Hi, just looking for some opinions from anyone experienced in making raised beds (can't do hugelculture on this plot ) in a rather wet climate much like the US Pacific Northwest.
I'm filling the beds lasagna style with aged sheep manure, hay, more manure, straw, more manure, then a top layer of bagged organic compost + about 10-12% charged biochar.
With our variable climate (rain, drought, hot and cold are all possible at almost any time, but rain and fairly moderate temperatures are "business as usual") I'd like to do all I can to promote good drainage without subjecting the beds to drought when it doesn't rain. Tragically, I'm gone 3 weeks in August every year and on the very off chance it's 35°C/95°F and cloudless every day for 3 weeks while I'm gone, I'd like to have a fighting chance of coming back to at least some live plants. PS Yes I will mulch.
So rain is business as usual, but I don't want to get the drainage so good as to make for disaster when drought strikes. No irrigation is permitted where I have my garden.
So, on the balance...
-- Would you add 10, 20, 30% sand to the compost mix to improve drainage? (Award-winning carrots would be nice too)
-- Would you add some of the aged manure into the compost/biochar mix? If so, what percentage? I'd love to use a lot because I've got it free, but I don't want to overdo it.
I would add some sand to your beds. It'll improve your drainage and add what I feel is some needed mineral content to a soil that is otherwise mostly organic.
I also live a wet cool climate (SE Alaska). We don't get much for heat and typically not much concern about drought. Except this spring it didn't rain for a month and everything is very dry.
Anyway I always add sand to the beds I build. I've been building beds in much the same manner as you. We don't have anything much for soil here. It's all either peat bog or rock and sand. So to make soil I've done the lasagna bed method. I pretty much add whatever materials I have at had. Straw and chicken bedding, grass clippings, weeds, composted kitchen waste, some local "soil" which is mostly peat, and sand. I live on an island so I also add materials from the sea, seaweed, crushed shells, fish waste, crab and shrimp shells. The sand improves the drainage quite a bit. I don't really measure how much I add and it likely varies with each bed to some degree. I'd guess maybe anywhere from 10-25% by volume.
My main concern is drainage not concern about drought. However, one of the beds I build this way last year and planted with garlic last fall. I mulched it heavily then and have pretty much ignored it since. We had a record breaking dry May...no rain the entire month. We did have a really wet April though so the ground was well saturated at the start of the month. I pretty much ignored this bed and didn't water it at all. Last week I finally got around to giving this area some attention. I was pleased to find that under the mulch these beds still retained quite a bit of moisture in the soil. And the garlic looks fantastic so clearly the bed retained enough water for its needs. So these beds don't seem to dry out too fast.
Could you build a hugel bed, then hide it inside a conventional raised bed? I enclosed a 3-year old hugel like this in the garden here at my parents request, they just didn't like dealing with the sloped sides on the hugel. Seems to be working well. Of course if it's a prohibition on digging below ground-level this won't work as well, and if it's lack of woody materials then it's no help at all.
What's the soil like below these raised beds? If there is good mineral availability in the soil below, many plants should be able to get to that, depending on bed height... If it's got a decent sand content, maybe that would suffice, added sparingly to the beds? Otherwise I'd tend to agree with Chris that some sand wouldn't hurt. Actually, since you specified multiple beds, I'd probably use a different amount in each, to see how it impacts things. Because everyone always needs another variable to track!
Adding some sand is fine, but here's a couple things to consider from my experience:
If the raised beds have drainage ports at the bottom, gravity and the sun hitting the beds from all sides is going to accelerate the drying/evaporating process. If you are planting water loving veggies in the beds like tomatoes, i would want to hold as much moisture as possible without puddling.
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