Found this article hope it might help others In future.
I’ve grown cannabis more or less continuously since the Summer of 2004. During that time I have learned a great deal about this crop and how to grow it successfully indoors.
One thing that I have learned is that there are virtually no reliable resources when it comes to diagnosing leaf and stem symptoms in container grown cannabis plants. I have come to believe that most of the leaf and stem symptoms we encounter in this crop indoors are caused by soil pH that is out of range.
The two leading causes of pH problems are source water and ones choice of fertilizers. Both of these problems can readily be solved with some investigation and a well founded fertilization plan.
During a recent fertilizer trial I had the opportunity to observe the physical effects of very low soil pH on cannabis plants and to confirm the presence of those same symptoms in two additional plant species grown under nearly identical conditions.
A small group of cannabis plants were fertilized using a high potential acidity fertilizer. At 60 days the soil pH ranged from 5.68 down to 4.93. To varying degrees, all of the plants showed the following symptoms:
- necrotic or burnt leaf tips
- downward hooking and cupping of the leaves
- spots turning yellow progressing to ‘hole shot’
- purpling in petioles progressing to stems
- interrupted flowering
Here is a photo of a cannabis plant grown for the trial. The symptoms are quite visible.
The soil pH for this particular plant measured very low at 5.16. The hooking and cupping are clearly seen as well as the necrosis of the leaf tips.
Flowering in this plant had stalled almost completely and the foliage had a mottled appearance that was tending toward yellow. New leaves were pale and small.
Plants in this condition are compromised. They will finish flowering, but the yields and quality of the flowers will likely be diminished.
At the same time that I began this fertilizer trial I also started some New Mexico chiles and broad leaf sage plants from seed. I grew these in precisely the same media, fertilizers and water as the cannabis plants.
I thought it would be interesting and perhaps valuable to grow different species of plants alongside the cannabis, apply the same stress and see what could be learned. As it turned out there are very strong similarities in the leaf and stem symptoms I observed in all three species.
Here are some photos of a chile plant that was grown in the trial.
The soil pH in this plant measured 5.11 at 60 days. The most notable common symptoms were necrotic leaf tips, spots turning yellow progressing to ‘hole shot’ lesions.
The plants developed purpling in the stems and at the nodes above the fruit. This progressed downward to the main stem.
A moderate amount of downward cupping and hooking appear in the oldest leaves. The plant stopped blooming completely and the developing fruit was comparatively small for this variety.
Measures were taken to quickly raise the soil pH for these plants and the fertilizer was switched to a basic reaction type.
Within a week the chile plants began to develop new leaves and the overall color of the foliage improved dramatically. Soon after, the plants began to flower heavily and have since set well developed fruit.
As another comparison I chose broad leaf sage. Here are some photos of a sage plant grown for the trial.
The soil pH for this plant measured at 5.24. The most notable common symptoms were downward cupping and hooking of the leaves and purpling along the stems and at the nodes.
While each of these species is very different, I was able to observe very similar leaf symptoms when applying a common stress across the group.
This exercise has confirmed to me which leaf symptoms are most likely related to low soil pH in cannabis.