plants Article Seedling Responses to Organically-Derived Plant Growth Promoters: An Effects-Based Approach Simon Hodge 1,2,3, * , Charles N. Merfield 1 , Wendy Y. Y. Liu 2,4 and Heng W. Tan 2,5   Citation: Hodge, S.; Merfield, C.N.; Liu, W.Y.Y.; Tan, H.W. Seedling Responses to Organically-Derived Plant Growth Promoters: An Effects-Based Approach. Plants 2021, 10, 660. https://doi.org/10.3390/ plants10040660 Academic Editors: Andrea Ertani and Ivana Puglisi Received: 29 January 2021 Accepted: 26 March 2021 Published: 30 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 The BHU Future Farming Centre, Lincoln 7640, Canterbury, New Zealand; charles@merfield.com 2 Faculty of Agricultural Sciences, Lincoln University, Lincoln 7647, Canterbury, New Zealand; wendy.liu@qiup.edu.my (W.Y.Y.L.); hwtan@stu.edu.cn (H.W.T.) 3 School of Agriculture and Food Sciences, University College Dublin, Dublin 4, Ireland 4 School of Biological Sciences, Quest International University Perak, Ipoh 30250, Malaysia 5 Department of Cell Biology and Genetics, Shantou University Medical College, Shantou 515041, China * Correspondence: simon.hodge68@gmail.com Abstract: Organically-derived biofertilizers and biostimulants, developed from harvested materials such as seaweed and waste from animal and fish processing, are currently the subject of much fundamental and applied research. These products have significant potential in reducing synthetic fertilizer inputs to horticultural, arable, and pasture-based agricultural systems, although there is frequently some ambiguity over the magnitude and consistency of any positive effects these products may have on plant performance. This study examined the effects of organically-derived plant growth promoters (PGPs) available in New Zealand on the early vegetative growth of 16 plant species maintained under glasshouse conditions. When applied as a root drench to low nutrient potting mix, the effects of the PGPs on seedling shoot growth were strongly related to the NPK contents of the applied solutions. Any positive effects on shoot growth were, on average, reduced when the seedlings were maintained in higher nutrient growing media. Applying the PGPs at concentrations twice, and four times, the recommended concentration, only caused further growth responses when the PGPs contained high levels of nutrients. Applying the PGPs as a foliar spray had negligible effects on shoot growth. Overall, the results of these trials suggest that the positive effects of applying some organically-derived PGPs on seedling growth are a function of the PGP nutrient content, and not due to any indirect effects related to phytohormone pathways or modification of rhizosphere microorganisms. Keywords: algae; biofertilizers; biostimulants; New Zealand; organic farming; plant nutrition; seaweed extracts 1. Introduction A wide range of organic supplements, fertilizers, plant growth promoters [PGPs], and ‘biostimulants’ are now available to commercial horticulturalists and home gardeners. These products are frequently based on harvested organic materials, such as seaweeds, byproducts from fish processing and animal slaughterhouses, and the composting or fermentation of humic substances and compost ‘teas’ [1–4]. There are now numerous reports and reviews in the scientific literature which describe how such products increase plant growth, improve plant health, and enhance yield and quality of the final produce e.g., [1,5–8]. These PGPs have a general appeal to organic, sustainable, and regenerative growers on multiple levels: the biological origins of the product, the reduced reliance on synthetic fertilizers and pesticides, and, often, secondary claims of improving soil properties and microbial health [1,3]. Global sales of these products are, therefore, responding positively to a growing awareness of organic farming and the environmental harm caused by intensive farming, together with legislation restricting the use of inorganic fertilizers such as urea and ammonium nitrate. It Plants 2021, 10, 660. https://doi.org/10.3390/plants10040660 https://www.mdpi.com/journal/plants