Soil for our Rose Gardens

By Cindy Crookston, Consulting Rosarian

Roses depend on soil for water and nutrients; it is critical for the health of the plant. So it is necessary for us to understand our garden’s soil in order to manage it well to grow healthy and beautiful roses. There are a few fundamental principles that if understood will help us to grow our roses successfully.

Soil Structure

A good soil is made up of four major components:

1.Inorganic materials, comprising about 45% (often called mineral material)

2.Organic materials, comprising about 5%

3.Air, comprising about 25%

4.Water, comprising about 25%

We do tend to think of soil as a solid, but it is interesting to note that the top layer of our soil, several inches, is mostly comprised of about half air and half water. This makes clear how important water and air are to proper soil function.

Water is essential to all plants rooted in the soil. A rose consists of 50-90% water, so sufficient water is critical. Water keeps roses turgid, is used for transpiration to cool the leaves, and is also a vital component of photosynthesis. This is how the plant takes the energy of sunlight and converts it to CO2. Water is also used to transport nutrients and carbohydrates throughout the plant.

Air is required for respiration for all living organisms in the soil; ie, roots, earthworms and micro- organisms. The oxygen in soil is an important element needed by roses, thus they cannot grow in water-saturated soil. They need oxygen. This being said, most air that a rose receives will come via the leaves.

Soil Texture

Our soil texture is composed of inorganic and organic materials. The inorganic part of soil, tiny rock particles, is made up of sand, silt and clay.

1. Sand is a very large particle (1/20mm to 2mm) that creates large pores in the soil that allow for good drainage and space for roots to grow. The drainage of water allows oxygen to penetrate the soil.

2. Silt is a medium-sized particle (1/500mm-1/20mm) formed as sand being broken down into clay. Silt is the most important source of available water in the soil. Sand on the other hand, allows the water to drain away too quickly, and clay holds the water too tightly for the plant to use.

3.Clay is a very small particle (less than 1/500mm) that is in a crystalline lattice form, meaning that it has a net negative charge. This means that clay soil is sticky and holds onto water and nutrients. Sand and silt have no charge, and don’t hold on, but release water and nutrients. When clay holds onto water and nutrients it is held in reserve for the plant.

The proportions of sand, silt and clay are shown on Figure 1, The Soil Texture Triangle produced by the USDA. When all three inorganic materials contribute to the soil’s property, it is called loam. This is the ideal soil texture for roses. This is a sand/silt/clay ratio of about 40%/40%/20%.

The American Rose Society recommends: “..that you incorporate copious amounts of finished compost to loosen your soils especially if you have clay based soil. The texture of the soil determines many of the properties of the soil. For example, sandy soils require more frequent watering and fertilizing. This is because the water flows through so quickly and the nutrients are quickly leached from the soil. Clay soils tend to have extremely poor drainage, but require less fertilizing and watering. Loam soils have the best properties with both good water retention and drainage, while also retaining nutrients.” (emphasis added)

Organic Material

Soil is comprised of about 5% organic material, and even though this is a rather small amount compared to the 45% inorganic material,” it nevertheless plays a vital role in nutrient cycling and building and maintaining good soil structure, “ says Dan Mills, “ The living portion of soil organic matter, mainly the microorganisms and earthworms, is constantly at work breaking down the dead portion (organic waste) into: 1. carbon dioxide and water, 2. mineral nutrients, 3. a wide variety of organic compounds and 4. humus.” This is called the carbon cycle, nitrogen cycle and sulfur cycle. During this decaying process the released mineral nutrients are taken up by the roots and utilized by the plants. Some will be held in storage by the clay particles or by the humus substances. And others may yet be leached out by the water. The decomposing organic matter that is now dead is called humus. Humus builds and survives under a thick blanket of mulch in the rose garden. Humus particles are dark brown to black. Humus plays an important part of healthy soil:

1. Humus breaks down inorganic matter into nutrients available to the plant,

2. Humus and clay particles together hold onto nutrients,

3. Because humus plays a part in the formation of soil aggregates (grouping together of individual particles called peds) it is critical to the formation of good soil structure.

4. Humus along with other organic matter increase the soil’s ability to hold water. Thus available water is higher in soils with high levels of organic matter.

Now that we have learned about the structure and texture of soil, let’s hear again from the American Rose Society: “The balance between air and water is important for healthy soil and healthy plants. The best soil structure has the air spaces created by the larger particles of sand and peds (soil structure) in uncompacted soil, the available water holding capacity of silt and organic matter, along with the nutrient holding capability of clay and organic matter. Soils that have all the characteristics of sand, silt and clay are called loam. Thus the ideal soil is a sandy loam with plenty of organic matter- rich sandy loam.”

References

American Rose Society, Consulting Rosarian Manual 3rd Edition Revised, July 2016

Mills, Dan, “Soil and Water and Roses”, The Sustainable Rose Garden: A Reader in Rose Culture, edited by Pat Shanley, et.al. 2010, Newbury Books

Stock, Dr. Melaine Assistant Professor/ Extension Urban & Small Farms Specialist USU, Certified Soil Scientist, SSSA, Soil and Water Management for Roses, June 2019

Figure 1