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Trench Mulching Experiments


You can never get enough of a good thing.

I may have blanketed my garden in mulch but there's never enough of the stuff. When last I addressed this mulch topic, I explained the method I was exploring (see drawing above).  I've been stifled by the fact that because we haven't had much rain, the grass doesn't grow, the lawns aren't cut ...and I don't get many loads of grass clippings which I use for mulch.

But , touch wood, I'm reaching that blanketing threshold that encourages me to start experimenting. This week I've begun the process of watering only the  pathways between the garden beds. These are sponge deep in newspaper and woody offcuts atop a sliver of something impermeable -- smothered in grass clippings.

I'd prefer that this sponge was thicker/deeper but I make do with what I've got. Everyday I check the garden beds for their moisture content and ask,
  • Does water move laterally from these wet, spongey paths/trenches to the vegetables a'growing in the garden beds?
  • Does the water saturating these sponge channels evaporate or seep  into the sand so that it isn't on hand to irrigate my plants? In effect , does it go up and down rather than sideways?
I've been throwing everything I can get at these paths. My ingredients for the mulching recipe are eclectic: 
  • stuff that will rot: twigs, banksia cobs, rags, newspapers, cartons, cardboard,branches...
  • stuff that won't rot: aluminium foil, plastic bags, wine cask bladders...which I use to lay down a narrow underlying gutter along the length of each path. These semi impermeable  gutters supposedly prevent moisture from seeping quickly into the sand below.
I've already worked out that by hand watering the paths I can quickly 'top up' the moisture content without using much water to do so. I've also worked out that it is best to water late afternoon or at night so that there is much less evaporation while the water seeps in. On sand, trying to  fill up these trench pathways as though they are damns is wasting water as the water will drain away as soon as it hits the sand underneath.

'Deep' watering is a fool's errand on sand.

I'm hoping to add a layer of woodchips to these paths -- if I can get a supply from the local council. I don't want too much of this stuff because I may be leaching Nitrogen from the beds. But like peanuts in a cookie, the woodchips will add 'texture' and bulk.

Underneath my trellis,where it is shady, the paths stay moister longer than in open country. Nonetheless, I am surprized how much dampness I can feel below the garden bed surface, even in full sun. I suspect that at this early stage the main game is to train the soil to readily share moisture through capillary action. 

My set up is complicated by the fact that my original soil is sand and the larger particles in sand mean that their  mechanical tension is less and the distance of potential water transit is shorter. Similarly, since I have covered my  beds and pathways with a lot of mulch there may not be sufficient particle contact for water to travel laterally (see reference)  which means the whole experiment will be a failure.

However I have been using a Leeaky Hose system to good effect along the centre of each garden bed and below the mulch layer . So here's hoping. I am in fact pursuing the same hydraulic  principles.

Ultimately, what I'm doing is trench mulching and the irony is that the more I raise up the level of the pathways -- my trenches -- closer to the level of the garden beds, the more efficient I suspect the system will be. 

So while I mulch the beds to suppress weeds, protect the soil from erosion, lessen evaporation, cool the earth, build biomass and encourage carbon break down to increase fertility, etc -- all the reasons one mulches -- I'm 'mulching' the pathways primarily  to create water storage  channels. The tragedy of heavy mulching -- this you learn as you go -- is that a thick layer of mulch atop a garden bed can prevent precipitation  getting through to the soil and plant roots below. There are ways around it -- but it's like having a wet blanket on a dry bed. 

The question that nags me is what element in the mix is supposedly going to be the most active in transporting water: the mulch layer or the sand or the transitional zone where mulch, manures and sand have fused already, creating a sandy loam. In my soil there is no clay. 

I know I can store water but will it move? Will my water travel sideways...?

Wicking bed theory suggests it will. 

Thie system I'm after is modeled on one advocated by Tiny Eglington. But since my garden is only 12 months old it will take time to mature it and culture the soil.

I'm thinking that if it works -- and I mean  if it works well -- I can develop the principles involved by cross thatching the garden beds with  cross channels of rolled up newspaper such that I divide the  beds into smaller squares or rectangles so that groups of  plants  are moisture sourced on four sides. Burying rolled up newspaper -- either vertically or horizontally -- has proven a powerful -- and very efficient, water saving -- means to keep water up to my fruit treess, bananas and pawpaw.

In fact my garden has so much paper buried in it in it you could say it has been wallpapered or papier mache-ed!


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