Mulching

Introduction

Mulching is effectively a natural practice. Photosynthesis assimilates inorganic atmospheric carbon combining it with minerals and water into organic living tissue releasing oxygen. These mineral nutrients enter the fine roots of deep rooted trees in solution. The tree transports these minerals upwards from deep underground subsoil where other plants cannot penetrate. These minerals from deep underground are eventually incorporated into living leaves as they develop….

Leaves carrying these minerals fall and form a leaf (natural mulch) layer on top of the soil. Microorganisms like bacteria, plus earthworms, microarthropods, ants. beetles etc all play a part to decompose the leaf (natural mulch) layer releasing minerals to the top of the soil where other shallow rooted plants can access them.  Soil moisture is conserved and a rich ‘O’ horizon is formed as part of the soil profile. This is the process of natural mulching, and nutrient cycling. This natural system works in-situ and functions perfectly because it is undisturbed, and in general it takes 2 years or more for decomposition of an annual leaf fall..

Selecting just one natural example, eg: Prunus padus (Bird Cherry) to illustrate this natural ‘mulching’ system we can see that it is an ever continuing cycle…

1. In spring as temperatures rise slowly soil microbiota become active and start to finish the decomposition of leaves that fell the previous autumn. In doing this they use Nitrogen from the leaves and surface of the soil in their cell walls as they reproduce.
2. The tree commences growth and it too takes nitrogen from the soil. A bit later the tree flowers and its fragrance attracts bees and pollinating insects to gather nectar in April (UK). After flowering the petals fall to the ground.
3. In summer soil microbial activity is at its peak. They are decomosing the remaining leaves and the fallen petals. Black and bitter fruits form and on the tree these are eaten by birds. The birds deposit nutrient rich dropping around the tree augmenting the surface organic matter and topping up the nutrients for microbial activity.
4. A bit later, on the ground badgers and other small mammals eat the fallen fruit and deposit additional dropping rich in nutrients.
5. Microbial activity is prolific in summer because temperatures in the soil are high, they have plenty of organic matter to decompose and their need for Nitrogen is supplemented on account of the droppings from birds and mammals.
6. In autumn temperatures are falling so microbial activity is declining, but new organic matter is being deposited from the leaves which fall to the ground.

This is not the same as what takes place in a productive garden or orchard. In horticulture for the purpose of production man interferes and breaks the natural mulching cycle. By removing the fruits from the tree man takes away the bird and mammal food and thus the bird and mammal droppings! This loss of additional nutrient is important and detrimentally influences microbial activity. Below is a generalised table of leaf nutrients…

What is a Mulch Horticulturally?

Horticulturally and in landscape a mulch is anything that is laid on the surface of the soil, and there can be one or more functions to the selected material and operation. The landscape designer, landscape manager and gardener choses the material and thus the function/s according to the design and management objective/s. These materials can mimic the natural ‘mulching’ process or not according to the material used.

Functions of Mulching

There are many beneficial functions of mulching:

• Weed Control;
• Soil Water Conservation;
• Prevent water splashes;
• Can minimise disease spread;
• Cools the Root Zone or Warms the Soil
• Feeding the soil ‘Naturally’;
• As a Decorative ‘Foil’.

Mulching Technicalities

Mulching is a very popular horticultural and gardening practice and it’s very beneficial providing it is carried out correctly. Carried out incorrectly and it can be very detrimental indeed, Many plants are killed each year through incorrect mulching practices, and many plants are rendered weak and without vigour because of an oversight in mulching practice.

The critical issues with mulching are as follows:

• NUTRIENT LOSS: from surface soil through decomposition;
• SOIL MOISTURE: content at the time of mulch application;
• WEED STATE: of the soil at the time of mulch application;
• LAND RECLAMATION: of certain sites possess potential and serious problems related to mulching;
• MULCH DEPTH: around the stem-root union of plants.

A deeper discussion of each of the above points now follows…

Nutrient Loss from Surface Soil: can and does occur when a mulch is laid on the surface of a soil. The extent to which it occurs depends on the existing nutrient status of the underlying soil, the thickness of the organic mulch and the state of decomposition of this mulch material at the time of application.

This happens because the bacterium which are a fundamental part of the decomposition cycle require Nitrogen for their cell walls, so as they multiply during the active cycle, (ie in late spring and summer,) Nitrogen is stripped from the soil surface – read more…

https://landarchconcepts.wordpress.com/nitrogen-deficiency-by-mulching/

This example in the link above was an extreme case, but it shows that mulching has to be done with respect to the Nitrogen requirements of the bacteria that play such an important role in decomposition.

Ideally the soil needs to be analysed and the nutrient status corrected accordingly prior to planting. Then prior to mulching an additional dressing of slow biological release organic fertiliser should be applied to the soil surface and this should NOT BE WORKED IN. The optimum choice for this organic fertiliser is Hoof and Horn Meal (13N), spread at the rate of between 30g – 40g/m2. The great thing about Hoof and Horn Meal is that it will not release its N until bacterial activity commences, thus there is no danger of an early spring flush of growth which later gets damaged by a late frost, such as would ne the case with slow release fertilisers. To review nutrients click the link below…

https://landarchconcepts.wordpress.com/plant-nutrition-and-fertilisers/

So in the final analysis the popular notion that mulches feed the soil are only correct up to a certain point. Certainly throwing uncomposted bark and lawn clippings for example on to the soil as a mulch is not necessarily going to feed your soil as much as it is deemed, indeed it is more likely to be detrimental from the perspective of plant nutrition, because it will strip Nitrogen from the soil. There are numerous ‘Quick and Dirty’ horticultural practices and garden myths like this, which are detrimental in the long-term – click the link below to read more…

https://landarchconcepts.wordpress.com/quick-and-dirty-lazy-and-amateur-horticultural-practices/

Soil Moisture Content: at the time of mulching is very important. Mulch should not be applied to a dry soil. Before the application of the mulch the soil should be irrigated to field capacity – to review more about this click the link below…

https://landarchconcepts.wordpress.com/soil-texture-ph-cultivation-and-drainage/

Applying a mulch to an irrigated field capacity soil means that walking on the soil to apply the mulch should be minimised or eliminated altogether. The use of boards to distribute weight is necessary unless the beds/borders/plantings are small enough to employ the shovel flick distribution method with final raking to finish off.

Automatic drip and seep irrigation lines should be below the mulch layer directly on the soil surface. These irrigate the planting in the deeper layers and maintains moisture at the mulch-soil interface where the bacteria are active. This is the best way to conserve soil moisture, ie by keeping the surface of the mulch as dry as possible until natural rain falls. If the mulch-soil interface and the soil itself at deeper levels are moist, plant and bacteria activity can continue, but the transpiration stream is interrupted by a dry layer of much on the exposed surface. Obviously if you do not use drip or seep irrigation this is a mute point, but if you do its an important point. Moreover it is far more aesthetic to not have irrigation pipes on display.

The Weed State: at the time of mulch application is very important. It should go without saying that applying mulch to ground infested by vigorous perennial weed is not at all wise. At the time of mulch application all perennial and annual vegetative weed growth should have been removed. Annual weed and perhaps some perennial weed seed will of course be present on the surface of the soil, but the mulch suppresses light and so inhibits weed seed germination.

THE LAND RECLAMATION ISSUES: are very specific. As a case study in point I was once involved with some gas board land reclamation sites at 2 different geographical locations. On both sites that were miles apart spontaneous combustion occurred and the entire planting areas were burned severely. These were not small areas of planting, but extensive plantings.

The planting zones were mulched with forest bark. On behalf of the landscape architects I worked for I brought in the Forestry Commission Research Station at Alice Holt Lodge and an investigation was carried out. What materialised was that the forest bark which had been applied was pine bark and it was releasing turpentines into the soil. the soils had residual methane being land reclaimed from gas board storage tank zones. The turpentine was reacting chemically with the methane and soil temperatures to cause spontaneous combustion.

Following these incidents all Texaco filling station who were also clients for these same landscape architects had the bark removed from the planting areas around their forecorts and mulching was struck out of the specification for future projects for these particular clients. As with the previous case study involving Nitrogen deficiency the issue revolved around the use of undecomposed bark. This was a rare anomaly, but it serves to emphasise the importance of bark quality for mulching, specifically the composting.

Mulch Depth: is the most misunderstood and abused practice of landscaping and gardening. The depth of a mulch can depend on which mulch material is selected. For example forest bark would typically be specified between 50 and 75mm deep, but there are some provisos with this. You should not place bark around the base of plants: trees, shrubs, heraceous or annuals at 50mm deep. The bark mulch must taper down to nothing at the point where the plant enters the soil….

The reason why this is so important is because of the TRANSITION ZONE. The Transition Zone is an area of differentiated cells progressively intermediate between the aerial anatomy of the plant (trunk, branches, shoots,) and the subterranean anatomy (root system) of the plant as in the illustration below …

The design of the plant is such that the vulnerable vascular system, (the ‘lifeblood’ of the plant,) transporting manufactured sugars in solution, moves from the peripheral extremes of the airiel trunk/shoot to the centre of the root where it is protected from soil borne pathogens by hard toughened cells.

So the root system is designed to be subterranean and the shoot system is not. Therefore pilling soil and mulch around the transition zone puts the plant at risk from pathogen attack. The further up the transition zone of the shoot or bole of the plant soil and mulch is placed, the more potential there is for the plant to be attacked and die. Plants can die suddenly as a result of this or it may take many years.

It is particularly important with trees and shrubs, and other permanent plantings. Thus the plant must be planted at the correct height in the soil and any much must be graded downwards towards the trunk or shoot as in the earlier mulching detail image. There are other potential problems of planting too deeply also, like girdling..

In commercial landscape and amateur gardening there is the notorious practice referred to as ‘volcano mulching’!

Image ‘A’ though not a heavy example of the notorious ‘volcano mulching’, it is still too high. Images ‘B’ and ‘C’ are what is usually meant by volcano planting.

NB: All images A, B and C are detrimental to long-term plant health.

While one (B) has no mulch at all, all the images (A, B, and C) below even though there is no ‘volcano mulching’, the trees have been planted too deeply…

In all images above the entire transition zone and some of the tree bole are below the soil – these trees will eventually die unless remediation is carried out. Below is an image showing what a tree should look like at the soil interface…

The root flare is evident and the complete transition zone is ariel and above the soil. This image shows grass, but it could easily be soil or mulch. NB: You should always be able to see the start of the root system disappearing into the soil. If you look at trees in the wild this evidence of surface roots can be extensive.

The Pitfalls of ‘Quick and Dirty’ Landscape Management…

https://landarchconcepts.wordpress.com/quick-and-dirty-lazy-and-amateur-horticultural-practices/

Why Employ a Professional Landscape Manager?

https://landarchconcepts.wordpress.com/why-employ-a-professional-landscape-manager/

What is Landscape Management?

https://landarchconcepts.wordpress.com/what-is-landscape-management/

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Graham Slocombe © LandARCHConcepts 2019