Cover your bases to boost moisture, nutrients and soil texture
A thick layer of organic mulch is the best way to keep your plants happy, save water and improve your soil. Donovan Kirkwood gives us the lowdown.
Yes, it is summer and yes, in a water-scarce, drought-prone country like South Africa the water-saving benefits of mulch are immense and well known. We all need to use a lot less water in our gardens, which means we need to use a lot more mulch.
However, few gardeners appreciate just how much plant-based mulch can benefit soil texture and nutrient status, make weeds easier to manage, and provide low-maintenance paths and beds. Carefully swept, “clean” soil is still deeply ingrained in our South African gardening and farming cultures. It shouldn’t be.
Reduce moisture loss
Any porous material that traps a layer of still air at the soil surface dramatically slows evaporation and reduces soil moisture loss. Mulch needs to be applied thickly to achieve maximum benefit – a layer at least 10cm-15cm thick will reduce soil evaporation loss to around half that of bare soil. That translates directly into less watering needed.
The moisture benefits to mulched plants are more profound than reducing watering needs, though. Soil moisture content and temperature fluctuate less with mulch, especially in the surface >
layers where most plants have dense feeder roots. For vegetable growers, this improves plant health and prevents spongy or split fruit and veg, which happen when soil dries out too much or gets dry and is then watered.
For some reason, even enthusiastic mulchers tend not to mulch pot plants, containers and raised beds. With a relatively small volume of soil relative to the surface, this is actually where the benefits of mulch are the biggest, reducing watering needs dramatically and keeping soil temperatures from shooting up on hot days. Container gardening with mulch is not just for urban dwellers but can be a very water-efficient approach even for plattelanders with lots of space.
Mulch is also a remedy for those of us who struggle with so-called “oily” sand – water-repellent soils due to the presence of plant waxes and specific fungi. The mulch layer will allow soil to become and stay moist, and support beneficial fungi that, over time, degrade the waterrepellent layers around sand grains.
Keep your soil healthy
Mulch is the ultimate soil structure and fertility builder. The protected moist interface between mulch and soil is rapidly colonised by soil microorganisms, earthworms, isopods and insects that eat dead plant matter. Not only do they unlock nutrients steadily as they slowly consume the mulch, but, helped by gravity and water movement, they also transport soil-building organic matter downward into the soil, improving texture and aeration, reducing compaction and increasing water-holding ability.
The myth that woody mulch robs the soil of nutrients is just not borne out in reality. Yes, those decomposing bacteria and fungi can withdraw nitrogen from their environment to do their work, but measurements show that as long as wood chip isn’t covered with soil, the nutrient-trapping effect of woody mulch extends a few millimetres at most, with no meaningful loss of soil fertility in the root zone. Over time, as the mulch decomposers are themselves returned to the cycle, the nutrient content of the soil below steadily increases. Even pure wood chip with minimal initial nutrient content is colonised by bacteria that actively convert nitrogen from the air into plantusable forms. So even very low-nitrogen mulch increases soil nitrogen over time.
One typical scientific trial showed that after five years, trees planted in degraded soils with 15cm-deep wood-chip mulch had substantially improved nitrogen and phosphorus levels compared to untreated soils. And although soil nutrient levels under plain mulch were a little lower than in areas composted every year, tree growth was 50% higher with just plain woodchip mulch compared with compost, and nearly three times higher than in bare soil with no treatment.
Of course mulch can also include some or all loose green, leafy material, including coarse cut grass. This has higher plant-food content than wood chip, and breaks down within a few months to a year. This is still slow release by compost or fertiliser standards, continuously feeding and improving soil texture in the critical plant-root feeding zone near the >
Is compost fast food?
It seems that modern gardening is all about instant everything. The internet is crawling with click-bait promises of quick, easy solutions. Even permaculturists often take pride in showcasing weekend installations yielding immediate vigorously growing food plants rather than emphasising the complex, slow business of creating a system of self-supporting nutrient and resource cycles.
Mulch does have immediate soil-water advantages but its true soil-building value takes at least a few months. On the flipside, mulch’s soil benefits are utterly reliable, easy, cheaper and more environmentally friendly than the alternatives.
Most of us garden organically, avoiding highly soluble commercial fertilisers usually considered to be damaging to soil health. But even the most popular natural soil builder, compost, can also cause big nutrient pulses, resulting in overfeeding and even environmental pollution, and then break down and disappear in what seems like no time. The intense biological activity of commercial hot composting means that a substantial amount of the macronutrients that it contains, especially nitrogen, is converted from being trapped in complex, stable molecules to more simple, volatile ones.
Moderate applications of compost or liquid feeds are of course useful for very nutrient-hungry plants and vegetables like chard, spinach and the various cabbages and Asian greens, but highly available forms of plant nutrients, in excess of immediate plant needs, just evaporate or leach away.
Freshly made manure-based composts can burn and kill plants, especially lownutrient-adapted indigenous plants. Furthermore, compost needs to be reapplied often, especially in sandy soils, and it’s expensive to buy. Making your own compost from garden waste is frankly a giant hassle, what with getting ratios and moisture just right if you want a hot, quick result – and then all the turning! Less rigorous cold composting still takes ages and lots of labour.
Why bother when shredding the same material and spreading it over the soil surface provides all the same nutrients in a perfect slow-release system that also saves water and does even more to build good soil structure?
surface, which is perfect for food gardens and fruit trees. One of the immense benefits of mulch is that it has the most impact on moisture, soil quality and nutrients close to the soil surface. And that is precisely where nearly all vegetables and most other plants have dense networks of horizontally spreading feeder roots, an adaptation to a natural world where most ecosystems naturally accumulate and decompose plant material at the soil surface. Pull aside mulch in a healthy vegetable garden and you will reveal a happy network of fine white roots right at the soil surface.
Most weed seeds are stimulated to germinate by sunlight. A covering layer of mulch dramatically reduces weed emergence. With less compacted soil, weeds are also much easier to pull.
In badly infested areas, weeds will just grow through normal mulch, but a technique known as sheet mulching can be effective – weeds are roughly slashed, given a decent sprinkling of compost or manure and a solid watering, then covered with several layers of overlapping cardboard and topped with a really thick layer of mulch. The cardboard provides a solid barrier to weeds, preventing regrowth and new germination until a lot of the seed load in the soil dies off. Desirable plants can be planted straight into holes punched through the cardboard, although in my experience it’s best to sheet-mulch a season ahead and be patient. Over time the cardboard decomposes, leaving improved soil with fewer weeds.
This technique doesn’t always manage to eliminate established kikuyu grass or the most pernicious weeds like false onion weed, but it’s a great way to establish a new garden on a neglected site. There is still no substitute for good weed management, especially not letting any go to seed, but mulch does a lot to reduce the problem.
Protect the soil
Woody mulch acts a bit like snowshoes, spreading the pressure of footsteps, and protects the soil below. Mulched garden paths can double the rooting space available to plants in neighbouring beds instead of being compacted dead zones.
The other protective effects of mulch are less obvious. Any but the finest water sprayer will damage bare soil structure by puddling and mixing with direct water streams, destroying loamy structure of healthy soil in seconds. Mulch acts as a permeable filter, spreading water so it percolates in, supporting building an aerated, crumb-like loam structure. This goes for sandy and clay soils, and the effect is particularly valuable in pots where >
There is still no substitute for good weed management, but mulch does a lot to reduce the problem.
frequent watering and small area mean soil mixtures are easily damaged by a too-strong jet of water. For gardeners in the Cape, and others with no choice but to pour grey water from buckets or watering cans around their most precious plants, this is useful in the rest of the garden too.
In windy environments mulch is a lifesaver, keeping sand and dust from blowing everywhere. As lawns are increasingly removed or reduced in size because they’re too maintenanceintensive, dead from drought, or the footprint for water, fertiliser and power for mowing no longer seems reasonable, mulch is the soft affordable alternative to permanent paving or impossible-toremove gravel.
The negatives of mulch
The vast majority of bugs and microbes that live in mulch are entirely beneficial, feed on plant matter that is already dead, and never attack living plants. Healthy populations of beneficial organisms actively reduce soil and above-ground pests and disease.
Unfortunately there is one pest that thrives in mulch: the vine weevil. It’s perhaps the only real negative of mulch for gardeners who want perfect-looking plants. These small chewing beetles shelter in mulch during the day and climb up at night to nibble new stems, flowers and leaves. Because they’re nocturnal, even a healthy population of insectivorous birds won’t control them. Fortunately vine weevils typically make plants and flowers a bit raggedy without killing them and, like most pests, they are only occasionally a problem. Small numbers can easily be found and handpicked using a torch at night. Bigger infestations might require one or two treatments with a spray that targets chewing insects. One effective and safe option for chewing pests is the organically approved Spinosad (marketed locally as Efekto Eco Insect Control SC). Just be sure to apply it on still evenings once bees and other beneficial pollinators have gone to bed.
Cutworms, the caterpillars of several moth species, also shelter at ground >
level during the day and kill whole seedlings and soft-stemmed plants by chewing through the stem. Although mulch could be blamed for providing shelter, cutworms don’t seem to be more of a problem in mulched systems. In fact, they’re easier to locate by pulling aside mulch. (In bare soil they burrow and can be hard to find.) Even slugs, those other soil-level nasties, don’t seem to be noticeably worse in mulched areas.
Mulch could theoretically harbour plant disease. Studies comparing no-till grain farming with straw left on the lands sometimes find slightly higher rates of fungal disease – but so little that the long-term farming benefits of retaining organic matter are still overwhelming. I have never noticed higher rates of mildew or other problems in mulched beds. If anything, fungus-prone crops seem to get fewer problems when protected by mulch.
The how to of mulch
The best time to apply mulch is five years ago. The next-best time is now. Really thick mulch can absorb enough water to slow down soil wetting at the start of the rainy season. If this is a worry for early planting, just pull mulch aside in key areas as needed and put it back once the soil is wet. • Make it thick To ensure maximum soil moisture retention you should apply a layer of at least 10cm, but 15cm-20cm thick is even better and means you won’t need to top up nearly as soon. This means 1m3 of mulch will only cover 7m2 of ground. It’s better to mulch target areas properly than to try make a small amount cover a huge area. • Top it up How long mulch lasts and how much it needs to be topped up depends on mulch type, climate and soils. A rough guide: 10cm of pine wood chip will usually last at least five years before showing signs of getting thin and patchy. Since mulch mostly decomposes from below, thicker mulch will last longer, and it’s likely to be more costeffective to buy in bulk once than to top up with smaller amounts frequently. • Planting Planting vegetables into mulched beds mostly means growing seedlings in plugs and small pots and planting them through mulch. Early and faster growth makes it worthwhile. Bigger seeded species like beans, peas, turnips and maize can be drilled directly into the mulch and will grow through it without trouble.
FROM LEFT Mulch provides a happy space for a range of organisms, such as these shaggy ink cap mushrooms; potted herbs love a layer of mulch; tasty spring onions from a mulched bed.
This healthy mulch garden attracted a western leopard toad. Rock mulch is well suited for use around succulents. A good example of rough mulch. The proof is in the pudding – this broadbean crop was grown in unimproved sand treated only with a two-year cover of plain wood chip mulch before planting.
FROM LEFT You’ll need a chipper to process bigger branches and prunings; pumice gravel in a pot; fallen leaves make excellent mulch around larger plants and trees.
Vine weevils, the one pest that likes to take shelter in mulch, are controlled relatively easily. RIGHT A small chipper may be the most cost-effective way to turn trimmings and other garden waste into this.
Another example of scavenged or rough mulch. Mulch around sown-in-place vegetables like carrots after thinning them. Used chicken bedding shouldn’t be applied fresh in a food garden. Mulch boosts moisture, soil quality and nutrients close to the surface – right where the feeder roots of most vegetables are concentrated.