Resources 2021, 10, 102. https://doi.org/10.3390/resources10100102 www.mdpi.com/journal/resources Article Distribution of Phosphorus Forms Depends on Compost Source Material Marge Lanno 1, *, Mait Kriipsalu 2 , Merrit Shanskiy 1 , Maidu Silm 1 and Anu Kisand 1 1 Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, 51006 Tartu, Estonia; merrit.shanskiy@emu.ee (M.S.); maidu.silm@emu.ee (M.S.); anu.kisand@emu.ee (A.K.) 2 Institute of Forestry and Rural Engineering, Estonian University of Life Sciences, 51006 Tartu, Estonia; mait.kriipsalu@emu.ee * Correspondence: marge.lanno@emu.ee Abstract: Composting is a sustainable method for recovering nutrients from various organic wastes, including food waste. Every input waste has different nutrient contents, in turn, suggesting that every compost has different fertilizer and/or soil improvement values. The phosphorus (P) concen- tration and relative distribution of P forms is related to the original organic material. The relative distribution of P forms determines how readily plants can absorb P from the compost-amended soil. The aim of this study was to investigate the content and relative share of P forms in composts made from fish waste, sewage sludge, green waste, and horse manure. Six forms of P (labile; bound to reducible metals; bound to non-reducible metals; bound to easily degradable organic material; and bound to calcium) were determined using sequential extraction method. The results indicated that fish waste compost had relatively high proportion of labile P, suggesting good biological availabil- ity. In comparison, sewage sludge compost contained the highest overall P concentration per dry weight unit, while labile P constituted only 6% of summary of P forms. The results indicate that the evaluation of composts as alternative P sources in agriculture should rely on the relative distribu- tion of P forms in the compost in addition to the typically recognized value of the total P. Keywords: organic fertilizer; sequential fractionation; fish waste; manure; sewage sludge; green waste; circular economy 1. Introduction With the growing human population, the need for more sustainable agriculture and food production is increasingly essential. Likewise, as the population grows, the need for fertilizers increases. To ensure the higher crop yield, nitrogen (N) and phosphorus (P) fertilizers are widely used [1]. In general, P is added to the soil with chemical fertilizers. As phosphorite is mined from Earth’s crust, our natural P resources become increasingly limited [2,3]. Similarly, significant quantities of P are being lost through the entire cycle of organic wastes. Fur- thermore, indiscriminate application of chemical fertilizers can exhaust the soil in the long term [4,5]. P run-off can result in environmental pollution and eutrophication in water bodies. To avoid increased P concentrations in streams and lakes, it is important that P added with fertilizers is assimilated by plants [2,4,6]. Alternative P sources are needed to ensure sustainable crop production. To close the P cycle and promote food security and environmental integrity, the European Commis- sion has announced the preparation of an Integrated Management Action Plan (INMAP) with the purpose of reducing the nutrient losses and stimulating the market for recovered nutrients [7]. Options for reducing P losses include the redistribution of P inputs, P recov- ery and recycling, and redefining P in the food web [8,9]. Organic materials that could Citation: Lanno, M.; Kriipsalu, M.; Shanskiy, M.; Silm, M.; Kisand, A. Distribution of Phosphorus Forms Depends on Compost Source Material. Resources 2021, 10, 102. https://doi.org/10.3390/ resources10100102 Academic Editors: Konstadinos Abeliotis and Katia Lasaridi Received: 31 August 2021 Accepted: 5 October 2021 Published: 8 October 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (http://cre- ativecommons.org/licenses/by/4.0/).