Some Final Thoughts

Even today you have to be a library sleuth or avid magazine subscriber to stumble across really good composting and soil building information. Articles outlining steps to creating compost and extolling the benefits of composting are common. But they all tend to assume that all composts are equal – that all composts are acidic due to the production of humic acids as they decompose. But are they?

This article was prompted by information presented in a British book (Perfect Plants for Problem Places by Gay Search) read over the Christmas holidays. It’s not a book about composting, but it is full of composting tidbits worth noting - especially in chapters on gardening in acidic and alkaline soils.

In alkaline soils, Ms. Search cautions, you can’t simply dig a hole, fill it with lime-free compost and plant acid lovers and expect thriving plants forever onward - especially if you water acid-loving plants with water which is alkaline (our local geology tends to give us alkaline water). In a relatively short time the environment around the root hairs tends to revert to an alkaline environment and root hairs of acid-loving plants will become unable to absorb nutrients in the balance essential to their needs. The result: sickly, yellow-foliaged plants soon to wither away. The problem: lack of iron and magnesium. The cure: Add some sequestered iron to your watering can or sprinkle sulphur pellets on the soil and then fork in these nutrients to a depth of a couple of inches. Take care not to injure the plant roots!

In neutral or alkaline soils, Ms. Search recommends mixing your native soil with two of the most acid-yielding composts - pine bark or cocoa shells - to get acidic soils for acid-loving plants. Other composts, such as garden compost, leaf mold and rotted manure, yield less acidic byproducts and you would have to use larger amounts of these to achieve the same shift of acidity. How much compost to add depends on the alkalinity of your the soil. She suggests leaving the soil unplanted for a month and then taking follow-up soil samples. It takes about a month for the soil to reach a stabilized pH from compost additions. It is quite likely that further additions of the appropriate compost will be necessary to get to the desired level of acidity – often a pH of 6.0 to 6.5. (It takes much less time if you use inorganic acidifiers such as pelletized sulfur or ammonium sulphate.) Once the soil is sufficiently acidic, then you plant. It is worth spending this extra time getting your soil right. How much money and effort are you willing to waste on expensive plants that will sicken and die if the soil isn’t right?

She reminds us that organic mulches decompose slowly. When completed, the alkalinity of alkaline native soils will gradually return unless fresh acidic compost is applied on a regular basis. This is usually an annual chore, best done in the spring and may require annual soil tests to monitor your efforts. Keep in mind that if you want garden plants to self-seed successfully, you should delay mulch/compost application until after the soil has warmed up and your seedlings are up and growing. Mid June is soon enough for mulch/compost application.

Mushroom compost is one of the most widely available composts available in the Ottawa area. We’ve used truckloads of it in vegetable gardens and full sun beds with excellent results. But our developing shade gardens (where many of the plants are wanting acidic soils) have never really prospered with mushroom compost applications. I’ve tended to pass off this lack of plant performance as a problem of tree root competition for available moisture. But Ms. Search warns that spent mushroom compost contains lime and therefore tends to create alkaline soil conditions! Obviously I don’t want to go in this pH direction, I want a product which maintains acidic soil conditions.