## 4 The Impacts of Microalgae/Cyanobacteria and Their Extracts on Crops' Production
### Soil Improvement
#### 4.1.2 Nutrients' Availability in Soils  
The use of microalgae and cyanobacteria in soils can also increase the availability of other essential nutrients for plants. As photosynthetic microorganisms, microalgae and cyanobacteria uptake nitrogen and phosphorus from different environments (even from sites where these nutrients are limiting) and store them in their biomass. As these nutrients are essential for the development of higher plants, microalgal/cyanobacterial \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mode of Action** & **Alage/Algal Extracts** & **Target Crop/Soil** & **Observed Improvements** & **Ref.** \\ \hline \multirow{8}{*}{\begin{tabular}{} \end{tabular} } & \multirow{8}{*}{\begin{tabular}{} \end{tabular} } & \multirow{8}{*}{\begin{tabular}{} \end{tabular} } & Increase in zinc, iron, copper and manganese \\  & & & Availability in the soil \\  & & & Increase in organic carbon content in the soil \\  & & & Increase in grain yield \\  & & & Improvement of plant nutritional value (increase \\  & & & in grain micronutrient contents) \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \hline \multirow{8}{*}{\begin{tabular}{} \end{tabular} } & \multirow{8}{*}{\begin{tabular}{} \end{tabular} } & \multirow{8}{*}{\
\begin{tabular}{} \end{tabular} } & \multirow{8}{*}{
\begin{tabular}{} \end{tabular} } \\  & & & Increase in nitrogen availability in the soil \\  & & & Availability in the soil \\  & & & Increase in organic carbon content in the soil \\  & & & Increase in grain yield \\  & & & Improvement of plant nutritional value (increase \\  & & in grain micronutrient contents) \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Cont. biomass can be considered as an important source of these nutrients, avoiding the use of the traditional chemical fertilisers [4; 8; 97]. Other essential microelements for plants' growth and development (e.g., potassium, magnesium, sulphur and iron) can also be found in microalgal/cyanobacterial biomass. These elements are typically involved in redox reactions, playing an important role in plants' metabolism. Therefore, the use of microalgae and cyanobacteria in soils can improve the availability of both macro- and micronutrients for plants' growth. This improvement in nutrients' availability has already been reported in the literature [88; 89; 91; 92]. [PERSON], et al. [88] applied microalgal-bacterial floos and \(N\). _culata_ biomass to a tomato culture and evaluated their impact on soil composition and fruit development. The authors concluded that the presence of the photosynthetic organisms increased the availability of nitrogen, phosphorus and potassium in the soil and improved the fruit quality, by an increase in the sugar and carotenoids contents. However, a lower tomato yield was obtained when compared to the one achieved using an inorganic fertiliser. In the study performed by [PERSON], et al. [89], the use of cyanobacterial-bacterial biofilms and consortia to okra crops resulted in an increase in zinc and iron contents in the soil and contributed to an increased root yield and weight. More recently, [PERSON], et al. [92] evaluated the influence of applying a _Chlorella sorokiniana_ culture in a vineyard soil on its composition, concluding that the presence of this microalgal increased the concentration of nitrogen in the referred soil. Microalgae and cyanobacteria can also act as a source of organic matter to higher plants, which should be supplied in adequate concentrations for a proper plants' growth [96; 97]. Firstly, these photosynthetic microorganisms fix atmospheric CO\({}_{2}\), converting it into organic biomass. Moreover, these organisms tend to excrete extracellular polymeric substances, increasing the organic carbon levels in soils and promoting the growth of other microorganisms.