Sandy soils are characterised by low water and nutrient retention and thus are considered marginal for agricultural production. However, with increases in global population, continual efforts are required to increase agricultural production. Thus, improving productivity of sandy soils is in line with United Nation´s sustainable development goal 2: End hunger, achieve food security and improved nutrition and promote sustainable agriculture. The potential for increasing crop production on sandy soils may be achieved with the use of biochar as a soil amendment. Therefore the overall objective of this study was to investigate how biochar affects sandy soil properties in two contrasting agroecological zones; temperate and tropical, and its potential to ameliorate plant abiotic stresses such as drought and salinity. To ahieve this, four major experiments were conducted in Denmark and Ghana. In Denmark, two-greenhouse pot experiments were conducted with maize whereas, two field experiments were conducted with shallot in the Keta Sand Spit of Ghana. In both places, planting of crops was carried out a few days after biochar application to soil (fresh biochar) and again after the biochar had aged for a year (aged biochar). Another aspect forming part of the study was to develop a new and affordable tool (Rootsnap sensor) for observing growth of roots in situ. The Rootsnap sensor was used to investigate the effect of fresh biochar on maize root growth and root development of grass in a minor experiment.

The soil properties that were investigated varied slightly between the two locations due to different constraints. General properties such as water retention and bulk density were measured at both locations. In addition, site-specific properties, which were of specific relevance to each study area, were also investigated. For instance, subsoil mechanical resistance to root development were investigated in Danish subsoils, whereas in the Keta Sand Spit, Na adsorption capacity was of more interest due to the use of saline groundwater for irrigating vegetables. Crop phenological and physiological processes were measured during and after the experiments. In Denmark, maize response to four concentrations of wheat straw biochar (0, 1, 2 and 3%) incorporated in the subsoil from 20 to 50 cm depth was investigated under two irrigation schemes; full irrigation and drought. In Ghana, rice straw biochar was applied at two concentrations (0 and 3%) to investigate its effect on dry season shallot cultivated under full irrigation with saline groundwater.

The results indicate that the application of wheat and rice straw biochar improved sandy soil properties such as water retention, decreasing bulk density and increasing porosity in both study areas. In Denmark, wheat straw biochar reduced the arrival time of roots to the 50 cm depth as observed by the rootsnap sensor, suggesting a reduction in subsoil mechanical resistance to root penetration. Furthermore, maize physiological processes such as photosynthesis; leaf water potential and transpiration were enhanced during drought. In Ghana, rice straw biochar increased soil cation exchange capacity, which enhanced the Na adsorption capacity of soil and could theoretically adsorb the sodium load from the irrigation water. However, the positive effects observed in soil and crop physiological processes with fresh biochar did not translate into significant improvement of crop yields in both study areas. This was due to suspected phytotoxic effect of fresh biochar exhibited as negative root geotropism in maize and malformed bulbs in shallot. With aged biochar, the positive effects on soil properties where maintained after one year indicating that the effect of biochar may be durable at least for some time. Furthermore, there was no observed negative effect on either of the crops. While there was no significant effect of aged biochar on maize under full irrigation, the highest biochar concentration significantly increased yield under drought conditions. With shallot, there was no significant effect with aged biochar although the Na adsorption capacity of the soil increased with ageing.

The findings of this study suggest that biochar can improve the physico-chemical properties of sandy soils in both temperate and tropical regions. However, the mostly negative effect on crop growth observed with fresh biochar suggests the need to allow phytotoxins from fresh biochar to dissipate before usage. The positive effects seen with aged biochar suggests that benefits may take time to accrue as seen with other studies showing incremental positive effects with time. The significant increase of maize yield under drought conditions shows biochar has the potential of ameliorating drought stress in the short-medium term. Under irrigated conditions, short and medium term benefits such as possible savings of irrigation water and fertilizer amounts need to be further explored. The findings also highlights the need to combine management measures with biochar amendment in efforts to ameliorate salinity stress. From an environmental conservation perspective, benefits such as carbon sequestration, reduced nutrient leaching and reduction in agriculture water use may be obtained, however its adoption in commercial cropping system may require some incentive to farmers at least in the short to medium term.