## 4 The Impacts of Microalgae/Cyanobacteria and Their Extracts on Crops' Production
### Soil Improvement
#### 4.1.3 Soil Physical and Chemical Amendments  
Microalgae and cyanobacteria also play an important role in the recovery of damaged soils, as they are able to: (i) modulate soils' pH; (ii) control soils' salinity or protect plants from high salinity levels; and (iii) remove heavy metals and other contaminants from soils [7; 85; 97; 98]. Regarding the pH control, several studies have reported either an increase or decrease in soils' pH when using microalgal/cyanobacterial biomass. However, it is important to highlight the beneficial effect of this biomass in controlling the acidic/basic nature of different soils [7; 99]. In terms of soils' salinity, several studies have reported an improvement in plants' growth in soils presenting high salinities [97]. This improvement may be a result of (i) a reduction in the soils' salinity (by increasing the water retention capacity of soils); or (ii) an increase in plants' tolerance to high salinity levels (which may be achieved by gibberellic acid, a phytohormone commonly found in microalgae and cyanobacteria). Finally, microalgae and cyanobacteria are able to sequester trace elements (e.g., iron, zinc, copper and manganese), heavy metals (e.g., cadmium, lead and chromium), hydrocarbons and other compounds from the surrounding environment. Therefore, when applied to soils, microalgae and cyanobacteria may contribute to its remediation and to an increase in soils' quality and fertility [7; 8; 98; 100]. [PERSON], et al. [95] evaluated the potential of inactive _Spiulina platensis_ cells and phycocyanin extracts from the same species on hydrocarbons removal from a soil contaminated with diesel and biodiesel. Compared to the control, phycocyanin extracts and inactive cells have effectively removed hydrocarbons from the contaminated soils, with the extracts being more effective in biodiesel removal (88.8%) and _S. platensis_ inactive cells in diesel removal (63.9%).  
Besides their importance in the enrichment of soils with essential nutrients and in the reclamation of soil properties, microalgae and cyanobacteria can also improve some soil characteristics, such as aggregation, porosity, permeability, ventilation and humidity level [6; 7; 97; 101]. Typically, the improvement of these characteristics is associated with the polysaccharides excreted by several microalgal/cyanobacterial species [6; 97]. When excreted to the soils, these polysaccharides tend to form an adhesive and gelatinous sheath that contributes to soils' aggregation and prevents them from erosion [6]. These polysaccharides also play an important role in soils' aeration and in their ability for water retention, which is crucial to maintain soils' temperature, pH and salinity [6; 7; 97; 101]. The improvement of these soil properties significantly enhances crops' productivities, as they stimulate root growth and boost the soil microbial activity. Therefore, microalgae and cyanobacteria have been widely used in the restoration of soils' properties and in the improvement of soils' fertility. [PERSON], et al. [93] demonstrated that the resistance to wind erosion of unconsolidated sand was increased in the presence of desert soil algae, such as _Microcleus vaginatus_, _Phormidium tenue_, _Scytonema jaanicum_, _Nostoc_ sp. and _Desmococcus olivaceus_, concluding that this increase was associated with higher crusts' cohesion in the samples treated with the referred algae. Similarly, [PERSON], et al. [94] inoculated a poorly aggregated soil with _Nostoc_ strains, achieving a higher degree of aggregation in the inoculated soil when compared to the non-inoculated one.