Impacts of Soil Structure on Photosynthesis

January 23, 2023

Good soil structure is needed for optimal photosynthesis.

From a minerals perspective, the rule of thumb is that calcium loosens soil and magnesium tightens it; that is, if kept in balance. It is also true that soils with very low magnesium can also have poor structure because, technically, the structure was too loose and collapsed.

An explanation of Total Exchange Capacity (TEC) or Cation Exchange Capacity is in order. Sandy soils typically have a low TEC, in the range of 3-5 approximately, while good clay soils are more like 15-30. Why is this important? Because sometimes clay soils test as having a TEC of only 7-10. When that happens, you can expect that the clay has aged and collapsed, making a tight sticky soil.

In these cases, if calcium and magnesium seem to be in balance from a base saturation perspective, it is typically helpful to apply a calcium source to help structure the soil. What has happened is that the clay has become so compacted that it has lost its TEC, its holding capacity, and it needs help to become a productive flocculated soil again. Productive soils need pore space so that the aerobic microbes can breathe and so the roots can grow. The average roots will not grow more than 1 inch into compacted soils.

If you want to imagine what soil should look like, think of a chocolate cake; you want a well-structured and spongy soil. The chocolate part might be overstated, but you get the idea. Another way to describe the ideal soil would be to imagine a used deck of cards, where there is air space in-between the cards. A new deck of cards, which still stands flat and has almost no air between the cards, would be similar to a compacted soil.

We want the soil to look similar to the used deck of cards, with plenty of pore space present so that the roots can grow and the microbes can breathe.

There is something else that is important as well: when we get our soils properly structured, we will have much better potassium release. Potassium has a unique structure that very easily gets stuck when soil are not properly structured.

In addition, when soils are compacted, and especially when they are high in magnesium, it is harder to have adequate nitrogen coming from the soil. Why? Because first of all, most of the nitrogen that is provides naturally in our soils comes from the atmosphere, which is 78% nitrogen, or approximately 32,000 tons of nitrogen over every acre of land. To utilize this nitrogen bank, it must be drawn into the soil and fixed by either nitrogen fixing bacteria or nitrogen fixing plants (legumes) that have nodules. Both of these need oxygen, or say it another way, soils that have good structure and plenty of pore space. In addition, we need good pore space for the soil to be able to inhale the nitrogen in the first place.

The double whammy in compacted soil is that, in addition to not fixing nitrogen as just described, the soil actually gasses off nitrogen, through the action of anaerobic bacteria.

Now it’s time to connect a few dots: How does soil structure impact photosynthesis? Last week we discussed minerals and how they impact photosynthesis. But did you realize that when you ash a plant, only about 5% of it is minerals? The rest, approximately 95% of plant weight, is composed of carbon, hydrogen, and oxygen, which are all elements that heavily rely on good soil structure for release.

Am I saying that minerals are not important? No. But that soil structure and the free resources that are available to us have a much larger impact that we can grasp.

What do I want you to get from this article? Am I saying that you need to apply lime to structure the soil? No. Calcium is really important for good soil structure, but remember that not all calcium is lime; there are other calcium sources that may work better in a high pH soil, such as Aragonite or gypsum, or a blend of both. One reason why I prefer Aragonite over lime in high pH soils is because it releases much faster and it has similar calcium content.

For a summary, in clay soil like mentioned above, where calcium levels are ideal but the soil has a low TEC for a clay soil, it is helpful to apply calcium. When done correctly, it will structure the soil and increase the TEC without increasing the pH; as long as magnesium is still kept at a decent balance.