Sunday, April 18, 2010

Soil Myths 2: Mulch Ain't Mulch

Not all mulches are good for every garden.

There are many gardening and composting books out there that suggest all different combinations of mulches for organic gardens. Some mulches include newspaper, shredded paper, woodchip, hay, lucerne, leaves, clipped grass, compost, manure, mushroom compost etc. Gardening books may suggest that you use one, two, three or more as a combination to get the most perfect vegetables or flowers. However, both my own and other Australian research shows that different mulches have different interactions with soil. Some are high in nitrogen, others high in carbon, some can cause changes in pH; this may limit available nutrients for plants. This may affect the productivity of your garden.

Dana and her veggie garden

Before choosing mulches for your garden, firstly think about your soil. I am using my friend Dana's vegetable garden as an example for determining the perfect mulch for her soil.

Step 1: Is your soil stable? 

Stable soil means that it has good water retention and space for root growth, meaning plants will thrive! If it is stable, you won't need to add any mulch to improve the soil structure.

To determine if your soil is stable, get a shallow container with some bottled water. Pick up 3-5 pieces (peds) of soil from your garden and gently place into the water. Leave for 10 minutes. Look at the soil and see if there are any changes. Has the water become cloudy? Has the soil broken apart? Is it still the same?

Emmerson Aggregate Test: No changes to peds = awesome!

If any changes have occurred, that is when you need to think about adding lots of mulch to your soil. You will need at least 3kg/m2 of mulch, and make sure you dig it in to 5-10cm. If you don't have any changes, you just need a top dressing of 1.5kg/m2, and no need to dig in. Just cover up your garden bed to protect it from rainfall impact. Dana will be adding a top-dressing around her plants.

Step 2 : What is your pH?

Your pH can help you determine the type of mulch you need. If you have a high pH, you want to add compost with a fresh mulch (hay/non-composted mulch). If it is low, only use fully-composted mulches, for example composted manure with composted hay.

Dana is lucky to have a pH of 6.5-7. She just needs to add a fully composted mulch.

A pH Kit: pH of 6.5 - 7 is perfect!

Step 3: Do I have good Plant Available Nutrients?

In Canberra, we are typically low in Nitrogen and Phosphorous. Adding a manure based compost will increase nitrogen. Rock phosphate can be added in an organic garden to increase plant available P.

Dana's Organic Veggie Garden Mulch Mix: 
After planting, she needs to spread out 2 handfuls of rock phosphate. Dana will then spread mulch over the surface of her garden using 2:1 mushroom compost with composted grass clippings. This will increase her Phosphorous levels, protect her garden from rainfall impact, promote soil biology and nutrient cycling, and provide slow release nutrients.

Some helpful hints: 
* Never add too much carbon. High carbon mulches include woodchip, paper and hay/grass. These mulches are not a very good food source for soil biology. They have to work extra hard and as a result, you end up with less other nutrients available to plants. Only use high carbon mulches if you want to suppress plant growth.
* Manures and composts can be high in nitrogen. Try to mix in some high carbon mulches, for example 2:1 compost with carbon mulch.
* Do you know your pH? Kits are available at most nurseries and garden shops.
* Soil testing can be expensive and prohibitive. However, you can look up ASRIS online and get a general idea of the soil fertility in your area, or try Googling soil fertility for your area (i.e. soil fertility Canberra). 

Have any questions? Leave me a comment! I am always happy to help.


  1. Which has lower pH, shredded pine bark mulch or horse manure compost

  2. Hi Roman! It depends on what soil type you have and what you are using the mulch for as to the best option. Where do you live and what do you want to use the mulch for?!?

  3. Hi Jess, I know this is an old post but I thought I'd ask anyway. What is the value of sugar cane mulch? Is it too high in carbon to be useful around veggies? I live in SE Queensland where sugar cane is cheap and plentiful, but I don't want to risk adding too much carbon to my heavy clay soil. Thanks, Alison

  4. Hi Alison,

    Go for it! As long as you mix it with some manure, or your own compost, or use it to make a compost (with food scraps and manure), I don't see why not! :D You just need to get a bit of a balance.

  5. We have just shifted into a newly built house and have no topsoil whatsoever. The ground is rock hard clay and I will hire a rotary hoe to dig it, what do you reccomend I add as a mulch or compost before any topsoil is added?