Fertilizers Notes: By Cactus Joe
- Organic fertilizers need time to release their “goodies”. The organic nitrogen is not available to plants until they are converted to soluble inorganic forms. Microorganisms in the soil do this job. You, therefore, get a natural form of slow release fertilizer. The release of nutrients is dependent on the population density of the microorganisms that are around to do the job and how active they are. And they work at different rates depending on the soil conditions – temperature, availability of moisture, aeration, etc. Which is just about perfect, since plant growth and nutrient needs increase as the temperature rises, just at the same time when microbial activities increase. These fertilizers, therefore, need to be applied early in the season, because it will take some time for the nutrients to be made available to the plants. How long? This depends – on the type of fertilizer, the soil type, and the ambient temperature. A good practice is to apply a mixture of bone meal (for phosphate) or super phosphate, blood meal or fish meal (for nitrogen) and alfalfa meal in early spring, generally when the first Spring pruning is done and the winter protective mulches are pulled back.
- Synthetic fertilizers in general release their ingredients relatively quickly. Unlike organic fertilizers, whose ingredients are stored in the organic non-soluble form until released by microbes, the synthetic fertilizers have ingredients that are soluble, and tend to leach out quickly with the rain. Thus application of these after the roses are showing unfurled leaves is recommended.
- The prices of the different synthetic fertilizers depend on the percentage concentration of the nitrogen, phosphate and potassium in the fertilizer and on how the ingredients are packaged. The cheaper ones probably are dissolved and released relatively quickly, whereas some of the mid-range priced ones are a “slow release” granular (but not in the encapsulated form of Osmocote, which is very expensive). Keep in mind that the faster the fertilizers are released, the more likely they are to cause damage due to root burn, and the more quickly they get leached out with rain and watering.
- In plant growth and health, both non-mineral and mineral nutrients are essential. The non-mineral nutrients of carbon, oxygen and water are freely available in air and water. These are fixed by the process of photosynthesis, using the sun’s energy. There are 13 mineral nutrients of which there are three primary macronutrients – nitrogen, phosphorus and potassium (the NPK in your fertilizer label) – and three secondary macronutrients – calcium (Ca), magnesium (Mg), and sulfur (S). The secondary macronutrients are usually available in sufficient amounts in most soils, but are more likely to be deficient in sandy soils. When we add lime, we are adding calcium and a bit of magnesium. When we use organic materials – compost, grass clippings – we provide sulphur. The remaining 7 mineral nutrients are micronutrients. These are essential elements that are only needed in small to minute amounts – boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn). If you use lots of organic material, notably from the compost pile, you will be providing plenty of these micronutrients. But some fertilizers are packaged with small amounts of these, just to be sure. In general, if you have great, organic rich soil, additional application of micronutrients may not be necessary. It’s a different story, however, with container growing – over time, these minerals will be depleted with repeated waterings (and rain). Regular application of a soluble fertilizer is recommended as it has these micronutrients for container grown plants.
- Until the soil has built up enough of organic components to its composition, the virgin soil is usually lacking or short of the mineral macronutrients. When plants first grow on those soils, their growth “sucks” those macronutrients out of the soil, depleting the soil quickly. That’s where the bulk of fertilizing comes in – to constantly replenish the soil of these nutrients, especially at times of vigorous and rapid vegetative growth. Of these macronutrients, nitrogen (the N in NPK) promotes top growth – stems, foliage. It contributes to chlorophyll production, which gives the green color of healthy plants. More chlorophyll means increased photosynthetic capability therefore more plant growth. (Memory Note: N = UP, P = DOWN and K = ALL-AROUND)
- Phosphorus (the P in NPK) promotes root development and increases flowering ability and bloom size. This is useful when you are planting that new mail order bare root rose in the garden, or when you are transplanting. It helps the plant establish faster by helping it grow a good root system quickly. Once established, it continues to help the plant maintain a good root system and promote flowering.
- Potassium (the K in NPK) is multi-functional. It promotes disease resistance and improves drought and cold tolerance. It helps in improving root development and promotes photosynthesis. I apply high-potassium fertilizer in late fall/early winter to improve cold hardiness. Some would recommend an application in early summer to improve tolerance to excessive summer heat, or for recovery from pest damage.
- Most of you probably know (but some may like to know) that the three numbers, e.g. 6-8-6, refer to the percentages by weight respectively of the elements nitrogen, phosphorus and potassium in the fertilizer. In another example of NPK of, say, 20-10-4, the respective compositions of nitrogen, phosphorus and potassium are 20%, 10% and 4%. A laterally thinking gardener would jump out of his chair and exclaim, “Hey! That’s only 34%!” Well, yes. Because the rest is made up by other ingredients, largely fillers. The fillers help you spread the fertilizers more evenly. Moreover, the choice of the filler determines how quickly the fertilizer gets released into the soil.