Final answer:
The most substantial contributors to organic matter in soils are vascular plants, which create organic material through photosynthesis and contribute to the soil through their waste, remnants, and tissues.
Step-by-step explanation:
Most organic matter found in soils results primarily from vascular plants. These organisms are prolific producers of organic material through the process of photosynthesis, during which they convert sunlight, water, and carbon dioxide into glucose and oxygen, providing the foundation of most terrestrial food webs. The remnants and waste of vascular plants, as well as their living and dead tissues, contribute to soil organic matter significantly more than other primary producers like cyanobacteria or photosynthetic protists.