Composting and Fertilizers

Dahlias have different needs depending on their life cycle, discover the differences between using compost and mineral additives.


Soil composition

The primary source of nutrients comes from the 'organic matter' layer of the soil, found in the upper layer. So when we seek to optimize our soil for our dahlias, we must first ensure a good structure of the latter in order for the microorganisms (first 5 "to 7" of the upper soil layer) to  be able to ingest the organic matter to feed our plants. But before we go into that, let's try to understand the composition of our soil:


Element name Percentage of soil Composition
Organic Matter
5% "Organic Matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds. The definition of organic matter varies upon the subject for which it is being used." - Wikipédia





Carbon(C), Oxygen(O) and Hydrogen(H)





  • Nitrogen   NH4+ NO3-
  • Phosphorous   H2PO4+ HPO4-2
  • Potassium   K+
  • Calcium   Ca2+
  • Magnesium   Mg2+
  • Sulfur   SO4-2

Minor (Oligo-elements)

  • Iron   Fe3+ Fe2+ chelated form
  • Manganese   Mn2+ chelated form
  • Copper   Cu2+ chelated form
  • Zinc   Zn2+ chelated form
  • Molydene   MoO42+
  • Borium   H3BO3
  • Chlorine   Cl-


Nutrients must be in the form of ions (cation and anion-+) in order to be absorbed by the plant. To get this form, soil organisms (insects, worms, bacteria, fungi, algae) must slowly degrade the minerals and soil's organic matter.

Depending on our soil quality, it may not contain the necessary amount of nutrients required by your plants. In that case, it may be necessary to amend your soil with compost or fertiliser in order to provide the nutrients your plants may require.


Plants need to absorb the 16 elements (indicated above) in order to thrive.


Using Compost for you dahlias

Compost is primarily an amendment, the first step in restructuring our soil. It improves the
physical, biological and chemical composition:

  • Helps improve soil structure gradually (aggregation) and structural stability (compaction, erosion).
  • Improves drainage of clay soils.
  • Increases the capacity of sandy soils to retain water.
  • Increases the soil's capacity to retain nutrients.
  • Feeds soil organisms (algae, fungi, actinomycetes, bacteria, worms etc). Feeding the soil organisms (algae, fungi, actinomycetes, bacteria, worms etc) allows mineralisation, which in turn feeds the plant.


The quality of the compost depends on the following:

  • Materials used: manure, leaves, wood shavings, organic waste.
  • Type of composting: green/brown, aerobic/anaerobic, vermicomposting, etc...
  • Compost maturity: A young compost is generally more abundant in nutrients, but it comes with a risk; decomposition sometimes produces toxic substances which may hinder the roots of your plant.


Can we use too much compost?

Yes… compost should amend the soil, not replace it. Too much compost, especially in clay soils, can lead to drainage problems and eventually to roots rotting or aquiring other fungal maladies. 


Can we rely only on compost for fertilization?

That depends on a few factors: A) the quality of the compost.  B) the quality of the soil. C) the needs of the plant. D) our needs and objectives.

The main drawback to using compost is that it is impossible to determine the amount of nutrients that are released, and the timing of their release in order to meet the needs of the plant . Early in the season, the needs of the dahlia are not too high, but when it begins its rapid growth about mid-June, the needs increase rapidly. In addition, to maintain flowering, energy demand is high.

I personally do not believe that only using compost is enough for raising dahlias, at least not with the objectives we have set for ourselves.


However, fertilizers should be used in a RATIONAL WAY.


Using fertilizers for your dahlias

If your flower bed has been amended with compost for several years, you should have a sufficiently rich soil except perhaps for nitrogen. If your leaves appear too light, especially at the base of the plant or if your flowering diminishes, a slight nitrogen fertilization may be necessary. Products such asurea (46-0-0), ammonium nitrate (32-0-0), blood meal (12-0-0) or other products with low phosphorous and potassium are recommended. Remember that too much nitrogen fertilization can lead to elongation of the plant with slender stems, smaller flowers and increased difficulty in storing bulbs or tubers.

If you start a new flower bed, even by adding compost, it would be better to fertilize your soil with inorganic fertilizer whose nutrients are released more quickly. We would suggest a balanced fertilizer containing all three elements, for example; 15-20-20, 6-12-12, 5-10-15, etc.


NB   The calculations made ??in this article are intended as an example, and not a recommendation. No research on the nutrient requirements of the dahlia, according to an analysis of soil, has ever been made. The purpose of this article seeks only to understand that the use of fertilizers must be done rationally.

Containers of fertilizer always display three digits: the first shows the %nitrogen (N), the second %phosphorus (P2O5) and the third, %potassium (K2O) content in the mixture. The majority of articles on fertilization seem recommended to use fertilizer with low nitrogen (eg, 5-20-20, 8-16-16, etc..). This is a way to limit the application of nitrogen (it can elongate the plant too much) because people often put too much fertilizer at the bottom of the hole, usually under the tuber.

Take for example the recommendations for the potato in a mineral soil. (The Spanish imported the dahlia for early consumption). Recommendations for an average terrain is about 150kg/ha of nitrogen, phosphorus 150kg/ha and150kg/ha of potassium fertilizer; thus a 20-20-20 might do the trick but at what dose? Here's the math:

1 ha = 100 meters side X 100 meters side = 10,000 m2

150kg/Ha =    150kg/10,000m2 =   150,000gr/10,000m2 =   15gr/m2

Therefore:  15 grams nitrogen, 15 grams of phosphorus and 15 grams of potassium per m2


As stated previously, the numbers on the container indicate a percentage. So with our example, 1 gram of 20-20-20 contains 0.2 grams each of nitrogen, phosphorus and potassium. How much 20-20-20 can meet my need per m2?

How many grams/m2 of 20-20-20 will give 15 grams each of nitrogen, phosphorus and potassium per m2?

?? grams/m2 X 20% = 15 grams/m2 --> 15 grams/m2 ÷ 20% = 75 grams/m2

75 grams of 20-20-20 contains  15 grams each of N, P2O5, K2O.


If you plant your dahlias every 61cm (2 ft) your plant covers .37 square meters.  Therefore the needs to match your plant are as follows:

.37m² / plant  X 75 grams/m²  =  28 grams per plant


This 28 grams / plant corresponds to the needs for the entire season. (One ounce = 28 grams) If you decide to make multiple applications, you must divide this amount by the number of applications.

Four applications seem to be an acceptable approach. A first fertilisation at planting (10-20 May), which must be mixed very well at the bottom; and the other three around;  15-20 June, 15-20 July, 15-20 August. Finally some application will be needed around the base of the plant. It is important not fertilize after August 20, specifically nitrogen, since this increases the problems of conservation, especially rot during the winter.

Another resource is available with comments from Marc Boyer on the American Dahlia website. He has written two articles on fertilization:   Part 1 Part 2


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