If organic matter makes soil retain more moisture, then why doesn't compost retain moisture as well?

I have a vegetable bed in Taiwan. It is usually 30-35 degrees in the afternoon this time of year and there is high humidity. There is relatively little wind on site due to a few natural and man-made windbreaks. That is all the relevant information about where i'm growing.

Just to give you an idea of what it looks like, one could break the bed into 4 sections: section 2 has garlic and green onions that we haven't used up yet, section 4 is new and is growing food for the cabbage moths (the row cover blew off one day and the little *******s got into my brassicas). I'll get to sections 1 and 3 in a bit. The whole thing is a lasagna-style bed, built with newspaper on the bottom then some grass and leaf litter that collects at the edge of the park, decomposes, then is gathered by myself.

Long story short: i'm growing in 100% compost.

Up until recently, i thought i was doing pretty good. I noticed that the lettuce seeds i'd sown germinated pretty poorly but I just told myself "Made in Taiwan. I'm lucky ANY of them germinate" and now i sow pretty thickly and thin later. Besides, the red radish and white radish seeds i sowed in there have germinated pretty reliably and the: garlic, green onions, plantain herb, aloe vera, cat nip, mint, rosmary and "unidentified plant #1" all did pretty well.

Then i got the idea to dig a big hole in section 1 and bury some fish in there (the water here is too polluted to eat the fish but I get no satisfaction from catch-and-release so I can put them to good use this way). After this, I raked sections 1 and 3 flat and sowed two kinds of lettuce and some green onion seeds. Section 1 is mostly native soil on top and section 3 is 100% compost on top. Each section gets water daily either from my watering can in the morning or from the rain these past two days. On day 3, section one (soil) started germinating. It is now day 5 and section 3 is germinating nothing but crab grass.

The only difference i can conceive is that section 1 has soil on top and section 3 has compost on top (the fish are too deep to be making a difference yet). On observation, i noticed that section 3 is dry almost as soon as the rain stops, as far down as my second finger joint. Section 1, on the other hand, is retaining moisture all the way into the next day, just centimeters beneath the surface.

As i'm typing this I keep thinking of 2 foot tall lasagna gardens and straw bale gardens that seem to do just fine, presumably because the organic materials retain moisture well enough to support growth.

Does organic matter only help SOIL retain moisture by bonding particles together? Or can it, in itself, retain water as well?

On that subject: heads up to any of you who are preparing to do what I am doing, expect poor or slow germination if you are sowing small seeds into a lasagna garden. Perhaps you might benefit from spreading a layer of soil on top before sowing.

Dan Good soil should have so much organic material in it that you can quickly dig in it with just fingers, still there should be at least as much dirt as Organic matter !
Having said that, yes, soil with a good amount of organic mater in it will hold a lot more water than soil without Organic Matter !

I would let the fish Dehydrate before I placed it in the ground so that it can't rot (anaerobic bacteria make the soil sick) and it can't release to much Nitrogen too
quickly !

I think i will set back and watch the rest of the comments! For the Good of the Crafts ! Big AL

+1 - Compost is great but does not contain all of the nutrients required for healthy plants and fruits/vegetables. You need a good mix of compost and soil and perhaps even augmented with some organic fertilizer and/or rock powders.

Perhaps I didn't explain the allotment clearly.

The bed is made of compost on top of soil. It is not worked into the soil. Seeds are sown into the compost and then the roots are able to work their way down to the soil as they mature. The compost is not tilled, spaded, or otherwise incorporated into the soil except for zone 1.

The seeds i sowed into the exposed soil in zone 1 have excellent germination. The seeds sown, fromt he same packet on the same day, into the compost have very poor germination rates, likely due to poor water holding capacity. Has anyone else expereinced this? Is there another solution besides tilling the compost into the soil?

Compared to soil, compost is light and airy and on the surface (above soil) would likely tend to dry out quickly which could explain a poor seed germination rate. Is there a reason you didn't mix it in the top layer of soil? Don't get me wrong, compost is also a great mulch, but I'm thinking (as you discovered), perhaps not the best for seed starting unless kept very moist.

dan long wrote:
Perhaps I didn't explain the allotment clearly.

The bed is made of compost on top of soil. It is not worked into the soil. Seeds are sown into the compost and then the roots are able to work their way down to the soil as they mature. The compost is not tilled, spaded, or otherwise incorporated into the soil except for zone 1.

The seeds i sowed into the exposed soil in zone 1 have excellent germination. The seeds sown, fromt he same packet on the same day, into the compost have very poor germination rates, likely due to poor water holding capacity. Has anyone else expereinced this? Is there another solution besides tilling the compost into the soil?

I like to think of compost as a living breathing organism. The compost has all kinds of fungi, bacteria, and mesoorganisms that eat organic matter and drink moisture and breath out water vapor.

When I put compost in the soil I try and mix it with the dirt to slow down the respiration rate of the composting organisms. If they work too fast they drink all the moisture and breath it out as water vapor. Composting is our way of accelerating natural processes. To slow the process down break up the compost (often tearing the fungal mycelium), disturb the bacteria, and knock around the mesoorganisms.

Now the compost chemistry (the water holding molecules) are in the dirt but the respiration rate of the the composting process has slowed down.

How hot is your compost layer in your gardens? Are you heat sterilizing seeds and slowing plant growth?

As for mixing the composting and soil:
double digging
tilling
making compost tea and using that to water the garden instead of regular water, use the compost tea grounds several times and then add them to the top of the garden as mulch

Thanks Brett. That makes a f*** of a lot more sense than anything else i have read so far.