If you are serious about gardening, you need to get as much organic material into your soil as possible. All things being equal, soils with high levels of organic matter will have less impact from pests and diseases, particularly soil based ones (such as root knot nematodes).


Don't Be Fooled

All too often people will purchase bagged soil and expected to get good results, particularly when they have purchased a so-called premium organic mix. Without life supporting organic matter, the fertilisers in the soil will quickly be used up or washed away. Once this occurs plants may struggle to survive, let alone thrive.


Organic Matter

Incorporating organic matter into the soil can have several outcomes, affecting the physical, chemical, and biological balances in the soil. It can change:

All of these changes are related to the way organic matter is decomposed when it is incorporated into the soil and to the particular type of organic matter used.


The Process

When organic matter is incorporated into soil, the larger organisms like mites and soil animals break it into smaller pieces. Then the fungi and bacteria start to decompose it by secreting enzymes. When the enzymes break the molecules into smaller sections, the bacteria and fungi can use some of the energy or nutrients released for their own growth.

If there are nutrients that the microbes do not use, they will be available for other soil organisms or plants to take up and use. When microbes die, their cells are degraded and nutrients contained within them become available to plants and other soil organisms.

Microbes can access nitrogen in the soil more easily than plants can, so the plants sometimes miss out and the plants can become nitrogen deficient. This is why incorporating organic matter into soils can change the amount nitrogen (and other nutrients) that is available to plants. This will be a short-term effect that happens when soils do not have high levels of organic matter and soil microbes.


The Benefits

If the number of fungi and bacteria associated with the breakdown of organic matter increases, there may be some improvements to the soil structure. Adding organic matter can also increase the activity of earthworms, which in turn can also improve soil aggregation. If organic matter is retained in the soil, the number of microbes in the soil increases. This is because the microbes can use the organic matter as a source of energy and so they can grow and multiply.

Soil pH

The pH of your soil is critical for your success in growing healthy plants, particularly food plants. For most vegetables, the ideal pH is around 6.4. At this pH, the minerals required by plants are most available.

There are a number of plants which require a pH level different to the 6.4 ideal. Blueberries are a classic example, generally disliking a pH over 5.5. Potatoes are also known to prefer a slightly more acidic soil.


Changing Soil pH

To change the pH of your soil, you want to use something that has a pH higher, or lower, than where you would like to end up. While there are products available on market that already have the correct/ideal pH, if you start applying them at the start of the pH changing exercise and your pH is significantly off the mark, it will take you a long time and many dollars to get where you want to end up.

A pH testing kit it essential, when thinking about modifying the pH of your soil.


Raising pH

Assuming that you are trying to reach a pH of around 6.4 and your pH is below that (say 5.5) the following products/materials will help achieve your goal:

  • dolomite (pH around 8 to 9)
  • wood ash (pH unknown, test it before you use it)
  • garden lime (pH higher than dolomite) - this can cause problems for the soil life, so only use it in extreme or special cases.
    NEVER use hydrated lime as it has an extremely high alkalinity which will likely cause harm to your garden.
  • gypsum (pH around 6.5) - it can also be used to raise pH if only minor adjustments are required (and the soil has a pH below 6.5)
  • compost - if you soil pH is quite low (e.g. 5), compost will help raise it overtime (composts are naturally acidic, but not highly acidic)


Lowering pH

The following products will help lower the soil pH:

  • sulphur (very low pH) - a little goes a long way. Also remember that it acts slowly, so give it about one to two months before retesting the soil pH and making further adjustments
  • gypsum (pH around 6.5) - it can also be used to lower pH if only minor adjustments are required (and the soil has a pH above 6.5)
  • compost - if you soil pH is quite high (e.g. 8), compost will help lower it (composts are naturally acidic, but not highly acidic)