Soil Management

by Ronald G. Schwerdt
3019 N. Major Ave. | Chicago, IL

Are your roses growing as well as they can? There may be many reason why they are not performing as they should. Some problems have simple solutions, while others are a bit more complex. Many are physiological and not related to infectious diseases or plant pests. Most arise with the plant’s environment, such as the soil. Too much or too little water, poor drainage, insufficient soil aeration, or a nutrient deficiency, etc. may be the cause. These are conditions that need to be identified and eliminated in order to grow healthy roses.

It is a well-known fact that insects and diseases thrive on stressed and unhealthy plants. Start with a biologically active soil rich in humus. Under proper conditions of moisture and soil temperature, most species of soil organisms in the process of decomposition produces substances such as antibiotics, enzymes and trace elements that provide optimal plant growth, also help fight off diseases, resulting in a natural immunity for the plants – before insects or diseases can get established.

Not all problems related to unhealthy plants are caused by environmental conditions. Some problems can be traced to various insects that inhabit the soil, such as nematodes, fungus, gnats, etc. They can feed on plants’ roots and spread diseases showing symptoms similar to plant dehydration, and yet not be caused by a lack of moisture in the soil.

While rosarians will agree that water is of the utmost importance for growing roses, so is proper soil management. Part of this “management” is conserving soil moisture for as long as possible, by any and all means. The best method of conserving moisture is to incorporate organics regularly, like sphagnum peat moss or compost, to build up a live healthy soil. This results in improved soil aeration as well as improving the tithe (workability) of the soil. Opening up the soil for greater air infiltration makes for quicker decomposition of organic matter. This organic material feeds the soil’s active microorganisms as it decomposes, thereby building the soil structure, ensuring a constant supply of nutrients and humus, which acts as a reservoir for storing soil moisture.

During hot, dry periods, with a breeze, the loss of moisture will be greatly increased. Having a limited root system and large top growth during these hot dry periods causes some plants to dry out quickly. Plants sometimes lose more moisture through the leaves, than the roots are able to absorb from the soil, causing a temporary wilting of foliage. Water, along with dissolved nutrients, are pulled upward by capillary action under tension. Moisture flows from the roots through the xylem tubes to leaves, where it evaporates into the atmosphere (leaf transpiration), producing a cooling effect in hot weather. As long as the capillary action remains unobstructed, the plants receive an adequate supply of water. Repottimg a plant in an incompatible soil may result in a break of continuity of gravity’s pull of water by the roots.

Soil is the key to growing healthier roses. A garden soil is nothing more than a combination of rocks, worn down over the years by nature, into various-sized particles. These particles, from silt to coarser sand to very fine sand are combined with a various amounts of organic matter, such as peat moss, decayed leaves, etc.

There are basically 12 recognized types of soil structures that make up the physical structure of a soil, with the percentage of sand, silt and clay determining the class. These are light, medium, and heavy soils, depending on the degree of air infiltration, water and nutrients absorption. A basic formula is one third coarse sand, one third clay, and one third decomposed organic matter. When mixed together, these elements provide a soil mixture that is light and friable, doing what you want it to do. This is known as loam, and is what you should try to change your existing soil to, if need be.

A light soil is generally sandy. While providing excellent drainage and aeration, light soil also warms up quicker in spring; it is often on the acid side; and it lacks the ability to retain moisture or nutrients. To overcome these problems, a large amount of organic matter must be worked into this kind of soil.

The soil structures in many areas are clay or silty clay loam – a heavy soil. This makes it ideal to absorb and retain moisture and nutrients, but it suffers from poor drainage. To correct the drainage problem of a heavy soil (which you must do to get oxygen down into the root system), you must work in coarse sand (torpedo) and decomposed organic materials.

These facts stated, it becomes apparent that a medium-type soil structure is best – one which incorporates only the best features of clayey and sandy soils into one that is fertile, and well balanced. Any soil can be conditioned into one that will produce healthy roses by incorporating into it what the soil is lacking. Do not try to condition a soil that is too wet to work. All you will do is compact it, making it extremely difficult to condition afterwards.

Fall is often the best time to condition the soil, as there is still sufficient time to incorporate anything required to improve the soil , letting it mellow over the winter months for next year’s spring growth.

Soil aeration is necessary for soil’s microorganisms to function properly. This is vital in the utilization of plant fertilizer as they feed on the organic matter as they condition the soil. Over a period of time this organic matter is depleted and needs to be replenished. The more chemical fertilizer used, the quicker this will happen. For this reason, it is a good practice to incorporating some organic matter into the soil every year. Without these organisms and enough organic matter for them to feed on, any fertilizer you give your plants will be of little value.

You should have a professional soil test made at least every four years, sooner if you have problems with the roses. During the growing season, check the soil’s pH yourself. Soil testing kits and meters are available that are easy to use. Even though there are indications that the soil is lacking in a certain element, the element may be present in the soil, but because it is locked up by a too high or too low pH, it is unavailable to the plant. Adding fertilizer to a plant suffering poor growth is of little use unless a soil is first conditioned to a level where the plant can take up and utilize the available nutrients. The pH in a soil is commonly referred to as the acidity/alkalinity balance. On a scale of 0 to 14, 7 is neutral. Above 7 is alkaline, below 7 acidity. For roses, the pH range should be between 6.0 and 7.0, with 6.5 ideal.

Sulfur is usually used to lower the soil’s pH, while dolomite lime is used to raise it. Dolomite lime is also a source of calcium and magnesium. The quantity to use to raise or lower the pH to the desirable range is on the box. Claims are made by many rosarians that Epsom salts (magnesium sulfate) agricultural grade encourages basal breaks. There is a great percentage of sulfur (S) 12.9 percent in a 50# bag, and magnesium (Mg) 9.8 percent. If you already have a soil on the acidity side, adding sulfur will lower your pH further! Be wary of the ashes from some types of briquettes used for outdoor cooking. While many are a source of potash, some contain a high level of sulfur. This, when combined with water, forms sulfuric acid, which again lowers the pH of your soil.

Gypsum (calcium sulfate) does not change the soil’s pH and has the ability to loosen up a heavy clay soil. Soils high in excessive amounts of sodium salts sustain damage to their structure. Having an alkaline condition can cause the plant leaves to burn. Adding gypsum provides calcium that replaces the sodium in the soil’s structure. This breaks the sodium’s bond, allowing water to flush out the soil’s salt buildup from too much chemical fertilizer.

It is important to understand the relationship between water, soil and maintaining the proper soil pH. Make adjustments if need be, and when the new rose season is in full swing, take inventory of how the roses are performing in this new changed environment. If rose are still not up to your expectations, what are the problems? Is new growth stunted and puny looking, or are blooms or foliage of abnormal size?

Do you have healthy looking foliage, but no flower buds? How about color of the bloom? Are they washed out and faded looking, instead of deep vivid colors? Are leaves lime green or yellowish (chlorotic) color instead of the dark green which is normally associated with healthy foliage? Maybe something is missing or not being utilized in the soil.

There are times when a plant may show signs of a nutrient deficiency even after being fertilized. These signs include a lack of color in the blooms, stunted plants, some leaves turn light green, white, dull gray, yellow, etc. All these symptoms can be caused by a lack of one or more trace elements (not all fertilizers contain trace elements.) A trace element can provide a plant and the soil bacteria with a balanced nutrition. When using a trace element, only a very small quantity is necessary.

One example of a nutrient deficiency occurs when trace elements are used up in the soil, causing the leaf to turn yellow while the veins of the leaf remain green. This condition usually occurs when there isn’t enough magnesium which the plant uses to manufacture chlorophyll in the soil. Yellow leaves (chlorotic) are a good indication the plant has some type of a physiological problem, a nutrient deficiency, or a problem with the plant’s environment.

While there are many reasons that cause leaves to yellow, their symptoms can be confusing. Often leaves look like the discoloration could be caused by a shortage of several different elements.

A nutrient deficient plant is plagued by insects and diseases more often than a healthy plant.

This article was provided to the TVRS as a courtesy by the American Rose Society.