Digging It!
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| Kwan Yuen surrounded by native honeysuckles and agaves |
My recent garden visit from Les and Jay was an eye-opener from the start, because the conversation began with talking about one of my favorite topics: soil. My childhood and teen years of gardening in the heavy clay of the Midwest, then spending 40 adult years gardening in the saturated sands, glacial till, and clay mixes in the Northwest, taught me a lot about the importance of creating and maintaining healthy soil. However, neither of those experiences prepared me for gardening in hot desert soils, especially in the blazing months of summer!
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| Food-digester compost being harvested |
When we first began gardening here in Arivaca in the fall of 2016, we bought two barrels of aged alpaca manure to add to our concrete block raised bed veggie garden, along with some store-bought compost. Every fall since then, I have added the contents of our 3 small “food-digester” composting bins to it before planting. (Food-digesters are an easy DIY project to make, and ensure that rodents are kept out of your food waste while it's composting) Last fall, when I added 2 steel stock tank raised beds to my studio porch, I used good quality potting soil with lots of organic matter in it to fill the beds before planting my veggies. I also installed micro-drip irrigation to all the beds for consistent root-zone watering. That all sounds good, right? And a bounty of fall and winter veggies from November- May has come from both garden areas. But…I made big mistakes in both garden areas when planting and growing my summer crops:
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| No organic matter left from my fall additions! |
· In my existing concrete block raised bed, my main mistake was not adding enough organic matter each year, due to how baked the soil gets in the summer by the sun and the radiant heat from the concrete blocks. These conditions make organic matter get used up much quicker than in the Northwest. I should have been adding at least 2-3 times as much compost each year to counteract the baking of the soil and the washing away of the organic matter by irrigation and rain. When digging into this concrete block raised bed soil after talking with Les and Jay, I found almost no trace of organic matter left in it!
In my new stock tank beds, my main mistake was adding too much drainage, as if I was still gardening in the rainy Northwest. I had added a layer of crushed rock in the bottom (after drilling many drain holes in each stock tank) and mixed a bunch of extra perlite into the potting soil. I’m used to doing that in the Northwest to prevent the soil from becoming a sticky gooey mess, but it was not a good idea at all here! In fact, it counteracted all the benefits of the organic matter that was in the soil to start with. Plus, the summer sun-baking and irrigation in both beds had used up the organic matter there too.
Jay also reminded me that during the hottest months of the year, some soils can dry out enough to resist water infiltration, so it’s best to do “pulse watering”, even when using drip irrigation: Turning it on for 30 minutes, then letting the soil sit for 15-20 minutes to start absorbing the water, and then turning the irrigation back on for another 30 min. In all my years of designing gardens in the Northwest, I had seen hydrophobic (water resistant) soil in only one client’s property, which had extreme levels of silty sand. The minute I started thinking about how fast my irrigation water was running through all of the raised beds each day, Jay’s words rang so true: without enough organic matter in the soil, the water was simply not able to be absorbed enough to nourish the plants!
No wonder most of my summer veggies couldn’t survive! In all the sustainable garden classes I have taught for the past 20 years in the Puget Sound area, I always lead off by saying, “healthy soil = healthy roots = healthy plants”. Yet I had not really noticed this summer how my own soil-building and watering choices had negatively impacted my garden during the blazing heat. Humbling and enlightening at the same time.
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| Soil transformed with compost |
OK, now that I knew the problem, how did I go about fixing it? My first step was to remove enough of the native soil in the concrete block raised bed to add almost ½ yard of organic compost. My second step was to ask our neighbor Brad if we could dig up some of his clay soil to mix into the stock tank beds to increase water retention, which was another recommendation from Les and Jay. I also mixed in several inches of compost to each of the stock tank beds, and after planting a variety of starts, added a two-inch layer of compost as a top mulch.
Though it was hard work, these soil fixes were fairly easy to do in 100 sq. ft. of raised bed vegetable gardens.
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| Wood chip mulched planting bed |
However, I would really be kicking myself if I had made these mistakes in the 2000 sq. ft. ornamental garden in my front yard... but thankfully, I had been smarter with my soil-building and maintenance there from the start: In 2016, after removing a choking layer of landscape fabric and crushed rock (thank you Jerry and Tom!) and building a series of curving, mounded berms throughout the garden, we mixed in approx. ten yards of aged horse manure from the Arivaca Boys Ranch to the soil. After planting dozens of native trees, shrubs, cacti, and perennials in the beds, we added a two-inch top layer of arborists’ wood chips as mulch over all the soil. In my experience over the past 30 years, arborists' wood chips are the best mulch for woody shrubs and trees, as well as perennial plants, because they:
- Conserve soil moisture;
- Foster the growth of mycelium, which forms a symbiotic relationship with the plant roots;
- Suppress weed seed germination;
- Prevent soil compaction and retain air in the soil even when people or animals walk through the beds;
- Slowly feed the soil and beneficial microorganisms with organic matter as they decompose;
- Will not lower nitrogen levels as long as they are laid on the soil surface and not dug into the soil.
(Beauty bark, bark dust, or other manufactured woody mulch have none of these benefits, and in fact typically become a compacted, plastic-like barrier to air, water, and beneficial microorganisms)
In the 3+ years since our initial wood chip mulching, we have only had to replenish the chips in a few areas, but sometime this winter, I plan to get another good sized load of chips to make sure that all the mounded beds have a two-inch layer again. These soil-building methods, coupled with the fact that native desert plants require much less organic matter to thrive than annual vegetable crops do, have added up to healthy and sustainable soil conditions in the front garden.
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| The new veggie bed |
Thanks to Les and Jay’s inspiration and wisdom, and a lot of mulling, we're building a new mounded bed in the back yard for growing more veggies, where there is shade from Mesquite and Hackberry trees. So far, we've used a combination of our own native soil along with more
clay soil and aged cow manure (thank you Brad!), and plan to add some more organic compost and potting soil mix to increase water retention even more.
From now on, I plan to add plenty of compost twice a year in every one of our veggie beds to keep the soil healthy, and to do “pulse-watering” in each of them, especially during the heat of summer!
