Three minerals are necessary for any plant's healthy growth – nitrogen, phosphorus, and potassium, which comprise the main ingredients in most commercial fertilizers. Nitrogen, in particular, is required to make amino acids and proteins crucial for healthy foliage and lush blooms. Unfortunately, almost 80% of earth's nitrogen is in the atmosphere, a form plants are unable to assimilate. Instead, most plants acquire nitrogen from the soil and/or water in which they live. For outdoor plants, bacteria converts nitrogen released during decomposition of dead flora and fauna into a form plants can absorb. As housplants are confined to containers they will, over time, deplete vital elements from their growing medium, having no carrion nor enough decaying vegetation to feed them.
Signs of soil-exhaustion will become particularly evident at this time of year. When nitrogen-deprived, chloroplasts in a plant's cells begin breaking down causing older leaves to turn yellow or brown, newer growth to look stunted or paler than usual, and/or the under side of the leaves to turn a red or purple hue. Easier wilting, even when watered as usual, is another sign the soil's viabily is deteriorating.
Transplanting can solve the worst cases of soil malnurishment. However, some plants favour meagre mediums and others are persnickety, prefering minimal handling or confining containment. Transplating these types of plants can cause even greater problems. Instead, you can sustain nitrogen levels with regular feedings. Commercial fertilizers are potent and some can burn more sensitive plants. For those plants a slow-release nourishment, applied regularly – from once a month to twice a year depending on each plant's nutrient requirements – works better. I use a solution of plain gelatin and water, and that keeps our houseplants flourishing. Here's how to make that solution:
Dissolve one tablespoon of powdered (plain) gelatin in one cup of boiling water; when thoroughly dissolved, add 3 cups of cold water; when solution is completely cooled, feed plants sparingly and store the remainder in a glass container; stir well before each use.
All plants need nitrogen, but houseplants are truly reliant on us to supply it. Spring is a good time to check for soil depletion, although houseplants, with longer and more varied growth cycles than outdoor plants, are best sustained by regular feedings. If you're looking for a good low-dose, slow-release, non-toxic source of nitrogen for your hungry houseplants, gelatin makes a thrifty solution!