Horticulture/The Benefits of Compost

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<Horticulture/Soils and Composts

Community composting area

Compost is a true “magic bullet” in the garden, fixing many problems in the soil that are reflected by unhealthy plants. The benefits of compost are many, and can sometimes seem inconsistent, as in some cases it achieves two goals that appear to be opposites. The start of any good garden is good soil, and adding compost is the easiest and most long-lasting way to improve soil conditions.

Drainage and Water Retention[edit]

Pick up any good garden book, and you’ll notice that most garden plants like a “moist, well-drained soil”. This means that the soil should hold plenty of water for plants to use as they need it, while at the same time draining quickly after heavy rainfalls or overly enthusiastic irrigation. This seems like a tall order for a soil, but is easily achieved by adding compost.

Compost improves water retention because of its sponge-like nature. Mature compost (also called humus) can hold up to 200 times its weight in water! This means that rainwater or irrigation water stays in the soil, rather than leaching out or running off. Soils rich in humus therefore need less frequent irrigation, saving time, worry and money for the garden-owner.

Compost also improves drainage, which is important because roots need oxygen to survive. This improved drainage happens because compost binds up the soil into little crumbly bits, leaving good-sized air spaces in between. It might seem a bit strange that it allows more air in at the same time as holding more water, but the water is held inside the crumbs, while the air moves between them.

Fertilization and Nutrient Retention[edit]

Most garden plants prefer a balanced, nutritious soil (though there are a few that don’t like a lot of nutrients, so be careful). Compost helps create and maintain a nutritious soil solution for plants to use.

When compost is added to a soil, it brings nutrients with it. Compost is, after all, what’s left after bacteria, fungi, and other creatures get done breaking down dead tissues, and so contains the basic components used by plants to produce new tissues. Compost is a slow-acting fertilizer, and lasts a long time in the soil, especially in comparison to salt-based synthetic fertilizers.

Compost also improves the soil’s ability to hold nutrients, as it has many “pockets” where nutrients can be held ready for the roots to use. This soil property (called the “Cation Exchange Capacity”, or CEC) means that most nutrients added to the soil stay bound up, rather than leaching out. Compost also helps keep Nitrate in the soil (which is an anion), which is important because Nitrogen is the most easily leached soil nutrient.


Aside from good drainage, water retention, and fertility, plants also benefit from a soil that is stable. The “moist, well-drained” property of a good soil is one sort of stability, as is the slow, constant supply of nutrients available in humus-rich soils. Another kind of stability is also quite important, namely the stability of the soil’s pH value. The soil’s pH (the power or activity of Hydrogen ions) is a measurement of acidity or alkalinity. Different nutrients and toxins become available to the plants depending on the pH value of the soil, and if the pH varies every time there is a rain event or irrigation, the plants find the available nutrients out of balance. In soils with a low CEC, the pH can be changed quite dramatically by acidic rainfalls or alkaline irrigation water.

Compost stabilizes the pH of the soil because it significantly raises the soil’s CEC. The acids and alkalis that arrive with those life-giving waters are taken up in those “pockets”, rather than floating around in the water held by the soil. This really helps plants live a healthier life.

The one thing to keep in mind is that if there is a high humus content in the soil, it will be more difficult to change the pH when you want it changed. One should never try to change the soil’s pH without a soil test in any case, as the amount you’d use to slightly alter a humus-rich soil would drastically alter a humus-poor soil.


Compost is living, breathing stuff. Composts contain residues (which have not yet been completely broken down), humus, and a “microflora” made up of many species of bacteria and fungi. These bacteria and fungi are important for maintaining the stability of the soil, and help break down and organic fertilizers that might be added to the soil when necessary.

Keeping the soil alive helps nutrients spread out for an even distribution, and helps fight plant diseases by filling the “pockets” where disease organisms might find a home. Some of these “good guy” organisms will even eat the bad ones, turning them into more nutrients for your plants. This property, called “antagonism”, is very important in keeping plants healthy and strong.

Aesthetic Benefits[edit]

Aside from all the hard-science benefits, compost also makes the soil more pleasant to be around and work with. The soluble parts of compost turn the soil a rich, dark color, which is just as nutritious as it looks. Compost also provides that wonderful “earthy” smell after a rain, which some folks find even more invigorating that the aroma of a good cup of coffee. For the gardener, compost-rich soils have a nice, crumbly feel between the fingers, and are easy to dig in.