The yellow leaves of a nitrogen deficiency may show signs of brown, and they will usually become soft and sort of "fold" in, before possibly turning crispy but ultimately falling off on their own. If the yellowing leaves are at the top of your plant or the yellow leaves are mostly new growth, then you probably don't have a nitrogen deficiency. Nitrogen deficiencies always affect the oldest, lowest leaves first. If new leaves aren't getting enough nitrogen, the plant will start to "steal" nitrogen from the older, lower leaves, so that it can give it to newer leaves. This is what causes the yellowing and wilting of a nitrogen deficiency.
It's relatively normal for your cannabis plant's leaves to start turning yellow towards the end of your flowering cycle as the plant becomes nitrogen deficient while creating buds.
However, if your cannabis plant is losing lower leaves fast due to yellowing (if yellowing and dying leaves is "climbing" up the plant from the bottom), especially in the vegetative stage before plant is making buds, you have a problem that you will need to fix as soon as possible. Got this info from grow weed easy check it out. Also have you checked for pests ? What are your temps, and humidity looking like ? Whats the size of the pot the sickly ones are in?