New orchard on old mulch?

We have a ~2000 SF area planned for a fruit tree orchard.  We have minimal soil on fractured limestone.  There is very little growing in the location now beyond some scattered native grasses or weeds and wildflowers.  It has been covered with mulch for 5 years.  The mulch is primarily mountain cedar (or juniper) and has not been renewed since it was initially spread.  Since there is limited soil, and the land slopes slightly, we intend to build up the area 6 to 12" with good soil (enclosed in a block retaining wall).

Should we be trying to remove the mulch prior to adding additional soil?  It's 1 to 3" deep.  I think I understand the issues with nitrogen, but since it's been decaying for years, I think the larger issue might be that the mulch is cedar and might not fully decay during the current century.  Any advice?

Will

Keep the mulch in place and just keep adding on top. My mother took the time to research how long the alleopathic properties of cedar take to break down and it's only one year. That's about the same amount of time it took for grass to start infesting our pure wood mulch walkway, so I don't think nitrogen tie down is a concern after five years.

Most cedar isn't as rot resistant as people think, My understanding is that it is the heartwood of the really old trees (which there are hardly any left now) that has the really impressive life span. Buried in soil most cedar will break down a lot faster than you think. During the decaying process it will do a good job of controlling moisture in your soil both by absorbing excess and then slowly releasing it during dry spells.

If you're  not comfortable with that, you could scrape it up and reapply it to the top again. Until it actually finishes breaking down it's still a very effective protective layer for your soil.

Thanks for the detailed answer, Casie!  You forgot to mention that it's a lot easier to leave the mulch in place!  I had not thought about the aspect of the mulch potentially slowing infiltration and holding moisture which, in our climate, is a bonus - almost like a hugelkutur.

Just be sure that if you go so far as to build hugelculture on your land, you plant your trees off to the side. I have a black cherry tree is rocketing up with no extra care because it's planted next to our hugelbed. If it were planted on the hugelbed it would suffer from the steady sinking of that much organic matter. It would even be more likely to blow over because it wouldn't be able to establish stabilizing roots until it was well away from the trunk.


edit: even the computer doesn't know how to spell these words.

It looks like you're planning what is essentially a really big raised bed, so my question is - what provision are you making for rainwater harvesting, or do you plan to irrigate for the lifetime of the trees?  I planted a fruit orchard in some pretty nice deep prairie soil and the trees all died in the drought a few years ago.  I won't plant more trees without "planting the rain" first.

I've been on my property five years. It had piles of cedar in 5 spots just sitting.  2 days ago I got my tractor and spread the piles out. While the top was branches, it was crumbly humus on the bottom. I was surprised.

All organic carbon is good carbon.  I wouldn't touch it, other than to pile another 10 inches of new mulch right on top of it.

If it's been down on the ground for 5 years, I'm sure that you've got a fungal network established below.  Do mushrooms pop up after you get a good hard rain?  You don't want to disturb all that wonderful fungi growing under the mulch layer—you want to feed it.  New mulch will break down more quickly due to the microbial community that is already established due to that aged mulch.

I'm convinced that one of the reasons my soil has become so rich and my orchard is doing so well is that when we re-mulch it every year with a new layer of wood chips, we are putting down chips from dozens of different kinds of trees.  Early last year I got two huge loads of pine chips.  Pine needles are wonderful to work with.  They lay down on the ground so nicely and age to a beautiful nutty brown color.  Later in the summer, I got a load made up of a variety of different kinds of trees:  Carrot Wood, Cloud Fern, African Sumac, Ficus, and others.  Every different kind of tree offers a different chemical make-up, and brings different trace minerals and elements to your soil.  Trees are the ultimate dynamic accumulators, and putting wood chips down on your soil brings that amazing variety of minerals and such to your garden.  I take what they give me, but I love it when its a mixed load with a variety of kinds of trees in it.

Everything but stupid Brazilian Pepper --- you end up pulling seedlings out of your orchard for months.  Oh well -- even those seedlings become additional chop and drop mulch.  Pulling tree seedlings out is easy enough due to the thick mulch layer.

In my clay and rubble filled urban lot, I tend to plant the trees kind of high and then mound up the soil/ mulch around them.
I don't want them to drowned, and I do intend to rcontinue to add mulch, which I don't want to be too high around the trunk.
My climate is wet but intermittently so.
In the places where I have built soil by adding leaves and wood, I never need to water. The spongy mass holds water from one rain event to the next.

Tyler Ludens wrote:It looks like you're planning what is essentially a really big raised bed, so my question is - what provision are you making for rainwater harvesting, or do you plan to irrigate for the lifetime of the trees?  I planted a fruit orchard in some pretty nice deep prairie soil and the trees all died in the drought a few years ago.  I won't plant more trees without "planting the rain" first.

That's a good way to describe it.  We'll probably have to use city water to irrigate initially, but I hope to incorporate something like a watson wick, open wicking bed, or similar during site work to help manage rainwater and irrigation.  We already have rain catchment but that water supplies household needs.  There will come a time when family water demands are reduced, at which point some rainwater can be used for irrigation.  I'm also looking in to rain gardens.  Final result will incorporate several methods to reduce dependence on irrigation.  Long term I hope for a situation like William describes, where the trees do not need supplementary water (except in extreme drought).  I haven't found any books that specifically address artificially improving water retention (beyond using organic matter or volcanic rock).

Wayne and Marco - the mulch has decayed to a crumbly mass in thicker areas where I pull back the top layer.  I'd prefer not disturb the soil beyond holes for tree planting. We don't have too many local options for types of mulch, but I suppose that any type makes a difference.

It's difficult to collect enough rainwater in tanks to irrigate with it effectively. Our neighbors have a 20,000 gallon rain tank for irrigation and they quickly use up that much water irrigating their trees.  Much more effective to store it in the soil.

The best reference I know of for rainwater harvesting is the book Rainwater Harvesting for Drylands Volume 2 by Brad Lancaster.  His website also has lots of info:  http://www.harvestingrainwater.com/