Chapter 10 Water

If we understand how irrigation water behaves in soil and in containerized nursery stock we can better understand how to manage bonsai stock in the ground and in pots.

10.1 How Water Behaves in Our Local Soil

This section is based on a 2016 article by James Hartsig, Soil Scientist, Duraroot Environmental Consulting, LLC, in Soils Matter (https://soilsmatter.wordpress.com/2016/05/15/how-does-water-move-through-soil/

The inorganic portion of soil consists mainly of sand, silt, and clay. The relative proportions of these particles determines overall soil texture, which in turn determines how fast and far water moves in it. Sand-sized particles are the largest of the three and bind together very loosely. Clay-sized particles are the smallest and bind together tightly. Silt-sized particles are intermediate both in size and cohesion.

Soil pores are formed by gaps between these soil particles. The pores allow water to infiltrate the surface of the soil initially, then move both laterally or upward by capillary action and vertically downward due to gravity. The size and number of pores in soil determines how far and fast water moves.

Soil that is granular or crumbly (like you might find in the mountains) or very sandy (like that near the coast) has mostly large particles and so large pores. These let water enter easily then drain downward once the surface zone becomes saturated. The red clay soil (ultisol) that dominates the Piedmont Plateau region differs in it has extremely small pores. North Carolina red clay soil averages 16% pore space, 2% organic matter and 82% inorganic matter. For comparison, optimum garden loam averages 50% pore space, 5% organic matter, and 45% inorganic matter. Small particles and pores means water does not enter easily, nor does it move laterally or vertically very quickly.

We can see evidence of this limited water movement if we dig down 2-3 feet in an undisturbed area or look at the soil profile in a recently dug house foundation. Usually there will be a layer 3-12 inches below the surface where the soil changes color; there may be additional layers even further down. These layers form as water travels down from the surface through soil pores carrying clay particles, iron oxide, and other insoluble pigments. The materials accumulate in different sub-surface layers, coinciding with the depth at which most surface water slows or stops moving downward.

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Figure 1. Schematic diagram of the normal layers in natural soil. Figure 2. Cross section through Cecil soil, the type of soil that makes up most soil in Piedmont NC. Link to originals: Image 1; Image 2

In extreme cases the clay particles that wash down can form a claypan layer that further hampers movement of water into the deeper soil layers. This traps water so the soil layer above it is nearly saturated while deeper soil layers remain too dry to support root growth. This is more likely to occur in soil that is frequently flooded or saturated (say from over-irrigation).

These hydrological characteristics of our local soil are not a major problem for farmers growing shallow-rooted annual cash crops. They also do not create major problems when growing pine or other timber crops because larger tree roots can penetrate the layers and create paths for water to permeate further into the soil. However the hydrology of our native clay soil DOES create problems when we are growing trees in open ground to fatten their trunks.

There is a temptation to plant trees in fairly shallow soil to make them easier to lift and root prune later. Yet when trees are planted on top of the clay boundary layer, they are more likely to experience “drought-and-drown” cycles. During heavy seasonal rains, the upper soil layer remains saturated and oxygen-depleted, stressing the roots. During dry periods, the upper soil does not maintain a consistent moisture level, again causing root stress.

When first planting trees in open ground, it is a good practice to dig through the clay boundary and ensure there is crumbly, well-drained soil extending well below the deepest roots. I’ve not read any good guidelines for how far down to dig, but I know from past digging that my plot has a heavy clay layer about six inches below the surface. I try to dig out another 12 inches below that, or about 18 inches in all. My goal is to create a reservoir space for water to move PAST the existing roots of the trees. This reduces the likelihood that trees will have to endure overly wet soil, and provide a better buffer against drought.

Rather than try to amend the clay subsoil I dig out, I have started replacing it. I collect and save my old soil when I repot trees. Most of the time the mix still has good structure, and I can add some fresh bark soil conditioner to replace the decomposed organic phase if needed. Then I use the recycled soil to fill the space in the hole beneath the root ball of the tree I am planting out. I use my regular soil mix or a 1:1 blend of my soil and commercial topsoil to fill the rest of the space around the root ball.

10.2 How Water Behaves in Containers

Water in a nursery pot or bonsai container behaves differently from water in open ground. Its rate of movement still depends on porosity and relative particle size of the potting mix. The key differences are that there is much less lateral capillary action, and gravity plays a much larger role in how much water a container holds.

Imagine a nursery pot filled with a 1:1 mix of bark nursery mix and 1/8” grit particles. As we add water it starts moving into and through the pores between the soil mix particles. Once we add enough water, all of the available pores will fill up temporarily with water.

When we stop adding water, the liquid in the pore spaces will continue moving downward due to gravity. As the water drains from the bottom of the container, pore spaces in the upper part of the container reopen and fill with air. Only a thin film of water will remain on the particle surfaces. As water evaporates from the thin film, more water is drawn back in to replace it by capillary action. At some point the combination of cohesive forces holding water on the soil particles inside the container and capillary action drawing water upward into the pot will becomes equal to the force of gravity pulling water down and out of the pot. At this point the water stops draining from the pot, and soil pore spaces below this balance point stay mostly full of irrigation water. This feature of potted stock where water does not drain fully from the container is called a perched water table.

The height of the perched water table in a container depends on many factors, but the most important is particle size. Soil mix with large particles will have large pores and less total surface area than soil mix with smaller particles. This produces less force to resist gravity, and the water table will lie close to the bottom of the container. Soil mix with smaller particles and smaller pores has more total surface area to attract water, so the water table will be higher in the container.

Particle size also affects the speed water moves through soil. When particles are large the excess water drains quickly, but when particles are small, any excess water will drain down more slowly. This is not a problem for most species in cultivation for bonsai, but it does pose a challenge for anyone who grows fussier species like Serissa or Bougainvillea that do best in moisture-retaining soil that still drains easily. The easiest way to maintain drainage and high moisture for these species is to add water-retentive materials like coconut coir or whole fiber sphagnum moss to the basic bark-grit soil mix.

The height of the perched water table inside of a nursery pot is determined by the properties of the potting mix (materials used, particle size, inherent moisture-holding capacity, etc.) not the depth of the container. If the perched water table forms in the lowest 2 inches of potting mix in a 12-inch tall nursery pot, it will also form in the lowest 2 inches of a container just 3 inches deep (assuming the same kind of mix is in both containers.)

The perched water table works to our advantage in bonsai. The water table in a bonsai container perches much closer to the soil surface than it does in a taller nursery pot. In other words, under ideal conditions a low bonsai container actually holds more water in the root zone of trees than does a taller container.

There are some important caveats to this blanket statement though.

• A perched water table does not form if there is not enough irrigation water to fill most or all of soil pores (at least initially). This is one reason why so many books say to water until it runs out the bottom holes of a pot.
• A perched water table is not permanent. Water can be lost through uptake by the roots or through evaporation.
• While a perched water table sits higher in a bonsai container, that container also has more open surface so water will evaporate faster. A low flat container heats up more quickly in the sun as well, further accelerating evaporation.
• Taller containers like those normally used for nursery stock have less open surface so water does not evaporate as quickly, and they hold less heat than ceramic containers. This is partly why nursery stock in standard containers tend to show less summer heat stress than trees in bonsai containers.

The easy way to know whether you are irrigating thoroughly on a routine basis is to check the root balls of each tree you are repotting.

• Look for a tangled mass of hair-like roots forming a web or mesh on the bottom of the soil mass when you first un-pot the tree. Fine roots grow where the water is. If you have a net of fine roots (1/16th inch or smaller) holding the bottom of the soil ball, you are irrigating enough for the perched water table to form.
• Dry soil in the bottom of a recently watered tree suggests you may not be applying enough water to create a perched table.
• Gaps or voids in the root ball that contain dusty soil suggest chronic under-watering.
  • The soil in the bottom of a pot can dry out, either due to insufficient water for an extended period, or as a result of smaller soil particles washing down from higher in the pot and forming a poorly porous barrier layer.
  • Once the area where the perched table should form has dried out, water can cut small channels where it flows through to the bottom of the container. This makes it more difficult still to keep the lower layers of soil mix properly wetted.

One nursery text I read suggested keeping sentinel trees next to your bonsai trees. These are cheap junipers or seedling trees planted in the same soil mix you use normally, and watered at the same time as styled trees are. The sentinel trees can be un-potted occasionally and checked to see if they are getting enough water without disturbing the root balls of more valuable trees. (If you pick a sentinel species that is prone to a particular disease or insect pest, it also will give you an early warning when conditions are favorable for an infestation.)