Book Review Compost Science for Gardeners, by Robert Pavlis


Compost Science for Gardeners by Robert Pavlis

Compost Science for Gardeners, Simple Methods for Nutrient Rich Soil, Robert Pavlis,

New Society Publishers, January 2023. 224 pages, 6 x 0.52 x 9 inches, charts, diagrams and photos. $19.95.

Robert Pavlis has been a gardener for over 45 years, and is a very good science writer. I have previously reviewed Soil Science for Gardeners and Plant Science for Gardeners. This is a valuable, concise, accessible book for home gardeners, and also homesteaders, market gardeners, small-scale and large-scale crop farmers.

The book gives an introduction to the what and why of compost, and then covers the role of compost in the soil, the science of composting, compostable materials, managing the composting process, the options of piles, bins and tumblers, easy composting methods, vermicomposting, Bokashi composting, eco-enzyme composting, buying compost, compost tea, choosing a composting method, using compost and discarding compost myths. There are some almost cult-like groups with beliefs about how to make compost that are not science-based. The author has formed a Facebook group, Garden Fundamentals, you can join, to learn science-based information and help bust some myths yourself.

Garden Fundamentals logo

The introduction explains the importance of high organic matter in the soil, and the cycling of carbon and energy that can increase soil OM. Soil with more organic matter holds onto nutrients better, leaching less, keeping more nutrients in place for the crops. As gardeners we can help the process. We can leave extra plant material where it falls, spread tree leaves around our plants, and establish bug hotels. We can make compost from inedible and unaesthetic plant matter and use the mature compost to fertilize and build the soil, and feed the soil biology.

Nationally, 30% of garbage is yard waste and food scraps. At the landfill, these produce methane, a greenhouse gas with 25 times more global heating power than carbon dioxide. And then add on the pollution from the garbage trucks. It’s plain to see that making compost benefits everyone (except companies selling fertilizer).

There are quick and easy ways of returning garden waste to the soil (chop and drop), and there are labor-intensive ways that kill some weed seeds and diseased plants, and break down woody fibrous materials. You choose. When you do it right, there are no noxious smells. There are even methods you can use inside your house.

Screening a large pile of compost for the greenhouse beds. Photo Wren Vile

The Role of Compost

  • Mulch: keeps soil cooler, damper; may or may not reduce weeds.
  • Fertilizer: contains lots of plant nutrients including micronutrients, that are slowly and steadily released. Because compost has a high Cation Exchange Capacity (CEC, meaning it can hold lots of nutrients), nutrients stick to it, reducing the leaching rate.
  • Soil Builder: adds organic matter, improving aggregation in both sandy and clay soils, helping soil ingredients combine into larger soil particles, upgrading soil structure.
  • Water retainer: a 5% increase in organic material quadruples the soil’s ability to hold water.
  • Toxin remover: compost attracts and binds toxins, preventing plants absorbing them.
  • Acidity buffer: compost brings soil pH closer to neutral, from either side of the scale.
  • Microbe feeder and habitat: More microbes = healthier soil. Add compost, don’t add microbes! Provide the right conditions and they will multiply.
  • Reducer of landfills and greenhouse gas.

If you haven’t already read the author’s earlier book Soil Science for Gardeners, I recommend that. There is a brief description in this book, and charts and a diagram of the soil nitrogen cycle. Plants cannot use large pieces of organic matter, but need it to have it decomposed enough to release the ions. Ions from organic sources or bags of chemical fertilizer are identical. What is different is the ratios and mixtures of ions you are applying.

Tilling is controversial, because it destroys soil aggregates, and brings up weed seeds from lower in the soil profile. Tilling adds air to the upper level of soil, increasing the rate of decomposition of the organic matter. However, it has been found that the organic matter deeper in the soil increases after tilling, so that the total in the top 12” (30 cm) remains unchanged. This may be due to plants making deeper roots (roots are OM!). If you don’t need to dig it in, it is better for the soil and the planet to add materials to the surface.

Digging compost into our cold frames in preparation for fall planting.
Photo Wren Vile

What happens during the composting process? Understanding this helps you make informed decisions in selecting materials and managing the process. Larger life forms play a role in composting by mechanically reducing material to smaller pieces, making the foods accessible to microbes. These macroorganisms do not thrive in hot conditions – they leave when things heat up, and return later. It doesn’t work to second-guess what the compost pile needs. Adding earthworms doesn’t help. They’ll make their own way when conditions are right.

Composting happens when microbes decompose organic matter. Different microbes will self-select for the conditions you provide, just as macroorganisms do. They will multiply and thrive if temperatures and moisture suit them.

Plants are made of cells, and cells are made of molecules. Manure is plant matter that an animal has started to decompose. It’s still made of molecules, mostly unchanged as yet. After composting, all the large molecules have been changed into smaller, simpler molecules: amino acids, simple sugars, the stuff that microbes seek out for dinner. After dinner, and also after life, microbes release nutrients that plants can use. Compost happens!

Here is a clear explanation of the various stages of hot composting and the various kinds of microbes active during the different phases of the process. Worth buying the book for this alone! If you have a lot of seeding weeds, choose a hot composting method, but also chose better gardening techniques that control weeds!

At a garden scale, it will take at least 3 warm months to complete a hot-composting process. In colder climates, it will take 4-6 months or more. Cold compost piles can take 1-2 years. Commercial compost can be “made” in two weeks, but it needs another month to mature. “The first phase of bokashi is done in two weeks, but it is not really composting,” says the author.

We are encouraged to understand the C:N ratio of the materials we use, rather than use the simplistic (and confusing) Browns/Greens concept. Mostly we are feeding microbes, and their perfect food has a 24:1 C:N ratio. There is a helpful table of the C and N content of various compost materials you can use to calculate a recipe, using what you have available. The author offers an Accurate Calculation Method, an Easier Method, and an Even Easier Method.

Don’t worry about the pH. You don’t need to add lime. Home-made compost is usually in the 7.0-7.5 range initially, and drops a bit once in the garden.

A large proportion of the nutrients in finished compost are only released slowly (over maybe 5 years). If you run short of compost one year, don’t worry, your soil probably has enough from previous applications. When is a pile of compost ready to use? Try the Seedling Test. Put some compost mixed 50:50 with soil in a pot. Compare with a pot of just soil. Sow some quick-germinating seeds in each pot. As they emerge, compare the seedlings in each pot. If the plants grow equally well in both pots, your compost is fit for purpose. If the seeds in the compost/soil mix don’t emerge, or are stunted, allow the compost to age longer.

Robert Pavlis has three lists of compostable materials: Good, Bad and Controversial. He addresses antibiotics in animal manures, herbicides in animal fodder and yard trimmings; unnecessary worries about coffee grounds and various food scraps; eggshells, woody branches and used potting soil (OK but pointless); paper and its possible included toxins (not enough to worry about, in the author’s opinion, but not a particularly useful compost ingredient anyway).

The controversial ingredients include diseased plant material (know which disease, advises Pavlis); “compostable” plastics (they don’t break down using garden compost methods); human waste (too much sodium in urine, maybe transmissible diseases in feces); pet waste (maybe your own dog or cat, whose parasites and diseases you already risk, but not other people’s pets, unless you do a good hot composting method); toxic plants (diluted by other ingredients, digested by microbes, don’t worry); weeds (if you don’t let weeds seeds, you won’t have to worry about the compost; if they do seed, you’ll need hot composting the render them unviable; pernicious perennial weeds are best killed before composting); wood ash –  no point as it doesn’t compost, useful to add directly to acid soil.

Bad composting materials include “disposable” diapers (contain plastic and hydrogels); thorny plants; dryer lint, carpet fluff, vacuum cleaner bag contents, fabrics (these days these are largely synthetic fibers or cotton treated to be crease-resistant).

Compost-making is both art and science, and individual gardeners have their preferred twists on the basic method. Basic principles include

  • Location: indoors or outdoors? Indoors limits your options of method. Outdoors, you’ll need a water supply, proximity to the garden and other sources of materials (truck access?), a flat area with drainage and sunshine, and distance from neighbors. Check local laws.
  • Storage of ingredients: to use a hot method, you’ll need to store ingredients until you have a quantity and balance of inputs to build a pile. If you can’t do this, go for a cold system.
  • Air: turn the pile, or stir with an aerator, or make the pile with vented piping included, or layer the materials as you make the pile.
  • Water: add some every few inches as you build the pile. When you turn the pile, squeeze some of the material – it should be damp like a sponge, not sodden, not dry. Add more as needed. In wet climates, add a roof or a tarp over the top. If the pile gets too wet, turn it and fluff it up.

Get a compost thermometer, and once the pile reaches 145ºF (63ºC) – possibly as often as once a week – turn it to mix things up and start it reheating. Compost tumblers make turning easy. Three-bin systems give you somewhere to turn the pile into. A fork is the best tool for turning compost. One way to speed things along is to pre-shred your ingredients, perhaps by spreading them on a patch of grass and mowing them.

You do not need to add compost activators. These may be just microbes, which you should have plenty of. Or they may contain some fertilizer, in an expensive form. You don’t need to add phosphorus or potassium, but you might benefit from adding some nitrogen. Urine diluted 10:1 with water can provide that.

Once the pile no longer reheats, you have reached the curing stage, which takes about two months. Immature compost has a high C:N ratio, and is acidic. It could damage plants. If you are spreading your compost on the soil rather than incorporating it, you don’t need to cure it. This minimizes leaching. If you plan to dig it in just before planting, better to fully cure it first.

There is a troubleshooting chart of things that can go wrong and how to fix them. There are some photos and tables of pros and cons of each outdoor composting system. Resist any urge to turn your compost tumbler more than once a week, or you will disrupt the heating cycle and slow things down.

Pit and trench composting are methods involving gradually filling holes in the garden with compost materials and then covering with soil. This method works well for kitchen winter and spring, when there are fewer other ingredients. A keyhole bed with a central cylindrical cage for compost materials is a version of this idea.

There are electric “composters” which grind and dry kitchen scraps. They do not actually compost the food scraps, or even finely grind them. They use your money to produce dried food scraps, not fertilizer. And they use electricity doing it. When mixed with soil, the food rehydrates and grows mold.

Six worm bins.
Photo NCSU

Vermi-composters are bins holding worms, food scraps and other organic material, which together produce worm castings, which can be used as fertilizer. Vermicomposting is faster than traditional hot composting and can be done indoors, even in winter. The book contains enough detail for you to know whether this practice is one you want to try.

Microbes are the worms’ main food. Worms only digest 5%-10% of their food intake. The rest is excreted, along with a lot of microbes. Learn how much to feed, so that it has mostly gone by the next feeding. 1000 worms (1lb, 0.5 kg) will eat about 0.5lb (0.25 kg) each day. When food runs out, the worms will eat the bedding. Shredded newsprint, fall leaves, aged wood chips and more, make good bedding. There is a trouble-shooting chart.

The worm bin material (“vermicompost”) is a mix of worm castings, bedding material, uneaten food scraps, nutrients, worms, worm egg cocoons and microbes. While the worm bin is active, leachate liquid drops out from a drain hole in the bottom of the bin. This contains organic matter, nutrients and microbes. It can get smelly. It can be diluted 10:1 with water, to use on potted plants, or to water the garden.

Vermicompost will be ready to harvest 6-24 weeks after setup. Ideally, about 50% of the bin will be bedding and compost, and 50% castings. You can use the castings and compost mix in your garden right away. Worm castings have 10-20 times as much microbial activity as soil or most other forms of organic matter. They compost themselves once in the garden. To use for potting soil store it for two months, then mix 20% by volume in your potting mix.

The composition of vermicompost is a little higher in N than regular compost made from the same materials, and the potassium is the same, but the phosphorus is almost four times the level in the regular compost, which can be a problem if your soil is already high in P.

Next the mysteries of Bokashi fermentation (often called composting) are revealed. It’s an anaerobic process, similar to making silage, but usually done in small batches for small gardens. Most pathogens are killed. Waste organic matter is combined with sweetened “Bokashi bran” which contains fermentation microbes in a lidded bucket. Bokashi can be made indoors, and is a useful option in climates with cold winters. All kinds of food can be included with no worries about rodents.

Ignore the claims about “Effective Microbes,” because a study has found them unnecessary. Farmers often start Bokashi with naturally occurring microorganisms rather than buying the 80-microbe EM-1 product. One study found. no difference between using indigenous microorganisms, yeast or nothing at all.

A dark liquid leachate is drained out of the bottom and used as fertilizer, diluted 1:50 with water. It is low in nitrogen, but high in phosphorus, sodium and chloride. See the photos in the book for proof that it is not a good solo fertilizer.

The full bucket is left for a couple of weeks, and then the material is either added to a compost pile or to a large worm bin. It is not ready to be used for plants. You could save time and just feed the raw materials directly to the worms. The pH of Bokashi is around 4 (quite acidic), whereas the pH of compost is about 7.

The Eco-enzymes process, also known as Garbage Enzymes, is similar to making Bokashi or sauerkraut, and is popular in some households in Thailand and nearby countries. It reduces the amount of garbage going to the landfill. The system relies on the microbes already on the surface of the foodstuffs. None are added. Anaerobic conditions prevent pathogens from growing.

Enzymes are proteins created in living organisms. They carry out most of the chemical reactions that take place in the cells, including digesting organic matter. To start an eco-enzyme process, combine 3 parts of chopped organic matter, 1 part of sugar and 10 parts of water in a closed container. Open the lid once a week to let excess pressure escape. Wait three months at room temperature. Then separate the liquid from the solid ferment.

The liquid contains acids, alcohols, bacteria, yeast and protease, amylase and lipase enzymes, as well as plant nutrients.  It can be used to fertilize plants, or as a cleaning product with antifungal and antibacterial properties. The solid ferment is similar to Bokashi ferment, and needs composting to be useful to plants.

A commercial compost windrow turner.
Photo by Pam Dawling

Not everyone can make their own compost in large enough quantities. Some municipalities collect compost materials and make compost, which is then sold locally. Because a hot composting process is used, there will be no pathogens or weed seeds. There are no national US standards for compost, so read the small print.

Bits of food packaging, microplastics (less than 5mm in length) and nanoparticles (smaller) will be in municipal compost. A 2021 US EPA study found 300 pieces of microplastics per gram (8,400 per ounce) of food waste collected from grocery stores. Make your own compost if you can!

Chemical contamination is another concern. Pesticide residues will mostly (not entirely) decompose during composting. Leaves from city streets may have picked up oils and car exhaust compounds. Heavy metal contamination can come in with industrial wastes, or sewage sludge (biosolids), and unlike pesticides, will not degrade. Class A biosolids are tested for fecal coliform, salmonella, heavy metals and a few chemical contaminants. They can be sold as suitable for garden use, although not on Organic farms.

Some herbicides are digested by grazing animals, some degrade if composted, some bind to the soil. Some plant growth regulator herbicides, however, pass straight through livestock and kill the plants they land on. The list includes clopyralid, aminopyralid, aminocyclopyrachlor and picloram. Manure, compost, straw or commercial organic fertilizer can be contaminated. Organically produced materials should not contain these toxins.

Mushroom compost is pasteurized after the mushrooms are grown, and sold to vegetable growers. The material does not contain the right nutrients to grow a second round of mushrooms, but is useful for vegetables, with an NPK of 1-0.7-1, with a high calcium level.

Compost tea comes in two main kinds: anaerobic and aerobic. The anaerobic method is to put some organic matter in a container of water. Wait at least a few days, then draw off the liquid. Microbes will have decomposed some of the material and released some nutrients. Some people call this type of tea a “watery extract” to distinguish it from aerobic compost tea, which has air bubbled through it until done. To make “tea” you can use finished compost, weeds, comfrey, vermicompost, manure.

Compost tea does have some benefits in the garden, but not as many as some wishful thinkers believe. An increase in plant growth has been shown in studies in comparison with water. Not surprising! No study has shown better results from compost teas than other plant fertilizers with similar amounts of the same nutrients. It’s cheaper for you to make compost tea than buy fertilizer. But it may be better value for your time and money to use the organic matter you would have made into tea, in some other kind of compost.

Does compost tea build healthier soils? It is mostly water, which doesn’t build soil. The solid matter left behind might improve soil structure if added directly. The tea does not. Adding microbes via compost teas does not increase the number of microbes in the soil, except very short-term. Microbes will die and feed each other, go forth and multiply until the number of various types have rebalanced, back to the same numbers before your intervention. In a 5-year tree study, compost contained 50% more microbes than compost tea did. Skip the tea-making!

If compost teas are sprayed on foliage, some of the microbes may outcompete some existing microbes, including plant pathogens. Some plant diseases (powdery mildew in some cases) were helped by compost tea. Others (gray mold, downy mildew) seem unaffected. Washington State University has done extensive research. Composts vary, tea-making varies, weather varies. Compost tea is far from a cure-all, that much is clear.

Which composting system suits your conditions best? The trench method works for gardens with no space for bins or piles. Eco-enzyme and compost tea are small scale activities. Vermicomposting and Bokashi can be done indoors in winter. A chart compares traditional composting, vermicomposting and Bokashi Ferment, and another chart compares six traditional compost methods.

If your compost is high carbon, with lots of visible leafy or woody particles, spread it on the surface. If dug in, it would need to find plenty of nitrogen to continue decomposing the carbon. If your compost is still “hot”, and not fully mature, don’t dig it in, as it could hurt your plants.

Robert Pavlis tackles the issue of using too much compost, leading to problems with very high phosphorus levels in the soil, by recommending 1” (2.5 cm) of compost a year as a sustainable amount. Commercial compost may contain an NPK ratio of 1-1-1, whereas plants use 3-1-2. Homemade plant-based compost might have an NPK of 3-0.5-1.5. If you use compost to supply all the P the plants need, you will need to supply more N. This is safer than using compost as your main source of N, and building up too much P in the soil.

Excess phosphate gets locked up in the soil, and it could make it hard for your plants to absorb iron and manganese. If this happens you may see interveinal chlorosis. Some crops are good at integrating and removing phosphorus from the soil, so the situation can improve over time, if you minimize the P you are adding each year. If you see no excess phosphate problems, I advocate for not worrying too much! Expect to get some of the nitrogen you need from cover crops, and added fertilizer.

As well as your preferences and conditions, environmental concerns are important to consider. How much carbon is lost as CO2 or as methane? How much nitrogen is lost? If you didn’t compost, what would your alternative be? Would that produce more or less greenhouse gas? Sending organic matter to the landfill is the worst option. In general, the more aerobic your compost process is, the better for the environment. Bokashi and vermicomposting only take the material halfway to being composted. Remember that producing your own compost helps your garden soil and crops, and your wallet.

Robert Pavlis has a new book Microbe Science for Gardeners, coming soon.