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The Role of Soil Fertility In Gardening

Reproduced from the April 2002, Minnesota Dahlia Society Dahlia Digest ©by the Author and Dahlia Digest, 2002

Plants consume a lot of different nutrients, most of which we never need to worry about too much. There are certain ones that must be replenished in the soil because they are consumed in fairly large quantities and are not returned to the soil under normal circumstances.

In unusual situations, certain soils are lacking specific minor nutrients and we may need to add these to balance the nutritional profile of the garden. Successfully using fertilizer in a garden is a matter of adding the right amount of whatever nutrient is in short supply. Guessing at what needs to be added can result in adding more of what we already have and still be missing what is in short supply. We may be experiencing what appears to be a nutrient shortage, when what is actually happening is a nutrient availability problem because the pH is out of whack for the plants we are raising.

Up to this point, a decent soil test may appear to be all we need to do to figure out what the garden needs to prosper, but sometimes other factors may blacken our green thumbs.

Sometimes we have too much or too little water. Sometimes the amount of organic matter in the soil is not right and creates problems. How much light is available? A real good way of looking at gardening is to ask one’s self, what is the greatest liming factor I have in this garden, everything considered? Low fertility may not always be our biggest problem — a gardener needs to view the big picture. Success may mean fixing the drainage, correcting the pH or making more light available before the plants will be able to even make use of any nutrients we add to the soil.


We may practically view pH as a way to measure how much acid or alkali is present in the soil. A pH of 7 is acidic while more than 7 represents an alkaline situation. The primary reason pH affects nutrient availability is that plants pick up nutrients which are dissolved in the soil water, and pH affects the degree to which most nutrients will be dissolved in the soil water.

A lot of plants, including Dahlias, do best on soils which are slightly acid; just a little less than a Ph of 7, such as 6.5-6.7. If the pH of the soil is much above 7, a number of nutrients will become less available because they will be tied up in compounds that are not as soluble in the soil water.

To a degree, we are victims of our location. Most garden soils in southeast Minnesota will have a pH of 8 or slightly more because the soil has a lot of carbonates in it due to limestone in the parent material our soil is composed of. Folks in northeast Minnesota will likely have slightly acidic soils. Generalities do not always apply to a specific garden plot.

Rather than guessing about soil pH, it can easily be determined when a soil test is done (everyone has their soil tested – right?). Soils that are drastically high in pH can be adjusted by adding sulfur, and it may need to be done each year because there may be a good supply of the stuff in the soil that tends to raise the pH anyway.

Yellow garden sulfur will have the greatest affect and will require less material than will other acidifying products. Low pH (high acidity) is normally corrected with lime additions. A point which is not usually recognized is that garden fertilizers may affect pH due to the ingredients contained in blended fertilizers. When a 10-10-10 or a 5-5-5 fertilizer is purchased, it is normally a fertilizer blend that has been diluted by additions of 50% to 75% carrier. The carrier is usually lime, which is just fine if your garden has a pH of 5.5 and needs to have some of the acid neutralized, but it works against you if the soil pH is already 8.4. The additional lime will only serve to maintain the already high soil pH. For this reason, I stay away from diluted low analysis fertilizers and use smaller quantities of higher analysis fertilizers. Other fertilizer ingredients, such as monoammonium phosphate, diammonium phosphate and triple super phosphate are somewhat acidic to begin with because of how they are manufactured, and frequently contain up to 1% sulfur. Knowing what your garden pH is through frequent soil testing is essential in order to know if the acidity/alkalinity needs to be adjusted, and by how much.


Understanding a few of the basics is key to understanding what a person is getting when purchasing fertilizers and ultimately knowing how to apply this information to raise some really great dahlias.

I think it is a fair statement that most consumers have at least a vague idea that there must be someone out there who looks out for the interests in the marketplace, but they might not know exactly who that is and what they do. The regulation of fertilizers, which includes definitions of terms, ingredients and labeling and many other issues, is done primarily by the states through their Agriculture Departments. Most states also do routine product control sampling to assure that what is being sold agrees with what it is represented to be. State fertilizer regulators belong to a national association, The Association of American Plant Food Control Officials (AAPFCO). A primary purpose of AAPFCO is to promote uniform fertilizer regulations, definitions and standards that ca then be adopted by the individual states. The net result is uniform labeling across geographic regions and among products so that a person can logically compare products.

The major plant nutrients that are present in most fertilizers we know by the familiar N-P-K designation we see on the fertilizer label. N being % by weight total Nitrogen, P is % by weight available phosphorus pentoxide (P205) and K is % by weight Potassium oxide (K20).

The secondary plant nutrients (used in smaller quantities than the major plant nutrients) are Calcium, Magnesium and Sulfur and each of these are shown on the label as % by weight.

Micro Nutrients include Boron, Cobalt, Chlorine, Copper, Iron, Manganese, Sodium and Zinc. As with the secondary nutrients, these are labeled as % by weight, or they may be labeled as PPM (parts per million). 1% equals 10,000 ppm, so if you see something labeled as 5,000ppm, it is the same as 0.5%.

When it comes to ingredients which provide nutrients in fertilizer, there are officially 37 ingredients which are used to provide Nitrogen, 24 which provide Phosphorous, 11 for Potassium, 18 for Calcium and Magnesium, 9 for Sulfur and more than we care to talk about for the micro nutrients. Many of these substances provide more than one nutrient, such as potassium nitrate (N&K) and Diammonium Phosphate (N&P). A full discussion of fertilizer ingredients is more suitable for a separate occasion than this article , but a person can get an idea by reading the list of ingredients on commercial fertilizer labels.

For our immediate purposes, the issues of ingredient purity, controlled release characteristics, and the use of feed ingredients as fertilizers have considerable general interest.

Fertilizer grade ingredients are not required to have the same purity as products intended for animal or human consumption. A person needs to understand that the label guarantees on a fertilizer container are telling you what you are getting. The guarantee does not tell you if there are other nutrients in smaller quantities, nor does it provide information about heavy metals or other substances you may wish to avoid. A lot of fertilizer products do contain small amounts of sulfur, iron, magnesium, manganese, zinc, etc. even though it does not say so on the label. The incidental inclusion of micronutrient is one of the reasons we do not normally need to add micro nutrients to the soil. As far as contaminates are concerned, certain fertilizer ingredients, such as urea, ammonium nitrate and potash are known to be relatively free of contaminants. Other fertilizer products, such as “Ironite” (mining waste for a particular mine) and “Milorganite” (dehydrated sewage sludge from the Milwaukee municipal sewage system) are essentially waste products and are being sold more as a matter of disposal than because they are the finest fertilizer available. In both of these cases, you are buying some heavy metals, and possibly other nasties, along with a small amount of plant nutrients. Normal agricultural and garden fertilizer avoid significant amounts of stuff you do not want. “Miracle Gro” and similar products cost about five times as much as agricultural or garden fertilizer because they are general composed of “technical” grade ingredients that have a high purity, and are usually 100% water-soluble. These are fine for specialty use, but may not be economical or necessary for large quantity garden use. A little care in label reading will normally provide a person with sufficient information to know if they are purchasing a product of appropriate purity for the intended purpose.

Slow release products have been marketed for the purpose of supposedly doing one fertilizer application rather than several over a period of time. The way these things work is by: 1) Having a coating (such as sulfur coated urea and osmocote), 2) Having a low solubility in water (like isobutylidene diurea) 3) Being an organic product (for example alfalfa meal) so that the material has to decompose over time to be available to the plants. Sulfur coated urea is one of those things that works better on the drawing board than it does in the garden for slow release characteristics because the coating is almost always perforated to begin with or cracked in handling–maybe a good source of nitrogen and sulfur, but it will not do the slow release thing very sell. Most of the remainder of the slow release products will work to a degree, but it is hard to know how long a period of time the release takes place over. People who want to be sure of delivering a set amount of nutrients over a period of time either need to experiment a lot with the slow release products to find out how they will deliver, or simply do frequent small applications of standard fertilizer products so they know for sure how much the plants are getting over a set amount of time.

Feed ingredients may, and frequently are used by some people as a source of fertilizer for gardens. Feed grade products, unlike fertilizer ingredients have to meet standards on contamination including removal of fluorides and heavy metal limits. If you think about this for a bit, both plants and animals have a lot of similar nutritional requirements, with some obvious differences about how they get it. Inorganic feed ingredients (which are all “quick release”), such as feed grade urea, feed grade monoammonium phosphate, feed grade diammonium phosphate and feed grade potassium chloride are very nearly the same chemicals as are the equivalent fertilizer ingredients by the same name except that they are of a much higher purity and can be considered to deliver a slightly higher level of N-P-K than will the fertilizer ingredients because of the increased purity. The down side is that they can be a bit hard to get unless you happen to live next to a fed mill that carries these products. Organic feed ingredients are more commonly used in gardens and are more easily available. Any feed product that has a protein value can be used to supply slow release nitrogen. To determine how much nitrogen alfalfa meal, corn gluten meal, brewer’s dried grains, soybean meal or meat and bone meal will provide, simply divide the protein value on the feed ingredient label by 6.25. All of these products will provide a small amount of phosphorous, potassium and micro nutrients, however meat and bone meal will provide significant amounts of phosphorous and calcium.

Traditionally, people will use steamed bone meal for gardening, which provides slow release phosphorous and calcium as the bone powder decomposes. At the present time it is easier to find this material in a garden center than in a feed mill. People can successfully use feed ingredients as fertilizers where either high purity with respect to avoiding contamination or slow release of nitrogen is desired.

Editor’s Note: Dave Syverson is the husband of Minnesota Dahlia Society Bronze Medal winner Dianne Syverson. He graciously contributed his thoughts on soil fertility to our club for publication in our newsletter. Thank you, Dave for the fabulous information. This will help all of us to grow better dahlias.

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