For our minimal budget garden (in LA) we have been using the city's organic compost as a soil amendment. This compost is made from yard trimmings. It's mostly broken down, but there are small sticks in it, so it's somewhere between a mulch and a compost. Are there drawbacks to using this as a soil amendment in our garden?
There shouldn't be any drawbacks, provided it was turned into compost using a hot, aerobic system. Many yard trimmings contain pathogens - fungal infections, insect infestations, so it needs to be processed in such a manner to ensure those things aren't still in the compost when you get and use it. The small twigs are often a component in compost you produce yourself at home via a bin or heap - you can either sift those out, or pick them off, or just leave them in if they're not large and continue to use it as a soil emendment - or a mulch. Either way, it will increase the humus content and the biodiversity in your soil, all of which is great for plants.
What you are buying are other homeowner's 'clean green'...normally lawn clippings are dumped at a separate location or at least not mixed with woody debris. There ARE problems with this stuff, I know because I've worked with it.
Homeowners are notorious for buying pesticides and never reading the labels. Homeowner debris can be so chemically infused with pesticides that you can't grow a petunia in it. Topsoils sometimes are mixed with this compost to be sold already mixed...and many times that stuff wouldn't even allow weeds to grow. Serious.
They are also FILLED with weed seeds. The process of decomposition, as you've seen, creates heat but not enough to get rid of weed seeds, pesticide residues and the pathogens Bamboo talked about.
Your instincts are correct however. The BEST AND ONLY way to improve any soil is with decomposed organic matter...not sand, not gravel, not gypsum, not lime (mixed with clay and water is the recipe for concrete btw). Making your own is the best way to make decomposed organic matter where you know what it contains. But unless you own a restaurant or an organic golf course, it'll take ages to make enough to use with any results in this lifetime, grins!
My favorite and soon was my ONLY compost I'd use isn't that available everywhere, much to my dismay. Most of my time spent as a professional in this field was in the Seattle area. There's a company called Sawdust Supply that used human sewage mixed with sawdust and completely decomposed and tested 5X (federally mandated). They called it GRO-CO. The most miraculous compost ever!
No pesticide residue, no weed seeds, lots of nitrogen and stunningly beautiful...dark taupe, FINE textured, no sticks, stems, clumps, rocks...nada. This stuff fed the soil organisms like crazy. All one had to do was use it as a mulch on top of the soil. Within a year or two one would need to reapply because the soil organisms would eat this stuff and then they'd go back down into the soil and poop it out. The organisms would mix this compost into your soil profile FOR YOU. That is the main difference between decomposed organic matter and raw organic matter. Decomposed is readily available to ingest by the macro and micro organisms. Raw or non-decomposed matter has to be decomposed FIRST and those organisms need NITROGEN to do the work. That is what you've heard. Depending on the type of organic matter, take bark chips for example...takes years to decompose. Clean green, vegetable matter only weeks or months. Clean green actually comes with lots of nitrogen (as well as pesticide residues and weed seeds). I used this kitty litter made from alfalfa...great nitrogen, in my compost piles.
There is a draw back with GRO-CO. Well, first drawback is availability. The stuff in bags at the nurseries is or at least what I've opened is disgusting...heavens...it wasn't fully decomposed!! EEEuuuuwww. GRO-CO smelled sweet and earthy (honest injun), if it smells at all like the original ingredient not good at all. Second drawback is heavy metals. Not that bad but in an ideal world I'd never use this on plants meant to eat. Nowadays, there is more heavy metals in our tap water than there is in this mulch.
Don't know where you live but I would check out your sewage facilities to see if anyone is making this stuff near you. Lucky you if so. They give you a print out of the tests that show EVERY chemical in this mulch. Not going to find that with clean green or other commercial facilities making mulch.
I'd recommend getting your soil tested by your Extension Service of whatever University closest to you. Talk to the Master Gardeners there...they'll be able to hook you up to super compost. Don't worry too much about weed seeds...ie) decomposed horse poo, chicken poo makes great mulch...lots of weed seeds but weeds are like the easiest things to deal with. Pesticide residue not so much. Use those Master Gardeners!! They know your area, your soils and places to get large amounts of good compost. What plants work...that kind of thing.
There could be a problem depending on the collection and separation practices at the composting facility. If your municipal compost includes garden waste from people's green bins there is always the chance that plastic, metal or other contaminants including pesticide residues are in the finished mix. Compost facilities do a great job of hot sterilising organic material but usually cannot separate plastic, beer caps or any of the other junk people throw in their green bin.
Our local recycling facility used to accept sod at no charge as it went straight into the compost stream. Samples are taken at regular intervals of the end product and they found "hot spots" consisting of motor oil. People were using the sod as a free sponge to get rid of something they would normally pay to get rid of.
Here are two tests that you can do yourself on compost:
- physical check: do you see plastic, cigarette butts, pull tabs and other contaminants?
- if you are using compost on areas where you will be growing food sow some lettuce seed at the start of the season. Lettuce is very sensitive to herbicides and pesticides. It sprouts fast and failure to germinate and grow naturally is an indication of problems
Timing will help too. Apply compost in the fall after you have harvested and any problems will have had a chance to be washed out before spring planting
See here for a similar question about using leaves. You have a simple question but the answer is not necessarily yes/no. It depends....