Fertilizer
Are Your Roses Overfed?
When it comes to our rose gardens, many rosarians can relate to the motto “go big or go home!” After all, we grow roses for their blooms and we expect those blooms to be large, lustrous and abundant. To this end we amend our soil and fertilize our roses at regular intervals throughout the year. When our fertilized and well-watered roses don’t produce the blooms we want, our knee-jerk reaction is often to apply even more fertilizer, but this can be a misguided and even dangerous approach.
What is Essential?
Basics We Need to Know About Fertilizer
Plants need certain elements to grow. The primary or macronutrients which are essential for plant growth are nitrogen, phosphorus and potassium (NPK). The remaining 10 essential plant nutrients—boron, calcium, chlorine, copper, iron, magnesium, manganese, molybdenum, sulfur, and zinc are needed in much lower quantities. These nutrients are often present in our soil and become available to the watered plant to uptake through its roots.
The Label
By law, fertilizer companies must disclose the amount of nitrogen (N), phosphorus (P) and potassium (K) on their products’ labels. A 10-20-10 fertilizer includes by weight 10 percent N, 20 percent P and 10 percent K. In a 10-10-10 and 20-20-20 fertilizer all three nutrients are balanced and in the same ratio, but the 20-20-20 fertilizer has twice the concentration of NPK and smaller amounts should be used. Micronutrients if included are listed on the label, and the rest of the bag is filler, which gives the fertilizer bulk and makes it easier to spread.
Different plants often have different nutritional requirements. As a result, there are many varied NPK fertilizer blends available in garden centers. If N, P or K is absent from the blend, then the corresponding number in the ratio will be replaced with a zero and the product is an incomplete fertilizer. For example, a 21-0-0 fertilizer (ammonium sulphate) contains 21 percent N and no P or K. The directions for use on the label specify how much fertilizer to apply. Following directions that specify the amount in tablespoons and cups per plant is easy. Calculating how much nitrogen to use when a soil test recommends fertilizing your garden in pounds per 100 or 1000 square feet requires some math. That 21-0-0 product contains 21 percent nitrogen, so a 10-pound bag contains 2.1 pounds of nitrogen. (10 x 0.21 = 2.1).
Organic and Synthetic
Organic fertilizers such as blood meal, bone meal, cottonseed meal, and fish emulsion are derived from the remains of living organisms. These fertilizers work in concert with soil microbes which break fertilizers down. Organic fertilizers don’t add excess salts to the soil, they improve the structure of the soil, and they are beneficial in encouraging healthy soil biology rich in microbial activity. Because these fertilizers act slowly, they are less likely to ‘burn’ our plants which is easily done when overdosing with synthetic fertilizers.
Synthetic fertilizers are lab-made and faster acting so they boost and expedite plant growth and bloom rate. However, these fertilizers are high in salts and do not improve our soil’s health, texture, or long-term fertility. They can be detrimental to populations of beneficial microorganisms and too much can ‘burn’ our plants. Synthetic fertilizers are typically more cost effective since they generally have higher NPK numbers so gardeners can apply less and deliver an equal amount of nutrients.
Fertilizers can be granular, slow-release or liquid
Granular and slow-release fertilizers are solids that must be scratched into the soil and release nutrients over time. The nutrients decompose and are released slowly for plant uptake with water and an increase in soil temperature. Slow-release fertilizers can last between 3 to 8 months depending on the formulation. The fertilizer in the slow-release formulations is released by water penetration, weathering or microbial action.
Liquid or soluble fertilizers have the advantage of supplying plants with a rapid release of nutrients although if drainage is too fast, for instance after a heavy rain, nutrients and especially nitrogen can leach below the roots before plants are able to utilize them.
Fertilizers Can Be Problematic
…when we use more than we need
Proper fertilization enhances plant growth without polluting the environment. Too much unnecessary fertilizer can injure, burn and kill a plant and mess up our soil. It can also run off into the ocean, lakes and streams.
… even if they are organic
Linda Chalker-Scott Ph.D. associate professor and extension urban horticulturist at Washington State University writes in her article The Myth of Organic Superiority that the bottom line in applying fertilizers and amendments is to be “conservative in what chemicals you add to a landscape, regardless of their source. Any organic substance, natural or synthetic, can cause environmental problems when added in excess of what a landscape system can absorb and utilize.”
…when a complete fertilizer isn’t needed
Using a complete fertilizer such as a 10-10-10 seems sensible, but, in fact, this isn’t always the best choice since plants don’t use nutrients at the same rate. If our soil does not need to be replenished with a nutrient, we should not use it as it can harm our plant and our soil.
…when they are unnecessary
Phosphorus and Potassium
To get big blooms, rose gardeners tend to apply fertilizers with high amounts of phosphorus on the premise that this nutrient improves flower growth. The problem is that phosphorus and potassium stay put in the soil and applications can remain in the soil for years. When we apply and then reapply for instance a 10-20-10 product, there is a good chance we are oversupplying our soil with phosphorus and potassium. Healthy soils are naturally rich in mycorrhizal species, but we jeopardize these mycorrhizal associations by excessive fertilizer application and especially the excessive use of phosphate.
Epsom salts
There is a belief among rosarians that the addition of epsom salts (magnesium sulfate) promotes new basal breaks. This long-held belief is disputable as researchers have found no benefit to this practice in controlled and replicated trials. Like the addition of phosphorus to a non-deficient soil, this practice can be problematic. Why? Because the accumulation of too much of one nutrient, such as phosphorous or magnesium can cause other necessary nutrients to become unavailable to the plant.
What to know about Nitrogen
Nitrogen is the most important element to a plant’s growth and should be added to the soil periodically throughout the growing season as nitrogen unlike phosphorus and potassium is very mobile and depletes quickly. Lighter green leaves or an even yellowing of leaves indicate a lack of nitrogen. However, we do need to be careful not to over fertilize with nitrogen as this promotes vegetative growth, encourages infestations of pests such as aphids and decreases plant resistance to disease. When our soil contains sufficient phosphorus and potassium and is deficient only in nitrogen, we should use an incomplete single ingredient fertilizer that provides nitrogen alone such as ammonium sulfate 21-0-0, urea 46-0-0 or blood meal 13-0-0.
How Do We Know What Our Plants Need?
Soil Test
The best way to know what our plants need is to do a soil test every 2 to 3 years. The results of the test include both an analysis of our soil and recommendations for fertilizer applications and rates to safely correct deficiencies.
A soil test also checks pH
An incorrect pH prevents our plants from properly taking nutrients from even a nutrient rich soil. This is frequently referred to as the nutrients being ‘locked up’. The ideal pH for roses is a soil that tests between 6.0 and 6.5. Generally, soils in the western part of the USA are alkaline. We can decrease the pH by adding elemental sulfur.
We love to feed the ones we love but over feeding is problematic not only for us but also for our plants. When our plants have been fertilized and adequately watered but are looking ‘not quite right’ check your soil’s drainage and get a soil test.
