Based on the structural characteristics of the polyfunctional groups of humic acid (HA), a HA water-retaining agent (HAA) was prepared by the polymerization of an aqueous solution using potassium humate, acrylic acid, sodium carboxymethyl cellulose, and bentonite. HAA was obtained by the polymerization of an aqueous solution using an assembly ligand, and a polyurethane (PU) sponge was used as a carrier to mix HAA in the foaming process of the PU sponge to obtain the HAA-modified PU (HAA-PU) sponge matrix. The HAA-PU sponge prepared with the HAA-to-PU mass ratio of 1:4 had the strongest foaming volume and water absorption. Fourier transform infrared spectrometry, scanning electron microscopy, and thermal stability analysis showed the successful preparation of the HAA-PU sponge substrate, which allowed for the slow entry or storage of moisture and nutrients. The results of the pea seedling growth test on the HAA-PU sponge and soil CK (control check) group showed that the emergence rate of the HAA-PU group was>60. It was not affected by the amount of watering and provided plants with long-term water and nutrition. The chlorophyll, and vitamin C content of pea seedlings grown in the HAA-PU sponge group were higher than the soil CK group.
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