This part of cannabis research is dedicated to cannabis development and the effect of microelements as N, Cu, K, and P.
What is the effect of copper on cannabis roots?
A research team from Italy has investigated the effect of cooper on cannabis root development. They have used 150 ppm CuSO4. Copper is an essential trace element in plants but it becomes strongly phytotoxic at high concentrations. Although the root system is genetically determined it can be affected by a number of environmental factors, including metals. Cannabis is well-known to be able to accumulate heavy metals, such as lead, nickel, cadmium, zinc and chromium. Previous research shows that cannabis is able to accumulate copper in the whole plant.
Copper stress induced a significant reduction in shoot and root dry weight. Also aerial part of the plant (measured as leaf area and shoot length) was significantly reduced after copper treatment. Treated plants did not show clear symptoms of chlorosis. Roots length, diameter, surface area and volume were significantly reduced after copper treatment. Less branched root system is less efficient in absorbing nutrients. 
What is the effect of phosphorus and potassium on cannabis dry matter and nutrients accumulation?
Hungarian research teams has investigated the effect of P, K and N in outdoor fiber hemp grow. Their results show that nitrogen has effect on dry matter accumulation but this was not the case with phosphorus and potassium. Increased nitrogen fertilization was associated with increased stem yield, however increase above 160 kg/ha of N has no significant effect. Higher P levels do not increase the stem yield significantly. Increased K levels had positive effect on stem yield. 
What is the effect of nitrogen on THC content in cannabis leaves?
Research team from Hungary has studied the effect of nitrogen on THC content in cannabis leaves. They have observed that N supplementation increase the fresh weight of shoot and plant height but THC content decrease in response to N fertilizer. The decrease was significant with the higher dose of nitrogen. 
- Influence of nitrogen supply and P and K levels of the soil on dry matter and nutrient accumulation of fiber hemp (Cannabis sativa L.)
- Effect of nitrogen on tetrahydrocannabinol (THC) content in hemp (Cannabis sativa L.) leaves at different positions