NC State Floriculture


Low Substrate Phosphorus Corrective Procedures

Brian E. Whipker, James L. Gibson, Paul V. Nelson, Todd J. Cavins, and John M. Dole

Disclaimer: Growers should read and follow all label directions. Test the corrective procedure on a small number of plants prior to applying it to the entire crop.

Possible Causes
Corrective Procedures
The amount of fertilizer supplied is lower than the crop's nutrient demand (crops are categorized as light, medium or heavy feeders), or at a particular stage of development (actively growing plants have greater nutrient demands than young or flowering plants). 1. Increase fertilization frequency (use more frequent fertilization irrigations instead of clear water irrigations).
2. Increase fertilizer rate.
3. Avoid leaching the substrate with clear water irrigations.
4. Recheck the EC values to make sure they are within the acceptable range.
P lacking in fertilizer program (eg. no P starter charge used or dark weather feed [calcium nitrate + potassium nitrate] used which does not contain P). 1. Use a fertilizer that contains P.
2. If injecting acid to neutralize alkalinity, consider supplying P from phosphoric acid (upper limit of 10 to 15 ppm P).

Leaching too great.

1. Avoid leaching the substrate with clear water irrigations.
2. Recheck the EC values to make sure they are within the acceptable range.
Injector malfunction. 1. Check the EC of the clear water and the fertilizer solution. Fertilizer solution EC minus clear water EC = EC contribution of the fertilizer. Compare this value with the tables on the fertilizer bag to determine the fertilization rate being applied.
Fertilizer mixing rate error and too little fertilizer used. 1. Check if the amount of fertilizer used, the size of the concentrate bucket, and the proportioner setting are correct.
Don't forget to check out the Plant Root Zone Management (PRZM) Manual for more information! Want to know more? Click here!

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