Dahlias prefer soil with a balanced supply of nutrients.
Based on the interpreted results of a soil test, nutrients (fertilizers) should be added to the soil when preparing the dahlia beds.Most people cannot interpret the results of a soil test. They will need the help of someone who can determine the amounts and kinds of nutrients to be added to the soil to bring those nutrients to a level that dahlias prefer. Even then, for those of us needing help, it is not an exact science. A specified nutrient, like nitrogen, may be in the soil, but may not be available to the plants in the quantities required. Temperature, moisture, pH, and soil type all influence the amount of nitrogen that will be available to a plant at any particular time. The interrelationships are complex. That is one reason why there are so many commercial greenhouses. In a greenhouse, those variables can be controlled–outside in the field they cannot. In the beginning the amateur dahlia grower either accepts the interpretation of the soil test results and adds fertilizer accordingly, or adds fertilizer based on other information or hunches. Over time, the amateur finds approaches that work for him/her. Perhaps the particular set of conditions in his/her garden has been augmented to suit the dahlias preferences. Soil test based action is the best approach for the beginning grower. If the soil test indicates the need for very small amounts of fertilizer, it is feasible to use natural organic materials (compost, manure etc.) to meet those needs. On the other hand, if the amounts of nutrients that must be added to the soil are large, inorganic chemical fertilizers will be needed. Natural organic materials are not a significant source of nutrients, with the exceptions of bone meal and dried blood. See Tables I and II. The amounts of organic fertilizers required to provide plants with their needs are enormous. Table II shows this rather dramatically. For 100 square feet of soil, 3.2 ounces of superphosphate 45% is equivalent to 60 pounds of manure. See Soil and Bed Preparation Check List for discussion of when to fertilize. |
Table I – Nutrient Content of Some Organic Materials
| Materials | %Nitrogen | %Phosphate | %Potash | Availability |
| Steamed bone Meal Dried Blood Compost Cattle Manure Horse Manure Sheep Manure Swine Manure Poultry Manure Peat Sawdust Milorganite Sewage Sludge (activated) Sewage sludge (digested) Wood ashes |
0.7-4.0 12 1.5-3.5 0.25-2.0 0.3-2.5 0.6-4.0 0.3 1.1-2.8 1.5-3.0 0.2 0.5 2-6 1-3 0 |
18-34 1.5 0.5-1.0 0.15-0.9 0.15-2.5 0.3-2.5 0.3 0.5-2.8 0.25-0.5 0.1 2-5 2-7 0.5-4.0 1-2 |
—— 0.57 1.0-2.0 0.25-1.5 0.5-3.0 0.75-3.0 0.3 0.5-1.5 0.5-1.0 0.2 2 0-1 0-.05 3-7 |
slow-med med.-fast slow med. med. med. med. med.-fast very slow very slow med. med. slow rapid |
Source: Colorado State University Extension Service in Action Bulletin no. 7.217
Table II – Equivalent amounts of fertilizing materials (phosphate) per unit area.
| Material | Acre | 1,000 sq. ft. | 100 sq. ft. |
| Super Phosphate 45% | 87 lbs | 2 lbs | 3.2 oz |
| Bone Meal 22% | 175 lbs | 4 lbs | 6.5 oz |
| Sewage Sludge | 300 lbs | 7 lbs | 11.0 oz |
| Manure | 13 tons | 600 lbs | 60 lbs |
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