Mixing Fertilizer for injection into a drip system using a proportional injector.

When mixing up a solution of fertilizer for injection into a drip system the higher the concentration of the fertilizer in solution the longer the container will last before having to be refilled. If the fertilizer is inject continuously with the water using a proportional injector then the concentration of N in the water should be about 20-25ppm. The solubility of fertilizer compounds differ greatly in their solubility in water.

Solubility of Some Fertilizers (Pounds Soluble Compound in 1 Gallon of Water
ammonium nitrate 16.17
urea 9.02
ammonium sulfate 6.23
diammonium phosphate 5.74
monoammonium phosphate 3.12
muriate of potash 2.83
potassium nitrate 2.63
sulfate of potash 9.2

There will be impurities in the fertilizer that will not dissolve. So, 16 lb. of ammonium nitrate contains 5.44 lb. of nitrogen (34% N). The amount of nitrogen in ammonium nitrate can be determined from the chemical formula but the amount is less than calculated using the formula because of the impurities. The amount of nitrogen per pound of fertilizer is written on the fertilizer bag. Having added the fertilizer to water which weighs 8.6 lb. per gallon, you have 5.44 lb. of nitrogen per gallon but the percentage is now 22.% nitrogen which is the maximum concentration achievable.

16.17 lb amonium nitrate+8.6 lb of water = 24.77 lb of water+ fertilizer

concentration 5.44lb of N/ 24.77lb= 22.% nitrogen 

This translates 220,000 lb. N per1,000,000 lb. of N+H2O or 220,000 ppm of N.  If the injection ratio of a proportional injector is set to 1:500 the delivery water concentration would be 220,000/500= 440 ppm. If the dial could be set to 1:10,000 the delivery water would be 22 ppm. To set the concentration of nitrogen in the irrigation water using a dosotron set to 1:500 dilevery rate , the amount of ammonium nitrate mixed in a gal of water should be 0.3 lb which would make the concentration = (0.3*.34/ 0.3+8.6)/500= 23ppm. With the injection unit set at 1.500 a 5 gal bucket of solution will last for the time to apply 2500 gal of water. A garden hose has a flow rate of approximately 5 gpm which is around the maximum flow of a drip system connected to a outlet faucet. The flow rate can be considerable less then this if the drip system does not have many emitters connected to the line. Assuming a maximum flow rate of 5 gpm, the fertilizer 5 gal bucket will last. for 8.3 hr of running the drip system and the fertilizer can be applied throughout the total irrigation time.

Another approach is to put more than 16 lb of amonium nitrate in a 5 gallon bucket and then the concentration would be injected a 440 ppm so the injector should only run for 5% of the irrigation time and because the injector has no time control system to shut it off a selonade would be put into system to prevent injection of fertilizer after the needed time. The other solution is to only run the injector fertilizer 5% of the total irrigation time for the month. Consequently if the irrigation system runs for 30 hr per month and is irrigated three time a week or 7.5 hr a week or 2.5 hr a irrigation time then onece a month the injector should be run for 1.5 hr  or over fertilizer and run it for 2.5 hrs or one irrigation cycle. This would put 60% mor nitrogen than needed on the landscape which is about the normal fertilizer efficiency for a landscape. 


What about the other fertilizers? In 9 lb of urea there are 4.14 lb. N (46% N). Dissolving the material in 1gallon gives you 4.14 lb. of N per gallon but is now 23.5% nitrogen, not 46%. This yields 235,000 ppm solution.For ammonium sulfate:Six pounds per gallon yields 1.26 lb. N per gallon.The solution would then be 8.6% N.Per gallon this yields 86,394 ppm.