Photosynthesis and Plant Geometry

As farmers and ranchers, we are essentially in the photosynthesis business, and the shape of our crops can increase how much they photosynthesize

In ecological terminology, productivity is the accumulation of energy and matter in the form of biomass: living plants and animals. As we all know, the first step in this process (primary productivity) happens mostly within green plants. Green plants, algae and certain cyanobacteria are capable of converting the radiant energy of sunlight into carbon compounds. The elements within these carbon compounds are “glued” together by light energy and become chemical energy. 

The process by which this happens is, of course, photosynthesis: the synthesizing of carbon compounds from air and water using the energy from the sun. The fact that this happens never ceases to amaze me. A little water, a little sunshine, and plants can make sugars, enzymes, acids, bases, starches, proteins, oils, resins, waxes, wood, flowers and a host of other things, all out of thin air!

The fundamental process of photosynthesis, whereby energy and matter combine, is described in the basic photosynthesis equation (leaving several things out to simplify matters):

6CO₂ + 6H₂0 + energy ???????? C₆H₁₂O₆ + 6O₂

C₆H₁₂O₆ is glucose, a basic sugar. Some of the energy stored in the carbon bonds of glucose is used by the plant itself to keep its cells alive, to build its own body and to create all of the secondary materials that we all depend on for food. 

Corn, beans, pumpkins, grapes, bananas, etc., are all manufactured by the plant using the energy released during respiration, which is essentially photosynthesis running right to left instead of left to right, as written above. The matter and energy accumulated during photosynthesis is termed the gross primary productivity, which can be measured at the individual plant, the population, community or ecosystem level. The remaining energy and matter of gross primary productivity after the plant uses what it needs to grow and reproduce (respiration) is called the net primary productivity. 

Water, sunlight and carbon dioxide are the fundamental necessities that our plants need, and we should have at least a basic understanding of this as ecological farmers. Yet notice that none of the basic elements necessary for photosynthesis and respiration need to come from the soil. The leaves and stems of plants have openings (stomata, lenticels, etc.) to allow gases and liquids to pass into and out of the plant without the intermediary of soil. This is one of the reasons feeding plants nutrients foliarly actually works. 

Secondary productivity is when organisms consume the primary photosynthates and convert them into other forms. Folks who grow and sell vegetables, grains, fruits, etc., are farming the net primary productivity of the site. Those who raise animals (from cattle to earthworms) or mushrooms, are farming the secondary productivity. The more primary productivity on your site, the greater the secondary yields can be. 

Chlorophyll, the green in our green plants, is found mostly within the leaves of plants, but also in their stems, flowers and even internal cells. Firewood cutters have all seen green wood in small-diameter stems, and that green is chlorophyll. Not much radiant energy penetrates three years deep into wood, but some does — leafless, woody stems actually photosynthesize. 

As farmers and ranchers, we are essentially in the photosynthesis business. One of our aims is to have as much photosynthesis happening on each acre as practically possible. The more net primary productivity happening on our properties, the more harvestable yields we can have. 

If sunlight, water and the atmosphere are the essential ingredients for photosynthesis, it is in our best interests to strive to maximize the capture of sunlight, air and water — and since I’ve already mentioned that photosynthesis can happen without soil, let’s just focus on the aerial portions of our crops. 

Most of us have seen mile after mile of soybean or wheat fields. They are incredibly uniform and of a relatively short stature. They are essentially a flat surface that is collecting CO₂ and sunlight from the atmosphere, along with dew, humidity and precipitation on its leaves. One of the reasons corn has higher net primary productivity (yield) than small grains and beans is its architecture. Just by being taller, it can capture more sunlight and air than a soybean. More leaf surface area and more stem surface add to this. 

An almond tree has more total yield (crop, leaves, wood are part of the total yield in ecological terms) than corn. It is easy to imagine how a 60-foot-tall oak tree can capture more air and sunshine than a two-foot-tall bean plant, but what is not so easy to imagine is how much moisture it can scrub directly from the atmosphere. In several texts I’ve read that it takes a half-inch rain just to get the exterior surfaces of a mature oak tree wet. This wetting of leaves, stems and trunk, whether we’re talking about corn or an oak, is called interception. 

Now let’s put all of these basic ecological principles together and begin to think about how we can use them to improve our ecological farm. As an example, consider the work of Jason Mauck (interviewed in the December 2023 issue of this magazine) and others. By planting alternating strips of corn and beans, the canopy architecture of a farm field changes. Instead of being a flat, uniform green surface, each acre of land now has a radically increased surface area. The crop alternates from short beans (or wheat) up to the tops of corn, then back down to beans. In addition to receiving sunlight on top, the corn now receives rays on its sides as well. 

This is one way nature optimizes the exchange of nutrients and gases between things — the design it uses is a deeply convoluted shape. Just think of the insides of your lungs. Instead of being smooth sacks for air, your bronchi (a similar root word as branches!) subdivides smaller and smaller from your single windpipe. Similarly, the insides of your intestines are lined with tiny villi, which increases surface area and facilitates the passage of fluids and nutrients. 

Growers like Jason Mauck who intercrop tall and short plants get higher per-acre yields because every acre has more exposed surface area. Think of the agroforesters who grow rows of trees between crop fields. Jason’s canopy height may only reach up to eight feet, but some agroforesters are working with trees 60 feet or taller. The increased per-acre yield, in part by increased photosynthetic surface area, is well known in agroforestry circles as the land equivalent ratio. 

What if we combined strip-cropping/relay-cropping such as Jason practices with an alley-cropping system? How much sunlight, atmosphere and moisture could we capture on each acre of ground? How could we cost-effectively design, install, manage, and harvest these systems in order to convert as much sunlight into useable biomass while living a good life? 

This is the art and science of ecological farming! Mark Shepard is a land designer and consultant and is the author of Restoration Agriculture, Water for Any Farm and the Water for Any Farm Technical Manual.