The method of growing determines when the fertilizer should be changed. Outdoors in soil, the fertilizer is switched when the plants are in transition from vegetative growth to flowering. The dates differ by variety, since each has its own critical dark period that triggers flowering.
Indoors, the time to change formulas depends on method of cultivation. Plants in systems where there is a high level of residual nutrients in the planting medium, such as soil and soilless mixes, should be switched to the flowering formula at the same time as the lights are changed. They retain high levels of nitrogen (N), which the plants use in the early part of flowering. The switch to a high phosphorous (P) or high P and potassium (K) formula will result in the use of the residual N so that little will be left at the end of flowering when the plants require more P.
With hydroponic systems in which there is no nutrient build-up in the media, the plants should receive N during early flowering. This is used for tissue building, that is, to grow flowers. Flowering formulas sometimes contain little nitrogen. Look at the list of nutrients on the label. If the flowering formula doesn’t contain at least one third the N as P, mix it with the vegetative growth fertilizer during the first two thirds of flowering.
If there is no nitrogen available, the plant transports it from lower leaves to new growth. As a result the lower leaves turn yellow and die. Only in late flowering should the N level be lowered further.
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