Sponges in the Desert
I didn’t know that there were sponges in the desert! They don’t grow here naturally,
but we can plant them. We can use absorbent material to passively store moisture from the
episodic rains and snows of the high plains deserts around Albuquerque and hold it in the
ground next to plants (1). A sponge is typically a bathtub-sized hole dug into the ground
and filled with waste materials that will absorb water, store it, and slowly release this
valuable resource to nearby plants over time. The sponge material is carbon waste that will
break down naturally and recycle carbon back into the surrounding plants and soil life.
Eventually the resulting exuberant plant growth will shade and enhance soil life.
My interest in sponges began while I was trying to grow a windbreak on a
sloping property line between me and too-near neighbors in Edgewood, New Mexico (2). I
wanted a sight and sound break from them. This was my third attempt at a windbreak. As
the windbreak began to grow, I observed that the small basins I had left around each tree to
harvest rainwater had filled with silt from splash erosion during our brief and heavy rains.
This would cause future rains to flow past the trees, so I was missing this valuable
rainwater harvesting opportunity.
Often desert soil has a humus content of only 1 to 1.5 percent, enough to store water
for only a few days. I decided to dig the basins deeper and place straw in them, something
that would hold the moisture longer and keep the basins from silting in so quickly. This
would allow the moisture to be slowly released over time, rather than evaporating quickly
from the silt in the basin. The fertility of ideas shared in the Albuquerque Permaculture
Guild encouraged several of us to begin experimenting with sponges in a variety of ways, in
gardens, in front yards, and around thirsty trees.
Most desert plants have roots that are shallow and quickly absorb water from the
surface soil during our brief rains. I want to grow a wide variety of perennial plants and
trees, many of them food-producing. By concentrating and storing water in sponges next to
these non-native plants, I hope to introduce species that will thrive in the desert and
develop deeper roots than many desert plants, someday creating a food forest. Plant roots
also drill through the desert hardpans that can keep water from penetrating deeply into
desert soil.
Small sponges: I dig a small sponge on contour 6 to12 inches uphill from a sapling
(3). These sponges can be fairly small, like the size of a folded newspaper (14” x 14”) and
12 to 18 inches deep. You can dig these sponges anytime after you are finished planting the
saplings.
Large sponges: I dig large sponges above older trees with enough absorbent
material that the moisture from the summer rains will last as long as possible toward the
winter snows, and vice versa. Above larger trees, I like digging these large soil sponges
uphill from the drip line of the tree where the roots are actively growing. Some root
pruning seems to be okay, but I prefer that less than 15 to 20% be disturbed. I dig the
sponge only 12 to 18 inches deep, since 90% of tree roots live in this topsoil. The
proportions of sponges vary greatly, but sponges are usually around 5 to 7 feet long and 14
inches wide. I throw the dirt from the hole directly downhill to form a low level ridge, or
berm, below the tree, helping concentrate more water on the sponge and around the tree
basin itself. If I have enough time and energy, I dig wings slanting uphill from the ends of
the sponge, guiding even more water into it. This multiplier effect insures that the sponge
will get saturated whenever there is a significant downpour.
Sponge materials: Any material that absorbs water and naturally decomposes can
be used to fill a sponge. I use carbon and protein materials, but avoid all synthetic materials
because they may not break down. I use carbon material such as newspapers, cardboard,
junk mail (the cellophane windows are okay because they are cellulose), magazines, phone
books, hardcover books, envelopes, old clothes, confidential documents, and any number of
other clean, biodegradable materials. I put noxious weeds in the bottom of the hole. I also
put my old clothes at the bottom so they stay wet and don’t get dug up by our dog, coyotes,
or our bear neighbors in the East Mountains near Albuquerque. Protein materials such as
wool and leather also naturally degrade. Neighbors gladly save all of these materials for me,
provided I pick them up regularly. Concerns expressed by an academic horticulturalist
about nitrogen deficiency in the plants around the carbon sponges have been observed to
be unfounded.
There is no need to tear or shred the materials; the worms will do a great job! A
friend of mine insisted that it would take forever for water to soak into the middle of a
stack of magazines. I had to agree. And he agreed with me that it would also take a long
time for it to leave, creating a fertile, moist spot for roots to penetrate and for worms to
feast. The glossy pages (clay) of magazines dissolve and the reds (copper and iron), dark
blue (cobalt) and other colors are so dilute that they have little impact and probably benefit
our depleted alkaline soils (4).
This carbon sponge material looks like garbage to most people, so top-dressing the
sponge with a thick layer of straw (expensive), grass clippings, pine needles, leaves or
woodchips gives it a natural appearance that melds into the landscape quickly. Every fall,
when I was commuting regularly, I used to harvest hundreds of bags of leaves off of the
streets of Albuquerque, hauling them 25 miles to mulch the trees and keep the grass
competition down around the saplings. There is a rich and varied resource of clean organic
“waste” that can be harvested from our urban neighbors before it is dumped in our
overwhelmed landfills, so I try never to drive from the city empty. Time and material
permitting, I like to armor the downhill berm with woodchips to slow the slumping of the
berm over time. Straw can work, but needs to be carefully scattered in an interlocking
manner, and I add whole flakes of packed straw on top of the berms to keep it from blowing
away.
I bring this top-dressing mulch to the site before digging the hole so I can add it
before the unsightly “garbage” in the sponge can blow away. I add the sponge material to
the hole immediately after digging, so the hole will not lose any of its precious moisture and
the plant roots won’t dry too much. It’s fun to get in the hole and slide the materials around
into the corners of the hole and stomp down the cardboard.
Then I mulch both the sponge and the tree’s basin to help it look nice and simple. If
we are in a severe dry spell or the tree is in trouble, I soak the sponge with stored
rainwater or groundwater until it overflows into the basin around the tree, thoroughly
soaking the tree’s roots and giving the tree a boost until the next rain or snow. This can
bring a dramatic recovery in a drought-stressed tree.
At this point I am completely done making the sponge. Once a sponge has been
planted, I may never have to water that plant again! It’s fun to dig fingers into a mature
sponge and find worms wriggling in the moisture, leaving their rich castings. The sponge
can be re-dug years later if needed and new material added, giving us a chance to see what
really happened! This all depends on the site, the volume of rainwater, and how much silt
and detritus the rainwater will carry and deposit.
Cautions: The multiplier effect helps us design how large a surface area is needed to
collect sheet flow rainwater from a larger area and focus it into a sponge so it can become
thoroughly saturated with water once in a while, ideally once a season and at least once a
year. However, a sponge can also drown a young tree or a recovering tree with a fragile
root system (5), or wreck a building’s foundation if too close (sponges should be at least 10
feet away). It is important for the ground at the base of a tree trunk to remain at its original
level: bark will rot in many species if soil builds up around it.
It is helpful in designing the placement of carbon sponges to first understand the
topography of the total site as well as possible, where the rainwater falls initially, where it
runs, and where it slows and collects.
I used to recycle the carbonaceous wastes from five to ten households into my
sponges. It’s fun to show visiting friends and relatives how these wastes of theirs can be
utilized with such wonderful consequences. It is a surprisingly meaningful way for them to
feel a connection to the earth. I invite friends to bury their personal letters and
photographs in a sponge. I have seen this have an amazing effect upon people. Rather than
watching their memories get shredded or burned, they see them laid to rest in an organic
way, transforming some of their past into a beautiful tree. This can be done with absolute
confidentiality as the donor can water the sponge while contemplating this life transition.
I’ll typically even dig the hole for them.
The situations where sponges can be of benefit are varied and I appreciate hearing
about new applications. Please let me know what you discover.
Christian Meuli
4-16-14
1. The original version of this article was published in part in Rainwater Harvesting in
Drylands and Beyond, Vol. II, by Brad Lancaster (2008)
2. I published “Sponge Ladders” in the Permaculture Drylands Journal, #25, in April
1996
3. The contour gives us the information we need to design catchments to efficiently and
safely soak rainwater into the ground. The contour can be found with an A-frame,
which is three pieces of wood that form an isosceles triangle with a level attached to
the level horizontal member so it looks like a large “A.” When the feet of the triangle
are level we have identified the contour of the land; water flows downhill
perpendicular to the contour.
4. Max Lindegger, an Australian permaculture teacher did a literature review on this
topic in the 1990’s, but I have yet to locate this reference
5. Sponges can hold water too close to roots for too long, causing root rot. Many plants
will prune their roots back from too much water; also root hairs extend out amazing
distances and remodel themselves daily. See Robert Kourik, Roots
Demystified…change your gardening habits to help roots thrive (2008)