Mulch and moisture retention

The highest cost in growing one’s own fruit and vegetables in Adelaide (South Australia) is the cost of water, where the new water prices for the 2011-12 financial year were:
1) Quarterly water supply charge of $58.65
2) $1.93/kL for water use from 0-30 kL per quarter
3) $2.75/kL for water use from 30-130 kL per quarter
4) $2.98/kL for water use above 130kL

During the summer months (December through to the end of February) I’ve used 166kl (166 000 litres) of water from the mains supply. Almost all of this water went onto the garden, with household use being negligible after all the practice we’ve had conserving water during the recent decade of droughts.

Nor does this figure include the 60 kl of rainwater carried over from last winter in our giant tanks and now almost gone. All this, despite cutting back on the size of the garden planted out this year as a consequence of the gardener’s absence during the critical spring planting season…

So the cost of water for this past summer has been: -

$58.65 + 30kl x $1.93 + 100 kl x $2.75 + 33kl x $2.98 = $490.

Not far behind the cost of water is the cost of mulch to protect the soil, prevent evaporative losses and to keep the soil cool and the worms near the surface. We use pea-straw for this, purchased in bulk at about $6.60 per bale, and needing about one bale per square meter to apply.

The cost of buying water and mulch heavily outweigh the cost of seed and seedlings, new tools and various stakes and fencing materials in the gardener's budget. I’ve never had the courage to do a more careful analysis, but I reckon the cost of operating this 1000 square meter garden runs to some thousands of dollars each year, putting real pressure on the gardener to produce edible food sufficient to offset the costs of shopping at a greengrocer.

So each year I try some new method of making water and mulch go a bit farther.

This year I’m laying down about $500 worth of pea-straw (about 75 bales) along the dirt paths surrounding the fenced growing beds. This layer of mulch will allow winter rainfall to percolate through for safe storage well into next Spring, give the chickens somewhere to scratch for earwigs, and provide a mulch in a more broken-down form for surrounding small plants come next Summer’s heat waves. (Mulch for seedlings is collected off the paths, as it is softer than new straw and less full of moulds and other stuff from the farm where it grew).  Each bale is broken up into ‘bricks’ about 10cm wide; these are as easy to lay down as carpet squares, and are just dropped on the soil and kicked into place.

Over the next five years, these constant applications of mulch will build up the carbon content of the soils and make them more friable for tiny roots to grow through. In the orchard (below) those new grapevines are doing well in last year’s mulch, now broken down by the constant contented scratching of the five hens who hang out here.

In the long-term though, I’d like to reach a place where I can grow my own ‘green cover’ during winter, cut it in Spring, use it for mulch over Summer, and grow all crops on the 75 000 litres of water captured from winter rainfall on surrounding roofs and plumbed into those tanks.

Mulch in a ten-minute vineyard

Anyone who experienced the fortnight of straight daily maximum temperatures over 40 degrees C (104 degrees Fahrenheit) in the drought-stricken summer of 2009/10 on the Adelaide Plains will now be preparing for a summer kitchen garden with a whole new sense of caution.

Top of the gardener’s list is to lock winter rainfall into the soil against evaporative forces over the next few months, and to keep that soil cool enough to bring various crops through.

  

 

 

 

 

A couple of other nasty things rolled over us as a consequence of that drought-stricken decade; fresh food prices went through the roof and so (much less visibly) did the cost of pea-straw mulch. Worse was to follow: water was rationed to the point that one had to live outside the law to grow one’s own food. The 60 000 litres of winter rain water that I am holding in my tanks will be enough for only a month’s supply in this large garden if the heat turns back on and spring rains fail us. To eke out that supply, I have to invest instead in bales of pea-straw laid down in slabs across the whole garden. Each bale costs A$7.00, and longer-lasting barley or lucerne straw costs significantly more.

So now in my Spring garden I plan for these harsh conditions, while hoping for the best. Every shower of rain is a blessing from now until next April. I’m paying for pea-straw mulch for this year, but next year’s winter garden will grow my own mulch in-situ while adding some soil nitrogen fixed from the atmosphere by broad bean crops grown this past winter for bulk seed; kilograms of it. Broad beans shade the soil long into late Spring, and can be cut down in place as mulch. I’m growing two types; the large heritage Aquadulce broad bean for eating, propagation and seed, and the smaller Coles Prolific as mulch.

And so here in our fenced orchard, under a $100 coat of pea-straw mulch, are 16 Isabella table grapevines grown in a pot over winter and costing nothing at all (they were given to us by seed-saver friends). These grapes will provide fresh fruit on our breakfast table in year’s to come, even during drought and times of high food prices.

And yes, we did plant the grapevine cuttings in only ten-minutes; preparing the area and laying the mulch took up much more time…