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Sponge Hole Irrigation

I've been experimenting with the mulching technique I've previously called a Honey Hole or  Fertility Trench.

It is a very simple method that is a useful for  irrigating and introducing  fertility into the soil. I refer to these holes as 'worm takeaways' because they concentrate a lot of organic matter in the one spot, and once settled, garden worms will move in and feast up.

But these holes' primary function -- while their contents breaks down -- is as a water sponge.

So they serve two functions:
  • a vertical fertility mulch
  • a water reserve/irrigating sponge
The trick is in the mix that you fill the holes with.

The Sponge Mix

After some experimentation I'm currently working from this recipe:
  1. Throw old newspapers, junk mail, old phone books, cardboard, etc into a large container -- I use a wheelbarrow -- and steep  them in water.
  2. When thoroughly wet, shred the papers and cardboard by  tearing the pieces apart with your hands. You should end up with lots of shredded paper and cardboard strips, something like papier mache
  3. Put on some rubber gloves and with a sieve or just your hands break up a quantity of manure and sprinkle it into the paper mix, tossing, stirring  and blending the manure in as if you were making a muffin mix. The paper strips should all be  well coated with the manure. Delicious-- but don't lick the spoon.
  4. Depending on your soil, the nature of your manures and so forth a good rule of thumb is a mix of 30-40% manure to 60-70% wet paper.
  5. If the mix is too dry -- add more water -- and let the manure/paper mix marinate for 5  to  7 days, stirring occasionally so that you brew a dense manure tea.
Once you have prepared your sponge mix, you can dig your holes.

The Hole
  1. With a hand spade dig a hole roughly half the depth of your forearm. I dump the soil I dig up into a 3 litre plastic pot so that I can keep each hole I dig the same depth and volume. Using a pot also keeps the soil off the garden bed, and any plants,  nearby.
  2. Fill the hole with a generous amount of sponge mix and ram the mix down into the hole with your (gloved) fist or/& a mallet. You want to really stuff the mix into the hole so that you end up with a convex depression on top with the mix rising up on the sides of the hole. That's your mini-billabong.
  3. Now place a stick or rod in the centre of your hole  and tip your bucket of dug up soil on the top of the hole. Pat the soil down and wiggle the stick to create  a crater --something like a hole on a putting green.
  4. Mark your crater with a stick or label rod so that its location is flagged in your garden bed.
When watering the garden, direct a spray at the flagged crater you have marked so that the the mix below engorges with water and pools on top, like a miniature pond. If you dug each hole approximately the same depth and diameter, you should be aware how large is your sponge bowl.

Location. Location.

I use these sponge holes as a supplement to my terracotta pot irrigation system.

I position them in sections of the garden bed where I think the water from the pots isn't reaching. Since water is its own conductor, these sponge holes can act as channeling stations for water across the distances between the pots.

The large paper content serves as a sponge that absorbs water when  wetted and, as the sponge mix breaks down, the elevated carbon in the soil increases its water holding capacity, protecting the soil from  future arid conditions.

Sponge holes are also very effective when dug next to newly planted trees and perennials. They are also much less disturbing to soil structure than the excavation required to  dig elongated mulch trenches. While the manured  'goodness' is concentrated in one spot, soil biota and worms will do all the work of distributing the sponge mix more widely in the bed neighbourhood.

As the sponge holes sweat moisture into the surrounding soil the water merges with the fluid content emanating from the terracotta pots, thus promoting greater holding capacity and broader spread per litre of irrigation. In my sandy soils both methods ensure I keep moisture in my top soil longer rather than have it quickly drain away.

You need to be aware that the hole's contents are initially manure 'hot' so you need to consider what you plant nearby. Heavy feeders should do alright.

I treat these holes as a shadow version of my terracotta pots. They are roughly the same volume and depth  while serving a similar function. So when I come to fill the pots every 3-4 days, all I need do is direct my hose spay at the marked sponge holes as well...at least for the following few months. Rather than hose the garden beds -- wasting water and encouraging weed growth -- I concentrate my water usage in spots that store it underground.

In my imagination I envisage these sponge holes as a key element in my worm neighbourhood. In hot weather I discovered that garden worms will gravitate around the terracotta pots because of the coolness and moisture in the soil. These properties are replicated by the sponge holes with the added feature that they are also serve as a restaurant and takeaway. 


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