C.S.I. in the garden: Nutrient deficiencies and toxicities
By: Paul Josephs
When plants start to look distressed in the garden growers often utilize C.S.I. tactics to identify the problem; in this case C.S.I. stands for Cannabis Symptom Investigation. Ideal nutrient levels allow Cannabis to grow vigorously and easily if proper light and temperatures are maintained. If all the environmental factors are normal but the plants show unusual growth or leaf discolorations and spotting, a nutrient problem is most likely the cause. Determining the cause involves observation and investigation to solve the mystery.
Goldilocks and Cannabis
The relationship between plants and nutrients is very much like Goldilocks and her porridge; the nutrients, like the porridge, must be “just so.” Instead of too-hot or too-cold, Cannabis likes the levels of nutrients to fall into a middle range between too much and too little. The deficiency of a particular nutrient prevents the plant from performing certain functions at an optimal rate, or that essential functions may cease. An excess of many nutrients can interfere with plant growth, or with the plant’s ability to utilize other nutrients. Unlike porridge, however, plant nutrient problems involve more than a thermometer to determine whether things are just right.
Similar symptoms, different causes
Like a good C.S.I. episode there is a bit of old-fashioned investigation involved, and what may seem apparent at first might not turn out to be the real root of the problem. This is a somewhat complicated topic to cover exhaustively and, to be honest, even professional plant pathologists sometimes have a difficult time determining what the underlying cause of certain symptoms are. For example, one common symptom seen in Cannabis leaves is called interveinal chlorosis. This is when the leaves exhibit pale-green or yellow tissue between the main leaf veins, which remain green. This symptom can have multiple causes — nitrogen deficiency in its early stages can exhibit this same discoloration as can iron, magnesium, manganese, molybdenum, and sulfur deficiencies. To further complicate your investigation, an excess of one nutrient can cause other nutrients to become unavailable so the symptom may not at all reflect the actual root cause.
An ounce of prevention
The best way to manage nutrient issues is to avoid them altogether. Be proactive by maintaining pH levels within the correct ranges. In soil cultivation, the pH should fall between 6.2 and 7.2. In this range, all nutrients are available to the plant. In soilless or hydroponic cultivation the pH should range between 5.2 and 6.3. When pH is below the normal ranges many trace elements can become overly available and be toxic to the plant; some major nutrients, especially phosphorus, can become unavailable. If pH levels become too high some nutrients become unavailable, especially iron and phosphorous.
Another proactive practice is using fertilizer applications conservatively. Make sure that in hydroponic applications the fertilizer program you are using has all the major, secondary, and trace nutrients in it. If growing in soil read the package information to determine if nutrients have already been added. Consider whether or not your soil mixes has slow-release fertilizers added before applying additional fertilizers. Some organic mixes have quite a bit of nitrogen- and phosphorus-providing ingredients added to them. Composts, manures, guanos, and kelp extracts can provide nutrition that may not need much supplementation from additional fertilizer. While Cannabis is a fast-growing plant and uses a lot of nutrients, it is best to err on the side of slowly upping the levels as it is easier to correct a slight deficiency than to deal with a multiple-nutrient overdose problem.
Problems with Cannabis nutrition fall into four main classes: chlorosis (leaf yellowing), abnormal coloration, necrosis (dead spots and areas on leaves), and stunted or deformed growth. Where these symptoms occur on the plant is a good indicator of the type of nutrient that is to blame. Mobile nutrients that are easily transported throughout the plant usually show their deficiencies on lower and older growths and move up the plant. This is due to the fact that they can be “pulled” from older growth to become available to newer growth in the absence of a readily available source in the soil or nutrient solution.
Immobile nutrients are not easily transported within the plant from place to place and problems usually appear in new leaves or growing tips.
The most common nutrient problems are listed below with suggested remedies.
Nitrogen: This is a highly mobile nutrient and a deficiency manifests itself by progressive yellowing of the lower leaves that moves up the plant from the bottom. If the plant is several weeks into flowering, this can be a normal occurrence with some strains. If a deficiency is indicated adding a nitrogen-containing fertilizer will green up most affected parts, unless the yellowing has become severe. A nitrogen excess will appear as very dark-green lush and soft drooping growth. This can increase susceptibility to plant pests and diseases.
Phosphorus: Deficiencies in phosphorus manifest as slowed growth, with purpling of the underside leaf veins and sometimes as an intense blue-green pigmentation of leaves. As the deficiency advances purple blotching that turns necrotic may appear. Excesses of phosphorus are uncommon as the plants use a lot of this nutrient, but if it occurs it will usually manifest as a trace element deficiency because phosphorous can bind with them to form compounds that lock up the trace elements. Deficiencies can be treated with a phosphorous-containing fertilizer; however, the symptoms evident in the plant will not be reversed although new growth will improve.
Potassium: Deficiencies in potassium will be seen as red or purple stems and necrotic spotting in older leaves along with some chlorosis. It is important to note that some strains normally have a lot of red and purple pigments in them and colder temperatures or maturation can increase this coloration. Excess potassium can interfere with the absorption some other nutrients, such as magnesium.
Calcium: Calcium deficiencies usually appear as stunted growths and may also be seen as distorted new-growth tips that die off. Calcium acts as a pH buffer and too little can cause low pH levels to occur with resulting additional nutrient problems. Hydrated lime in very small quantities can be added to soil, or a fertilizer with calcium added; both will help prevent calcium deficiency problems.
Magnesium: A deficiency in magnesium is usually evidenced by lower-leaf interveinal chlorosis. Severe deficiency causes the leaves to curl upward and some growers refer to the curling as the plants “praying for” magnesium. Adding a teaspoon of Epsom salts per gallon of water will alleviate the problem and often the plant will green up from the top down in response to this treatment.
Sulfur: New growths will exhibit yellowing if sulfur is deficient, and growth will slow. Epsom salts added as described above will improve the problem.
Boron: A deficiency in this trace element will exhibit as distorted and necrotic new growth. The whole plant may be stunted. Use of a trace element supplement containing boron will improve new growth.
Copper: This is rarely seen as a deficiency in either soil-grown plants or in hydroponic setups, unless purified water is used and no copper is included in the fertilizer. Excess copper will result in interveinal chlorosis similar to that seen with other trace elements with the exception that copper is very toxic and can sometimes kill plants if not treated by flushing the soil or nutrient solution.
Iron, zinc, and manganese: Deficiencies in these trace elements cause problems that are very similar in appearance. New growth will exhibit increasing interveinal chlorosis. All three elements may be deficient at once. Zinc may be the more common culprit and it will exhibit more necrosis if severely deficient. Iron may be the only culprit if a high pH is indicated. Trace element supplements can be used to improve plant health. But be careful, they are referred to as trace element for a reason and some are toxic to the plants if over-applied.
It is relatively easy to add a deficient nutrient if needed, but what about toxic nutrient levels? Flushing is the best way to eliminate excess nutrients and salts from soils and nutrient solutions. For both soil and soilless containers, run the volume of the container of plain water through the container several times. This should leach excess nutrients from the soil. Replacing the nutrient solution in water-culture methods will eliminate the problems of an overly salty nutrient solution.
Preventing these nutrient problems from arising in the first place is the best approach. This is done by monitoring pH levels and avoiding over-fertilizing. Remember to take small steps when changing fertilizers or amounts to prevent many nutrient problems even before they occur. Eventually as Cannabis becomes more accepted and legal, it will become possible to utilize local agricultural extension resources for tissue analysis of affected leaves. For now a good tool to use when troubleshooting nutrient problems is to nutrient-deficiency flow charts easily found on the internet, simply type in “plant nutrient deficiency flow chart,” into your favorite browsers. Cannabis is like most vegetables when it comes to nutrition-related symptoms and their appearance. There are some excellent graphical charts specifically for Cannabis found online that are extremely helpful in diagnosing problems.