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.

Man­age weeds

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.

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