# Soluble Elements Released from Organic Wastes to Increase Available Nutrients for Soil and Crops
## 3 Results and Discussion
### Nutrients for Crop Nutrition and Human Health  
As previously stated, K, Ca, Mg, Fe, Mn, Cu and Zn are essential nutrients for people and plants, and Na is beneficial for plants and essential for humans [29,37]. Our results indicate that it is possible to enhance the availability of all these nutrients, mainly Na, K, Ca, and Mg, in soil solution, as well as in micronutrients. Ca, Mg, Cu, Fe and Zn are one of the most lacking in the human diet [39]; therefore, if they are in optimal forms and conditions for plant uptake, this could be beneficial for human nutrition. The K needs of plants are higher than existing reserves in an assimilable form of primary elements in soil, so it is necessary to make contributions using fertilizers [17,34]. K is necessary for plant nutrition because it increases disease resistance and is involved in photosynthesis, carbohydrate metabolism and translocation of starches, seed quality and fruit formation. Humans take K for muscle and nerve activity and for proper fluid balance [5]. This element is critical for the adequate development of people.  
Ca and Mg are secondary macronutrients, and the contribution of Ca from soil solution for the growth of crops is usually sufficient [16]; however, deficiency can be frequent in soils on highly weathered tropical soils [17]. This could have serious negative implications related to harvest because Ca stimulates microbial activity, reduces plant respiration, and promotes plant growth and fruit formation. Ca ingestion is vital to ensure muscle and nerve activity, immune system health, blood clotting, pressure regulation and healthy bones [5]. Mg lack in plants is observed all over the world, aids in enzyme functionality and plant use of Fe and P, mainly on strongly acidic soils [17,36] and PP could be an interesting option due to its high solubility index (Table 7) and its almost neutral pH (Table 2). Fe, Cu and Zn deficiencies usually occur in plants growing on soils on calcareous or alkaline soils (covering 25 to 30% of land surface), especially in arid and semi-arid environments, so lowering soil pH can improve its uptake [39]. Fe, Cu and Zn are involved in photosynthesis and promote plant growth. Fe acts as an O\({}_{2}\) carrier, Cu promotes plant reproduction and fruit flavor, and Zn takes part in seed formation. The human body needs Fe to deliver oxygen to the tissues and for brain and muscle functioning; Cu for Fe metabolism; and Cu, as a component of enzymes, DNA, RNA and proteins, promotes immune system health, as well as contributes to the perception of taste [5].  
PP is the residue with the highest amount of K, Ca, Mg and Mn in aqueous extraction (Tables 5 and 6) and the one that obtains the highest solubility index for these nutrients and high solubility rates for the rest of the nutrients (Table 7). OP can provide an extra provision of Cu due to its significant concentration in soluble form and high solubility (Tables 6 and 7). SC and PN achieved the greatest amounts of Fe in the aqueous extract (Table 6). However, the Fe solubility index is 0 and 2, respectively (Table 7). Consequently, it is convenient to prioritize the use of VP, which provides the highest I\({}_{\rm Fe}\) (Table 7). Soluble Zn can be increased mainly with PP application (Table 6); although it shows a high I\({}_{\rm Zn}\), AP obtains the highest (Table 7). The importance of an efficient nutrient supply to crops is undeniable, as well as its connections to soil, ecosystems and human health [83, 84, 85].