Water & Nutrients
Water provides a medium to transport the necessary nutrients for plant life and makes them available for absorption by the roots. Water quality is essential for this process to work efficiently. Microscopic root hairs absorb water and nutrients from the growing medium and then transports them up through the stem to the leaves where they are used for photosynthesis. The flow of water from the soil through the plant is called the transpiration stream. Only a fraction of the water is processed during photosynthesis, the excess water evaporates into the air via tiny pores located on the undersides of the leaves called the stomata in a process called transpiration. The main factors which govern this process are water quality, pH, and nutrient levels.
Nutrients are elements that the plant needs to live. Carbon, hydrogen, and oxygen are absorbed from the air and water. The rest of the elements (nutrients), are absorbed from the growing medium and nutrient solution. Supplemental nutrients supplied in the form of a fertilizer allow cannabis to reach its maximum potential. Nutrients are grouped into three categories: macronutrients, secondary nutrients, and micronutrients. Each nutrient in the above categories can be further classified as either mobile or immobile.
Mobile nutrients include: nitrogen, phosphorus, potassium, magnesium, and zinc, are all able to translocate, or move from one part of the plant to another as needed. For example, nitrogen in the older, lower leaves can translocate to younger leaves to solve a deficiency issue. The result of this translocation will result in deficiency symptoms appearing in the older, lower leaves first.
Immobile nutrients include: calcium, boron, chlorine, cobalt, copper, iron, manganese, molybdenum, silicon, and sulfur, do not translocate to new growth when needed. They remain deposited in their original place in the older leaves. This is the reason deficiency symptoms appear first in the upper, new leaves on top of the plant.
pH - Nutrient Uptake
In order for plants to be able to absorb the nutrients contained within water, the pH will need to be within a certain range, as you can see in the image on the left. When growing organically in soil, you will want the pH range of your water/nutrient solution to be from 6.5-7 ideally. When growing hydroponically, the ideal pH range is from 5.8-6.5. Every full point change in pH signifies a ten-fold increase or decrease in acidity or alkalinity. For example, soil or water with a pH of 5 is ten times more acidic than water or soil with a pH of 6. Water with a pH of 5 is one hundred times more acidic than water with a pH of 7. As you can see, with a ten-fold difference between each point on the pH scale, accurate measurement and management is essential for a strong, healthy garden.
When the pH falls out of the range required by cannabis and most other plants, the nutrient salts within the water/nutrient solution become chemically bound and the roots are unable to absorb them, leading to deficiency symptoms and disease. When nutrients become bound within the growing medium, the resulting toxic salt buildup severely affects the plant’s ability to intake water. The pH of organic soil mixes is very important because it dictates the ability of beneficial pH sensitive bacteria.
There are different ways of measuring the pH of your water and nutrient solutions, but the most effective and reliable is the simple chemical test kit made by General Hydroponics. There are also digital meters available from a wide variety of manufacturers, but they are consistently unreliable and in my opinion not worth the hassle.
When growing organically in soil, you can adjust the pH up by using baking soda. 1/8th teaspoon of baking soda per gallon will adjust the water pH by 1 full point, so be careful when adjusting. To lower the pH of your water, you can use 1/2 to 1 teaspoon white vinegar per gallon for a 1 point adjustment downward. If you are growing hydroponically, I would recommend purchasing pH up and pH down for making these adjustments.
Different Plants have Different Nutrient Needs
The most important thing about nutrients, whether you decide to use chemical or organic based fertilizers, is how much to use. Most fertilizers purchased in specialty gardening shops will have recommended dosage instructions which may or may not be right for you. Different varieties of cannabis for instance will have different nutritional requirements with some being able to take significantly higher quantities than others. Using the instructions on the bottle may be toxic for your variety. So the best practice if you are unsure is to start light and use a mixture of around 150 ppm if using a chemical fertilizer and growing in an inert medium like peat and apply once a week. Then instead of increasing the ppm, increase the frequency if deficiencies start to appear. Also, be aware that different sized plants will also have different needs, a large plant will need more than a small plant.
Don’t try to wing it by scheduling feeding by memory, you will fail. Print out a 3-4 month calendar and hang it in the garden. Keep an exact accounting of what was done on each day. Keep track of things like if you used nutrient or plain water, if you applied a mycorrhizae solution, when you transplanted to larger containers, when you adjusted light schedules, and when you notice deficiencies or signs of over fertilization. Keeping a specific accounting day by day will make you more successful in future sessions.
Many indoor gardening problems are misdiagnosed as nutrient deficiencies. Often, disease and insects are the real cause of such problems. Other times, problems are the result of an imbalanced pH of the growing medium and water/nutrient solution. A pH between 6.5 and 7 in soil and 5.8 to 6.5 in hydroponics will allow nutrients to be chemically available; above or below this range, several nutrients lockup and become unavailable.
Incorrect pH contributes to most serious nutrient disorders in organic-soil gardens. Many complex biological processes occur between organic fertilizers and the soil during nutrient uptake. The pH is critical to the livelihood of these activities. When the pH fluctuates in a hydroponic garden, the nutrients are still available in the solution for uptake, and the pH is not as critical. EC (electrical conductivity) or the nutrient solutions TDS PPM (total dissolved solids, parts per million) is the most important indicator of plant health and nutrient uptake in hydroponics.
Once a plant shows symptoms, it has already undergone severe nutritional stress. It will take time for the plant to resume vigorous growth. Plants grown indoors are often harvested so quickly that the plants don’t have time to recover from nutrient imbalances. Correct identification of each symptom as soon as it occurs is essential to help plants retain vigor.
Do not confuse nutrient deficiencies or toxicities with insect damage or poor cultural practices. The basic elements of the environment must be checked and maintained at specific levels to avoid problems. Check each of the vital signs: air, light, soil, water, temperature, humidity, etc., and fine tune the environment, especially ventilation, before deciding that plants are nutrient deficient.
Diagnosing nutrient deficiencies can be difficult as the symptoms from one nutrient to the next can have a similar appearance. Below we have compiled some information about the most common cannabis nutrient deficiencies found in the grow room as well as over fertilization symptoms.