The variations of nitrogen release behaviors from the fertilizer could be attributed to the botanical origins of starch, which impart the starch-SAP with different water absorbent properties via the size (Rg) of fractal-like gels on nano scale and the diameter of grids on micron scale in the 3D network of starch-SAPs. Specifically, an increase in fractal gel size and a reduction in grid size were preferable for increasing the water absorbing capacity with a reduced absorbing rate and thus improving fertilizer slow-release property.