What is the single most important nutrient for growing plants? There are two possible correct answers, water or carbon. Both of these run in global, sun-powered cycles and provide renewable energy. Both are crucial for growing all plants, including our garden varieties.
First let me describe the carbon cycle, and then we can look more specifically at how it affects our gardens and how our gardens may affect this global cycle in a small way. The carbon cycle describes the process in which carbon moves from a solid or liquid to a gas and then back into solid or liquid forms again.
Carbon is found in three different major forms. In its pure form, carbon is a solid mineral, commonly found in various degrees of purity and hardness. Think of diamonds, coal, and pencil lead as examples. When carbon is combined with hydrogen, it is a liquid, primarily petroleum in its various forms, such as gasoline, diesel, engine oil, etc. Hydrocarbons can also be gases, such as natural gas, that have a liquid state under enough pressure. Combined with oxygen, carbon is a gas, primarily either carbon monoxide or carbon dioxide. Carbon monoxide is produced by incomplete combustion of burning carbon and is very poisonous. Carbon dioxide is produced by complete combustion and through many other biological processes.
Another important way to think about carbon is in terms of energy. Solid or liquid carbon is loaded with potential energy. Common examples of this include coal-fired power plants and gasoline-powered vehicles. Carbon in a gas form has very little potential energy for our direct use, but it is very important for the growth of plants. More than 90% of what you see when you look at a plant, whether it is the smallest of herbs or the largest of trees, is carbon that the plant harvested from the atmosphere through the process of photosynthesis.
We can see that photosynthesis is one of the most powerful forces on earth. It is the part of the carbon cycle that reverses the dispersion of energy and, using the Sun’s light, reconcentrates carbon into forms that are again usable to power our lives. Photosynthesis is one of the awesome functions God created to keep this planet inhabitable. So how does it work?
In the process of photosynthesis, plant leaves take in carbon dioxide from the atmosphere and combine it with water taken up through the plant roots. This process creates a simple sugar, glucose. Glucose is mostly carbon and hydrogen with a little bit of oxygen. The carbon comes from carbon dioxide, and the hydrogen comes from the water. There is more oxygen in water than what is needed to form glucose. Plants release the extra oxygen back into the atmosphere, supplying it for all of God‘s creatures that need oxygen for survival, including us.
Are you starting to understand why I call carbon the most important nutrient? The process of photosynthesis creates glucose, and a very important by-product, oxygen. What happens next?
Glucose is used in many ways. A good bit of it gets combined with minerals to create things like cellulose, which makes up wood and straw. Some glucose is changed to starch, which fills grains and vegetables like potatoes. Some becomes more complex sugars that provide the sweetness in various fruits and vegetables. Plant fats, that create the various plant-based cooking oils we’re familiar with, are formed from glucose. All these different plant materials start with a glucose base that is combined with many different minerals, mostly coming from the soil.
Another major use of glucose involves the plants feeding the soil. I’m sure you understand the concept of dead plant material rotting and becoming soil, whether directly on the ground or in a compost bin. But do you know that is a secondary way in which plants feed soil? Plants directly feed the soil in which they grow with something called root exudates. They do a lot of this; a healthy plant can push up to 50% of its total glucose production out through the roots. That means when you see a plant growing on top of the ground, you can visualize a heap of sugar equal to that plant’s size that it provided for the soil while it was growing. This would be on a dry-matter basis, so the actual heap would be the size of the plant with most moisture removed. If you’ve ever dehydrated food, you can visualize how much smaller this would be. Even so, healthy, growing plants provide the soil with a lot of sugar.
Why was a plant created to do this? The answer has several levels. The direct benefit to the plant comes from feeding microbes. Bacteria, fungi, and all the hundreds of other life forms in the soil depend on the sugars plants provide for an energy source. Many minerals are made available to plants by the soil microbes, so it’s a two-way street. Plants provide the sugar to the microbes; microbes provide minerals to the plants. Neither one can function without the other. With the use of soluble, man-made fertilizers, it is possible to grow plants without microbial-supplied minerals, but these plants are never as healthy or as nutritious a food source as those grown in healthy soil.
You may have heard of carnivorous plants, such as a Venus fly trap, which trap and consume insects. But do you know that almost all plants consume bacteria? This information was newly discovered less than ten years ago. Plant root tips take in whole bacteria, absorb most of the mineral nutrition from this bacteria, and then pop the shrunken bacteria back out into the soil. At each site where the bacteria pop out, a root hair grows. This probably explains the reason that plants tend to grow a lot more root hairs in healthy, living soil. This process is called rhizophagy.
Another reason plants were created to push carbon in the form of sugar into the soil is the importance of carbon within the soil. Carbon is the biggest part of what we call organic matter in the soil. Some organic matter comes from decayed plants, but, in a healthy system, a good portion of it comes directly from living plants in the form of root exudates. Organic matter is very important to soil structure and water-holding capacity. Organic matter is the difference between dirt and soil. In most areas, dirt (minerals without organic matter) can hold very little water, so the rainfall either runs over the surface and leaves, or soaks through, out of reach of most plant roots. The organic matter in our garden soil is crucial for holding moisture. It is also very important for providing habitat and food for all the microbes. As we’ve already mentioned, microbes are very important for feeding the plants. It all functions together in a beautiful system if everything is in place.
How can we be good stewards of the Creator’s intricate system?
There are several practical things that help build carbon in our garden soils. Number one is to keep the soil covered as much as possible with living, growing plants. Water as necessary and provide fertilizer if the soil has not yet reached a level of functional health to be self-sustaining. Gardens with living soil that has plenty of cycling carbon seldom need additional fertilizer inputs.
Tillage can be detrimental to building carbon levels in our soil. Disturbed soil rapidly breaks down stored carbon and releases carbon dioxide. This is not all bad when there are good-sized plants surrounding the soil to take the carbon dioxide back in through photosynthesis. Overall, it is better to use mulches instead of tilling for weed control. Some bare soil and shallow tillage may be necessary in northern areas to allow the soil to warm faster. The warmer and dryer the climate, the more carefully you should avoid tilling soil.