Relative effects of fertilizer nitrogen form and phosphate level on control of bedding plant seedling growth

Abstract

Fertilizers with a high proportion of nitrogen (N) in the nitrate (NO3-) form are used in the ornamental industry to promote compactness in plants. Although the common belief is that it is the high proportion of NO3- that causes compactness, these formulations also contain no or a low level of phosphate, which can also cause compactness. This study was conducted to assess the relative effects of NO3- to ammonium (NH4+) ratio and phosphate supply, as found in high NO3- fertilizers, on seedling shoot growth. A series of fertilizers was formulated in which the level of phosphate and proportion ofN in the NO3- form were varied factorially. Additionally, commercial fertilizers varying in these same two nutrient components were tested to verify the results obtained using the formulated fertilizers. Test plants included gomphrena (Gomphrena globosa L.), impatiens (Impatiens wallerana Hook. F.), petunia (Petunia×hybrida Juss.), marigold (Tagetes erecta L.), and tomato (Solanum esculentum Mill.) grown as plug seedling crops. A strong inverse relationship occurred between the supply of phosphate and extent of compactness. The relationship between proportion of N in the NO3- form and compactness was comparatively small. In most comparisons within the study, shoot size increased with increasing proportion of NO3-, contrary to common belief. These data indicate that it is the limited phosphate level in high NO3- fertilizers that accounts for compactness rather than the high proportion of N in the NO3- form.