Fertilizer Calculations: Understanding Parts per Million

While teaching greenhouse and nursery management classes, I have found that students often have the most trouble learning how to calculate fertilizer and plant growth regulator ratios. All of these are typically based on parts per million or ppm. Most fertilizer bags and PGR labels have all the calculations printed on them, but often a refresher on the calculations is in order.

Parts Per Million

The use of liquid feed fertilization programs in greenhouse and greenhouse crop production is the standard of our industry. Many growers use either a constant feed program fertilizing with each irrigation, while others use a pulse feed program fertilizing on a regular periodic schedule. The program selected is determined by crop requirements, available equipment, and personal preference. The most important concern, no matter which program is used, is accuracy in calculation of fertilizer concentrations.

Crop nutrition requirements and most published fertilizer schedules use the terminology "parts per million" or ppm. There are fertilizer tables provided by most fertilizer producers for easier reference. With a little knowledge, a calculator, and patience, the tables are not necessary.

Many growers are familiar with the Quick "75" Method for calculating ppm. To calculate the amount of fertilizer required, divide the desired ppm by 75 and then divide by the decimal fraction of the desired nutrient (such as nitrogen, potassium or phosphorous) contained in the fertilizer. This results in the number of ounces of fertilizer to use in 100 gallons of water.


To use this equation, assume that the fertilizer recommendation calls for 200 ]ppm of nitrogen from ammonium nitrate (33% N)-Using the above equation, divide 200 ppm by 75 resulting in 2.67,and then divide by 0.33. The answer is 8.09 ounces of ammonium nitrate which dissolved in 100 gallons of water will yield 200 ppm nitrogen.


Confused? Some examples of what one part per million represents under various conditions are: 1 crystal of salt in 5 lbs., 1 drop in 16 gallons, 1 inch in 158 miles, 1 minute in 1.9 years, 1 pound in 500 tons, and 1 cent in $10,000. Therefore, to calculate ppm in 100 gallons of water, first multiply 100 gallons by 8.34 pounds per gallon which equals 834 pounds. Multiply 834 pounds by 16 ounces per pound which equals 13,344 ounces per 100 gallons. Therefore, 13,344 ounces per 13,344,000,000 ounces equals 1 part per million, or more simply 0.013344 ounces per 100 gallons of water equals 1 PPM.

1. 100 gal. * 8.34 lbs./gal. = 834 lbs.


2. 834 lbs. * 16 oz/lb. = 13,344 oz.


3. 3,344 oz/100 gal.


4. 13,344 oz./13,344,000,000 oz. = 1 PPM


5. 0.013344 oz./100 gal. = 1 PPM
   
   

The next step is to multiply the desired PPM by 0.013344, which is 74.94. By rounding 74.94 to 75, it must by understood that the result will not be entirely nor mathematically accurate, but perhaps is close enough for practical purposes. Within the units of the ratios commonly used in fertilizer solution, the error will be 0.02 or less per 100 gallons.

For those who can think in metric terms, there is an easier way to calculate PPM By definition, 1 milliliter (ml) of water weighs 1 gram (g), therefore 1 liter (1000 ml) weighs 1000 g. Thus 1 liter (L) of water weighs 1,000,000 milligrams (mg). This tells us that 1 PPM equals 1 mg/1,000,000 mg of water or 1 mg/L of water. To calculate PPM in liters, simply multiply the desired PPM by 1 and divide by fraction of the fertilizer. This results in the number of mg of fertilizer to use in 1 L of water.


To illustrate this equation, use the same fertilizer recommendation as before, 200 PPM N from ammonium nitrate (33%N). Substituting in the above equation, multiply 200 by 1 resulting in 200, and then divide by 0.33. The answer is 606 mg of ammonium nitrate, which dissolved in 1 L of water will yield 200 PPM nitrogen.


To increase this to irrigation volumes, multiply this result by the required volume. Multiply 606 mg by 380 L (100 gal) which equals 230,280 mg or 0.507 pounds (8.12 oz).

Time to Check Your Heating Systems

In Colorado, it has been a beautiful indian summer, yet blowing snow and sub-freezing temperatures are just around the corner. It is time to make sure that your heating systems are ready to function efficiently.

Greenhouse growers who use boiler systems typically have had all their annual inspections and have completed their annual maintenance, but growers who rely on gas fired unit heaters are often not quite so conscience of the conditions of their heaters. It is way past time to inspect your heaters. Regular maintenance will easily pay for itself this season with high gas prices predicted.

Inspect the flue pipe. Wind is responsible for most damage to flue pipes, however, one can always expect some degradation of the joints in the greenhouse. Check for rusting unions and for any debris that may have collected in the flu pipe. If the flue pipe has an exhaust fan to move exhaust gasses through the flue, make sure that it is operating properly.

Inspect the heat coils. Greenhouses are humid environments and metal equipment is subject to rust. If the heat coils are rusted through, exhaust gasses can contaminate the greenhouse environment.

Inspect the gas manifold. Dirty gas orifices will cause incomplete combustion of the fuel, which will result in exhaust gasses that will contaminate the greenhouse environment.

Check the ignition modules and gas valves. These devices do wear out and require periodic service. These devices should be inspected and serviced by a licensed technician. Inefficient operation of gas fired unit heaters can lead to a lot of problems in the greenhouse. Primarily we think of carbon monoxide, which is deadly to the staff, but one must also think about ethylene gas as well.

Ethylene levels as low as 20 ppb (that is parts per billion) have been shown to damage Cattleya species and 500 ppb are sufficient to cause flower abortion in tomatoes. Concentrations of 50 ppb for extended periods (how long? two to four hours) are just as deleterious as high concentrations.

There has been some interest in using carbon monoxide (CO) detectors for estimating ethylene in a greenhouse. In Holland they have studied these detectors. They conclude that the ethylene level would be less than 0.1 of the critical 50 ppb if the CO content of the undiluted flu gasses did not exceed 50 ppm. The presence of CO, however, does not guarantee the presence of ethylene and vice versa. But they are cheap. Some growers use tomato plants underneath their unit heaters and if the leaves exhibit epinasty, they assume that there is ethylene contamination. Tomato plants are typically more sensitive than other floriculture crops to ethylene. (you can read more in Dr. J.J. Hanan's text, Greenhouses: Advanced Technology for Protected Horticulture)

Prevention is the key to ethylene gas control in the greenhouse. Maintain your gas-fired heaters in good condition. Clean the manifolds regularly and check for cracks in the heat exchangers. Flue pipes must be clean and free of debris. They must also have the correct clearance over the building if they are not connected to a forced air exhaust system. IR-radiant heat systems are not immune to ethylene contamination. Mount the exhaust fans as close to the end of the flue as possible to prevent any back pressure from a prevailing wind. Finally, make sure that your gas supply is adequate for the unit heater and that you are supplying adequate oxygen for combustion.

If you suspect that you have an ethylene gas problem, contact the Floriculture faculty at North Carolina State University for testing.

Greenhouse fuel economy during tough times

Heating greenhouses this next winter will remind many growers of fuel costs during the 1970s during the Arab Oil Embargo. I have found reports from the Department of Energy that forcast the price of natural gas to be at $12.31 per dekatherm. If we have a cold winter, there may be gas shortages. During September, Hurricanes Gustav and Ike shut down 32 million barrels of crude oil and 165 billion cubic feet (Bcf) of natural gas production in the Federal Gulf of Mexico. Recovery is ongoing and expected to continue at least through October.

According to the National Oceanic Atmospheric Administration’s (NOAA) most recent projection of heating degree-days, the Lower-48 States are forecast to be 2.4 percent colder this winter compared with last winter, but 1.7 percent warmer than the 30-year average (1971 to 2000). However, regional heating degree-day projections vary widely; for example, the West North Central region is projected to be almost 5 percent warmer than last winter.

What is the greenhouse owner/manager to do? Many buy gas on the open market as a coopertive to lock in prices. Others negotiate directly with their gas provider. Rregardless of what fuel you use and your fuel prices, we still need to make cropping decisions. Do we grow Easter lilies or just grow spring bedding plnats. Do we put in quick turn crop of cut flowers for Easter or do we start our own plugs and cuttings. These are just a few decisions a grower must make.

To ease some of these decisions and relate them to fuel, Jonathan FrantzUSDA-ARS@utoledo.edu of the USDA-ARS in Toldeo, Ohio has put together a modeling system that you can access help you make your decisions. Below is a discription of the product from the USDA-ARS website.

Virtual Grower is a decision support tool for greenhouse growers. Users can build a greenhouse with a variety of materials for roofs and sidewalls, design the greenhouse style, schedule temperature set points throughout the year, and predict heating costs for over 230 sites within the US. Different heating and scheduling scenarios can be predicted with few inputs.

You can download the current version of Virtual Grower at: http://www.ars.usda.gov/services/software/download.htm?softwareid=108

The Commercial Greenhouse, 3E

Finally, after more than two years of work, the third edition of The Commercial Greenhouse, 3E has been released. I was honored to have been asked to provide the updates of Dr. James Boodley's classic textbook on greenhouse production. I have included many new photographs and updated the material where relevant. If you liked the first two editions, the third is a must. Here is what the editors say:

The Commercial Greenhouse, third edition, is a complete reference for the modern commercial greenhouse grower, educator, and student. The book is a complete reference on greenhouse systems and technologies and the science of growing crops. The third edition is supplemented by new color photographs that provide modern images of greenhouse production, as well as updated information on pesticides, including updates on soil sterilants, and a new section on integrated pest management, that provides tangible guidelines for greenhouse operations. The clear and concise presentation of fundamental concepts in The Commercial Greenhouse, third edition makes this text a must for every greenhouse professional's shelf.

You can find it online at any of your favorite booksellers. Look for ISBN-13: 978-1-4180-3079-7 .