The Latest Science on Soil Health

Most gardeners now understand that a healthy soil is critical to good plant growth and a successful garden. Many of you are also beginning to understand that it is the soil microbes that make healthy soil. Unfortunately, that has lead to a lot of myths about how microbes should be used. Companies are making this worse by supplying microbial products that have not been shown to work.

The latest science has made it clear that it is even easier to build good soil than we thought. Some of the old techniques still work, but there are new methods that are even more powerful.

Soil Microbes are Always at Capacity

Microbes improve soil in several ways:

• They convert organic matter into smaller molecules thereby releasing nutrients for plants.
• They create soil aggregation – that nice crumbly soil we all wish we had.
• They help plants fend off pathogens.
• They dissolve minerals and convert them into plant available nutrients.

If some microbes are good for soil then more microbes are even better and that is perfectly true. The science on this is very clear. If gardeners increase the quantity of microbes in their soil they will have healthier soil, which in turn grows better plants.

This sounds so simple but how does a gardener increase the number of microbes?

The obvious answer is to add them and this has been the thrust of soil improvement techniques for quite some time now. Many gardeners are now adding compost tea, weed tea and even purchased microbes. None of these work. These techniques do not increase the quantity of microbes in soil because microbes are always at capacity which means that they naturally expand in numbers to use up all available food. Adding more microbes simply means the new ones die from lack of food.

The only effective way to increase microbes is to add more microbe food to your soil. If you add the food, the number of microbes increases very quickly and without any extra effort from the gardener. There are three effective ways to do this: add shoot organic matter, encourage plants to produce exudates and something new that scientists call necromass. Out of these, necromass is the most important.

Adding Shoot Organic Matter

Let’s clarify some terms. Organic matter is anything that is made up of compounds that originate from something that was living. Shoot Organic matter is organic matter that originates from the above ground parts of plants. This is the major component of compost. It also includes manure, which is mostly decomposed shoot plant material. It can also be fall leaves and various plant meals such as alfalfa meal.

The traditional way of making healthy soil is to add shoot organic matter to the soil, either as a mulch or as a soil amendment. This material adds carbon to soil and provides microbe food.

So-called finished compost is not really finished, it just looks finished to our macro eyes. On a molecular basis it is just starting to decompose and on average will continue this process in soil for another 5 years. This slow degradation process is what makes the material so valuable. It feeds microbes for a fairly long period of time.

Is this organic matter a good food source for microbes? It is readily available to gardeners which makes it an obvious choice, but it is not really a great food source. That will contradict what most gardeners have thought for years but the science is clear on this.

A lot of the organic material added to gardens consists of large molecules including lignin and cellulose which require a lot of energy for microbes to decompose it. That means microbes waste energy using this food source, which results in slower growth of the microbes. The net effect is that you need a lot of shoot organic matter to grow a relatively small number of microbes.

The degradation process also releases a lot of CO2 which is lost to the air, reducing the amount of carbon that remains sequestered in soil.

Using shoot organic matter is the most popular way to improve soil and it does work. However, it is not a very energy efficient method, nor is it the best option for gardeners.

Roots are Better Than Shoots?

We now know that living roots in the ground are much more important for soil health than the shoot organic matter discussed above. There are two main reasons for this: living roots produce exudates and root carbon lasts longer in soil.

An exudate is a chemical made by the plant and released into the environment. For the purpose of this discussion, they are chemicals that are excreted from plant roots.

The rhizosphere is a very unique region around plant roots that is more fully described in this video and also in my book Plant Science for Gardeners.

Plants use photosynthesis to capture energy from the sun and turn that into a wide range of chemicals which are collectively known as photosynthates. We now know that up to 50% of these photosynthates are excreted into soil through the roots. These are the exudates. That is a huge amount of energy and nutrients that are perfect for feeding microbes.

Exudates consist mostly of small molecules that are easily digested and absorbed by microbes. Unlike shoot organic matter, this is a very energy efficient food for microbes and on a pound for pound basis, it grows many more microbes.

Initially it was thought that exudates consisted mostly of sugars and other carbohydrates but it is now known that they include a wide range of compounds including amino acids, proteins, fatty acids, sterols, anthocyanins and flavonoids, to name a few. The composition of these compounds also changes as plants go through different stages of life.

How does a gardener use exudates to improve soil? It’s quite easy. More plant growth means more roots which means more exudates. By growing more plants and bigger plants you are increasing the microbe population which in turn builds healthy soil.

Here are some ways a gardener can maximize the use of roots and their exudates.

• Keep the soil covered with living plants at all times.
• Grow cover crops or even weeds any time the soil is not being used.
• Use more perennial plants which excrete 30 to 50% of their photosynthates as opposed to annuals that only excrete 20 to 30% of theirs.
• Increase plant diversity. Each species of plant cultivates a different population of microbes so a diverse plant population results in a higher microbe diversity.
• Use intercropping so that one plant follows another through the season to maximize root growth.
• Keep plants growing as healthy as possible by keeping them watered and fertilized properly. Healthy shoot growth produces more root growth and more exudates.

Some scientists estimate that living roots may be as much as 5 times more effective at building healthy soil compared to organic matter from shoots.

Other studies have shown that the carbon from roots remains in the soil longer than the carbon from shoots.

The bottom line is that living roots are much more effective at growing microbes and building healthy soil than traditional shoot organic matter.

Necromass – The Secret to Healthy Soil

The latest studies show that the best way to build healthy soil is with necromass. Necromass is defined as a collection of dead organisms, but in soil it consists mostly of dead microbes. Even though most gardeners have never heard of it, it is essential for increasing soil organic carbon and building healthy soil.

Microbes have short lives and naturally die off quickly. They are also very dependent on their environment. If it gets too dry, they die. If the food source runs out they die. If you pull the carrot out of the ground the microbes living off the carrot exudates die off. The net effect of this is the accumulation of a large amount of dead microbe bodies which are collectively called necromass.

Accumulated necromass is responsible for aggregate formation – that crumbly soil all gardeners want. Better aggregation results in more soil organic carbon, better drainage, better aeration and larger pore spaces for plant roots. These aggregates have been found to be very stable and seem to be the best way to sequester carbon in soil.

You might have heard that humus is the stable form of carbon in soil, but the new thinking about humus is that it does not really exist. Instead, carbon is stabilized by the necromass trapped inside soil aggregates.

Testing of the top layer of forest soil has found that microbial necromass accounts for 45% of the soil organic carbon (SOC) and this value climbs to 73% in meadow soil and 78% in wetland soil. “Fungal necromass contributed more to SOC in these habitats than bacterial necromass”. Over 50% of the soil organic carbon in grasslands and croplands is derived from microbial necromass.

How does a gardener increase necromass? Simple, grow more microbes. There are some techniques you can use to facilitate this, including cover crops and living mulches.

The Value of Cover Crops

Gardeners have known about cover crops for a long time, but they have not embraced their use. This should change because they offer the gardener a great way to improve soil health.

A cover crop is a plant that is grown for a shorter period of time while the ground is not in use. For example, there are several months of bare ground between harvesting garlic and replanting next years crop and that is a perfect time to add a cover crop.

Why are cover crops so valuable? The above ground material provides a good source of shoot organic matter that can be used as a mulch or soil amendment, but the real value is in the growing roots. Bare ground adds no exudates to soil, while an actively growing cover crop adds lots of exudates which in turn increases microbe populations and accumulated necromass.

There is another less obvious benefit to cover crops. Since they are almost always a different plant species than the main crop, they also increase microbe diversity.

A meta study from 2019 looked at 60 relevant studies and found that cover crops significantly increased the number of microbes, their activity level, and diversity by 27%, 22%, and 2.5% respectively, compared to bare ground. These effects are dependent on climate, termination methods, and degree of tillage. Cover crops also increase the soil organic carbon level.

Depending on climate and growing duration, cover crops may be better than compost for building healthy soil.

Rethinking Living Mulch

Living mulch is a plant that grows between other plants and provides a cover for soil. It is a live plant that functions as a mulch and works very well in an established ornamental bed.

There has always been some controversy over using a living mulch because the plant also competes with the permanent residents of the garden for both nutrients and water. This latter point is still true, but it is clear now that the roots of the living mulch also contribute a lot to the health of the soil.

Intercropping, which is a method of growing two species together, has been shown to significantly increase soil aggregation more than growing a single plant.

It is quite likely that the benefits of living mulch outweigh any negative aspects.

Change the Way You Garden

How should you use these new scientific findings in the garden?

The traditional way of improving soil, namely adding organic matter, still works but it’s not as effective as other techniques. That does not mean you should stop doing what you are doing. Keep adding compost and keep mulching. Mulch in particular has other benefits in addition to building soil.

The new focus should be directed towards growing lots of roots and keeping living roots in the soil all of the time. Here are some things you can do.

• Use cover crops any time the soil is bare.
• Leave non-seeding weeds unless they are causing a problem – weed roots are better than no roots.
• Fertilize and water properly so plants grow well.
• Be careful how much nitrogen you use because too much produces top growth instead of root growth.
• Use different species in a cover crop mix to increase the microbe diversity.
• Intercrop so that more than one species is growing in soil at the same time.
• Disturb soil as little as possible. Tilling destroys the structure of aggregates and speeds up the loss of necromass.

A gardener can directly influence root growth, but how do they increase necromass? The way to do that is to focus on root growth. Grow roots and you automatically increase microbe populations and necromass.