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Soil Macronutrients and How To Optimise Them In Your Compost

Written by Angus Stewart

We have all heard of nitrogen, potassium and phosphorus as plant nutrients, but did you know that plants require at least 16 different elements that are essential for healthy growth? A number of these elements are required in much greater quantities than others such that the essential elements are separated into two groups; macro- and micronutrients. Let’s look into macronutrients and how we can maximise these in your compost, to ensure that we recycle our household organics such as kitchen scraps into fertiliser to grow our own fruit and vegetables or fertilise our indoor and garden plants.

The nutrients that are needed in the greatest amounts are carbon (C), hydrogen (H), oxygen (O). This is because they are part of most molecules that make up plants, whether that would be the sugars in the fruits and flowers, the structural cellulose in the stems, or the leaf tissue that carries out photosynthesis. Fortunately for us, these macronutrients are supplied in abundant quantities for free from the atmosphere, as either oxygen or carbon dioxide gas, and are absorbed mainly through the leaves and to a lesser extent oxygen also moves into the roots from the air in the soil. In other words, these elements are something we don’t need to worry about supplying to our plants.

As well as the macronutrients supplied from the atmosphere, there is a very important group that plants absorb from the soil, and these are the ones that we need to work out how to replace in the soil, because every time we harvest edible plants or prune back plants, we are removing the soil-derived macronutrients from the soil ecosystem. The great news is that we can usually supply all our garden needs of these from home-made compost and help reduce organic materials going into landfill, where they produce the potent greenhouse gas: methane. In particular, our kitchen scraps are relatively rich in the essential elements that are delivered to plant roots as soil-derived macronutrients, namely nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg).

Our goal in creating and maintaining a healthy garden soil is to replace any nutrients we remove in the correct balance for what is being removed. For many gardeners, the vegetable garden is the place where most nutrients are being harvested where they then go to the kitchen. The bottom line is that any food wasted in your kitchen usually contains, on average, the sort of balance of nutrients you are looking to replace. When we compost these, we are able to recycle a significant percentage of these macronutrients instead of sending them to landfill. Whatever method of composting you use you will be making a significant individual contribution to sustainability when it comes to soil-derived macronutrients.

Soil for Composting

To optimise the supply of the macronutrients from your compost here are some key tips;

(1) Always use a percentage (30-40% is a good rule of thumb) of nutrient-rich ingredients as part of your compost raw materials such as kitchen scraps or animal manure.

(2) Use the compost as soon as possible after it is ready. A good test is to raise some fast-growing seedlings, such as radish or lettuce and if they grow normally the compost is ready.

(3) If you can’t use your compost straight away put a cover over it to stop the nutrients leaching out.

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