Identifying the kind of fertilizer your flowers need can be a trial-and-error experience. This is especially true if you are growing plants that don’t thrive in fertile soils like vegetable garden loam, which is rich in organic matter and major nutrients such as nitrogen. Understanding the difference between fertilizer and compost (organic matter) can also be perplexing. We hope to clear up some of the confusion in this Guide to Understanding and Using Fertilizer.

The guide covers topics such as:

• An elementary understanding of manure’s importance
• The history of fertilizer
• Differences between organic and synthetic products
• How fertilizer differs from compost
• Nutrients in soil and fertilizers
• What “complete” and “balanced” mean when describing fertilizer
• How a soil test can help in making decisions about amendments, and
• When and how much to fertilize Salvias and their companion plants.

Flowers by the Sea Farm and Online Nursery cultivates ornamentals for a wide variety of growing conditions, especially Salvias and plants that grow well with them. We also raise many kinds of Fuchsias. Our plants need soils ranging from lean, gravelly desert types (low in nitrogen and organic matter) to loam.

For example, our Fuchsias are native to Western Hemisphere settings that have soils rich in nutrients and organic matter. In contrast, our Salvias come from homelands worldwide, and their native soils vary broadly. Salvias originating in desert and semi-arid lands prefer soils low in nitrogen and high in rocky mineral matter whereas Salvias from woodlands — such as in Asia and South America — require rich soil.

Choosing fertilizer is easier when you’re familiar with the varying needs of your plants. To help you identify these requirements, FBTS plant descriptions indicate basic needs including soil types. If you feel perplexed by the topic of soil fertility, imagine how 19th century agricultural scientists must have scratched their heads as they finally began understanding the nutritional requirements of plants.  

Manure: It’s Elementary, Dear Gardener

Here’s something to think about the next time you see bags of composted manure at your local market or garden center. Animal manure — the combination of livestock feces and urine — has been used as a fertilizer since humans first adopted an agricultural way of life in Neolithic times.

Manure is still one of the first materials gardeners think of when supplementing soil nutrients, because it’s high in nitrogen, phosphorus, and potassium — three of the six primary elements (macronutrients) for plant growth. The other macronutrients are oxygen and hydrogen — both gained from water — and carbon, which plants absorb from the air through pores in their foliage. Nutrients that are less crucial yet important are the secondary elements and micronutrients.

Elements are substances that can’t be broken down into simpler forms. The Periodic Table of Chemical Elements uses one or two letters to represent each element.  Often, these abbreviations are the first letter of an element’s name such as “C” for carbon, “H” for hydrogen, “N” for nitrogen, “O” for oxygen, and “P” for phosphorus. But sometimes, elements are stuck with seemingly nonsensical letters like the “K” for potassium. It’s easier to read fertilizer labels and soil reports if you memorize some of these abbreviations, especially the N-P-K triad.

Before detailing the elemental nutrients plants require, we need to turn a few spadefuls of history about the transformation of fertilizer from a thick layer of dung to a variety of packaged products, including ones based on synthetic (manufactured) nutrients.

Early History of Fertilizer

A University of Oxford archaeobotanical study shows that farms in the UK and Europe fertilized crops with manure more than 7900 years ago during the Neolithic period. The study, titled Crop manuring and intensive land management by Europe’s first farmers, was published in Proceedings of the National Academy of Sciences of the United States of America (July 30, 2013).

The researchers based their conclusion on the fact that manure contains the rare stable isotope nitrogen-15 (15N), which gets stored in plants treated with animal waste. They analyzed remains of ancient crops — such as grains of barley and wheat as well as legume seeds — and discovered high levels of the isotope.

Ancient Roman Advice about Dung

Historical records concerning the use of manure as fertilizer appeared around 160 B.C. in De Agri Cultura, a sort of farmer’s notebook in Latin by Roman agriculturist, general, and statesman Marcus Porcius Cato, who historians refer to as Cato the Elder (234 -149 B.C.)

Cato stressed the importance of maintaining a “large dunghill” of livestock manure and detailed how to apply it to forage, grain crops, grapevines, meadows, and olive orchards.

The Theory of Nutrients

The next advance in soil supplementation occurred in the 18th century when farmers began using ground-up livestock bones (high in phosphorus) as a soil supplement. But at that time, nobody understood what manure and bonemeal contained that was so helpful in growing plants until German chemist Baron Justus von Liebig (1803-1873) published his “Theory of Mineral Nutrients” in 1840. Liebig identified nitrogen, phosphorus, and potassium as being essential for healthy plant growth.

Liebig also theorized that it doesn’t matter how much of these macronutrients a plant receives if it isn’t getting its least available yet necessary nutrients. This is called Liebig’s “Law of the Minimum” and is partly responsible for the long list of ingredients in fertilizer mixes. For example, a plant needs far more nitrogen than iron, but it still needs iron for top growth.

Furthermore, Liebig found that because plants deplete soil of nutrients — particularly nitrogen — replacement is necessary. This led to his creation of the first nitrogen-based fertilizer and earned him the title “father of the fertilizer industry.”

Beginnings of Synthetic Fertilizer

Air is about 78 percent nitrogen, but the element doesn’t exist in its pure form in soil. Even if it did, plants can’t access it unless it forms a compound with oxygen (NO3- or nitrate) or hydrogen (NH+4 or ammonium). The process is called fixation.

Some fixed nitrogen makes its way underground via rainwater during lightning strikes.  A lot is grabbed from the air by soil bacteria, which make it available to plants. Livestock digestion of plants also fixes nitrogen in manure.

By the early 1900s, agricultural chemists had long pondered how to make more nitrogen fertilizer at decreased cost. Through a discovery called the “Haber-Bosch process,” they succeeded by getting nitrogen and hydrogen to chemically react and create ammonia (NH3). This was the first step in creating what are now known as synthetic fertilizers.

We’re not chemists, so we won’t go crazy here detailing all the many forms of nitrogen. However, we think it is best to avoid fertilizers high in ammonium (also called ammoniacal nitrogen) because over-reliance on it damages soil structure.  Ammonium products don’t provide the organic matter that soil fauna, like earthworms, and beneficial microbes need to survive and aerate the ground. This ultimately leads to compaction, which means that plant roots can’t get sufficient water and air.

Fertilizer Today

Both organic and synthetic commercial fertilizers come in various forms. They may be:

• Dry, pulverized materials to incorporate into soil or mix with water
• Liquids to dilute and pour into soil or spray on plant foliage, or
• Nutrients that are pelletized (organic products) or beaded and coated (synthetic fertilizers). These are handy for enriching the potting mix of containerized plants. Pellet or bead fertilizers usually are described as being “slow-release” products. Organic fertilizer, in general, is absorbed slowly by plants as they need it. Beaded synthetic fertilizer gradually becomes available to plants as the coating of its beads decays.

Commercial fertilizers, whether organic or synthetic, are mixtures of active and inactive (inert filler) materials.  The active substances are nutrients that plants absorb through their roots or through their foliage when sprayed.

Active materials in organic fertilizer come from plants, animals (blood meal, bone meal, and manure), and ground mineral matter like rock phosphate. Aside from granular minerals, the active ingredients of synthetic fertilizers are manufactured from chemicals and petroleum waste, such as sulfur.

Inactive materials are ones the plant doesn’t absorb. They eliminate clumping and improve spreading of dry fertilizer. Fillers also reduce the concentration of active ingredients, such as nitrogen, to avoid burning or otherwise harming roots and leaves. Water is the main filler in a bottled liquid mix.  Common fillers in dry mixes include granular limestone, sand, and sawdust.

To calculate how much of a product is filler, add up all the percentages of active materials and deduct them from 100%. A classic example is a bag of dry mix marked “10-10-10” to indicate the percentages of nitrogen, phosphorus (potash), and potassium. Let’s say small quantities of other nutrients add up to 5%. In that case, 35% of the mix is active, and 65% is filler.

By the way, when a fertilizer is labeled with three numbers, the first one always represents “N,” the second “P,” and the third one “K.”

Compost Isn’t Fertilizer and Fertilizer Isn’t Plant Food

Sometimes it helps to define something by saying what it isn’t. For example, bags simply labeled “compost” contain decomposed plant material. These products generally aren’t considered fertilizer because their N-P-K levels usually are much lower than those of manure-compost mixes. However, some plant composts — especially homemade kinds containing lots of grass clippings and green kitchen waste — have higher levels of primary nutrients.

Here’s another clarification: Fertilizer provides nutrients for plants but isn’t plant food even if that’s what the labels on commercial products say. Plants are factories that manufacture the glucose they consume and which helps them grow. During photosynthesis, the chlorophyll in their foliage absorbs sunlight, which interacts with carbon dioxide from air and hydrogen and oxygen from water to make this sugar.

Yet plants need more than glucose to produce healthy leaves, stems, and blossoms. A wide range of naturally occurring nutrients in soil support this growth. When there is a deficit of one or more necessary nutrients, fertilizer can replenish shortfalls.

But what do all those bags of plant compost do?

Role of Plant Compost

Although plant compost doesn’t provide significant amounts of N-P-K, it aids uptake of these nutrients, improves soil drainage, and makes it easier for air and water to reach roots.

Similar to composted animal waste, plant compost feeds tiny, beneficial creatures and microbiota in the soil. These organisms include earthworms and pillbugs as well as microscopic bacteria and fungi that work together to decompose organic matter and make its chemicals accessible to plants.

How Organic & Synthetic Fertilizers Differ

Aside from their ingredients, two key differences between organic and synthetic products concern how quickly and how long their nutrients are available to plants.

Unlike synthetic fertilizers, organic types don’t dissolve in water for rapid uptake, and their nutrients don't dissipate quickly. Instead, soil bacteria gradually break down the components of organic fertilizer, which causes slower, steadier delivery of nutrients.

Synthetic fertilizer’s speedy delivery of plentiful nitrogen is helpful if quick turnaround of a deficit is necessary. But this characteristic can overwhelm plants if too much of a nutrient is applied. For example, an overabundance of nitrogen can burn roots and cause foliage growth to outpace root growth. Without strong roots, plants can’t maintain healthy top growth.

Also, because the nutrients in synthetic fertilizers are water soluble, they are highly mobile. This means that if your plants don’t consume them quickly, the nutrients may cause pollution by getting into groundwater and sewers. This causes problems such as algae overgrowth in lakes.

Concerning use of synthetic fertilizer for flowers planted in ground, remember to accompany it with plentiful plant compost. As previously mentioned, synthetic products don’t increase the organic matter that earthworms and microorganisms need to live and improve soil.

Whether planting in ground or in containers, one final but important point is that although calibration of N-P-K is more precise in synthetic fertilizers, organic products encompass a broader range of nutrients. Synthetic fertilizers usually don’t include micronutrients because supplementation of these elements generally isn’t necessary due to most soils containing sufficient amounts. However, deficits can occur, and the availability of a micronutrient via fertilizer may be what a plant needs to improve performance.  

Categories of Soil Nutrients

The three categories of elements plants need from soil to thrive are macronutrients, secondary nutrients, and micronutrients.

MACRONUTRIENTS

• Nitrogen (N) aids creation and transfer of energy in a plant through its role in aiding photosynthesis. It helps build protein for construction of plant cells and is essential for vigorous growth of foliage and production of seeds and fruits.
• Phosphorous (P) also supports photosynthesis and is vital to flowering and root growth. It aids plant maturation and resistance to stress.
• Potassium (K) is derived from a variety of mineral salts called potash. Like nitrogen, it builds protein. Similar to both nitrogen and phosphorous, it aids photosynthesis. Potassium also helps reduce plant disease and increases fruit quality.

SECONDARY NUTRIENTS

• Calcium (Ca) is part of cell walls and encourages the movement of other elements within plants. It increases plant strength and binds roots to soil.
• Magnesium (Mg) is essential to photosynthesis and activation of enzymes promoting growth.
• Sulfur (S) builds protein and helps form chlorophyll. It aids plant vigor and resistance to cold as well as supporting root growth and seed production.

MICRONUTRIENTS

“Micro” refers to the fact that, although important, the following plant nutrients are only necessary in trace amounts. All may exist naturally in your garden soil. When a soil report identifies a deficit of any of these nutrients, slight amendment is the rule to avoid harming plants.

• Boron (B) aids development of strong plant cell walls, flowers, and pollen. It’s easier for plants to absorb when soil is neither too acid (low pH) or too alkaline (high pH).
• Copper (Cu) is necessary for plant reproduction and use of proteins.
• Chlorine (Cl) supports the chemical transformations of plant metabolism.
• Iron (Fe) helps form chlorophyl and helps transport oxygen from roots to stems and leaves.
• Manganese (Mn) activates enzymes and strengthens cell walls.
• Molybdenum (Mo) produces enzymes that help regulate plant functions. It aids nitrogen fixation.
• Zinc (Zn) helps regulate a plant’s use of glucose and its growth.

Complete and Balanced Fertilizers

Complete fertilizers contain N-P-K, the three soil macronutrients. Incomplete fertilizers lack one or more of these nutrients. For example, bone meal contains phosphorus and calcium, but not nitrogen or potassium.

Balanced fertilizers are ones that have equal parts of nitrogen, phosphorus, and potassium. Back to the 10-10-10 example: If fertilizer packaging identifies its product as being “10-10-10” it contains 10% each of N, P, and K. 

But balance is a term that can be misleading, because not all plants need equal quantities of the three nutrients and not all gardens have the same nutrient levels or deficits. The best way to identify what would be balanced for your garden is to obtain a soil test.

Soil Testing

Types of soils and nutrient profiles vary from one region to another and even between neighboring yards. A soil test obtained through a private lab or your regional USDA agricultural extension office is the best way to determine nutrient levels and amount of organic matter. It can tell you if there is too little or too much of a good thing. A test can also identify any toxins in soil, such as persistent herbicides and heavy metals like lead.

Sometimes soil reports don’t contain a measurement of nitrogen, because the amount in your soil may be highly changeable from day to day. The nitrogen level of soil alters rapidly by moving back into the atmosphere as gas, leaching away with rainwater, and getting gobbled up by plants.

Fortunately, if your report indicates there is enough organic matter in the soil — say about 4% — then the nitrogen level is probably sufficient. Remember that plenty of organic matter — both plant and animal waste — means your soil can feed the tiny creatures and microbes that break organic matter into a form of nitrogen plants can consume.

When organic matter or a nutrient needs supplementation, most lab reports will indicate how much to add to correct the deficit. Based on your soil test’s measurements and suggestions, as well as the needs of your plants, you can decide whether fertilizer and compost are necessary and what kinds. Here’s a good article from the University of Illinois Extension Service about how to gather a sample.

Just as soils vary from one state to another, so do soil tests. Consequently, reading a soil report may be confusing. Don’t be shy about asking an agent at the private or extension service lab that is testing your soil to explain the abbreviations and measurements.

How and When to Fertilize FBTS Plants

The information in this guide doesn’t solely apply to Salvias, but they are the main kind of flowers we grow. There are more than 1,000 species worldwide, which come from native lands with differing growing conditions and types of soil. Consequently, their needs for fertilizer vary broadly.

More sages and their companion plants suffer from over-fertilization than from lack of soil nutrients. For example, a variety of difficulties arise if species native to the sandy, low fertility soils of desert and semi-arid lands are over-fertilized. These problems include decreased flower production and reductions in winter hardiness and resistance to diseases and pests. These kinds of plants usually don’t need fertilizer. In our FBTS online nursery catalog, we classify them as being Blue Tag xeric.

At the other end of the spectrum, FBTS Fuchsias and many of our Salvias prefer loam. Our catalog descriptions identify them as needing “rich” soil. You may need to fertilize the Fuchsias once a month for optimal growth. But unless you are container growing a Salvia that prefers rich conditions or your soil is naturally lean, you may only need to fertilize once a season and early in the plant’s growing cycle.

In between these extremes, many Salvias adapt well to varying types of soil and need little to no fertilizer unless growing in containers.

So, what is a guideline for amending the soils of potted Salvias? Potting mix is significantly different from garden soil. Containers are limited growing environments that don’t have the complex resources of garden soil, so their nutrients need to be replenished regularly. Fresh potting mix may include some fertilizer, and the plants you place in it may have some fertilizer in their growing medium. But these resources run out.

You need to fertilize most potted plants periodically for best growth. If you are using liquid or water-soluble granulated fertilizer, that likely means a half-strength dose every two to four weeks for Salvias that need rich fertility. For others, it may mean a weaker dose once a month or perhaps none at all.

We find that the best and easiest way for home gardeners to fertilize container Salvias and their companion plants is with slow-release fertilizer pellets (organic) or beads  (synthetic) when first planting them. Sprinkle the pellets or beads on top of the potting mix surrounding the plants but not touching stems or leaves to avoid burning foliage. Then firm in the fertilizer and cover it with about an inch of potting mix.

How much fertilizer should you apply whether in ground or in containers? Base that decision on what your packaged product suggests but remember that it is a one-size-fits-all figure for many kinds of flowers. Based on your observations from one season to the next, you may want to adjust the quantity — especially for Salvias that need little to no fertilizer.

Any Questions?

Less is often more. The rule of thumb for watering Salvias is to err on the dry side. A good plan of action for fertilizing most annual and perennial flowers is to err on the side of applying less. Another good rule of thumb is to ask questions when in doubt. At FBTS, we’re always glad to share gardening knowledge, so please contact us if you have questions or need additional advice.