Is compost a good Soil Amendment? can every soil benefit?

compost used as a soil amendment

We live in the wet tropics of northeast Australia, in the northern zones of sugarcane country. We have contact with cane farmers and their families all the time, and during one of these times, we were discussing our permaculture process and how we had become fans of the composting process.

The elderly cane grower scoffed at the notion, believing it is a very dangerous substance. For context, tomato growing was the topic and not sugarcane. The comment was directed to growing tomatoes with composted materials, and he had come to the conclusion that it was a waste of time, and dangerous as well.

This was counter to all the training and research we had undertaken, but it got us thinking about how beneficial change can be slow to happen.

This brings us to this article, and why we have written it.

Is Compost a good Soil Amendment?

Compost is a great soil amendment if applied to soils that are either low in organic matter, have poor drainage, poor soil structure, have a pH imbalance, or are low in soil life.

There are very few gardens where an application of rich organic material will not help plants, and by giving plants what they require we can also convert dirt into a medium full of nutrients and organic matter that will also feed us.

compost added to soil
Compost to feed black pepper plants

Compost is so important to our system and fertility management that we wrote this article dedicated to making compost for self-sufficient gardening.

What is a amendment?

The word Amendment means ‘a change’; it does not include the speed or type of change. We use extra terms to describe the kind of amendment taking place, be it a Constitutional, Judicial, or soil amendment.

Soil amendments come in many forms and are the elements that create or cause the change to the soil. These can include fertilizers, mineral and rock dusts, processed animal waste, composts, and organic matter.

These can all play a part in altering the soil in our garden, hopefully to our benefit.

Why apply an amendment to your garden?

We apply an amendment to soil when we wish to alter the soil profile or soil conditions that are not suitable for growing an intended crop or plant.

A few examples…

Clay soil is most often very rich in minerals, but they are locked up and are not able to be accessed by most plants. Applying gypsum as an amendment to the clay breaks this lock which then allows humus and life to continue the process of turning the clay into a productive fertile soil.

Compost is the soil amendment that contains the humus and life.

compost with clay soil
Clay soil with compost and biochar

Sandy soil is a great subject to apply amendments to, because the soil already has adequate drainage, and the gardener can target how to keep the nutrients in the root zone before they fall deeper into the sand.

To achieve this, an amendment that has physical structure that can also include life and nutrients is best, and compost as an amendment fills this need well.

Both of the examples above are related to relatively fast change with the following note. Gypsum should be forked into the clay, allowing the gypsum access to do its thing. Sprinkling the gypsum on the surface is a waste of time and money, as it will wash off with the next rains.

Is compost the only amendment needed?

Compost is decomposed organic materials, and arguably the best soil amendment we know, so there are times and situations where the soil type and conditions are suitable for compost to be the only amendment needed.

There are also times where multiple soil amendments are needed to allow healthy plant growth unimpeded by nutrient lockout or poor soil drainage.

The typical vegetable garden will already have tilled soil, or loose friable conditions that allow compost to help without much else.

Sometimes after growing a heavy feeder like corn the soil can do with a light dressing of organic fertilizer like manure alongside the compost to ready the soil for the next planting.

A good organic fertilizer or manure can work well with compost as they both rely on biological activity to deliver benefits. We use two types of manure here, being chicken manure harvested from our chickens roost, and cow manure that has been composted.

To help make sense of whether to use fertilizer or compost in your situation, this article may help you make your mind up.

composted cow manure
This compost has cow manure as well as biochar

How often are amendments needed?

Soil amendments should be applied on an as-needed basis which is determined by the plant varieties you intend to grow and the soil condition where you wish to grow them. There are two kinds of applications with soil amendments.

  1. Initial soil change through applying amendments.
  2. Applying amendments as a maintenance program.

We expand on these below to explain the differences.

1. Applying amendments to change soil conditions.

As we mentioned above with the two examples of clay and sand, compost can be used to create structural long-term change to a garden. There are other times where applying compost is less structural and can take some time to take effect, such as moving soil pH.

Using an soil conditioner like compost to change pH is good because it can take some time to progressively move the pH towards neutral. This slow rate of change matters where sudden pH changes can harm plants in the area to be treated.

2. Using compost to maintain soil health.

In the eyes of organic food growers, compost is the go-to soil conditioner to maintain soil life and nutrient availability over time. This is most prominent in backyard food growers who refuse to apply chemical based fertilizers to the garden. We are one of these growers.

Using compost this way ensures the garden is as healthy as we can keep it, with the produce grown being the best possible. Applying compost on a regular basis can maintain the fertility and soil life, allowing us to grow without many costly inputs. We think this matters.

When we apply compost, it is beneficial because it is a natural buffer to change and can provide long-term stability if a quality mulch is placed over the compost.

cutting mulch for the garden
Banana fronds make good mulch.

This layer of mulch helps the compost because a well-made compost is teeming with microscopic life that is very beneficial to our gardens, and the mulch acts as a protective blanket for these lifeforms.

Mulch can be a reliable compost alternative in the wet tropics if the mulch contains a good variety of plant material, as the often-constant wet conditions allow micro-organisms to thrive. These make nutrients available to the plant.

To show how effective good mulch can be for your harvest, this article details one event we had with a ginger crop. It was a bumper.

mulch growing on location
Bananas give us plenty of leaf matter to use as mulch.

Can topsoil and compost be mixed together?

An amendment like compost can be mixed into the topsoil and used to inoculate the soil with beneficial microbes. We suggest covering with mulch to stabilize the soil temperature and the moisture content.

If you are familiar with the composting process you would be aware that the primary lifeforms in the compost are aerobic, meaning they require oxygen.

By mixing the compost into the soil you can run the risk of smothering these helpers, however if you till the soil as you apply the compost, there will be pathways for oxygen to reach into the soil naturally, so this mitigates the potential problem.

We mix compost into our raised garden beds every time we replant the beds, and we can share the observation of elevated worm activity in these beds where the compost has been worked into the soil.

Does compost need to be mixed with soil?

Compost does not need to be mixed into the soil and is able to be applied on top of the soil; it can even have seedlings planted in handfuls of compost surrounded by mulch. Covering compost with mulch is important because sunlight and heat will kill the beneficial microbes in the compost.

compost covered by banana leaf mulch
Black pepper vines with banana leaf mulch over compost.

Is there a best time to treat the soil?

Compost can be added to garden soils in spring to prepare for the growing season. We have our best vegetable growing season in the winter as we live in a tropical climate, and the wet season is too humid and wet for soft leaf vegetables.

We do have a select group of plants that do well over our wet summers. We treat them with composted manure and wood shavings, and these organic soil conditioners improve our returns consistently and reliably.

Are organic soil conditioners worth using?

To get straight to the point, we find compost is the best all-around soil conditioner, it is surprisingly easy to make and is well worth the effort. There is really no natural organic alternative if you think about it because growing vegetables is draining on soil, especially when you plant heavily as we do.

The likely intention of the average gardener is to either make the soil fertile or keep the soil fertile, so amendments and conditioners are both applied to achieve or maintain fertility.

A quick note: Soil conditioners are typically applied to change the soil structure, and in our eyes, this makes a soil conditioner an amendment.

Our recommendation; make compost bins or learn how to make soil amendments from waste materials like plant pruning’s and trees that have been trimmed. Try to get close to or more than a cubic yard or meter of bulk material, because a large pile breaks down faster.

What is garden fertility and why do we need it?

Fertility is a description for a soils ability to grow a plant. It was once considered to be measured in how many earthworms were in a defined area of soil.

Earthworms are an indication of how biologically active a soil is, and we know that earthworms do a great job of soil aeration and maintaining a soils ability to both hold moisture and to allow excess water to drain. The humble earthworm can be considered a natural way to amend your garden.

These are the attributes we try to emulate with soil conditioner and amendments, and while many store-bought products will help in the short term, compost is the natural way of achieving what the worms do.

Having fertile soil allows us the leverage to plant more heavily where appropriate, and to use less water and amendments over time. It is a more natural method of growing.

How does composting help fertility in soil?

Compost is a mix of organic matter, nutrients, and microbiology all in the one package. The organic matter allows us to target pH stability and moisture holding capacity.

During the composting process many nutrients have been transformed into plant-available forms that can immediately feed your garden.

The introduction of microbiology will serve several processes with the main two being mineral and trace element conversion into plant-available forms, and the second being a food source for earthworms and similar higher lifeforms in the soil.

The activity of the worms has already been mentioned above, and as we now know, fertility can be measure by worm numbers.

Much of the above depends on having moisture present, and protective mulch placed above the soil surface, even if the compost is not mixed into the topsoil.

Is there a best ratio of compost to soil for a vegetable garden?

There is no hard ratio as far as we know, and to explain I can share a story.

If you have visited our About Us page, you will have seen that we are qualified Permaculture Designers. We completed our deign certificate (PDC) in 2015.

During that course, the lecturer who owns and manages one of the Permaculture Research Institute demonstration sites here in Australia explained the process of growing the food for the on-site students.

During a typical PDC course students are shown and take part in building a compost pile and how to manage and maintain it from start to completion. As you can imagine, multiple consecutive courses all year, each year, generates a lot of compost, and this has to be used somewhere.

The compost is applied to the vegetable crops and over time, the nutrients have been constantly topped up, again and again. The lesson here is that it is difficult to apply too much compost to your garden soil.

To add to this, we don’t apply any more than about 40-50% composted material to soil as a top-up in our metal raised garden beds and have found this to be ok. We generally run out of compost before we hit any hard ratio limit.

Will chemical-based soil conditioners affect my garden?

There is a place for these kinds of products however long term use may create issues in time. There are studies that suggest there may be issues with some heavy metal accumulation in plants like ginger. Arsenic (As) and mercury (Hg) are noted as potential problems so our recommendation is to stick with organic matter.

Closing comments

Amendments or soil conditioners are the tools we can utilize to improve and maintain soil quality, fertility, and productivity. To be able to achieve these with one material we can make on site with little to zero cost is great, and to be able to grow food in a self-sufficient manner using this compost made on site is the best.

Article by Tim Blanch for He is a qualified Permaculture designer.