Healthy soil does not need chemical fertilization, and the preceding information, as well as the section on compost, may well eliminate the need for fertilization in the garden. Chemical fertilizers typically lack many essential trace minerals, may harm soil microorganisms, and can be detrimental to soil health. Use them sparingly if at all.
Most houseplants, however, and especially cacti and succulents, will benefit from regular fertilization during their growing periods, as well as from the use of well-rotted compost. Do not overfeed, and use all fertilizers as instructed by the manufacturer. Newly potted plants, or compost-fed plants, require no fertilization in the first few months.
Fertilizers come as liquid concentrates (again, dilute as instructed), as granules, or as slow-release plant spikes. Foliar feeding, while often highly effective, is reserved to specific types of plants, and is discussed further below.
Keep in mind that different plants differ in nutrient requirements. Fertilizers are formulated with this in mind, and will usually list its nitrogen-phosphorus-potassium content, and state specifically the type of plant it is intended for. Though concentrations may be high, these are often much less bioavailable than organic sources of the same compounds.
For a simple, organic fertilizer, try mixing 1 part each of blood meal and bone meal with 2 parts of woodash or 3 parts of peat moss, and with four parts compost. Keep in mind that your conditions, or your plants, may benefit from varying the relative components, and that wood ashes will make your soil more alkaline. Other organic alternatives are compost tea and green manures, both described in their respective sections.
Often plants such as Rhododendron or blueberries require a pH that is far more acidic than the local soil; other plants may prefer alkaline conditions. As mentioned, peat and potash can render soil more acidic. Composted pine needles or oak leaves can also help drop the pH. If an alkaline soil is desired, use wood ashes or lime, as discussed above.
Cacti and succulents tolerate higher concentrations of fertilizer without ill effect; fertilize these often. Try tomato or rose fertilizer (rich in magnesium and phosphorus) with flowering plants, half-strength on young plants. For young seedlings, use only diluted solutions. Fertilize in the evenings.
Bonsai as a rule do require fertilization, but again, only if diluted beyond its normal strength. Small amounts are used frequently during the growing season, i.e. from early spring to early fall. Start off with a high-nitrogen fertilizer, i.e. a 10-6-4; after midsummer feed with a low-nitrogen and high-potassium mixture, i.e. 4-7-10. As with other flowering plants, flowering bonsai benefit from tomato fertilizer used in late summer or early fall. Organic formulations may of course be substituted.
Orchids typically require frequent feeding, up to weekly or every few weeks during the growing season, with diluted fertilizers. See individual species for recommendations. Note however that plants potted in Osmunda require no additional fertilization, though this growing medium is now quite rare. It is also possible, using compost tea and diluted seafood extracts, to grow orchids very successfully organically.
If plants are grown in inorganic media, then essential nutrients must be provided as well as fertilizer, i.e. requiring hydroponic feeding. While beyond the scope of the current program, this topic may be addressed in future versions, if enough interest is shown.
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