You can put in anything. Large seeds (mango pits) don't break down.

The time to breakdown depends on your local climate, and the amount of stuff you are trenching. Typically trench composting is used in a garden, with the trench from the previous year being the planting row of the next.

I havent planted anything in the bed I'm putting it in. Just trying to get decent soil for spring planting. I have hard clay and read that dig and bury method would help the soil. I live in zone 10,San Diego. Had a heat wave for a couple of weeks but now overcast and 70� Thanks lazygardens!

How long it takes to break down depends on several things. An avocado pit will take a lot longer (maybe forever - I'm still testing that!) than soft things like carrot peels. Citrus peels take a long time but less if they are chopped if you want to bother. Eggshells take awhile but faster if crushed and are valuable. I've again started burying my compost as don't want to start another pile this late in the season and today chopped the cantaloupe rinds in small pieces as they take awhile if left whole.

It also IMO depends on how many good soil organisms there are so the more you feed the organisms the faster they will multiply and there will be more to do the work. It's also faster if the soil is kept moist which makes a better environment for the soil organisms.

I put in anything I'd put in my compost pile.

You can put most anything you want into your compost, whether trench or pile. Somethings will take longer to be digested or broken down, as some say it) then others unless they are broken into small pieces. Things such as corn cobs, stone fruit pits, egg shells can be easily identified for a very long time. You can compost Guava and citrus fruits although they may well take quite some time, I find orange and lemon peels in my compost even a year after they are put in.
So how long will it take the Soil Food Web to digest what you put into the trench? Depends on how healthy, how active, the Soil Food Web is as well as what you put there and the size of the particles. As a general rule smaller bits and pieces are better, more easily digested, then are whole fruits and vegetables. Corn cobs shredded fairly fine will be digested much faster then a whole corn cob would be.

Do not bury the material too deep. There is more air in the soil closer to the surface and so will hasten decomposition.

Thanks everyone for the info,one last question. Could I add earthworms to the mix? I've seen just a couple while I'm digging holes,would it hurt or help?

I think you'll be amazed at how quickly it breaks down, especially if you get some rain. I did this over the past winter, covered the trench with 3 inches of soil and planted beans directly into this soil. Great success and a super crop.

No rain yet dotty,maybe I should water it every so often? And how deep should I water?Thanks again for the comments and info. I love this site,everyone is so informative!

You need not concern your self about adding earthworms, they will soon multiply on their own. Earthworm populations rise when the food supply, organic matter in soil, increases enough to support them. Adding to the population before the food supply is adequate to support them means thye will either leave or go dormant until there is enough food to support them.

The worms will find the compost on their own.

One of my childhood garden mentors used to trench compost in the paths between rows, then move the rows over the trench areas the next year. She grew great veggies.

For me trench composting between the rows is the easiest and least unsightly way of composting, however im luckey enough to have ample room to space my rows 40" apart (with corn I space 12") When my seeds have germinated Ill run my tiller between 1 row to loosen the soil scrape back about a 6" deep 1' wide trench scatter food waste and clippings and horse maure and cover it right away.It takes about an hour a week while Im in the garden putzing and watering. By the end of the summer I have put OM in between almost every row. I dont have to worry about extra watering as that end of the garden I water over head and the other end of the garden where all my tomatoes cucumbers melons ect I water via dripline. This a really good way to add to the soil and get rid of all my neighbors clippings and aerate the soil and kill weeds. Its suprising how fast the OM breaks down,in the fall when I clean my garden up in the middle of Sept. I till in about 100 bags of my neighbors leaves and water it in well (2") and let nature do her thing untill the following May. I dont have to add a thing to my soil as Im adding on a weekly basis.

I use the long trench method.

https://www.redwormcomposting.com/?s=Trench+composting