Weed Fertilizer Recipe

Fertilization management can be considered both a science of what plants require and an art of how nutrients are supplied. The major nutrients taken up by plants are nitrogen (N), phosphorus (P), and potassium (K). With these three elements being the heavy lifters, most fertilizer programs focus on delivering NPK to the plant. The other elements included in fertilizers are still essential but generally are provided at constant levels. We evaluated two commercial cannabis fertilizer programs developed by suppliers who were willing to share their recipes and compared each program to our cannabis fertility research program at North Carolina State University (NCSU).

Source: Landis et al., 2019. Values reported are for greenhouse-grown cannabis mother stock plants, 3 feet in height and just before the time when cuttings would begin to be harvested. Leaves sampled were the most recently matured. Ranges based on six cultivars.

Source: Short, M. and M. McGinnis. 2020. Unpublished research from field grown hemp of floral material harvested at maturity and analyzed for mineral nutrient concentration. Mean of treatments fertilized with N at 150 lbs. per acre and K

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Nitrogen is one of the most important elements for plants as it influences overall plant growth. If provided in insufficient levels, plant growth is stalled. If N levels remain too low, plants will be stunted. The other obvious visual symptom is lower leaf yellowing (chlorosis) (Fig. 1, above). In greenhouse or indoor cannabis production, the typical substrates used hold limited amounts of nutrients. Therefore, if N levels are too low, noticeable symptoms will readily occur on plants in just two or three weeks. Adversely, excessive N levels can lead to lush leaf growth at the expense of flower development.

Evaluation. When evaluating fertility programs recommended by two fertilizer suppliers, both Suppliers A and B offer similar scheduled feeding recommendations (Fig. 2A and 2B, below). Both suggest a lower amount of N during the plant establishment stage. Small, newly transplanted plants are primarily developing root systems and will not allocate energy into shoot growth until the roots reach the outside of the pot, which will occur during two to three weeks of growth.

Figure 2A (left). A recommended nitrogen (N) fertilizer program from commercial fertilizer Supplier A. Figure 2B (right). A recommended nitrogen (N) fertilizer program from commercial fertilizer Supplier B.

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While the cannabis plants are bulking up, both fertility programs recommend 150 parts per million (ppm) N. In our studies at NCSU, we also use between 150 and 200 ppm N at this stage of development (up to about week 10 of growth), and plant growth is robust.

Both fertility regimens remain the same until the late flower stage (after about week 10 of production), when N levels are reduced to between 25 and 100 ppm N. Both companies then curtail all fertilizer applications during the last two weeks of production. Decreasing and then ceasing fertilization allows plants to uptake any nutrients contained in the substrate. Also, if the plant requires additional N, the plant will translocate (move) it from the older leaves to the younger leaves if needed, as N is a mobile element.

Nitrogen uptake patterns by cannabis indicate that most nutrients are acquired during the first half of the growing season and then are translocated, if needed (Fig. 3). A comparison of N levels in leaf tissue with N levels in floral buds found that both were similar in concentration (Table 1). This suggests that providing excess levels of N late in the production cycle will stimulate additional leaf growth, which can lead to lower quality flower buds.

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Another major element that affects overall plant growth is P. Its major contribution is stem elongation, contributing to more branching and creating a fuller plant architecture and canopy. When P is limited, plants do not have an extensive branching canopy and exhibit stunted plant growth, resulting in less floral material.

When P is limited, leaf symptomatology may manifest in two different ways, the traditional presentation being lower leaf purpling. This occurs in many other species, especially when the plants are grown under cool and wet conditions, but it’s usually not found in cannabis grown in optimal greenhouse or indoor conditions. Overall purpling has been observed during late flower bud development, and may be attributed to either genetic cultivar differences or if P nutrient stress is occurring (Fig. 4).

The second symptomatology is slight plant stunting, which is followed by olive-green spots developing in an irregular pattern along the leaflets on the lower and older leaves (Fig. 5, at right). As symptoms progress, the spots grow larger and appear sunken and almost wet with some marginal necrosis (death). Over time, chlorotic leaves become severely olive spotted with large areas showing symptoms. Leaf drop occurs with advanced symptomatology.

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Figure 4. (left) Lower leaf purpling may indicate a phosphorus (P) deficiency. Note that magnesium (Mg) deficiency has similar symptomatology. Figure 5. (right) Lower leaf brown spotting leading to leaf chlorosis is also a symptom of phosphorus (P) deficiency.

Research conducted by Dr. Josh Henry during his master’s program at NCSU focused on optimizing P fertilization rates for ornamental and vegetable plants. In most cases, the optimal range was between 10 and 15 ppm P. Rates from 5 to 10 ppm resulted in more compact plants, while rates below 5 ppm P severely stunted plant growth and limited the overall plant quality.

Phosphorus is a major component of floral and especially the fruiting (seed) portions of the plant, meaning plants require a higher amount of P to meet those demands. Research tracking cannabis nutrient uptake with a seed crop in Manitoba, Canada (Fig. 6, below), identified very early accumulation of P into the plant leaves and stems. Near maturity, P, a mobile element, was translocated by the plant into the floral parts to meet the increased P demand of the developing seeds. This suggests that higher rates of P applied early in the growth cycle would be beneficial to cannabis. Late in the production cycle, plants require and take up limited amounts of P to meet the basic needs of maximizing plant growth and floral yield. This does not mean plants will not absorb the oversupply; but additional uptake is referred to as “luxury consumption, ” when tested leaf tissue values reflect those higher levels of P, but those higher levels provide limited, if any, additional benefit.

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Evaluation. Supplier A’s fertilizer program starts with lower levels of P (22 to 52 ppm) and then increases to 196 ppm P at bud set (Fig. 7A), providing lower levels when the plant’s root system is still developing and cannot take up a large quantity of nutrients. The one-week boost of 196 ppm P would load up the system and should ensure that an adequate supply of P is available to the plant for later translocation as the flower bud matures.

Fertilizer Supplier B’s program (Fig. 7B) provides a much higher level of 65 ppm P at the start to aid in root development, then decreases for two weeks to 43 ppm P during vegetative growth. Levels then increase for one week during bud set to 130 ppm P. During the five weeks of bud sizing, flowering is maintained at 196 ppm P, before decreasing toward maturity.

Figure 7A (left). A recommended phosphorus (P) fertilizer program from commercial fertilizer Supplier A. Figure 7B (right). A recommended phosphorus (P) fertilizer program from commercial fertilizer Supplier B.

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A pre-charge of P boost during bud set makes sense to build up a reserve in the plant at a time when the root system should be able to utilize (uptake) the fertilizer delivered. Based on the nutrient uptake patterns from the Manitoba, Canada research, the continual high level during bud sizing and flowering does not seem necessary. A lower level as suggested by Supplier A seems more appropriate. A comparison of leaf tissue P levels with floral bud P levels found that floral material had a higher level (Table 1). This would support making sure that adequate levels are provided, but it does not suggest that P fertilization at excessively high levels (>100 ppm P) is warranted.

To take this one step further, recent unpublished research led by Paul Cockson at NCSU found that a continuous program of approximately 15 ppm P was optimal when considering plant growth, flower bud yield, and cannabinoid concentrations. Lower P levels resulted in smaller plants with lower yields. Higher P levels of up to 30 ppm P did result in larger plants and flower bud yield, but those flower buds had a lower cannabinoid concentration (most likely due to a dilution effect). We took the typical commercial floriculture approach to fertilization of making it simple by providing a continuous supply of P.

In our research studies at NCSU, we target about 20 ppm P as a continuous fertilization during the entire production cycle. No doubt further research will help determine if varying the rate at key points in the production cycle and providing P at levels of 30 to 196 ppm may prove beneficial. But at this time, based on our research, providing excessively high P levels above 30 ppm does not

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