My garden gobbles up compost faster than I can make it. While my compost bins and barrels have been multiplying, I still supplement with compost from my landfill’s composting program. Why do I use so much compost? It’s not just for the fun of shoveling and raking (although it really is much more enjoyable than lifting weights): compost is like a superfood for your garden soil.
I’ve used compost as long as I’ve had a garden, and it has visibly improved my soil and vegetable production. But recently I’ve been curious: is compost capable of meeting all of my plants’ nutritional needs? And are there any problems associated with using so much compost?
This 2010 review answered many of my questions. Compost-amended soil generally grows equal or greater amounts of vegetables than soil with mineral fertilizers, while untreated soil is less productive. This trend was observed for tomatoes, peppers, eggplants, onions, peas, snap beans, and broccoli, although gains weren’t evident until the second or third years when leaf mulches and plant-based composts (as opposed to animal manure composts) were applied.
Quality compost supplies enough nitrogen, phosphorous, and potassium (the major nutrients that plants need from the soil) for plants to thrive. Of these three nutrients, nitrogen is the most likely to come up short in compost, and plants can’t use nitrogen from compost as quickly as they can use synthetic nitrogen or nitrogen fixed in the roots of legumes. This nitrogen-delay is not typically a problem for plants that take the whole summer to grow, but it could affect fast-growing spring crops such as greens or radishes, which may grow better after legumes in your garden’s rotation plan.
Compost has variable effects on micro-nutrients, and adding it may either increase or dilute micro-nutrients like iron, zinc, copper, and magnesium. If you suspect a micro-nutrient deficiency, test your soil and amend it based on the results.
In addition to fertilizing plants, compost feeds soil-dwelling earthworms and microbes. This is a good thing: beneficial soil microbes keep plants healthy by out-competing pathogens. Compost-fed microbes can even target and kill certain fungi, including those responsible for root-rot. Healthy microbial communities in the soil also break down organic compounds, releasing valuable nutrients that plants can use faster.
And the downside to compost? Excluding wear-and-tear on your wheelbarrow and your back, there really isn’t one. Compost is safe for your garden and can be applied frequently. While synthetic nitrogen fertilizers often leach out of the soil and pollute downstream lakes, the nitrogen in compost is chemically bound to humus in the soil and doesn’t leach easily. Persistent organic pollutants and heavy metals were found in roughly the same amounts as those in background soil levels after many years of compost applications. Pesticide residues from yard waste are broken down by the high heat and microbial activity required to make compost.
When compost is kept above 65 degrees C (149 degrees F) for at least 7 days, it is sterilized, killing plant diseases and weed seeds. But these temperatures may be difficult to achieve in a home composting system. While temperatures in plastic, home composting bins in one experiment ranged from 20 to 65 degrees C (68 to 149 degrees F), high-quality compost was obtained.
Some caution is appropriate if your compost is very fresh, because compost, like chemical fertilizers, can chemically ‘burn’ plants’ roots if over-applied. My landfill suggests that compost should be applied in a 1:11 ratio with soil to avoid problems, although I admit I’ve never actually measured it and never had a problem.
So go ahead and feed your garden lots of compost. It improves nutrient levels, soil structure, and microbial communities, making compost the ideal nourishment for your garden. And you can even make your own compost from kitchen and garden waste.
How to Make Compost:
Compost piles are easy to make. Just add ‘brown’ and ‘green’ ingredients in equal parts and keep the pile from drying out.
- ‘Green’ ingredients include kitchen scraps such as vegetable peels, coffee grounds, and eggshells; grass clippings; and still-green plants from your garden.
- ‘Brown’ ingredients include dead, brown plants, dry leaves, straw, shredded paper, and paper towels.
- Larger piles generate more heat and compost faster. Periodically turning or mixing your compost aerates it, which also speeds up the process.
- Consider containing your compost in a plastic composter to deter rodents, skunks, and other wildlife from visiting and to conserve moisture in dry climates.
- You can sift your compost to separate it into smaller particles (which can be mixed into your beds) and larger chunks (which can be used as mulch). Mix the fine compost into your soil at the start of each year or prior to each new planting if you grow year-round. Or simply spread aged compost on the surface of your soil; if there is a thriving soil arthropod community, they will pull the nutrients down into the soil for you.
- If you don’t have space for a compost pile or you’re not producing enough for your garden, consider buying compost. It is available from plant nurseries, home-improvement stores, or possibly your local landfill. Our landfill diverts compostable waste into a giant pile, tests it, sifts it, and lets people buy however much or little they want for $35 per ton.
- If you notice that your garden is under-performing or showing signs of certain micro-nutrient deficiencies, analyze a soil sample so that you know exactly which micro-nutrient is needed before amending it. Check with your local extension office for soil tests.
Chan, K. Y., Dorahy, C., Wells, T., Fahey, D., Donovan, N., Saleh, F., & Barchia, I. (2008). Use of garden organic compost in vegetable production under contrasting soil P status. Crop and Pasture Science, 59(4), 374-382.
Colon, J., Martínez-Blanco, J., Gabarrell, X., Artola, A., Sánchez, A., Rieradevall, J., & Font, X. (2010). Environmental assessment of home composting. Resources, Conservation and Recycling, 54(11), 893-904.
EPA. (2017 March 20). Composting at Home. Retrieved from https://www.epa.gov/recycle/composting-home.
Trautmann, N., Richard, Tom. (1996). Composting in Schools, Frequently Asked Questions. Retrieved from http://compost.css.cornell.edu/faq.html.