# Soluble Elements Released from Organic Wastes to Increase Available Nutrients for Soil and Crops
## 3 Results and Discussion
### Physical and Chemical Characteristics of Wastes
Another factor is that increases in temperatures in summer affect soil microbiota, which reduces mineralization [63]. One of the key factors that determines soil agricultural potential, supply and nutrient solubility is soil pH, since it affects microbial activity, cation exchange reactions related to soil aggregation and the mobility of heavy metals [15]. Most of the residues studied (Table 2) have an acid pH (between 4.66 and 5.94), except SC which has a pH of 7.34. This result agrees with [PERSON] et al. [15], who state that the pH range of compost is between 7 and 9 (alkaline). SC can be an interesting option for managing acid soils, given that more than 40% of arable soils in the world are acidic [36]. Nevertheless, incorporating food waste compost into the soil can increase soil pH [59; 69] or decrease soil pH with PN [57]. Based on the work of [PERSON] et al. [56], needle pH can vary according to species from 4.00 to 5.32. The result obtained for PN pH (Table 2) is above this range (5.94). Therefore, based on [PERSON]'s et al. conclusions [56], these data confirm the acid reaction of pine needles, which can have consequences on the mobility of heavy metals. Results agree with those obtained by [PERSON] et al. [57]; soil pH near _Pinus halepensis_ showed a slight decrease related to litter decomposition. In addition, the allelopathic potential of PN should be kept in mind, as it can affect plant growth [70]. Moreover, the allelopathic interactions between crop residues and aqueous extracts can enhance or affect nutrient availability and yield crop [71]. The most suitable soil pH to enhance nutrient absorption by plants is usually close to 6.5 [72]; PN has a pH closest to that value (Table 2). In any case, PP, OP and AP have lower pHs than PN. Most olive orchards are grown on calcareous soils, with a pH higher than 7.0 [16], so to increase microelement availability, its pruning wastes (OP) and AP residues application might reduce soil pH. PP has a pH of 4.90 (Table 2), while data on date palm compost shows a pH of 8.38 and 7.6 [52; 53]. Previous work managed to reduce soil pH after applying date palm leaves compost [73]. The pH value achieved for VP (5.42) (Table 2) is like that (5.83) reported in other works [40], which led to a decrease in soil pH after the first year of application on alkaline soil. Therefore, AP, CP, OP, PN, PP and VP could be incorporated mainly in alkaline soils to control the pH. The proper choice of waste is key for crops since variations in soil pH can modify the availability of secondary macronutrients, micronutrients and trace elements that could be presented in the organic wastes and adsorbed in soil surfaces [12; 16; 28; 72]. Among the residues considered, PP and SC (Table 2) have the highest EC. AP and PN showed the lowest EC. EC from SC is higher compared to data provided by [PERSON] et al. [28] and like the one obtained by [PERSON] et al. [43] and considerably lower than the results obtained by [PERSON] et al. [61]. This means there is high variability in SC depending on the origin of wastewater and the treatments applied to wastewater and sludge. [PERSON] et al. [28] measured the increase in soil conductivity by application of SC, increasing soil EC according to the amount of applied SC from 750 \(\mu\)S cm\({}^{-1}\) to more than 1577 \(\mu\)S cm\({}^{-1}\). Despite this increase, they consider it far from a risk of salinization (\(\geq\)4000 \(\mu\)S cm\({}^{-1}\)). However, changes in soil pH, exchangeable sodium percentage and sodium adsorption ratio, among others, should be considered to assess crop vulnerability. In this way, PP (EC: 4358 \(\mu\)S cm\({}^{-1}\)) should be applied with caution in soils since it could increase EC, and soils can be salinized as well as SC (EC: 3380 \(\mu\)S cm\({}^{-1}\)) although EC is less than 4000 \(\mu\)S cm\({}^{-1}\).