#5 How do Plants get Soil Based Nutrients?

In this article we will talk about how nutrients in the soil get to the plant. In order to be absorbed by the roots, nutrients obviously need to come in contact with the root somehow! There are three main mechanisms for this to happen [1].

1. Root Growth - The root can grow and come in contact with new areas of soil that have nutrients in them. Based on the linked study, roots only are in contact with 1-3% of the nutrients in the soil directly [2]. 

Bonsai Comment: I believe this is a fairly low contributor to nutrient uptake in bonsai. The 1-3% soil volume contact quoted above was from a study of potted plants, so it is reasonable to expect that bonsai roots will not be orders of magnitude different. We also don’t actually have big pockets of nutrients hanging out in the soil since we use such inorganic/nutrient poor soils and it isn’t super common to mix in fertilizers into the soil. Akadama/Pumice/Lava don’t have large amounts of naturally occurring and available NPK/Carbon/Calcium/etc for roots to entrap.

2. Mass Flow - When the tree is thirsty, it starts that cohesion/tension conga line we talked about in article one which causes water to move through the soil solution towards the roots. As it moves, dissolved solutes (nutrients and other stuff) in the soil solution (made up of water/fertilize/soil) move with it to the root and can be absorbed. The amount of nutrients interacting with the plant depends on the amount of water used by the plant and the concentration of nutrients in the soil solution. The dissolved solutes being brought into the pot with the water are the main contributors to interactive ions in the soil solution.

Bonsai Comment: To me, this seems like a main mechanism in potted plants for nutrient uptake. Whether you are using a fertilizer that needs to break down before it’s water soluble (organic or solid ferts) or a water soluble fertilizer (like inorganic ferts), it ultimately gets dissolved in the soil solution and is pulled to the roots.

3. Diffusion - There is a natural process of molecules moving from areas of high concentration to low concentration. Roots take in nutrients near them, decreasing the concentration of these nutrients near the roots. Nutrients migrate from other areas of the soil to these low concentration areas and thus replenish the nutrients available to the roots.

Bonsai Comment: This undoubtedly happens in bonsai soil as well, but my thought is that we are watering so frequently that the volume of resources gained via diffusion is less than the fast, daily movement due to watering. When transpiration is high (water leaving the plant through its leaves) then mass flow is most likely a main mechanism for nutrient transport to the root, otherwise diffusion is the primary contributor.

Citations:

[1] Canadian Society of Soil Science. (2021, August 12). Digging into Canadian soils. Digging into Canadian Soils. https://openpress.usask.ca/soilscience/ 

[2] J. Agric. Food Chem. 1963, 11, 3, 204–207 https://pubs.acs.org/doi/pdf/10.1021/jf60127a017#