This is from a thread by Uncle Ben, you should read his threads they are great. Yellowing leaves is normal with AN nutes but not a good thing. Up the N in you feeding.
The never ending abuse of Phosphorous to enhance flowering
A common mistake for growers when they reach the flowering stage is to start hitting the plants with a high P fert like a 10-50-10, continuing to use this blend exclusively, and when their plants start experiencing a deficit of N, Ca, Mg or micros as reflected by the dropping of lower leaves and chlorosis, they wonder why. Plants flower as a response to long nights, not because of fert blends high in P. A ratio of 10-60-10 is WAY too high in P. The plant will only take what it needs and compete for other elements that may be more important at the time.
You may have heard that too much N can inhibit flowering. No question about it, exclusive use of a plant food that is rich in N such as blood meal, a 5-1-1 blend, or ammonium nitrate/sulfate may inhibit flowering especially if the phosphorous level is low, but most balanced blends have sufficient amount of P to do the job. The question is - "how much P is enough to support a good flowering response and still retain my leaves?"
Manufacturers/horticulturists will give you element analysis and what effect the elements have on plant growth, but remember this does not necessarily mean you will get better yields. Using a high P fert exclusively during flowering can actually work against you due to impending leaf drop. It's an abundant amount of healthy leaves going into 12/12 and maintaining their health that produces a lot of bud, not high P ferts.
I rotate fert blends as the plant *requires* them, not because it is "the thing to do." For example, when your plants are going thru the stretch phase during early flowering, they may need more N, especially if you're getting some yellowing in the lower leaves. Give up the cannabis paradigms and give them what they need. Go back to mild high P fert when the stretch ends, maintaining the foliage in a healthy state of growth until harvest for maximum yields. A 1-3-2 blend such as Peter's Pro Blossom Booster, 10-30-20, is one of the best flowering blends on the market because of several factors - it is higher in nitrate N and Mg. It is sold under the Jack's Classic label. An added benefit of Peters blends is their use of high quality, very pure salts that will eliminate root burn if used judiciously.
Uncle Ben
