Pansies require low fertility of 100 to 150 ppm N. Optimal substrate pH values for pansies range from 5.5 to 5.8. Pansies can develop both low and high substrate pH disorders. If substrate pH drifts lower than 5.5, iron (Fe) and/or manganese toxicity occurs and is observed as lower leaf black spotting. Substrate pH values above 6.2 to 6.5 inhibits Fe availability and induce interveinal chlorosis.
Read more at e-GRO (W. Garrett Owen, Josh Henry, Brian E. Whipker)