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The Loam Ranger – Mulches and soil improvers

The Loam Ranger

Dear Loam Ranger,

We have a problem in a landscape largely composed of Lomandra in traffic islands and roundabouts. The plants are yellowish and failing to thrive. We have mulched with woodchip and fertilised normally and are wondering why the landscape is not looking better.

It is quite likely that the cause is nitrogen drawdown by the woody mulch, a commonly seen problem.

The solution is to apply 50 g/m² of nitram to the surface mulch, water it in and assess the result in 1 month. Repeat if necessary.

Know the limits

Mulching and improving soil organic matter are important, with two provisos:

1. Don’t overdo it. For decorative and physical effects (such as reducing soil erosion), mulch needs to be no deeper than 75 mm, or problems will begin.

2. Use the right material for the right situation.

Problems

Using too much mulch or the wrong type can lead to a range of problems, including N drawdown, P toxicity in natives, anoxia, and either too much or too little nutrients. N drawdown occurs when microorganisms are presented with an energy source (organic carbon) but insufficient N to utilise it, so they raid soil N reserves, causing starvation in plants

SESL staff have seen many problems caused by inappropriate use of mulch. For example, a line of old Lophostemon (box brush) trees was killed by deep mulching (200 mm) with fresh (not composted) leaf and wood chip that came off the same trees during pruning. The decay of the rich, wet organic matter robbed the topsoil underneath of oxygen, suffocating the root system and turning the soil foul-smelling.

One of the worst cases we saw involved the death of grapevines as a direct result of the application of soil conditioner as a mulch to a depth of up to 250 mm beneath the vines. The first photo shows the soil conditioner piled way too deep and the rows of dead vines.

Dead grapevines

The second photo shows a close-up.

The mulch is too deep

Three mistakes are evident here. The first is that the soil conditioner was placed right up against the trunk. This is a sure way to encourage stem rot. The second is that the material is effectively soil, not a weed-suppressing mulch. The third is the extreme depth. What is not obvious is that the material still had a very high organic content and a high N content, so it continued to decompose, giving off heat, which the deep layer trapped. The decomposition, combined with the heavy loam soil beneath, poor irrigation practice and recent heavy rain, robbed the roots of all oxygen.

A coarse-textured mulch applied to no more than 75 mm would not have killed the vines.

This example illustrates the principle of horses for courses: A soil conditioner increases the soil's organic and nutrient contents, cation exchange capacity and moisture retention. It is, in effect, soil. A mulch, on the other hand, suppresses weeds, retains moisture and insulates against temperature extremes. They can't be substituted for one another.

Standards for mulches

Because types of organic matter differ hugely, AS 4454-2003 Composts, Soil Conditioners and Mulches was developed to describe them systematically. Apart from a list of requirements regarding the presence of foreign matter, pH, organic matter content and salt content, the standard specifies whether the products can claim a contribution to plant nutrition or not. For example, an uncomposted wood mulch containing 0.2% N (very low) and a C/N ratio of 150 (very high) could not claim a contribution to plant nutrition, whereas a green waste compost with total N of 0.9% and a C/N ratio of 18 could. A modest C/N ratio avoids N drawdown.

Properties of commercial mulches

Product	Properties	Precautions
Woody mulch, wood chip, sawdust, pine bark – uncomposted	Decorative and long-lasting. Severe N drawdown (pine bark less so). No nutrient contribution. Often acidic.	Apply nitram at 500 g/m³ before or 50 g/m² after spreading. Repeat every 3–6 months as needed. Use normal fertiliser applications.
Woody mulch, wood chip, sawdust, pine bark – composted	Dark coloured and long-lasting. N drawdown improved, but no nutrient contribution.	Use as is but assess the need for 50 g/m² nitram after 3–6 months. Use normal fertiliser applications unless already fertilised.
Green waste – uncomposted	Moderate nutrient content. Weeds and plant diseases a problem.	Do not use. All green waste should be composted to reduce the spread of weeds and diseases.
Green waste – composted	Coarse fraction similar to composted wood mulch. N drawdown reduced. Fine fraction usually low in nutrients. Use as soil improver.	Coarse fraction: assess the need for 50 g/m² nitram after 3–6 months. Additional fertiliser usually needed.
Sewage sludge – composted	Use as soil improver. Contains significant nutrient levels. No additional fertiliser usually needed.	Risk of P toxicity in P-sensitive plants. Excess nitrate possible. Excessive amounts in soil can cause anaerobic conditions. Add at not more than 20% by volume of soil.
Mushroom compost	Used as soil improver or mulch. Very high and unbalanced nutrient levels: very high in N, P, S and Ca.	Do not use on P-sensitive plants. Do not use additional fertiliser. Do not repeat application.
Poultry manure	Very rich in N and often P. Used as soil improver. No additional fertiliser usually needed.	Do not use on P-sensitive plants. Add at not more than 10% by volume of soil.
Other animal manures	Used as soil improver. Moderately high, well-balanced nutrient levels. Do not use additional fertiliser.	Do not use excessive amounts on P-sensitive plants. Apply at up to 20% by volume on other plants.

Organic matter

Generally, additional soil organic matter is not needed if the soil already contains around 5%. For many Australian natives, 2% is adequate. Do not add organic matter to more than 200 mm beneath the surface, as it increases the risk of oxygen starvation in wet conditions.

Woody or bark materials can be used uncomposted if extra N is added at planting and during maintenance. Green waste and leafy mulch should not be used uncomposted because, apart from the risk of spreading weed seeds and diseases, rapid decay can cause oxygen starvation and fungal growth. White fungal growth can waterproof the mulch and cause drought.

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