Notes on Fertilizer use, Calculations, pH and Alkalinity (getting down to the practical nitty-gritty)



Introduction
Our articles on water quality and mineral nutrition have generated quite a bit of interest, but also a fair number of questions. The questions tend to center around the practical interpretation of water quality reports and fertilizer concentrations and pH of fertilizer solutions. Units of measurement, particularly where the Metric and Imperial Systems clash, and volume versus weight interact, create a lot of problems. How do I convert parts per million into teaspoons per gallon, etc.? This section was created to try to answer most of those questions. Because of the complex inter-relatedness of the various issues in water quality and fertilizing, we have decided to treat this in an article format rather than a FAQ format.

Discussion of terms and concepts
        The first consideration is the plants themselves. What is the desired pH at the root level for a particular plant. Some plants are adapted to extremely acidic conditions, others and perhaps most of the orchids tend to prefer somewhere in the pH 5.6 to 6.6 range, while others that have adapted to growth in calcicolous or other ultrabasic rock such as serpentine seem to do better in neutral or slightly basic conditions.
        TDS = Total Dissolved Solids. Basically what we are talking about is everything that is dissolved in your water, either as it comes from the tap when we are talking about water quality, and what you dissolve in it when you mix up your fertilizer. There are three measurements related to the Total Dissolved Solids that are important to understand.
        PPM = Parts Per Million, a proportional measurement of solute in our water. It is important to realize that this is a weight/weight proportion. That is regardless of units one part per weight of a solute in one million parts per weight of water equals one part per million. It could be one pound of fertilizer in one million pounds of water, or one gram of fertilizer in one million grams of water. The usual expression of measurement for water quality and fertilizer is mg/L (milligrams/Liter). 1 mg/L is considered essentially equivalent to 1 PPM. Wait a minute, didn't I just say it was a weight/weight measurement? Isn't a liter a measure of volume? The answer is in the interelatedness of metric measurement. At 3.98 degrees C 1 liter of water weighs 1000 grams, and only varies about 4% across the entire range from 0 - 100 degrees C. It only varies about 1% within the range of temperatures we would be dealing with for irrigating our plants, so for practical purposes we may consider a liter of water to be equal to 1000 grams. Therefore, 1mg/L becomes equivalent to 1 PPM.
        pH may be thought of as the "potential for Hydrogen". It is technically the negative of the logarithm of the hydrogen ion concentration of a solution. It is a measure of how acidic or basic (another term often used for basicity is alkalinity, but this leads to confusion with another use of the term) a solution is, and is measured on a logarithmic scale of 0 to 14, with 7 being neutral, below 7 acidic and above 7 basic. It is important to understand that pH is a measure of intensity, not of capacity.
        After TDS, Alkalinity is probably the second most important number on your water quality report. Alkalinity is a confusing term, because we equate alkaline with basic, and therefore tend to think of it as pH. Alkalinity with regard to water is related to pH, but the two give us different important pieces of information. Alkalinity is the total of the carbonates and bicarbonates and hydroxides dissolved in the water. It is a measure of the buffering capacity of the water to neutralize added acid. It is sometimes also reported as the "carbonate hardness" of the water, and the generally accepted unit is mg/L (PPM) Calcium Carbonate (CaCO3) Equivalent (CCE), although sometimes it is reported as milliequivalents per liter (meq/L). PPM CCE = meq/L/0.02 .
        The formulation of a fertilizer will affect whether it will be acidic or basic in water. Most of the fertilizers we encounter in orchid culture will tend to be acidic. Pure water such as distilled (from a distiller or a dehumidifier or air conditioner), reverse osmosis, deionized or rainwater has very little if any alkalinity, and the fertilizer can tremendously affect the pH. Tap waters with low alkalinity, in the ranges of 25 to 50 PPM CCE, will generally yield a favorable slightly acidic irrigation water when mixed with typical fertilizer formulations used for orchid culture.
        The potting medium pH and buffering capacity produces a further effect. Many mediums are neutral with virtually no buffering capacity, and have little effect on the pH of the irrigation mix Other mediums, such as bark and sphagnum moss, can be extremely acidic and be quite resistantly buffered in the acidic range. Addition of supplements such as dolomitic limestone or crushed eggshell can sometimes be used effectively to create more basic conditions in neutral or acidic mediums if desired, but do tend to lead to a more rapid breakdown of the medium.

Confusion of weight versus volume and actual percentages of elements
        It is important to recognize that PPM is a weight/weight ratio. To add to the confusion is the use of ounce as both a unit of weight and a unit of volume (fluid ounce) in the Imperial System. Since many people tend to prepare fertilizer on a fractional teaspoon per gallon basis it is therefore necessary to know the weight of a teaspoon (or fraction thereof) of the particular fertilizer being used if you are interested in knowing TDS in PPM or the PPM of any particular component. It is also necessary to keep in mind that at least in the United States, the N-P-K numbers associated with the fertilizer have a further twist. While the N number represents Nitrogen as elemental Nitrogen, the P and K numbers represent the percent present as the oxide form of the element, that is P2O5 for Phosphorus and K2O for Potassium. So to convert the numbers to the elemental percentage in the fertilizer, you need to multiply the P number by 0.437 to get elemental Phosphorus percentage and the K number by 0.82 to get elemental Potassium.
To work an example for those, as above, who work in fractional teaspoons and gallons, you first need to determine the weight of a teaspoon of the particular fertilizer. Lets use grow-More 6-30-30 as an example. A dozen measurements of a level teaspoon yield a weight for this fertilizer of approximately 3.85g/teaspoon. A half teaspoon is therefore 1.93g (1930mg). A gallon of water is 3.785L (or 3785g or 3785000mg). 1930mg/3.785L = 510 PPM TDS from the fertilizer for 1/2 teaspoon per gallon, real TDS will also add what is already in the irrigation water. Of this 510PPM TDS, 31PPM is N (510 x .06), 67PPM is P (remember that at least in the United States the middle P number is not for elemental P but rather for P2O5, so the calculation is 510 x .30 x .437) and 130 K (again the K number on the fertilizer is for K2O, not elemental K so the calculation is 510 x .30 x .82). Because so much, including what we have put up on the subject of fertilizer is listed in PPM, and so many growers mix in the form of fractional teaspoons per gallon, we've compiled a database of several common fertilizers and the resultant values from 1, 1/2, and 1/4 teaspoon per gallon rates, as well as pH resulting from these rates in pure and 50, 100 and 200 PPM CaCO3 alkalinity water.

Comments on TDS and pH measurement
        Total Dissolved Solids are usually measured indirectly by measuring the conductivity of the solution across a 1 cm distance at a given temperature. I suggest that you buy a conductivity meter that reads in either micro or millisiemens. Conductivity meters that read in PPM are using a built in arbitrary conversion (usually 0.7 PPM = 1 microsiemen in the horticulture industry) that is considered an average encountered in horticultural use. The actual conductivity of various ions varies tremendously, and you can easily be getting converted readings that are as much as 40% away from the real reading. By way of example, for the Grow More 6-30-30 used as an example above gives a conductivity reading very close to 1 microsiemen to 1 PPM, so if you use a conductivity meter that has readings in PPM, it will give you readings that are significantly off. If you have a meter calibrated in micro or millisiemens you can use the values we have derived for different fertilizers to convert readings to PPM with considerably more accuracy. You will be interested primarily in readings in the range of 0 to 1200 microsiemens (0 to 1.2 millisiemens), so keep this in mind when making your selection. You also want to make sure that the particular meter automatically corrects for temperature. As an example of how temperature affects conductivity, a KCl calibration standard that conducts at 1413 microsiemens at 25C has a conductivity of 1147 microsiemens at 15C.
        Measurement of pH also presents a few challenges. pH measurement is also susceptible to temperature variation, and although the effect is not as serious as with TDS variation, it is best to buy a meter that automatically temperature compensates. The less expensive "pen" type meters seem to be reasonably reliable for measuring the pH of typical fertilizer mixes within a range of about pH 5.5 to 8.0 . Accuracy started to degrade outside of these ranges, and was also a problem with single compound solutions such as mixes of Potassium Nitrate or Magnesium Sulfate (Epsom Salts). Accuracy of both the meters and indicator papers and solutions can also be less reliable with very weak solutions.
        It is important to make sure that you have and use calibration solutions for both types of meters.

Some weight and volume equivalents
        The following charts are provided to be a convenient reference point for working both for scaling up or down fertilizers mixtures, but also for converting between Metric and Imperial Units.

  1 liter (L) = 1000 milliliters (ml) = 1000000 microliters (ml)
       
1 gallon = 3.785 L = 3785.412 ml = 3785412 ml
1 quart = 0.946 L = 946.353 ml = 946353 ml
1 pint = 0.473 L = 473.177 ml = 473177 ml
1 cup = 0.237 L = 236.588 ml = 236588 ml
1 tablespoon = 0.015 L = 14.787 ml = 14779 ml
1 teaspoon = 0.005 L = 4.929 ml = 4929 ml

  1 gallon = 4 quarts = 8 pints = 16 cups = 256 tablespoons = 768 teaspoons
             
1 liter = 0.264 gal = 1.057 qt = 2.113 pint = 4.227 cup = 67.628 Tbs = 202.884 tsp
1 milliliter = 0.000264 gal = 0.001057 qt = 0.002113 pint = 0.004227 cups = 0.067628 Tbs = 0.202884 tsp

  1 Kilogram (Kg) = 1000 grams (g) = 1000000 milligrams (mg)
       
1 pound (lb) = 0.454 Kg = 453.592 g = 453592.4 mg
1 ounce (oz) = 0.028 Kg = 28.350 g = 28349.5 mg

  1 pound (lb) = 16 ounces (oz)
     
1 Kilogram (Kg) = 2.205 lb = 35.274 oz
1 gram (g) = 0.0022 lb = 0.0353 oz
1 milligram (mg) = 0.0000022 lb = 0.0000352 oz

1 Liter (0.264 gal) of water at 3.98C (39.2F) weighs 1000 g (2.205 lb).
1 Gallon of water at 3.98C weighs 8.35 lbs (3785g)

Can I just add tap water back to my pure water source to replace calcium and magnesium?
        While this is a very common recommendation, we don't suggest this as a routine practice. Aside from the idea that you won't really know what you are adding back, the form of the calcium and magnesium is not desirable. Groundwater calcium and magnesium is generally derived from the carbonate forms. Carbonates are as a rule insoluble. They become dissolved in groundwater after long contact with slightly acidified water, acidified by either dissolved CO2 or humic acids, and go into solution in the presence of the acid as a soluble bicarbonate :

CaCO3(s) + H2CO3(aq) <---> Ca2+(aq) + 2HCO3-(aq)

When you water with this as the water evaporates you will end up with these elements precipitated out in the medium and on the roots again as the insoluble carbonate form, very hard to leach away, and although they will build up far more slowly because you have lowered the levels, it is in my opinion far more desirable, after going to the trouble to remove these ions, to add them back in a more soluble nitrate or sulfate form, any excess being easily leachable, in the form of a complete soluble fertilizer or by supplementing with calcium nitrate and magnesium sulfate separately.


Is it harmful for me to use a fertilizer that contains urea?
        The answer to this depends on several factors, revolving around what percentage of the total nitrogen in the fertilizer is derived from urea versus ammoniacal and nitrate nitrogen present, and what your water quality is. A little background is needed to better explain.
        Urea (CO(NH2)2) needs to be broken down into ammonium (NH4+) to be utilized by the plant. The conversion to NH4+ is accomplished by the enzyme urease. Urease is produced by many different kinds of bacteria, fungi, and actinomycetes, as well as by many plants themselves. These urease-producing microorganisms live on the plant's leaves and roots and are usually in highest numbers in the area of the plants' rhizosphere.
        It is a common statement to see that "it can take a year or more for urea to be broken down to ammoniacal nitrogen". While this statement can be true, it would be true only where conditions are such that urease activity is absent or highly suppressed. Such conditions would be a sterile environment, temperatures below freezing, a very high pH environment, or a very nitrate rich environment. These are not the conditions we generally grow our orchids in. Under warm, moist conditions with an adequate supply of urease present, the conversion of urea to NH4+ is quite fast, as we would expect of most enzymatically catalyzed reactions. In fact, if you search the literature you will find that most research surrounding the use of urea is in finding ways to slow the activity of urease. So the limiting factor is going to be how much urease is present, and that is going to depend on how the plant is being grown. Obviously a mounted plant with bare roots is going to have very little urease present, but a Phragmipedium growing in bark on top of a saucer of water is likely to have considerably more urease present. In fact, I have more than once heard people say that their Phrag pots "smelled like an aquarium", and indeed probably what they were smelling was the production of NH4+ from urea. Many of the same bacteria and fungi that produce urease are the same that cause the breakdown of organic mediums such as bark, so if you've ever had plants in seedling bark for more than six months you are very aware that they are present in quite large numbers, and very active, particularly in warm conditions.
        The second common argument against urea is that it is wasted because it will be washed out of the pot on your next watering before it can be converted to a useful form of nitrogen. While, firstly, as we just pointed out that may, or may not be true depending on how you are growing. Undoubtedly some amount will be converted. And equally undoubtedly some amount will be washed out. But this statement is also true of any other nitrogen source, any that is not taken up by either the roots of the plant or the medium will be washed out at the next watering if you are watering your plants properly.
        So what it really amounts to is what percentage of the total nitrogen in your fertilizer is derived from urea, and what percentage of that is converted to a useful form between waterings. If you have an excessive percentage of the total nitrogen as urea (as a few are), then that can present problems. First, you probably aren't converting all of it to NH4+; so much of it is useless dissolved solid. As we all know, most of our orchids do best when we limit the amounts of dissolved solids at the roots, so we are essentially simply adding excess useless dissolved solids. This problem becomes far more serious if you have irrigation water that starts with a higher salt content. If you were converting most of the high levels of urea to NH4+, then you may be creating a problem in itself with excess NH4+ which can be toxic. Most plant recommendations are for a NO3- to NH4+ balance of 2:1 to 3:1 to avoid toxicity problems. Higher NO3- ratios are usually suggested in lower light conditions.
        The net lesson perhaps is that a fertilizer that has a small to moderate percentage of its nitrogen source as urea is fine, and we do use one as part of our rotation. Its contribution to the total nitrogen input may be modest, depending on your growing circumstances. One with a high proportion of its nitrogen as urea can be a problem, however, as you end up adding extra dissolved solids in your irrigation without any substantial benefit from it. Look for a fertilizer with low or no urea, and a nitrate to ammoniacal nitrogen source in the two to three to one range.

The fertilizer charts
        To aid those working with "teaspoons/gallon" type measurement, be it to mix one gallon at a time or to tank mix hundreds of gallons at a time, we have determined the density (weight per teaspoon) of several common fertilizers. We did this by weighing a dozen level teaspoons of each fertilizer on a microbalance in the laboratory. The weights are reported initially in grams, in fact all of our measurements were done in metric and converted, for the simple reason that most laboratory equipment capable of precise measurement use metric measurements. We have the capability of weighing to one microgram and measuring volume to 1 microliter with reasonable precision. Reproducibility of a "level teaspoon" measurement varies with how the fertilizer is milled. Very finely and uniformly milled fertilizers allow for very accurate and reproducible measurement by volume, while those with non-uniform or coarse milling, or many that contained calcium nitrate in a pelleted form gave slightly less uniform results. All fertilizers tested were from freshly opened packages showing no hydration.
        PPM values are calculated values based on the weight of fertilizer to weight of solute. All EC (electrical conductivity) and pH values are measured. Water used was reverse osmosis processed and registered less than 10 microsiemens. The Dyna Grow Protekt (potassium silicate) effect on pH is reported for various fertilizers in RO water as this is one product we use to adjust a very acidic pH to a range we prefer. If the correction is not possible using 1/2 teaspoon per gallon Protekt, then we also use potassium hydroxide.
        Fertilizer analysis is listed as on the fertilizer package. Frequently fertilizers will not list micronutrient analysis, but this does not necessarily mean they do not contain sufficient micronutrient levels. Values listed on the package must be tested and guaranteed within a fairly small percentage of the listing, so in some cases the micronutrients are simply not listed although present to avoid having to be held to exceedingly close tolerances on small percentages. It would, indeed, be difficult to compound these fertilizers without significant micronutrient content just from trace contamination of the macronutrient compounds used.
        The data presented is subject to numerous possible measurement errors, and should be considered approximate, and exactly how the fertilizer will react with your particular water cannot be precisely predicted. We strongly suggest confirming the EC and pH of any solutions made before using them.

Better Grow 20 - 14 - 13 Orchid Plus

Label Analysis
Total Nitrogen 20.00 %
Nitrate Nitrogen 10.59 %
Ammoniacal Nitrogen 9.41 %
Phosphate (P2O5) 14.00 %
Potassium (K2O) 13.00 %
Magnesium 1.00 %
Boron 0.02 %
Chlorine > 1 %
Copper 0.05 %
Iron 0.20 %
Manganese 0.05 %
Molybdenum 0.005 %
Zinc 0.05 %

Derived from: Potassium Nitrate, Ammonium Phosphate, Ammonium Nitrate, Magnesium Sulfate, Sodium Borate, Sodium Molbdate, Copper EDTA, Iron EDTA, Manganese EDTA, Zinc EDTA.

EDTA is Ethylenediaminetetraacetate, a chelating agent.

Lab Values
Density = 4.94gram/teaspoon (0.174oz/tsp)

4.94 grams/gallon (1305 PPM) yielded an EC of 1660 microsiemens so 0.79 PPM = 1 microsiemen or 1 PPM TDS = 1.27 microsiemen.

pH in RO water and RO water plus Protekt and water at various alkalinities
Fertilizer Rate ß RO water 1/4tsp/gal Protekt 1/2 tsp/gal Protekt 50 mg/L Alkalinity 100 mg/L Alkalinity 200 mg/L Alkalinity
1 tsp/gal 6.0 6.5 6.8 6.5 6.8 7.0
1/2 tsp gal 6.2 6.8 7.2 6.7 7.0 7.2
1/4 tsp/gal 6.4 7.2 7.9 7.0 7.3 7.5

Elements in PPM for various mix levels
  1 teaspoon/gallon 1/2 teaspoon/gallon 1/4 teaspoon/gallon
TDS 1305 653 326
Total Nitrogen 261 131 65
Nitrate Nitrogen 138 69 35
Ammoniacal Nitrogen 123 62 31
Phosphorus 80 40 20
Potassium 139 70 35
Magnesium 13 7 3
Sulfur 17 9 4
Calcium 0 0 0

What are some of the things we can do with this sort of data for Better Grow 20-14-13? Well, first, with regard to pH it would appear to be a very suitable fertilizer for use with RO water, the acid values obtained are in a reasonable range and if you do want to raise the pH slightly it is easily achieved using Protekt within the label recommendations. It also gives very reasonable pH values with low alkalinity water of say 100 mg/L Calcium Carbonate Equivalence and below. With higher alkalinity water it may be necessary to use acid to lower the pH to optimal levels.
It also appears that somewhere between 1/4 and 1/2 teaspoon per gallon gives us a feed range for every macronutrient except calcium that is within reasonable range for Slipper Orchids. The label recommends 1 teaspoon per gallon for steady feed, this may certainly be suitable for heavier feeding orchids, but I think 3/8 to 1/2 teaspoon per gallon would be most appropriate for most Paphs and perhaps closer to the 1/4 teaspoon per gallon range appropriate for Phrags. The Nitrate to Ammoniacal Nitrogen ratio may be a bit low for cooler or overcast periods of the year, so I would be cautious about using this fertilizer extensively in the winter here in Candor.
        If you have a target for Nitrogen it is also easy to calculate that either on a per teaspoon basis or conductivity basis. On a per teaspoon basis, for example, if you wanted to target 100 PPM total Nitrogen it would be 100/261 = 0.383 teaspoons per gallon, or approximately 3/8 teaspoon per gallon. To calculate the expected EC for 100 PPM total Nitrogen we would take the conversion of 1 PPM TDS = 1.27 microsiemen and divide by our elemental percentage of 20 % (0.20) Nitrogen and then multiply by 100. (1.28/.20)x100 = 640 microsiemens.
        The amounts are easy to scale up for tank mixing, let's say you want 30 gallons (roughly the size of the larger plastic trash cans used by many folks for water) of approximately 100 PPM Nitrogen. 30*3/8 = 11 1/4 teaspoons of this fertilizer. If you are using RO water you can expect approximately a pH of 6.3 in this mix, if you are using tap water with 100 mg/L alkalinity it will be around pH 7.1. You can also factor in your dilution rate to calculate injector or hozon concentrates. Let's say you want 100 PPM Nitrogen (3/8 teaspoon/gal), and you are mixing up 5 gallons for a 1/16 siphoner. 5x3/8x16 = 30 teaspoons or 10 tablespoons of fertilizer in your 5 gallons of water to make your concentrate. To mix 5 gallons a concentrate that will yield 100 PPM Nitrogen for this fertilizer for an injector set at 1:100, 5x3/8x100 = 187.7 teaspoons or 62.5 tablespoons or 3.9 cups of fertilizer in 5 gallons concentrate.

Data for other fertilizers
        We have compiled data for several fertilizers and will be adding them as we can prepare the charts, please check back in the near future for additional fertilizers.

Please also see the following for more information:
        Water Quality Issues for Slipper Orchid Growers
        Mineral Nutrition for Slipper Orchid Growers
        Limestone Supplementation of Calcicolous Paphs.

Bob & Lynn Wellenstein
AnTec Laboratory
PO Box 65
Candor, NY 13743 USA
607 659-3330
http://ladyslipper.com
Copyright 2002 AnTec Laboratory
May be reproduced by nonprofit organizations in their newsletters if presented in its entirety.

        

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