Crop yields and yield potentials on Danish coarse sandy soils are strongly limited due to restricted root growth and poor water and nutrient retention. We investigated if biochar amendment to subsoil can improve root development in barley and significantly increase soil water retention. Spring barley (Hordeum vulgare cv. Anakin) was grown in soil columns (diameter: 30 cm) prepared with 25 cm topsoil, 75 cm biochar‐amended subsoil, and 30 cm un‐amended subsoil lowermost placed on an impervious surface. Low‐temperature gasification straw‐biochar (at 0, 0.50, 1.0, 2.0, and 4.0 wt%) and slow pyrolysis hardwood‐biochar (at 2 wt%) were investigated. One wt% can be scaled up to 102 Mg/ha of char. After full irrigation and drainage, the in‐situ moisture content at 30‐80 cm depth increased linearly (R2 = 0.99) with straw‐biochar content at a rate corresponding to 0.029 m3/m3/%. The lab determined wilting point also increased linearly with char content (R2 = 0.99) but at a much lower rate (0.003 m3/m3/%). Biochar at concentrations up to 2% significantly increased the density of roots in the 40–80 cm depth interval. Addition of 1% straw‐biochar had the most positive effect on root penetration resulting in the highest average root density (54% coverage compared to 33% without biochar). This treatment also resulted in the greatest spring barley grain yield increase (22%). Improving the quality of sandy subsoils has global potentials, and incorporation of the right amount of correctly treated residues from bioenergy technologies such as straw‐biochar is a promising option.

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Sustainable agricultural production on coarse sandy soil is constrained by the restricted growth of roots, and poor water and nutrient retention. Amending the soil with biochar can reduce these problems, but the processes involved are not known in detail. We investigated in the laboratory the effects of two fine‐grained gasification biochars made of straw (LTST) and other materials (LTSN) and of one fast pyrolysis straw biochar (FPST) on pore‐size distribution and soil compressibility when added to coarse sandy subsoil. Water retention and therefore pore‐size distribution were affected systematically. All biochars converted drainable pore space with pore diameters in the range 60–300 µm into water‐retaining pores of size 0.2–60 µm, which was taken as an estimate of available water capacity (AWC). Effects were linear over the whole range of biochar (0–4% by mass). The effect of LTST and LTSN on AWC (3.6% by volume per % biochar) was about 70% larger than the effect of the somewhat coarser FPST biochar (2.1% by volume per % biochar). The compression index increased linearly with biochar content without any significant effects from the type of biochar. The common least squares estimate of the slope was 21.2 kg m−3 %−1 by weight. The results reflect a strong interaction at the microscopic scale between biochar and soil rather than intrinsic properties of the added biochar. Highlights How fine‐grained biochars added to sandy soil affect water retention and soil compressibility. Such combined effects have not been investigated previously in coarse sandy subsoil. Pore space was changed from drainable to water retaining, and the soil became easier to compress. Fine‐grained biochars were more efficient than a coarser biochar in changing the pore space.

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