Soils and Water for Roses – the strange nature of small things in the garden.

Jerry Cinnamon

Sand, Silt, and Clay

Soils are called on to provide many services for a growing rose bush. The soil provides a structural medium to support the rose, and holds nutrients, water, and air to supply the growing plant. As a structural medium, soil is composed of mineral grains of different sizes including those of clay, sand, and silt.¹ For practical purposes; sand is a soil particle that you see as an individual grain with your eye.²

Soil Textures

Soils textures are described in terms of the textural triangle below. The top point of the diagram is a point representing 100% Clay, while the other points represent 100% Sand, or 100% Silt. Most soils are a mixture of these size components. Within the interior of the triangle, the field of Loam serves as an important reference.

A loam is defined by “feel” by rubbing it between your fingers. To be a loam, a soil should feel as though it is made of equal parts of clay, sand, and silt.

In a mixture, the slipperiness of clay is prominent, so a loam actually has only about 20 % clay, and 40% of both sand and silt.

The best soil to plant a rose in is one that both gives structural support while allowing good drainage, air, and water to get to the plant’s roots. These conditions are provided by a soil texture with a bit more sand than a loam. That soil is a Sandy Loam, indicated in the triangular diagram. Most of us do not have such an ideal soil, so we need to amend it by adding organic matter and perhaps sand.

The best soil for a rose is a Sandy Loam that combines good drainage, support, and some clay and organics to hold nutrients. We amend our soils toward this ideal.

Water In Soils

The Nature of Water

Water is one of the most intriguing compounds on planet earth and in the garden in its properties and behavior, but yet we encounter this uniqueness everyday and accept it as normal. For a start, water is the only common material found on earth as a solid, liquid, and gas. We get a further sense of water’s uniqueness from observing the behavior of liquid water in lakes and puddles as winter approaches. As liquid water gets colder, it increasingly gets denser until at about 4 degrees C water begins to get less dense, and turns into hard solid ice that floats on top of liquid water. What allows water to do this is the shape and electrical nature of the water molecule.

The water molecule (H₂O) is very much smaller than that of the soil’s mineral grains, measuring on the order of 0.000,000,1 mm compared to the 2mm – 0.05 mm size of sand size particles. Because of its small size it takes millions of individual water molecules to make the smallest of water droplets.

The water molecule shape can be described as looking a bit like Mickey Mouse with small charge hydrogen ears and a face made of a single large charged oxygen.³ It would seem that the two hydrogen’s, each with a +1 charge, would balance out the single negative (-2) oxygen, and they do as a generality, but that does not take into account the asymmetry of the molecule with the hydrogen ears on one end and the oxygen chin on the other. The Mickey Mouse shape results in unbalanced electrical charge in space, so that one end of the water molecule is negative and one end positive. It is these charged ends that allow water molecules to join with other water molecules in a disorganized manner in liquid water, to join in a very patterned manner in ice, and to join with soil particles in the garden as described below.

Water Interactions with Soil Particles

The mineral grains of soils are packed together to form a structure something like marbles of different sizes in a shoe box, as indicated below. When water enters the soil, the charged water molecules, that are very much smaller than the mineral grains, cling to the mineral grains. Water molecules can then migrate from one part of the soil to another through capillary action.

The spaces between mineral grains are termed pores and can be filled with air or water. If all pore spaces are filled first with air, and water is pored into the soil, the air would be displaced and the pores filled with water so that the soil is saturated with water. If you then attempt to drain the pore spaces of water, it would behave in two ways. Some of the water would drain under the influence of gravity, but some water would cling to the soil particles because of the interaction between the charged water molecule and the soil particle. The percentage of water that clings to the soil mineral grains increases as mineral grain size decreases, because smaller particles have more surface area for the water to cling to. The water that clings to the mineral grains is “bound” or capillary water and is not available to plants, whereas the water within the interior of the pores is “free” water and is available to plants.

Watering Roses

When you water roses, the first water that goes into the soil clings to the mineral grains and becomes capillary water and is unavailable to rose roots. Once the mineral grains are coated with water, the pores fill with free water that is available to the plant root. It is for this reason that you water roses deeply, by one or more inches of water, each week.

Water roses deeply, by one or more inches, each week.

Thanks for hanging in there to the end of this discussion of small things.

Footnotes:

By size definition, sand is defined as the component of soil between 2mm and 0.05mm in size, silt lies between the size range 0.05 mm and 0.002 mm, and clay is the size fraction less than 0.002 mm.
When talking about soil texture, people normally do not consider the sand’s composition, although it is predominantly the mineral quartz (SiO₂), a hard and chemically inactive material that does not provide or nutrients to the soil.

3. Each charged hydrogen and the charged oxygen are called ions, indicating that they

either lost or gained electrons to gain their charges.