In the past I've always switched from a grow formula of nutes to a bloom formula as the manufacturers advise you to do when putting your plants into the flowering stage. But I have always ran into yellowing of leaves starting half way through the flowering phase. When I did a search for the cause of this, I found this: "The yellowing of plant leaves is simply due to a nitrogen (N) shortage which is a perfectly natural phenomenon as the plants nears the end of its life cycle." That left me with this question: Why do they reduce the nitrogen content so much in the flowering formulas of most (if not all) of the nutes that are on the market? Ed Rosenthal: "In the middle to the end of the flowering stage, plants frequently show a N deficiency. They’re using the nutrients that were stored in the leaves and dropping their oldest bottom fan leaves. To prevent the deficiency from getting extreme, switch over to bloom nutrients gradually unless the bloom fertilizer contains some N." He goes on to say: "The plants switch to flowering growth over a week. Then they need higher levels of P and K. But they still require N. For this reason, during the first week of flowering use 1 part each bloom and vegetative. The second week use 2 parts bloom, 1-part veg. During the third week use 3-parts bloom and 1-part veg. After that use just bloom formula. Although the plants still need N in flowering, it’s not near the amount they need when they are growing vegetatively. By gradually moving from grow nutrients the plants receive enough N to last through the flowering process." For example, take Botanicare's Pure Blend Pro. For their grow formula they show a 3-2-4 NPK ratio and for their bloom formula they show a 2-3-5 NPK ratio. Why not go with 3-3-5? I also found this on a growing website: "Roughly speaking, NPK ratios of 3:1:1 are best for any cannabis strain throughout the growth stage. Once the plants start to flower, nitrogen percentages should be lowered as potassium and phosphorus levels increase. Early flowering calls for a 1:3:2 ratio; late flowering requires 0:3:3 instead." This is from a High Times article: "However, sometimes the nutrient program is a little too deficient in N and it sounds like you may have a bit of this issue here as the tiny leaves of your buds should generally not be turning yellow. One or two here and there is OK, but if the yellowing is rampant on your buds a boost of N could help." If some (or a lot) of your leaves turn yellow due to nitrogen deficiency halfway into flowering, why would nutrient manufacturers reduce the nitrogen so much in their bloom formulas?
********* First thing I'd comment, is that yellowing of leaves in flower is also caused by K deficiency. Also, about NPK ratio, I'd shoot for something like 1-2-3 in flowering until the end. And you want more Ca than Mg. This is a matter of cations exchange. You don't want to run out of K. From experiences, if you see yellow on sugar leaves look for pH and Ca before adding N. Yellowing in flowering is something normal happening as the plant mature, just like it would do in nature in late fall. I have never run out of N, and N toxicity is way more common in flower than N deficiency.
Vegging plants need nitrogen to put more foliage (vegetave growth) .. Needs more N... however when budding and flowering not growing much in stature or putting leaves,, flowers consist of more P and a moderately more K. Simple