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To mitigate water runoff from arable land, a variety of measures can be applied. One of them is the amendment of highly porous biochar to increase the water retention capacity of soils and store the runoff water. Various types of biochar are made under variable production conditions and amended to soils of different quality in a wide range of quantities. Since reported data show ambiguities due to various feedstock, pyrolysis parameters, soil characteristics, and a highly variable change in water retention capacity (WRC), robust statistical methods were applied to compare the reported data from previous studies. The results revealed that biochar increased WRC by 0.26% t ha−1 of biochar added on average. In terms of WRC, biochar produced at lower pyrolysis temperatures functioned surprisingly better than that produced at higher temperatures, which was confirmed by a decision tree made by J48 algorithm. The increase of WRC was higher with sandy soils, whereas silty and loamy soils did not show a significant increase. Biochar made from easily biodegradable, post-harvest residues such as straw or rice husk showed higher impact than others made from wood that is appreciated for its high porosity. This is a groundbreaking argument for stopping the usage of biochar for energy purposes. The high organic content in soils was the major driver damping the WRC increase. The most effective measure for increasing WRC is the application of 90 t ha−1 biochar, preferably made of an easily biodegradable phytomass that is pyrolyzed at low temperatures. The presented study compares 166 specific combinations of various biochar and soil properties, shows the effectivity of biochar amendments for WTR increment, and reviews the impact of biochar amendment in terms of water retention in agricultural landscapes.