Producers of floriculture crops strive to cultivate compact and healthy plants that are considered high quality and attractive for consumers. The fertilization regimen has a significant role in the ultimate appearance and robustness of a crop. Many commercial fertilizers mixed at recommended concentrations for greenhouse production supply greater phosphorus (P) concentrations than required by plants, as is the case with 20 nitrogen (N)–8.7P–16.6 potassium (K). This fertilizer mixed at a concentration of 200 mg·L−1 N would provide 87 mg·L−1 P. One recommendation for greenhouse crops by McMahon (2011) suggests using concentrations of only 5–10 mg·L−1 P. Other studies have indicated that P concentrations of just 0.093–1.5 mg·L−1 can keep floriculture crops healthy yet compact (Borch et al., 1998; Hansen and Nielsen, 2000, 2001). This disparity among P recommendations and P concentrations supplied in common fertilizers brings into question what level of P is required to produce healthy floriculture crops.
Recent research investigated the minimum P concentrations required by herbaceous ornamentals (Borch et al., 1998; Hansen and Nielsen, 2000, 2001; Nelson et al., 2012). These studies were focused on the potential of using low P fertilization to control plant growth. Although the nitrate (NO3−) form of N has often been used to keep plants compact, it is the low P levels in high NO3−–N fertilizers that are responsible for compactness (Nelson et al., 2012). Most NO3− based fertilizers recommended for compact plant growth are also low in P. Erroneously, it was thought that fertilizer formulations high in ammoniacal nitrogen (NH4+) result in greater plant growth. Experiments conducted by Nelson et al. (2002) used constant ratios of N source, but varied P concentrations, and found that plant size increased with increasing P concentrations. Previously, it was thought that higher P concentrations would only increase growth until plant P concentration reached 0.25% of total dry matter (Nelson et al., 2002).
Potential issues with very low P fertilization result from the fact that the soilless substrates used in floriculture production have limited P holding capacity (Marconi and Nelson, 1984). Without adequate P, crops have the potential to deplete the initial P concentration in the substrate and may begin reallocating P from older plant tissues, leading to the development of deficiency symptoms on the lower leaves (Mengel et al., 2001). Deficiency symptoms associated with P are commonly observed when dry plant tissue comprises <0.2% or 2000 mg·kg−1 P (Mills and Jones, 1996). Typical symptoms are often described as a reddening or purpling of the lower foliage, an overall darker green coloration, stunted growth, delayed flowering, and greater root lengths (Epstein and Bloom, 2005; Marschner, 1995; Mengel et al., 2001). For leaf tissue concentrations, a range of 0.2% to 0.5% of total plant dry weight is considered sufficient P for most plants (Mills and Jones, 1996).
This study aimed to determine the P concentration required by several floriculture species to optimize growth. Determining optimal P concentrations will provide improved grower recommendations and limit commercial fertilizer waste.
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