In past posts I have talked about some of the benefits of compost. It improves soil structure and it adds nutrients to the soil. What about the other benefits like adding microbes to the soil, reducing diseases and eliminating the need for additional fertilizer? Are these real benefits or just gardening myths?
Benefits of Composting
The following are all benefits that have been reported for compost. I’ll review some of these in this post, and a few more complicated topics will be discussed in future posts.
- Adds microbes to the soil
- Improves soil structure
- Eliminates the need for commercial fertilizers
- Reduces plant diseases
- Increases water holding capacity of soil
- Adds enzymes and hormones
- Eliminates garbage
- Reduces fertilizer run off
- Increases soil biota
- Buffers soil pH
- Removes toxins from air and soil
- Wards off pests
- Acidifies soil
The benefits of compost seem huge. Don’t get me wrong, there are a lot of real benefits to compost, but not everything on the above list is a valid benefit.
Compost Improves Soil Structure
Compost, humus and microbes act like sticky glue that binds the mineral components of soil together. It sticks the very small clay particles and the sand particles into larger clumps called aggregates. These larger aggregates are responsible for making that soft crumbly soil you find in undisturbed woods.
Sandy soil is improved by adding water retaining organic matter. Clay soil is loosened by the organic matter.
The subject of soil structure is complex and warrants it’s own post at some point. For now I’ll just say that the improvement of soil structure is, by far the most important benefit of compost. More details can be found in these posts; What is Humus and Organic Fertilizer – What is its Real Value.
Eliminates Need for Commercial Fertilizers
This is not always true. In my last post Compost Fertilizer Numbers I discussed the fact that compost is a slow release fertilizer which adds small amounts of nutrients over time. If your soil contains sufficient nutrients for plant growth, then adding compost regularly will keep up the levels of nutrients and you won’t need to add commercial fertilizer.
If however, your soil is deficient in one or more nutrients, then the amount added by compost will not remedy the situation quick enough. The best alternative is to add commercial fertilizer for the missing nutrient. The only way to know if there is a deficiency is to have a soil test done.
Increases Water Holding Capacity
Compost and humus act like sponges. One report said that a 5% increase in organic mater quadruples the water holding capacity of soil. The actual number will depend very much on the type of soil you have.
Why is this important?
When more water is held in the soil it dries out more slowly and and you need to water less often. This saves you time and reduces your water needs from the city–a benefit to the environment.
From a plants point of view, it means that the amount of available water is more constant. Instead of wide swings in water availability, plants now have a steady flow of water as needed. This improves plant growth.
What about wet areas?
Should you be adding compost to soil that is always wet? If compost holds even more water does this not mean that the ground will be even wetter? Not really. The secret is in the aggregates mentioned above. As aggregates form they get bigger. As they get bigger, space between them increases which in turn increases the amount of air between them. Large air spaces help water run away and helps it to evaporate faster. The aggregates retain moisture, and the spaces between them stays dryer.
Composting Eliminates Garbage
Organic material sent to landfill sites still decomposes, but there it produces methane, a gas that contributes to global warming problems. Even if you don’t want to compost, try to use a recycling system, or give your food waste to a gardener who does compost. In a later post in this series I will show you my No Composting, Composting System that makes composting so simple every gardener will do it.
Compost Reduces Fertilizer Run Off
Nutrients that are water soluble (most of them) have a tendency to be washed out of the soil by rain, or your watering hose. Some nutrients, like nitrogen, are washed out very quickly which is especially a problem in sandy soil.
Compost and humus act like nutrient sponges. The nutrients stick to the organic mater and are washed away more slowly. The nutrients stuck on organic mater are available for use by plant roots and microbes. Reference 1 has a very good description of how nitrogen moves in the soil.
Compost Increases Soil Biota
Soil biota is a term used to describe all of the living things in soil including plants. Other similar terms include ‘soil life’ and ‘life in soil’.
Compost is mostly non-decomposed organic material (ref 2). It is food for microbes, which in turn becomes food for higher order animals such as earth worms. Between microbes and earth worms you will find hundreds if not thousands of different kinds of organisms. They all benefit from the compost, and as a result of it, their populations increase.
Compost Buffers Soil pH
Any organic matter has the ability to buffer pH, including compost. What this means is that the material absorbs salt ions which then have less effect on changing pH. This does two things for the garden.
The absorbed ions are held in place and are less likely to be washed away by water. Plants can use these nutrients as needed.
All soil life would prefer a stable pH. By buffering soil pH, compost makes the biota environment less harsh which is also good for plants.
However, there is one potential problem with buffering soil pH. If you are trying to change the pH of your soil it will be more difficult with compost in the soil because you will need to add more acidic or alkaline chemicals to make the pH change. I don’t see this as a big problem since it really doesn’t make sense for you to alter your soil pH except in extreme cases. Learn to grow plants that like your pH.
For a better understanding of Buffer pH see Liming Acidic Soil.
Removes Toxins From Air and Soil
I found the following quote “compost destroys over 99% of Volatile Organic Compounds (VOCs) in the air and it can remove heavy metals, solids, oil, and grease from stormwater runoff…, thereby keeping you and your family healthy”.
I doubt that compost itself will destroy VOCs. The microbes living in the soil and in compost will destroy VOCs as described in Plants Remove Pollutants from the Air. Microbes will decompose virtually all types of organic chemicals–here I am using the chemist definition (What does Organic Mean?). This will include pesticides and oil.
Compost will absorb heavy metals, for example lead and mercury, from water as it runs through the soil. However heavy metals don’t get decomposed like other organic molecules. They are elements that will remain for ever. So if compost is removing them from runoff, it means that they are accumulating in your soil which is an environmental problem you don’t want to have. If you grow crops in this type of soil you may be harming your family healthy. In most home environments, runoff containing heavy metals is not a problem and there is no need to be concerned about this problem.
Other Benefits of Compost
The other benefits of compost, in the above list, will be explained in subsequent posts.
1) Understanding Nitrogen in Soils: https://web.archive.org/web/20180408104953/http://www.extension.umn.edu/agriculture/nutrient-management/nitrogen/understanding-nitrogen-in-soils/
3) Photo Source: Joi Ito