carbon cycle Archives

Photosynthesis is not a ‘constant’

Photosynthesis does not occur at a constant rate of speed. It varies from moment to moment dependent on the availability of light, carbon dioxide, water, temperature, chlorophyll concentrations, plant nutrition and genetics. This seems obvious on the surface, yet is almost always missed during research.

We understand that limitations on water, or nitrogen, or temperature extremes can have a pronounced impact on photosynthesis and consequently on plant growth and yield.

In contrast to this ‘downside potential’ of photosynthesis limitations, there is also an ‘upside potential’.

When environment and nutrition is optimized, plants can photosynthesize much more rapidly than what is ‘common’ or ‘normal’ (depending on how you define normal).

An extreme example is tomato production in greenhouses in the Netherlands, where yields are reaching up to 100 kg per square meter, equal to 890,000 lbs per acre. (No, that is not a typo, and it does not include an accidental additional zero.) Field grown fresh market tomato yields in the US range from 30,000 to 50,000 lb per acre, or about 6% of the yields in the greenhouses. To produce those results, lighting, CO2, and nutrition are all being managed very tightly.

This perspective on managing photosynthesis is very valuable when we think about how to increase yields and crop performance, and is often overlooked.

Very importantly, photosynthetic variability is completely overlooked in carbon sequestration research.

Research reports that this or that ecosystem can sequester xx amount of carbon. Grasslands at a certain level, forests at a certain level, farmland at a certain level.

The research, and the predictions coming from that research, contain the flawed assumption that the rate of photosynthesis is a constant from season to season.

Some fields/regions will photosynthesize less and sequester less carbon than the research indicates, because of a challenged environment.

Some fields and regions have the capacity to photosynthesize and sequester carbon at rates multiples higher than the research indicates.

As photosynthesis varies, so does root exudation, carbohydrate partitioning, disease resistance, insect resistance, crop response to microbial inoculants, fertilizers, and sprays.

All research evaluating the performance of products or practices on crops should contain the parameter, “what was the rate of photosynthesis in the plants contained in the study?” When this highly variable parameter is ignored, research does not translate consistently to other fields and farms.

Foliars as a tool of soil regeneration

Without the contribution of plants, ‘soil’ is only decomposed rock particles.

Plants contribute sugars, organic matter, carbon, the energy that sustains microbial populations.

Plants, through photosynthesis, are the only way we have of bringing new energy into the system.

The photosynthetic engine of most crops is only running at 15%-20% efficiency. (Charles Tsai, et al.) It makes sense to increase the efficiency of this engine as much as we are able.

The first priority of a successful foliar application is to increase photosynthetic efficiency. A foliar application that only addresses nutrient deficiencies and does not increase photosynthesis will not be nearly as effective as a foliar which does both. In fact, a foliar which does not increase photosynthesis can facilitate more efficient extraction of soil nutrients and increase soil degradation. Foliar design matters.

The nutrients which need to be present in adequate supply to increase photosynthesis are nitrogen, manganese, iron, magnesium and phosphorus. Obviously, many others are also important, but these are key.

We can use foliars as a tool for soil regeneration when we use them to increase photosynthetic efficiency and transfer a larger portion of plant photosynthates to the roots to feed soil biology.

When a well designed foliar is applied, the spike in photosynthesis can be observed in sap sugar content and dissolved solids, or brix. (Measured actual sugars on a plant sap analysis is best by far. Brix can be highly variable because of environmental conditions.)

After a successful foliar application, the photosynthetic rate will gradually drop back down, but not quite down to the previous baseline. With each successive application spike, and return to baseline, the baseline level increases. When photosynthetic efficiency baseline improves to a high enough plateau plants contribute more carbon energy to the soil than they withdraw mineral energy and the entire ecosystem becomes self-sustaining.

The drop back to the new baseline can occur quickly or slowly, depending on the level of ecosystem health. In a compromised and degraded ecosystem, the spike may last for as little as 3-5 days before it drops back down. In a healthy soil, with good biology, the elevated spike may last for as long as 5-6 weeks or even longer.

The healthier soils and plants become the fewer foliars are needed until the point is reached where they are completely unnecessary to sustain a level of health where plants are completely resistant to diseases and insects.

While on the pathway to this point, we can still use the photosynthetic efficiency spikes to produce interesting and valuable effects. If we have the presence of larval or sucking insects, a spike in photosynthesis is often successful in giving them a dose of sugar they can’t tolerate.

A slide from an academy presentation. Academy.regen.ag