There is no one best single material from a generic crop perspective since different crops have different nutrient needs. So selection of individual amendments such as cottonseed meal, alfalfa meal, corn meal, seaweed and/or kelp meal, hay, leaves, grazing animal manures, etc. can be used to meet a variety of plant nutrition objectives. But NONE of such amendments provide ANY DIRECT nutritive effect since roots cannot utilize such, in presented form.
Changing the structure (molecular form) of such amendments, thereby making them accessible to plant roots as a food source is accomplished by means of enzymatic liquification or by microbial decomposition (or both). A combination of human gut enzymes and acids - in liquid form and in proper proportion applied to such amendments in either an aerobic or anaerobic environment, will speed the microbial process. An anaerobic artificial 'digestion' process (such as using Dr. Higa's EM fermentation technology) is sometimes used, but since a crop rhizosphere is an aerobic environment, general consensus recommends aerobic conditions be utilized for best results to eliminate the need for a lengthy anaerobic-to-aerobic microbial re-conversion process.
The difficulty with regard to an aerobic microbial decomposition process is in obtaining a highly-concentrated source of the full soil microbiological community to perform the decomposition process to attain nutrient-cycling - strictly an aerobic process and since the speed and quality of aerobic microbial decomposition depends on the concentration of microbiology applied to the process - the higher a concentration used, the better and faster.
A relatively new breakthrough in advancement of the old "compost tea" technology (that has never been accepted by the scientific community due to its lack of consistency of replication) is called Liquid Microbial Concentrate (LMC) that is capable of producing the full community of aerobic soil microbiology in 24 hours. A 48-hour brew time attains a bacterial / protozoan population, while a 72-hour brew can exceed ten to the 25th power per ml - much higher than the best compost tea was able to produce, and with a very high consistency of replication. Presently being prepared for presentation to the USDA as a viable product for approval for use by the global agricultural industry.
So general consensus nowadays is to increase soil nutrient cycling by means of liquid application, which is less expensive than adding a variety of solid amendments and waiting for in-soil biology to process amendments into plant food.
The humic acids produced by the soil biological community are known to play a part in the nutrient-cycling process, but science does not yet know how to qualify or quantify that process. What we do know is that humic acids play a role in certain aspects such as nutrient retention, mineral chelation, etc. So commonly, when liquid compost is applied, humic and refined fulvic acids are added as Fortifiers.
Soil fertility by organic means is a matter of microbial nutrient cycling (saprophytic process of turning dead organic material into organic matter with an increasing level of amphorous humus content). Adding organic material to soil does not increase soil fertility per se - because it requires that material to be broken down (decomposed) by the full community of saprophytic microorganisms (primarily aerobic) by their enzymatic activity.
There is one COMBINATION of organic materials that is highly effective in raising soil fertility in sand, silt or clay and ANY percentage combination of each, called COMPOST.
Composting is NOT a natural phenomenon - it is a man-created method of speeding-up nature's saprophytic process. A leaf falling from a tree may take up to a year (or more to decompose sufficiently, whereas that same leaf processed in a QUALITY (non-commercial) composting process (which I have practiced professionally for over 50 years) will only take 12-18 weeks to begin adequate nutrient-cycling function.
Secret ingredient..... water.