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horticulturae Article First Steps toward a Test Procedure to Identify Peat Substitutes for Growing Media by Means of Chemical, Physical, and Biological Material Characteristics Katharina Leiber-Sauheitl * , Heike Bohne and Jürgen Böttcher Institute for Soil Science, Leibniz University Hannover, 30419 Hannover, Germany; [email protected] (H.B.); [email protected] (J.B.) * Correspondence: [email protected] Abstract: Due to the major environmental impact of peat-based growing media production and the need of lowering greenhouse gas emissions in all sectors, a wider application of peat substitutes in growing media is requested. All peat substitutes under use have constraints associated with their properties. Therefore, a preliminary test procedure for identifying new raw materials as peat substitutes in growing media was developed and validated. By applying the preliminary test procedure, the potential limitations of cultivation of potential peat substitutes are indicated, and measures for cultivation regulation are recommended. For the development of the new preliminary test procedure, four raw materials were investigated: composted heather, alder, cattail, and reed. The preliminary test procedure comprises several material and technological criteria as well as aspects of plant cultivation, enabling the evaluation of the raw materials and the processed components for growing media. Results derived from the preliminary test procedure were checked and confirmed by Citation: Leiber-Sauheitl, K.; Bohne, H.; Böttcher, J. First Steps toward a experiments with horticultural crops in different sections of commercial horticulture. Within two Test Procedure to Identify Peat years, the identification of new peat substitutes was possible by the application of the preliminary test Substitutes for Growing Media by procedure and its test criteria, which provide a structure for the systematic investigation of potential Means of Chemical, Physical, and new peat substitutes starting with the raw material. Biological Material Characteristics. Horticulturae 2021, 7, 164. https:// Keywords: peat substitute; growing media; decision tree; feasibility; heather; cattail; reed; alder; doi.org/10.3390/horticulturae7070164 peat reduction Academic Editors: Nazim Gruda and Juan A. Fernández 1. Introduction Received: 20 May 2021 For several decades, peat has been the most important component for growing me- Accepted: 22 June 2021 Published: 28 June 2021 dia [1] due to its ideal chemical, physical, and biological properties in plant cultivation [2]. Globally, about 90 million m3 of peat are produced per year from which 40 million m3 are used in horticulture [3,4]. Starting as a by-product, it is often to be found as the only Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in component in a growing medium. The search for peat substitutes started in the 1980s published maps and institutional affil- due to rising awareness of the importance of peatland conservation especially focusing iations. on nature conservation [1]. Recently, peatlands came into focus in the light of climate conservation due to the large stocks of carbon contained in peatlands. At present, several peat substitutes are already under use such as coconut fibers, wood fibers, compost, and many other [5–10]. However, there are constraints for each of these peat substitutes that limit their usage in large quantities, or the available amounts of these peat substitutes Copyright: © 2021 by the authors. are limited (e.g. [2,11]). Therefore, new materials for growing media have to be found, Licensee MDPI, Basel, Switzerland. This article is an open access article which first of all have to be tested if they meet the material requirements. Up to now, distributed under the terms and a lot of plant trials with different raw materials and different combinations of growing conditions of the Creative Commons media components were conducted (e.g. [12]). First, these trials are cost intensive and Attribution (CC BY) license (https:// often cannot directly be compared to each other since e.g., experimental conditions differ. creativecommons.org/licenses/by/ Second, due to the mixture of three or more substrate components in plant trials, results 4.0/). cannot be clearly attributed to a single substrate component. Due to this, it is often not clear Horticulturae 2021, 7, 164. https://doi.org/10.3390/horticulturae7070164 https://www.mdpi.com/journal/horticulturae Horticulturae 2021, 7, 164 2 of 18 if a raw material per se is not applicable or if it might be promising to specifically modulate some of its unfavorable characteristics by for instance another processing or mixing with other materials. Due to these reasons, a new test procedure for raw materials focusing on material characteristics was developed, the application of which makes the trials of different ma- terials tested as growing media components comparable. Several raw materials (heather, cattail, reed, and alder) were investigated by this new test procedure, and their suitability as a growing media component was evaluated. By using a hierarchical combination of test criteria, the resulting test procedure helps to identify completely unsuited substrate com- ponents with simple tests at an early stage of the procedure and gives enough information on problematic characteristics of more promising candidates at later stages so that it can be decided if these can be overcome technically. Various chemical, physical, and biological analyses are applied at different steps of the test procedure starting with material and simple substrate analyses of a raw material, continuing with extended substrate analyses on the chemical and physical properties of a processed raw material and ideally finishing with the plant test under practical conditions. The possible limitations of properties of potential peat substitutes, such as the water-holding capacity and N immobilization, are indicated, and measures for plant cultivation regulation are recommended. In the future, the test procedure needs to be extended by economic and ecological criteria such as availability studies, cost effectiveness, and life cycle analyses. The selection of the raw materials under investigation created synergy effects among climate protection, nature conservation, and the interests of agriculture and horticulture. These raw materials were derived from landscape conservation of a terrestrial site (heather maintenance) and from water affected or rewetted sites (reed, cattail, alder). The new raw materials tested here were selected according to a possible cultivation on rewetted peatlands as paludiculture crop or in order to utilize material from landscape conservation. It was not possible to include availability aspects in the investigation. The replacement of peat in horticulture needs to be achieved by applying a broad set of peat substitutes of which each may be available in smaller amounts only. The objective of our study is to demonstrate the feasibility of the first steps of a test procedure for peat substitutes by presenting the results of several raw materials applying the test criteria regarding material aspects. In the future, economic and ecological aspects need to be added to the test procedure. We hypothesize that the test procedure enables a true-proof identification of peat substitutes for growing media in horticulture. 2. Materials and Methods 2.1. Test Procedure and Test Criteria In general, the test procedure starts with simple and quick tests and only if these are passed through successively, more complex and time-consuming tests are performed in the following steps. The aim is to assure that the materials show a high probability being a new peat substitute but also to identify and sort out unsuitable materials as early as possible. Furthermore, especially, the more complex tests will first give information regarding how to handle the material in practice. The test procedure for new components of growing media involves several consecutive test criteria, which are applied successively (Figure1). The investigation of a new raw material using the proposed test procedure is performed in two consecutive years or seasons, which are called stage 1 and stage 2. In stage 1, test criteria I–IV are applied to the same batch of raw material resulting in a first rating of the horticultural suitability in laboratory and greenhouse experiments. In stage 2, steps I to IV of the test procedure have to be applied again with a new batch of the raw material in order to check reproducibility of the results of stage 1. The implementation of adaptations, e.g., in the processing of a raw material or fertilization strategies are possible. Finally, stage 2 complements experiments under laboratory and greenhouse conditions by experiments under practical conditions (test criterion V). Horticulturae 2021, 7, x FOR PEERHorticulturae REVIEW2021 , 7, 164 3 of 19 3 of 18 Figure 1. DecisionFigure 1. Decisiontree for treetesting for testing raw materials raw materials as aspotential potential components components for for growing growing media. media. First, test criterion I (TC I) comprises material and simple substrate analyses of the not The investigationprocessed of a new raw material, raw material which include using the the determination proposed oftest nutrient procedure contents is (% per- N, P, K in formed in two consecutivedry matter) years as well or asseasons, C/N ratio, which volume are weight called (g stage L−1), pH,1 and and stage salt content

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