How IMPORTANT CO2 is....

Supplemental carbon dioxide (CO2) refers to the addition of concentrated CO2 to the greenhouse atmosphere to provide more raw material for photosynthesis. Light, water, and CO2 are used by green plants in photosynthesis to produce carbohydrates for growth and metabolism. The rate of plant growth depends on a balance between the manufacture of high-energy compounds (carbohydrates) from CO2 and water in photosynthesis and the utilization of those high-energy compounds by respiration. Growth is only possible when the balance is in favor of photosynthesis. Of the two raw materials needed by photosynthesis, water and CO2, numerous studies using a wide range of crop plants have shown that normal atmospheric levels of CO2 limit photosynthetic rate. Water probably is not a limiting factor for photosynthesis directly. When plants approach the wilting point, there is probably enough water in the tissues for photosynthesis. However, wilting causes the stomata to close, CO2 within the tissues of the leaf are quickly used up, and no new CO2 can diffuse into the leaf. So the effect of limited water on photosynthetic rate is probably indirect by restricting the CO2 supply.
Carbon dioxide is present at a concentration of approximately 350 ppm in the atmosphere. However, this is an average and the actual concentration in a given location can vary. Climatic changes can cause a 4 to 8 percent variation in CO2 concentration daily or seasonally due to increases or decreases in solar radiation, temperature, humidity and the passage of storm fronts. Concentrations of CO2 are also influences by human activity such as burning fossil fuels. CO2 concentrations are usually much higher close to cities, manufacturing, and combustion activities.
In a greenhouse filled with plants, CO2 concentration will closely follow ambient outside concentrations during the day as long as ventilation is needed. CO2 concentrations rise during the dark period because plants are not using CO2 for photosynthesis and respiration by plants and other organisms are generating CO2, e.g. fungi, bacteria. During light periods in which ventilation is not required, CO2 levels may fall below ambient.
Ventilation, where possible, is an effective way to maintain a constant supply of CO2 to plants. However, carbon dioxide concentration can fall below ambient conditions in a greenhouse filled with plants when light intensities are high but cold outside temperatures prevents ventilation. In a tightly closed greenhouse with plants, the CO2 concentration can drop to 150 to 200 ppm. This concentration is at or close to the CO2 compensation point or where CO2 produced by respiration equals the amount utilized by photosynthesis. Plant growth is reduced when the CO2 compensation point is reached even for brief periods.
Many studies have shown that CO2 concentrations well above ambient can benefit plant growth. Typically, a three- to four-fold increase in CO2 concentration yields a 10% to 25% increase in plant growth. Supplemental CO2 increases leaf area, dry weight, lateral branching, and in some cases decreases time to flower. In the greenhouse, supplementing CO2 to about 800-1000 ppm from an hour after sunrise to about an hour before sunset has been used to increase growth of many crops including pansy, geranium, impatiens, begonia, coleus, petunia, chrysanthemum, china asters, carnations, and roses.
In the greenhouse, there may be times when CO2 levels fall below outdoor levels and limit growth. Carbon dioxide levels can fall quite low in airtight greenhouses when vents are kept closed for extended periods during the winter. With little air exchange to the outside on cold bright days, photosynthetic rates can be high and deplete indoor CO2 levels below outdoor ambient, thus limiting photosynthetic rates.
However, CO2 is a gas, and like most gasses, is difficult to control. As soon as the greenhouse warms up and requires venting, supplemental CO2 is blown out the vent. For this reason, adding CO2 to the greenhouse atmosphere may be limited to cooler climates and times of the year.
Supplemental CO2 can, therefore, be viewed as an additional crop input, no different from light or nitrogen. In fact, some authors refer to supplemental CO2 as “CO2 fertilization” or “CO2 enrichment.” However, it is also possible to get CO2 levels too high. High CO2 levels (>3-5000 ppm) will cause an initial growth increase followed by a decrease in growth.

CO2 - Plant Age
The effectiveness of supplemental CO2 depends on timing, duration, and concentration. Applying supplemental CO2 to seedlings in plug flats generally results in reduced time to transplant, greater accumulation of dry matter, and larger leaf area than those under ambient conditions. Begonias given various CO2 levels and artificial light in growth rooms were transplantable four weeks from sowing when given 970 ppm CO2, moderate light, and warm temperature (80°F). This represents a 47% reduction in time to transplant compared to seedlings receiving no additional CO2. Recent work with geranium and pansy showed that 1000 ppm CO2 applied to seedlings at least two weeks old decreased time to transplant. Applying 1500 ppm was only slightly more effective than 1000 ppm, and four weeks were more effective than two weeks, but one week was insufficient. Two-week-old seedlings with one or two true leaves were as responsive as four-week-old seedlings with two to four true leaves. Therefore, plants are most responsive to supplemental CO2 when young and the timing can be important.

CO2 - Temperature
Temperature can have an important influence on the extent of response to supplemental CO2. In the table below, chrysanthemums were given ambient or 1000 ppm CO2 at either 70°F Day / 60°F Night or 80°F Day / 60°F Night temperatures. An increase in day temperature and CO2 concentration increased cut flower stem length and fresh weigh more than increasing either factors alone.

Day-night temperature and CO2 effect on relative fresh weight and stem length of Chrysanthemum.

A higher night temperature had very little effect on the plants response to supplemental CO2. Numerous studies have shown that the optimum day temperate for plant growth increases as CO2 increases. A good rule of thumb when using supplemental CO2 is to elevate the day temperature by 5-10°F. One consequence of raising the day temperature is that ventilation can be delayed and the CO2 enrichment period can be extended.
CO2 - Light
In terms of its effect on photosynthesis, each plant has a unique maximum light intensity above which the rate of photosynthesis cannot increase called the light saturation point. As light increases from a very low level, photosynthesis increases up to the light saturation point. However, if additional CO2 is added to the atmosphere, the light saturation point is reached at a higher light intensity and at a higher photosynthetic rate. In fact, studies have shown that enriching the greenhouse atmosphere with additional CO2 increases growth at all but the lowest light levels. This implies that even under low light conditions that may limit growth, the addition of CO2 can improve photosynthesis and growth. In the winter under very low light conditions, the effectiveness of supplemental CO2 may be limited by low solar radiation. The addition of supplemental light with supplemental CO2 can be used together to improve growth.

Effect of supplemental light and CO2 on vegetative growth of tomato.

The addition of supplemental light had a larger effect on increasing growth than CO2 but the two together gave the largest increase in growth.

CO2 - Nutrition
Rapid plant growth under supplemental CO2 and bright condition also means an increase in the rate of nutrient uptake and utilization by plants. Low concentrations of nutrients in the medium have been shown to reduce photosynthesis and growth and nutrient deficiencies can occur quickly under CO2 enrichment, especially when combined with supplemental light.
At present there are few recommendations to guide growers in adjusting fertilizer programs under supplemental CO2. It is generally believed that fertility should be increased under supplemental CO2. However, several studies indicate that some nutrients are depleted quickly while others change very little. The best recommendation to date is for growers to monitor nutrition closely using soil and tissue testing, then adjust fertility programs accordingly.

Carbon Dioxide Sources
Practical application of supplemental CO2 to the greenhouse usually requires a relatively pure source of CO2, a distribution system, and a monitoring and control system capable of maintaining set levels. The more practical sources of CO2 include pure tank CO2 or the clean combustion of fuels, usually propane or natural gas. The economics of equipment and fuel costs often dictate choices for a greenhouse size and location. Carbon dioxide is heavier than air. Therefore, distribution systems should maintain enough turbulence to keep added CO2 evenly mixed with the greenhouse air. Monitoring and control systems vary in sophistication but should be able to measure CO2 levels at several locations in the greenhouse, compare current concentrations to a set point, and adjust the concentration by adding CO2 when required. Many greenhouse environmental control computers have this capability already programed in.
CO2 Burners: These units burn propane or natural gas very cleanly so the products are CO2 and water. They consist of a housing, a precision calibrated gas burner, and a 24-volt gas solenoid valve. One unit cost $450 and can provide 1500 ppm over a maximum of 5000 ft2. The unit also generates about 60,00 Btu/hr and consumes about 60 ft3/hr of natural gas. These generators are hung above head height along the center of the greenhouse. Internal air mixing is needed to distribute the CO2 evenly. The gas supplied to these units must be of a high purity since sulfur contamination will be burned to sulfur dioxide. Calibration of the burners should be checked often because incomplete combustion cane lead to ethylene and carbon monoxide which are toxic to plants. The burner should be kept clear and adjusted to a clear blue flame. The plumbing should be checked carefully for any gas leaks since unburned fuel can harm plants. It is equally important to provide enough oxygen for complete combustion. In tight plastic houses or glass houses in areas prone to ice formation, provide one square inch of opening to the outside for every 2500 Btu/hr burner capacity.
Liquid CO2: Large greenhouse ranges that use large quantities of CO2 often find it more cost effective to install large pressurized tanks of CO2. The CO2 is then piped into the greenhouse area. Service companies replenish the liquid CO2 using tanker-type delivery trucks. One advantage of the method is that the gas is usually very clean and free of contamination.
Solid CO2: In this case, dry ice is placed in special cylinders and CO2 is produced as the ice sublimes. The amount of CO2 going into the greenhouse is regulated by gas flow meters. This method is most often used in small greenhouses.
It should be kept in mind that CO2 is heaver than air and, in the absence of air movement, it will sink to the lowest level and stratify in the greenhouse. Concentrations may also be higher close to the source of CO2. Therefore, it is important to mechanically mix the air in the greenhouse during enrichment. Heating systems, when operating, usually create enough convective air movement. Internal turbulators may be used at other times.

Control Systems
Controls for a supplemental CO2 system usually consists of a CO2 generator, a control system, and a “feedback” monitoring system. The monitoring device is usually an infra-red gas analyzer (IRGA). A sample of greenhouse air is pumped into the IRGA which determines the current greenhouse CO2 concentration. This information is sent to the control system that compares the current concentration to a set-point which is pre-set by the grower. If the current concentration is lower than the set-point, the control system activates the CO2 generator.
The introduction and wide-spread application of greenhouse environmental control computers has made automatic and continuous regulation of supplemental CO2 possible. Computers have been programed to modulate the CO2 levels in the greenhouse based on changing light condition. Plants make more efficient use of supplemental CO2 under bright condition and less efficient use as light levels decline. These programs provide more supplemental CO2 under bright conditions and a reduced rate as light levels fall. Supplemental CO2 is usually ceased at light levels below a minimum, (depending on the crop), often 500 foot-candles.

Economics
The cost of installing and operating CO2 generators is usually low compared to the potential gains in growth and plant quality. Investment and installation of the generators usually amounts to about $0.10 to $0.12 per square foot. The cost for fuel may be $0.10 to $0.15 per square foot per year. However, most of this cost will come during the winter months. It should be kept in mind that each CO2 burner contributes about 60,000 Btu/hr that the heating system will not have to generate.
The demand for quality crops, tight production schedules, and increases in the cost of production should encourage growers to take a careful look at supplemental light and CO2. Though little work has been done on plug-grown seedling, some evidence indicates the supplemental light combined with CO2 may have a synergistic effect on plant growth. The two combined might be a way to dramatically reduce time to transplant and increase quality of plug-grown seedlings.

Cannabis Nutrient Disorders

Nutrient disorders are caused by too much or too little of one or several nutrients being available. These nutrients are made available between a pH range of 5 and 7 and a total dissolved solids (TDS) range of 800 to 3000 PPM. Maintaining these conditions is the key to proper nutrient uptake. --------------------------------------------------------------------------------
Nutrients: Over twenty elements are needed for a plant to grow. Carbon, hydrogen and oxygen are absorbed from the air and water. The rest of the elements, called mineral nutrients, are dissolved in the nutrient solution. The primary or macro- nutrients (nitrogen (N), phosphorus (P) and potassium (K)) are the elements plants use the most. Calcium (Ca) and magnesium (Mg) are secondary nutrients and used in smaller amounts. Iron (Fe), sulfur (S), manganese (Mn), boron (B), molybdenum (Mo), zinc (Zn) and copper (Cu) are micro-nutrients or trace elements. Trace elements are found in most soils. Rockwool (hydroponic) fertilizers must contain these trace elements, as they do not normally exist in sufficient quantities in rockwool or water. Other elements also play a part in plant growth. Aluminum, chlorine, cobalt, iodine, selenium, silicon, sodium and vanadium are not normally included in nutrient mixes. They are required in very minute amounts that are usually present as impurities in the water supply or mixed along with other nutrients.
*NOTE: The nutrients must be soluble (able to be dissolved in water) and go into solution.
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Macro-nutrients Nitrogen (N) is primary to plant growth. Plants convert nitrogen to make proteins essential to new cell growth. Nitrogen is mainly responsible for leaf and stem growth as well as overall size and vigor. Nitrogen moves easily to active young buds, shoots and leaves and slower to older leaves. Deficiency signs show first in older leaves. They turn a pale yellow and may die. New growth becomes weak and spindly. An abundance of nitrogen will cause soft, weak growth and even delay flower and fruit production if it is allowed to accumulate.
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Phosphorus (P) is necessary for photosynthesis and works as a catalyst for energy transfer within the plant. Phosphorus helps build strong roots and is vital for flower and seed production. Highest levels of phosphorus are used during germination, seedling growth and flowering. Deficiencies will show in older leaves first. Leaves turn deep green on a uniformly smaller, stunted plant. Leaves show brown or purple spots.
NOTE: Phosphorus flocculates when concentrated and combined with calcium.
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Potassium (K) activates the manufacture and movement of sugars and starches, as well as growth by cell division. Potassium increases chlorophyll in foliage and helps regulate stomata openings so plants make better use of light and air. Potassium encourages strong root growth, water uptake and triggers enzymes that fight disease. Potassium is necessary during all stages of growth. It is especially important in the development of fruit. Deficiency signs of potassium are: plants are the tallest and appear healthy. Older leaves mottle and yellow between veins, followed by whole leaves that turn dark yellow and die. Flower and fruit drop are common problems associated with potassium deficiency. Potassium is usually locked out by high salinity.
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Secondary Nutrients Magnesium (Mg) is found as a central atom in the chlorophyll molecule and is essential to the absorption of light energy. Magnesium aids in the utilization of nutrients, neutralizes acids and toxic compounds produced by the plant. Deficiency signs of magnesium are: Older leaves yellow from the center outward, while veins remain green on deficient plants. Leaf tips and edges may discolor and curl upward. Growing tips turn lime green if the deficiency progresses to the top of the plant.
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Calcium (Ca) is fundamental to cell manufacture and growth. Soil gardeners use dolomite lime, which contains calcium and magnesium, to keep the soil sweet or buffered. Rockwool gardeners use calcium to buffer excess nutrients. Calcium moves slowly within the plant and tends to concentrate in roots and older growth. Consequently young growth shows deficiency signs first. Deficient leaf tips, edges and new growth will turn brown and die back. If too much calcium is applied early in life, it will stunt growth as well. It will also flocculate when a concentrated form is combined with potassium.
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Trace Elements Sulphur (S) is a component of plant proteins and plays a role in root growth and chlorophyll supply. Distributed relatively evenly with largest amounts in leaves which affects the flavor and odor in many plants. Sulphur, like calcium, moves little within plant tissue and the first signs of a deficiency are pale young leaves. Growth is slow but leaves tend to get brittle and stay narrower than normal.
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Iron (Fe) is a key catalyst in chlorophyll production and is used in photosynthesis. A lack of iron turns leaves pale yellow or white while the veins remain green. Iron is difficult for plants to absorb and moves slowly within the plant. Always use chelated (immediately available to the plant) iron in nutrient mixes.
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Manganese (Mg) works with plant enzymes to reduce nitrates before producing proteins. A lack of manganese turns young leaves a mottled yellow or brown.
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Zinc (Z) is a catalyst and must be present in minute amounts for plant growth. A lack of zinc results in stunting, yellowing and curling of small leaves. An excess of zinc is uncommon but very toxic and causes wilting or death.
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Copper (C) is a catalyst for several enzymes. A shortage of copper makes new growth wilt and
causes irregular growth. Excesses of copper causes sudden death. Copper is also used as a fungicide and wards off insects and diseases because of this property.
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Boron (B) is necessary for cells to divide and protein formation. It also plays an active role in
pollination and seed production.
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Molybdenum (Mn) helps form proteins and aids the plant's ability to fix nitrogen from the air. A
deficiency causes leaves to turn pale and fringes to appear scorched. Irregular leaf growth may also result.
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These nutrients are mixed together to form a complete plant fertilizer. The mix contains all the
nutrients in the proper ratios to give plants all they need for lush, rapid growth. The fertilizer is
dissolved in water to make a nutrient solution. Water transports these soluble nutrients into contact with the plant roots. In the presence of oxygen and water, the nutrients are absorbed through the root hairs.
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The above text is excerpted from George Van Pattens' excellent book "Gardening: The Rockwool Book".


Key on Nutrient Disorders

To use the Problem-Solver, simply start at #1 below. When you think you've found the problem, read the Nutrients section to learn more about it. Diagnose carefully before
making major changes. 1) a) If the problem affects only the bottom or middle of the plant go to #2.
b) If it affects only the top of the plant or the growing tips, skip to #10. If the problem seems to affect the entire plant equally, skip to #6.
2) a) Leaves are a uniform yellow or light green; leaves die & drop; growth is slow. Leaf margins are not curled-up noticeably. >> Nitrogen (N) deficiency.
b) If not, go to #3.
3) a) Margins of the leaves are turned up, and the tips may be twisted. Leaves are yellowing (and may turn brown), but the veins remain somewhat green. >> Magnesium (Mg) deficiency.
b) If not, go to #4.
4) a) Leaves are browning or yellowing. Yellow, brown, or necrotic (dead) patches, especially around the edges of the leaf, which may be curled. Plant may be too tall. >> Potassium (K) deficiency.
b) If not, keep reading…
5) a) Leaves are dark green or red/purple. Stems and petioles may have purple & red on them. Leaves may turn yellow or curl under. Leaf may drop easily. Growth may be slow and
leaves may be small. >> Phosphorous (P) deficiency.
b) If not, go to #6.
6) a) Tips of leaves are yellow, brown, or dead. Plant otherwise looks healthy & green. Stems may be soft >> Over-fertilization (especially N), over-watering, damaged roots, or
insufficient soil aeration (use more sand or perlite. Occasionally due to not enough N, P, or K.
b) If not, go to #7.
7) a) Leaves are curled under like a ram's horn, and are dark green, gray,
brown, or gold. >> Over-fertilization (too much N).
b) If not, go to #8…
a) The plant is wilted, even though the soil is moist. >>Over-fertilization, soggy soil, damaged roots, disease; copper deficiency (very unlikely).
b) If not, go to #9.
9) a) Plants won't flower, even though they get 12 hours of darkness for over 2 weeks. >> The night period is not completely dark. Too much nitrogen. Too much pruning or cloning.
b) If not, go to #10...
10) a) Leaves are yellow or white, but the veins are mostly green. >> Iron (Fe) deficiency.
b) If not, #11.
11) a) Leaves are light green or yellow beginning at the base, while the leaf
margins remain green. Necrotic spots may be between veins. Leaves are not twisted. >> Manganese (Mn) deficiency.
b) If not, #12.
12) a) Leaves are twisted. Otherwise, pretty much like #11. >> Zinc (Zn)
deficiency.
b) If not, #13.
13) a) Leaves twist, then turn brown or die. >> The lights are too close to the plant. Rarely, a Calcium (Ca) or Boron (B) deficiency.
b) If not… You may just have a weak plant.

Solutions to Nutrient Deficiencies

The Nutrients: Nitrogen - Plants need lots of N during vegging, but it's easy to overdo it. Added too much? Flush the soil with plain water. Soluble nitrogen (especially nitrate) is the form that's the most quickly available to the roots, while insoluble N (like urea) first needs to be broken down by microbes in the soil before the roots can absorb it. Avoid excessive ammonium nitrogen, which can interfere with other nutrients. Too much N delays flowering. Plants should be allowed to become N-deficient late in flowering for best flavor.
Magnesium - Mg-deficiency is pretty common since marijuana uses lots of it and many fertilizers don't have enough of it. Mg-deficiency is easily fixed with ¼ teaspoon/gallon of Epsom salts (first powdered and dissolved in some hot water) or foliar feed at ½ teaspoon/quart. When mixing up soil, use 2 teaspoon dolomite lime per gallon of soil for Mg. Mg can get locked-up by too much Ca, Cl or ammonium nitrogen. Don't overdo Mg or you'll lock up other nutrients.
Potassium - Too much sodium (Na) displaces K, causing a K deficiency. Sources of high salinity are: baking soda (sodium bicarbonate "pH-up"), too much manure, and the use of water-softening filters (which should not be used). If the problem is Na, flush the soil. K can get locked up from too much Ca or ammonium nitrogen, and possibly cold weather.
Phosphorous - Some deficiency during flowering is normal, but too much shouldn't be tolerated. Red petioles and stems are a normal, genetic characteristic for many varieties, plus it can also be a co-symptom of N, K, and Mg-deficiencies, so red stems are not a foolproof sign of P-deficiency. Too much P can lead to iron deficiency.
Iron - Fe is unavailable to plants when the pH of the water or soil is too high. If deficient, lower the pH to about 6.5 (for rockwool, about 5.7), and check that you're not adding too much P, which can lock up Fe. Use iron that's chelated for maximum availability. Read your fertilizer's ingredients - chelated iron might read something like "iron EDTA". To much Fe without adding enough P can cause a P-deficiency.
Manganese - Mn gets locked out when the pH is too high, and when there's too much iron. Use
chelated Mn.
Zinc - Also gets locked out due to high pH. Zn, Fe, and Mn deficiencies often occur together, and are usually from a high pH. Don't overdo the micro-nutrients- lower the pH if that's the problem so the
nutrients become available. Foliar feed if the plant looks real bad. Use chelated zinc.

Check Your Water - Crusty faucets and shower heads mean your water is
"hard," usually due to too
many minerals. Tap water with a TDS (total dissolved solids) level of more
than around 200ppm (parts
per million) is "hard" and should be looked into, especially if your plants
have a chronic problem. Ask
your water company for an analysis listing, which will usually list the pH,
TDS, and mineral levels (as
well as the pollutants, carcinogens, etc) for the tap water in your area.
This is a common request,
especially in this day and age, so it shouldn't raise an eyebrow. Regular
water filters will not reduce a
high TDS level, but the costlier reverse-osmosis units, distillers, and
de-ionizers will. A digital TDS
meter (or EC = electrical conductivity meter) is an incredibly useful tool
for monitoring the nutrient
levels of nutrient solution, and will pay for itself before you know it.
They run about $40 and up.
General Feeding Tips - Pot plants are very adaptable, but a general rule of
thumb is to use more
nitrogen & less phosphorous during the vegetative period, and the exact
opposite during the flowering
period. For the veg. period try a N:K ratio of about 10:7:8 (which of
course is the same ratio as
20:14:16), and for flowering plants, 4:8:8. Check the pH after adding
nutrients. If you use a reservoir,
keep it circulating and change it every 2 weeks. A general guideline for
TDS levels is as follows:
seedlings = 50-150 ppm; unrooted clones = 100-350 ppm; small plants =
400-800 ppm; large plants =
900-1800 ppm; last week of flowering = taper off to plain water. These
numbers are just a guideline, and
many factors can change the actual level the plants will need. Certain
nutrients are "invisible" to TDS
meters, especially organics, so use TDS level only as an estimate of actual
nutrient levels. When in
doubt about a new fertilizer, follow the fertilizer's directions for
feeding tomatoes. Grow a few tomato or
radish plants nearby for comparison.
PH - The pH of water after adding any nutrients should be around 5.9-6.5
(in rockwool, 5.5-6.1) .
Generally speaking, the micro-nutrients (Fe, Zn, Mn, Cu) get locked out at
a high pH (alkaline) above
7.0, while the major nutrients (N, P, K, Mg) can be less available in
acidic soil or water (below 5.0). Tap
water is often too alkaline. Soils with lots of peat or other organic
matter in them tend to get too acidic,
which some dolomite lime will help fix. Soil test kits vary in accuracy,
and generally the more you pay
the better the accuracy. For the water, color-based pH test kits from
aquarium stores are inexpensive,
but inaccurate. Invest in a digital pH meter ($40-80), preferably a
waterproof one. You won't regret it.
Other Things…
Cold - Cold weather (below 50F/10C) can lock up phosphorous. Some
varieties, like equatorial sativas,
don't take well to cold weather. If you can keep the roots warmer, the
plant will be able to take cooler
temps than it otherwise could.
Heat - If the lights are too close to the plant, the tops may be curled,
dry, and look burnt, mimicking a
nutrient problem. Your hand should not feel hot after a minute when you
hold it at the top of the plants.
Raise the lights and/or aim a fan at the hot zone. Room temps should be
kept under 85F (29C) -- or 90F
(33) if you add additional CO2.
Humidity - Thin, shriveled leaves can be from low humidity. 40-80 % is
usually fine.
Mold and fungus - Dark patchy areas on leaves and buds can be mold. Lower
the humidity and
increase the ventilation if mold is a problem. Remove any dead leaves,
wherever they are. Keep your
garden clean.
Insects - White spots on the tops of leaves can mean spider mites
underneath.
Sprays - Foliar sprays can have a "magnifying glass" effect under bright
lights, causing small white,
yellow or burnt spots which can be confused with a nutrient problem. Some
sprays can also cause
chemical reactions.
Insufficient light -- tall, stretching plants are usually from using the
wrong kind of light.. Don't use
regular incandescent bulbs ("grow bulbs") or halogens to grow cannabis.
Invest in fluorescent lighting
(good) or HID lighting (much better) which supply the high-intensity light
that cannabis needs for
good growth and tight buds. Even better, grow in sunlight.
Clones - yellowing leaves on unrooted clones can be from too much light, or
the stem may not be firmly
touching the rooting medium. Turn off any CO2 until they root. Too much
fertilizer can shrivel or wilt
clones - plain tap water is fine.

if everything you say is a lie (your signature) why should anyone take your posts seriously?

nice....... im copying that post, may come in useful someday... thanks..

jesus of Cannabis said:

if everything you say is a lie (your signature) why should anyone take your posts seriously?

Click to expand...

No one said you had too..

For a small closet grow, when is a good time to start using CO2? Immediately? At the moment my seedlings are just now starting to lose their bottom seed leaves (I forget what they're actually called. heh).

i would use co2 the second your light comes on. depending on the delivery method make sure it is off when the lights are off, plants don't use co2 in the dark.

Lighting and everything seems ok, I would definitely make sure you have your C02 and your lighting are on the same circuit and on the same switch. Obv photo sinthesis and the consumption of C02 only happends when the lights are on. Good luck.

Good to know, for sure. But I meant more in terms of the plants stages. Would the CO2 be wasted on seedlings, over a more grown plant?

no absolutely not, you should use co2 from the day they sprout. They benefit in all stages.

max316420 said:

no absolutely not, you should use co2 from the day they sprout. They benefit in all stages.

Click to expand...

Awesome. Also good to know. Heh. Being a 'the cheaper the better' personal use grower, I'll be going with the yeast/sugar idea for CO2. On an 18/6 schedule, that's six hours of CO2 going to waste every 24 hours. Hmm. But I guess it can't be helped. Can't exactly cap off a jug of yeast/sugar concoction. Heh. Oh well.

leave your lights on 24/0, faster growth rate and no wasted co2....problem solved lol

max316420 said:

leave your lights on 24/0, faster growth rate and no wasted co2....problem solved lol

Click to expand...

Oy. I don't even want to pretend to imagine what that electric bill would be like, down the road. lol