We always want to maximize the yields when harvesting, however, this may not be as easy as we think. Yields are decided by a variety of factors—light, water, and nutrients to name a few. Nutrients are a tricky thing: Cannabis can still grow buds without any nutrients but conversely, both under and overfeeding can do harm to your plants and even kill them. Therefore, we should take great care when fertilizing our plants, and know when to fertilize them, which nutrients to add, and how often to fertilize them.
What is NPK? -How Often to Fertilize during the Flowering Stage
NPK refers to nitrogen (N), phosphorus (P), and potassium (K) respectively. These are the main macronutrients that plants use for growth—plants use these nutrients to build chemical structures and amino acid compounds to create sucrose and other structures that are then used to build new cells and all of the other parts that make up plants on a microscopic level. The number of NPK means the percentage of nitrogen, phosphorus, and potassium (in this order).
For example, NPK 10-20-10 means the fertilizers contain 10% nitrogen, 20% phosphorus, and 10% potassium. Although they are the most common nutrients, they have different effects on plants.
- Nitrogen: Nitrogen is used by plants to produce more chlorophyll, which can help to grow green healthy leaves. Cannabis needs a large amount of nitrogen during the vegetative stage. Nitrogen is active and does not easily accumulate in the soil, so nitrogen needs to be added frequently. A lack of nitrogen will lead to the slow growth of plants. If leaves are small and light in color, not the bright green we usually see and the stems are also fragile and easy to break, you probably have a nitrogen deficiency.
- Phosphorus: Phosphorus can transfer energy to focus on the roots and also stimulates the development of flowers and fruits. Cannabis needs a higher percentage of phosphorus in the flowering stage than in the vegetative stage. When plants are deficient in phosphorus, the roots will be prone to insects, and they will not bloom, or they will grow very small buds.
- Potassium: Potassium has a significant effect on promoting photosynthesis in plants. It can make your plants stronger and enhance their ability to flower and bear fruit. A lack of potassium will cause the leaves to turn brown, curl up, and look burnt over time.
Of course, most fertilizers not only contain nitrogen, phosphorus, and potassium but also other substances, such as potassium salt, lime, ammonium sulfate, phosphoric acid, calcium, magnesium, iron, copper, etc. These are all to balance the lack of naturally occurring nutrients in potting soil (or natural soil).
How often should I fertilize in the flowering stage?
The nutrient requirements will change dramatically when the plants enter the flowering stage, and this may be a little hard for new growers. You can feed your plants 1-2 times every week, gradually increasing the nutrients, but at the same time pay attention to your plants’ reactions after each feeding.
It does matter how you fertilize (the amounts and percentage of nutrients) as much as how often you fertilize. We will summarize some recommended NPK percentages for the flowering stage. Plants go through several sub-growth periods during this stage, and you can make some changes based on the states of your plants.
Weeks 1-3: NPK 5-10-7 -How often to fertilize during the flowering stage
Cannabis requires a higher percentage of P and a lower percentage of N during the initial flowering stage. The exact NPK is up to you—keep an eye on your plants when you adjust your NPK formula and be careful not to burn the plants. The plants will grow rapidly and stretch their body during the first week. You can distinguish the sex in the second week if you have not yet done so, remember to separate the females from the males. By the third week, your plants will stop growing and focus on flowering and developing buds.
Weeks 3-4: NPK 6-15-10 -How often to fertilize during the flowering stage
Instead of stretching their body, plants will transfer all their energy to grow buds. You will notice a layer of trichomes begin to develop on the leaves and they begin to emit a certain aroma and need more nutrients in this period. You can increase the P content during this stage.
Weeks 4-6: NPK 5-12-9 -How often to fertilize during the flowering stage
You should observe big, thick, and dense buds during this period. The more mature the plants are, the more trichomes there will be. The aroma will reach its peak during this period, so you probably want a carbon filter handy. The plants don’t need much P at this time, but you can add more calcium.
Weeks 6–8: NPK 4-10-7 -How often to fertilize during the flowering stage
During the final period of flowering, the buds should be covered with trichomes, and the color of most trichomes should be white, while some of them will be amber. You can decrease the percentage of phosphorus, potassium, and calcium. You will also want to flush your plants with PH-balanced water so that they can absorb the leftover nutrients from the soil—the final week before harvest you should wash out all nutrients from the soil.
Your plants can only grow healthily with the proper amount of NPK—otherwise, it may lead to a series of problems such as nutrient burn, nutrient deficiency, or nutrient lockout. Be aware of the various signs of these symptoms so you can respond to your plants’ needs accordingly.
Pay attention when you feed them, your plants will tell you how they feel, and what they need—you don’t want your plants to die when they are close to harvest.
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