Organic mulches provide a roof over the factory of life – the soil.

April 26, 2013

in Composting, Gardening For Wildlife, Hedgehogs

Organic mulches provide a roof over the factory of life – the soil.

Edited version of an article published in ‘Simply Vegetables’ spring 2013, magazine of the National Vegetable Society

This article combines two of my passions, wildlife and composting. The value of mulching around trees, shrubs, hedges, herbaceous plants, etc., is well documented and should be encouraged. It is practiced by many gardeners for the benefits it gives to the garden and the plants planted therein.  “Compost? Mulch? Oh yes, I mulch around my  plants, its good for them”!

I also use it as a top dressing for my raised beds. I never mulch right up to the stems of my plants, leaving a couple of inches free around the plants to discourage any diseases from flourishing and allows the plant stems to breath. I remove weeds before mulching and compost them for next years mulch!

Benefits of compost mulching for the gardener 

Saves time and effort Keeps crops clean and dry Saves earthing up
Enriches soil Improves soil structure Prevents capping
Less watering required Prevents soil erosion Aids crops to ripen
Suppresses weeds/seeds Recycles waste materials Keeps roots cool
Retains moisture Aids crop production Enhances ornamental beds
Raises soil temperature-insulates Aids pests and disease control Protects soil, roots, bulbs, crops from frost
Absorbs liquid fertilisers and spray drips Slows soil nutrients leaching Rich in humus and humic acids

 Different types of mulch

I am not talking about the inorganic materials such as stones, plastic and other man-made woven materials often used as weed suppressants. I’m talking organic mulches here, such as leafmould, leaf litter, straw, wood chips, hay, grass cuttings from chemical free lawns, shredded newspaper, composted shredded bark, shredded cardboard, manures, worm compost and garden compost. Research is at long last, being undertaken to harness the properties and advantages of organic mulches, which is in effect a blanket of biofertiliser over the soil.

Minerals in soils

 Many of you know the difference between soils and compost. Fresh compost is in the main, depending upon how you make it and what you add, organic matter and living organisms. Soils contain living organisms, organic matter, humus and rock particles. The rock particles contain the mineral elements, such as iron and phosphate. It is known rocks are broken down into smaller particles by several processes, known as direct weathering. Prime examples are water freezing and cracking the rock, (physical weathering), and rock particles dissolving in acidic rainwater, (chemical weathering). Indirect methods of rock breakdown included biological weathering, whereby in moist environments mosses and lichens slowly break down the rock surfaces and acids released by plants or in leaf litter. Recent research however has found another direct weathering of rocks, undertaken by microbes, microbiological weathering! Bacteria and fungi have been found to use a variety of mechanisms to release minerals from rocks.

There is one huge aspect of organic material mulching that is so valuable, with many people unaware of these values. That is its value to wildlife.

A compost mulch – Food resource and home for wildlife and microbes

A whole new world under our feet!

There is a whole and surprisingly complex universe of life directly under your feet and in your compost bin! Microbial activities within the mostly mineral based soil is perhaps more complex than many other micro biological habitats, especially if plants, trees etc., are growing in it. Microbes, such as bacteria and fungi are absolutely essential to terrestrial ecosystems. Yet they are given very little thought by us or researchers in general. They are the motor for recycling nutrients and elements from plant material (such as carbon, phosphorous, sulphur and nitrogen). Some bacteria even recycle gases from the air! A prime example is the bacteria that take nitrogen from the air and deposit it via nodules on your runner beans roots.

Hot spots”

Plant root surfaces and rhizospheres (areas immediately around actively growing plant roots) are known as ‘hot spots’ of activity for microbes, particularly fast growing bacteria and fungi which have been called ‘fast flora’. There is a whole ecosystem around your plant roots! Moving up the roots, nearer the shoot, i.e. the older region of the plant, root hairs and root cells may be sloughed off where the grazers, e.g. protozoa and nematodes feed. Further away from the hot spots, >4 mm, slower growing bacteria, fungi, organic matter fragments and some mycorrhizal fungi hyphae are found. A hidden jungle world of predators and prey all live in soil and mulches.

How can this benefit plants, birds and other wildlife? 

The microbial activity within soils and compost heaps have similarities and some unique characteristics. Laying an organic mulch on top of soil marries the two systems to some extent, which in turn can benefit the larger wildlife, such as birds. In and around plant roots the area is teeming with microbes. Worms are attracted to such ‘hot spots’. Many people are aware that worms live in compost heaps and soil. They believe that the worms are eating organic matter. In reality, earthworms are predators of microbes and are swallowing the organic matter because that contains the microbes who themselves are consuming the waste material!

Hedgehogs love mulches!

Hedgehogs will find more food in a compost mulch than on your lawn

Hedgehogs and birds will hunt for earthworms in compost mulched areas and ‘hot spots’ of microbial activity. For example, I have seen blackbirds pull out newly planted onion sets from the soil, to seek out and eat invertebrates and earthworms that themselves were feeding on the soil microbes and detritus underneath the sets and amongst the roots. Compost heaps would certainly not have such ‘hot spots’ with them not containing living root systems. But garden compost, laid on top of soil around a plant would encourage and promote root growth, and therefore ‘hot spots’! The mulch will be colonised by many forms of wildlife who use it for dispersal, shelter, breeding or foraging. In fact, many invertebrates will complete their whole life cycle, a whole universe of  biodiversity right under your feet in your very own garden!

Mulch as a protector

Mulching protects the soil structure from heavy rains and frosts. It can help prevent the soil freezing to a greater depth, than would otherwise would have occurred. It helps to maintain a more even and stable soil temperature. Stability in an environment enables and encourages biodiversity within the mulched area and hence your garden. Heavy rains can easily wash away the finer soil particles and small weed seeds from the surface. Dunnocks, for example have a beak designed to pick up small leaves and twigs in search of seeds and insects which they pick up and eat. They, unlike blackbirds, cannot dig deep into the soil itself and grab larger prey items. The mulch supplies a constant food supply for the numerous small invertebrates underneath it and offers them egg laying or nesting areas, protects and shelters them from the weather and predators.

Benefits of a friable soil

As the mulch decays (i.e. consumed by the microbes) worms pull the mulch into the soil which in turn assists in making the soil loose and friable, allowing birds to hunt easily in it as opposed to having to chisel their way through a hard crust of surface soil or frozen soil. A more friable soil allows invertebrates etc., to burrow deeper in situ as opposed to moving out of the area altogether, like a cellar in your house! They can stay put instead of exposing themselves to predators and the elements. The friable soil allows ever precious air to find its way into the soil helped by worms as they create tunnels through it, these tunnels offer hiding places for invertebrates. The looser the soil, the more invertebrates can find places to live and eat. The more invertebrates, the more predators.  Soil life determines the structure and porosity of your soil. Just look at your allotment soil as opposed to hard clay which most soil life find difficult to live in.

Friable soils encourages and promotes root growth and hence earthworms! Mulches can also assist in preventing some weed seeds from germinating leaving them in the uppermost soil layers to be found by birds that feed on small seeds, such as dunnocks, as they search through the loose mulch material. They want to find seeds not young growing weeds! Soil organisms thrive under a mulch, improving soil biodiversity and eventually assisting in the release of useful plant nutrients. As mentioned above, mulches retain moisture which allows more small creatures to live in it. Dunnocks in particular enjoy eating enchytraeids, (pot worms), which need a moist environment to live. They need a watery film within the soil which has to be kept in direct contact with their body to stop them drying out and aids their mobility. They are known as ‘saprovores’, meaning they feed upon dead and decaying organic matter, just like the larger earthworms, which they resemble except they are much smaller and whitish. They love mulches, and  can be plentiful there. Mulches provide food, shelter, moisture and darkness, all of which they need. They are also preyed upon by many ground living beetles and their larvae, predatory mites, nematodes, fly larvae and centipedes.

What’s for lunch?

What tasty tit bits can birds and other wildlife find amongst a mulch? Besides numerous organisms again with their predators, which we nor birds can see with the naked eye, there are enchytraeids, earthworms, mites, springtails, numerous fly species and their larvae,(fruit flies in abundance), thrips, fungus gnats and their larvae, earwigs, caterpillars, beetles and their larvae, grubs, spiders, centipedes, leather jackets,  ants, woodlice, millipedes slugs, snails and many other bugs and invertebrates. I have watched on many occasions blackbirds finding small slugs amongst the mulch. They have to continually wipe their beaks on the nearby grass lawns to deal with the defensive slime before eating it. You may get lucky and find a pseudoscorpion!

Songthrush, snail and anvil

A song thrush using an anvil to crack open a snail- a natural form of pest control!

 Birds diet and calcium

Calcium is an essential element in the diet of birds, particularly when they are producing eggs which need calcium to be viable. Insects are low in calcium and in winter months insects are less available anyway. Birds that eat insects or seeds usually depend on a calcium rich food source to supplement that deficiency. Our garden songbird bird families do not ‘store’ the extra calcium needed for egg laying in advance in their skeletons. They have to eat it. Without sufficient calcium many birds can produce defective eggs and have a higher rate of hatching failure. Besides matching the breeding cycle with maximum food availability, temperature and day length, females have to have enough calcium in their gut to actually lay an egg. Female blue tits, for example, are known to stop their normal food feeding bouts in time for them to find a calcium resource, just prior to roosting. In effect a calcium feeding bout.

The importance of calcium 

If you live in an acidic area, it is likely that snails will be less abundant and birds need longer to find calcium, depriving them of food searching and feeding time. In fact great tits were noted to break away from feeding to double their calcium searching time when none was readily available for them, even burrowing in the soil to eat sand and small stones. All time consuming. Less time feeding can result in birds not receiving their full quota of nutrients required for daily living. This in turn can result in birds laying fewer eggs, smaller eggs, or just having one brood instead two or more. Calcium is also required for other larger birds as some of it may be taken from the skeleton when laying eggs and it would need to be replaced. Young birds growing rapidly need calcium.

Calcium sources

 Calcium, like other elements, can be leached from the soil by rain and particularly acid rain. Where do they find the large amounts required for egg production? Grit, eggshells, calcium rich sand, lime rich mortar and possibly the bones of dead small mammals and bones found in owl pellets. But who has such calcium rich sources in their gardens? Some bird food retailers add oyster shell grit to their bird foods to assist birds, at breeding time. There are records of song thrush or blackbird egg shells, and small snail shells found in blue tits nests. Birds are also known to eat their own shells when the young have hatched. Waste not, want not!

Different species find different sources

Robins, wrens and pied flycatchers will forage on the ground to obtain their calcium requirements by eating woodlice and millipedes, both abundant dwellers in compost and mulches, with 3 times higher the amount of calcium than snail shells. If you have many millipedes and woodlice in your garden it may well indicate a calcium rich soil although they require less calcium than snails. They are experts at finding them.

Whereas blue and great tits are canopy feeders and in general do not forage often on the ground. By having more tasty morsels for them to eat on the ground, particularly millipedes and woodlice, they may well serve as additional calcium sources for them or other small insectivorous birds if they do venture on the ground which they must do to search for snails shells. Young blackbirds are unable to get enough calcium for their growth from earthworms alone. But can from the soil found in the earthworms guts! Compost and compost mulches attract worms in abundance. Thus a nutrient rich compost may well provide enough worms and subsequently soil for young blackbirds to thrive.

Utilising snail shells for eggs

Blue tits have been found to eat a snail shells just before they go to roost overnight. Once inside a female blue tit’s gizzard when she goes to roost the calcium in snail shells would be utilised about 36 hours later for an egg to be laid. If she consumed it any earlier in the day, it would leave the gut by the time it was needed for eggshell formation. She only has a narrow timeframe and timing their calcium intake greatly affects the birds ability to collect enough calcium for eggshell formation. Lack of calcium is probably the most limiting macronutrient required by laying birds. There a reports of poorly developed young tits in pine woods possibly due to the fact that such woodlands are in general acidic where snails are rare and the parents may not be able to find enough calcium for their young to develop normally.

Snails, compost and song thrushes

Song thrushes are a snail specialist. Their behaviour is unique amongst bird species. In fact, when the ground is dry or frozen and earthworms, a key resource for them, are scarce, snails prove to be a life saver for our shy song thrushes. Hence they have evolved to predate upon them and have learnt how to smash open the shells. Song thrushes will diligently search through composted mulch patches in search of snails. The larger, more aggressive blackbirds are more catholic in their food choices and can find alternative food resources, when the ground is dry.  Unlike song thrushes they cannot make use of snails as a food resource. Although they do bully song thrushes who waste time and energy searching and cracking open the shells only for the blackbird to steal the snail when all the work is done! This is known as kleptoparasitism! Ordinarily birds do not eat living snails because of the hardness of their shells. They eat the empty snail shell. Snails will lay their eggs in the compost, whether it is in the garden compost bin or in the composted mulched borders. Snails try to avoid being eaten by making their shells stronger or more ornate. They hide until the shells are hard. This needs calcium, gleaned from the vegetation growing in soils with a calcium content and time for the young snail to grow. Young snails hatch as soon as the weather warms up and as they have a very thin shell it is very easily crushed hence are targeted hedgehogs. These young snails need moisture and food, protection from predators and the drying effects of the sun, all amply supplied amongst compost mulches and compost bins.

Slug pellets

To protect their beautiful flowers bought from the garden centre, many people use slug pellets. Besides the toxicity of slug pellets, killing snails with poisons reducing their numbers, can only deplete snail shell availability in our gardens, at this critical time for our birds. Snail shells are considered to be the main source of dietary calcium for them and song thrushes depend upon snails themselves through the dry summer and are an absolute bonus in the frozen winter months. They are steeply in decline and are red listed as a bird of serious conservation concern. Something to ponder upon!

Collect empty snail shells!

Many snail shells are found in winter when the weather has killed them. I do collect snail shells, crush them along with egg shells and mix the lot up with bird seed placed on a ground feeder tray. I leave a few larger snail shell fragments so that the birds can recognise the snail shells. I have seen blue and great tits, wood pigeons, collard doves, robins and blackbirds in my garden eating snails shells, all found amongst my composted mulched borders.

Snail shells are used eaten by birds for their eggs

Snail shells, a useful calcium source for garden birds

 Who may come for an invertebrate lunch?

What wildlife have I seen taking advantage of this feast found in my mulchings? Pied and grey wagtails on my mulched and open vegetable beds, which are next to my pond with the following birds foraging and eating invertebrates within my borders with trees, shrubs and plants;  robins, blackbirds, redwings, mistle thrushes, field fares, jays, magpies, wood pigeons, collared doves, chaffinches, dunnocks, very occasionally blue tits, coal tits and great tits, with starlings and house sparrows on the fringes of the borders, greenfinches and goldfinches looking for seeds and even the wren ventures down onto the mulch scurrying here and there picking off some tasty morsels with song thrushes eating snails. I have seen birds carry away mulched materials to make their nests with. I’ve observed the sparrow hawk as it walks in amongst the bordered shrubs to flush out birds or corner them next to the wooden fencing panels as they were busying themselves in the areas. Hedgehogs, wood mice, frogs, toads and newts have all been seen hunting in or feeding amongst my mulched borders.

But the bird that most satisfied me and gave the most pleasure was the chiffchaff in late December early January 2008. It stayed in and around my garden for a least 4 days. It was feeding upon and inside the flowers of a mahonia ‘charity’. What fascinated me though was it keep flitting and even hovering for very short periods, darting from one shrub to another close by. I saw it catch numerous small gnats as they danced their little dance a couple of feet above the mulch. The mulch was a mixture of garden compost, worm compost and leafmould all mixed together about 3 inches deep and was being used by the possibly to feed or rest.

Using garden compost or worm compost from your food waste is an excellent way to recycle nutrients into the soil. Mulching around your valuable plants, is an investment well worth the small effort in undertaking this worthwhile task. Besides an investment in your plants, treat it as an investment in wildlife too, much of which you will not yourself see! But be satisfied in the knowledge that many other creatures will! I would say that there would probably be more small invertebrates within your mulched areas than your lawns making it far more attractive to our garden wildlife with whom we share our gardens and allotments!

Soil life and food web jpeg

Download the benefits of mulching to wildlife and birds

Refs. Besides my own experiences and observations:

Graveland J, Berends AE. (1997), “ Timing of the calcium intake and effect of calcium deficiency on behaviour and egg laying in captive great tits, Parus major.” Physiol Zool. 1997 Jan-Feb;70(1):74-84.

Graveland. J & Drent, RH (1997) “Calcium availability limits breeding success of passerines on poor soils” Journal of Animal Ecology  66, 279-288, British Ecological Society

Jeffery. S et al (2010) “ European Atlas of Soil Biodiversity”, European Commission, Luxembourg

Perrins. C (1979), “British Tits” The New Naturalist, Collins, London

Pilkington. G (2006)” Composting with worms. Why waste your Waste?” Ecologic Books, Bath.

Toms. M, (2009) “Going for a song. Song thrushes” Bird Table 59, Summer 2009, British Trust for Ornithology

Photo of song thrush with thanks from Roy & Marie


{ 6 comments… read them below or add one }

roger wood April 26, 2013 at 11:10 am

very interesting article and one we can all learn from, thanks


nurturingnature April 26, 2013 at 4:06 pm

Thanks Roger, such a valuable resource made from waste whereby everything benefits, a real case of a win-win situation!


charlotte May 1, 2013 at 8:00 pm

Hi George,
I really enjoyed reading this article. We have a really big compost section in our garden and i regularly take out the slugs for our resident robin to feed on . He seems to know exactly when that lid comes off the composter ……….
As for the calcium…after reading your article if I find any empty snail shells I shall cruch them and put them on the bird table ……..I cannot think of a better way for those horrid smails to be recycled !!


nurturingnature May 2, 2013 at 5:06 pm

Thanks Charlotte, you only need put them on the table prior to breeding, although blackbirds nest for a long period and may have several broods….so do keep putting the crushed shells out. Cheers George


Ari August 23, 2013 at 7:39 am

Love your article, and I never thought to crush up snail shells! I often find them around the garden after the Song Thrush has been.

I crush egg shells and sprinkle around my plants to protect from slugs, it never occurred to me that birds would use these and snail shells for calcium. I will be amending my gardening routine to incorporate this.

Many thanks


nurturingnature August 24, 2013 at 7:09 am

Thanks. Seems to make sense re recycling snail shells eh?!!


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