# Soluble Elements Released from Organic Wastes to Increase Available Nutrients for Soil and Crops
## 1 Introduction
Plants are composed of 20 basic elements: C, H, O, N, P, K, Ca, Mg, S, Fe, Mn, B, Mo, Cu, Zn, Cl, Na, Si, Co, and Ni that are provided from soil solution (except for C and O) [37]. Thirteen of these elements (N, P, K, Ca, Mg, S, Fe, Mn, B, Mo, Cu, Zn, and Cl), apart from oxygen, carbon and hydrogen, are considered essential for the growth of all crops [28; 38]. In addition, Na, Se, Co, Al, Ni and Si are beneficial for plant growth and Na, Se, and Co are essential for mammals [29]. [PERSON] and [PERSON] [39] consider Fe, Zn, I, Se, Ca, Mg and Cu the mineral elements that are most lacking in human nutrition. In addition, worldwide, over 2 billion people are suffering from micronutrient deficiencies, and over the last 70 years, nutrients and vitamins in food have drastically decreased [5]. It also happens in developing countries due to over-cultivation on soils with reduced phyto-availability of essential elements to human nutrition. The release of nutrients in soil occurs from the solubilization of the parent rock or sediments and from the mineralization of organic matter. To benefit crop yield, the soil nutrients available and absorbed by plants must be presented in soluble forms in the soil solution [36]. The speed of waste decomposition and nutrient release is important for sustainable crop management practices, mainly influenced by climatic conditions (temperature and humidity), soil quality (properties, microbiological activity and aeration rate), residue composition (for instance: biochemical composition, nutrient concentration and type of structure: lignified or not), and the application method (more or less direct contact of residue with soil, above or underground), as well as recycling treatment (drying, composting, pyrolysis among others), and residue size and storage method [11; 13; 20; 23; 27; 40]. Moreover, [PERSON] et al. [16] found that seasonality (in which residues are applied) is important, especially in Mediterranean climates, to take advantage of the leaching effect of nutrients from rainfall at the end of winter and spring. The decomposition of organic matter occurs in two phases, an initial one characterized by the washing of soluble compounds and nutrients and by decomposing of labile materials (sugars, phenols, starch and protein), followed by decomposing of recalcitrant materials (cellulose, hemicellulose, tannins and lignin) [20; 42]. Although [PERSON] et al. [25] highlight the importance of organic residues to increase crop productivity, the bioavailability of micronutrients in soil, as well as macronutrients absorption by plants, [PERSON] [41] contemplates that not all nutrients in biofertilizers are immediately available, nor when they can become so. This initial contribution of soluble elements has been scarcer studied [23; 43; 44]since most references related to nutrients of residues focus on the decomposition of labile or recalcitrant matter. The objective of this research is to assess physical and chemical properties and nutrient solubility from several organic residues. From each organic waste, elemental composition and aqueous extractable content were studied related to needed nutrients for people and plants (Na, K, Ca, Mg, Fe, Mn, Cu and Zn). Therefore, we deal with a first approximation of the waste potential for formulating technosols considering nutrient supply. Knowing short-term soluble nutrients from pruning residues allows farmers to add the residues to the soil under the nutritional request of their crops.