#4 Soil Science Basics

Soil Solution

It is important to note that tap water doesn’t interact with the plant in a vacuum. Water saturates the soil and a mixture is created made up of soil, dissolved solutes, and water which is known as the soil solution. When we talk about parameters like pH and TDS for tap water, we need to not only think of the values in our water, but also how the soil and fertilizer interacting with the water impacts those parameters. For example, when you water, your tap water may have a high pH, but it can completely change when it interacts with ions in the soil or fertilizers which impacts the conditions of the soil solution that interacts with the plant.

This soil solution is so important because it is ultimately what impacts the nutrient uptake of the plant, and the parameters of the soil solution can be different than the water you use. The soil solution is so important because for nutrients to be taken up by the plant, they have to exist in a dissolved form in the soil solution. [1] When people say that nutrients aren't in an "available form" for the plant, they are just saying that for one of multiple reasons, the nutrient isn't in a dissolved form in the soil solution. In this article, we’ll talk about the basics of the soil solution and soil chemistry.

Soil pH

Soil pH is a measure of the concentration of hydrogen ions in the soil solution and is measured on the same pH scale you may be familiar with that ranges from 0-14, with 0 being very acidic and 14 being very basic. Soil is a solid and doesn’t have a pH, so soil pH is actually measured using different ratios of water and soil mixed together in a paste or a saturated solution of soil.

Fig 1: The pH scale with some common materials and soils. Attribution to Lewis Fausak.

Soil pH affects nutrient availability for plants. Soil pH values in the 6.5 to 8.0 range are considered optimal for plants. One reason for this is that the different substances have different solubility in water at different pH, and thus different amounts of availability to plants since a nutrient needs to be dissolved in the soil solution to be available to the plant. 

There haven’t been a lot of studies on the optimal pH for the species we use in bonsai that I’ve found yet, so this is just a general range. Very acidic soil solution (low pH) can increase the concentration of aluminum, manganese and iron, which can be toxic at high concentrations. On the Alkaline side (high pH), pH can lead to excess levels of calcium in the soil that can react with nutrients and make them unavailable for the plant.

Fig 2: This shows nutrient availability and the influence by pH. Attribution to Lewis Fausak. 

Cation Exchange in Soils

Soil particles typically have negative charges, so they attract and bind with positive ions that enter bonsai pots through water or fertilizer. Common cations are things like Ca2+, Mg 2+, K+, H+ and AL 3+. Different soil types, for example akadama vs potting soil vs pumice, can have different amounts of capacity to hold these ions which we refer to as the Cation Exchange Capacity (CEC) of the soil. [1] 

If CEC refers to the amount of charged particles a soil can hold, then cation exchange just refers to the process by which ions bound to soil particles can swap between the binding sites on the soil and the soil solution. The amount of cation exchange capacity is a function of the amount of small organic and inorganic particles, the type of clay minerals present and the soil pH. Anions can also be stored in soil in a similar fashion, though more frequently at low pHs. 

Conclusions for Bonsai

In bonsai, it’s so important to be cognizant of what nutrients we put into the system via watering because of this concept of Cation Exchange Capacity and the understanding that nutrients have to be available in the soil solution in order to enter the plant. We do a number of things with our soil that directly reduce the availability of nutrients to the plant compared to plants growing in nature.

1. We don’t use much soil. When plants grow in the wild, they can spread their roots out wide and deep in search of nutrients in the soil. They don’t have that luxury in a small bonsai pot and they also have smaller colonies of beneficial microbes to interact with and break down nutrients for the plant.

2. Many of the components we use for typical bonsai soil (pumice and lava rock especially) have almost no cation exchange capacity! [2] This means the plant's refrigerator is almost never stocked by the soil we use directly.

3. We use massive particle sizes compared to what you find in most natural environments. This just means there is more space for water and air and less binding sites for cations.

I’m not saying the above to suggest we change our soil components or strategy, but more to suggest that we put increased emphasis on the quality of our water since the system is so delicate and sparse nutritionally. My hypothesis is that nutrient uptake of the plant will be heavily influenced by tap water quality due to the relatively low amount of available ions in the soil. If your tap water is influencing the soil solution chemistry heavily, that not only means ions in your water are interacting with the plant, but also means that your tap water could be changing the availability of nutrients you add through fertilizers to your plant. I hope to do some tests in the future to look more into this for my soil and share those experiments with you all!

Citations

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

[2] Pumice. Botanicare. (2020, November 6). https://www.botanicare.com/hydro-101/pumice/#:~:text=Pumice%20has%20very%20low%20water,for%20plant%20uptake%20as%20needed