The protein content of vegetation is very similar to its ratio of carbon (C) compared to nitrogen (N).  Quick laboratory analysis of protein content is not done by measuring actual protein itself but by measuring the amount of combined nitrogen the protein gives off while decomposing.  Acacia, alder, and leaves of other proteinaceous legumes such as locust, mesquite, scotch broom, vetch, alfalfa, beans, and peas have low C/N ratios because legume roots uniquely can shelter clusters of nitrogen-fixing rhizobia.  These microorganisms can supply all the nitrate nitrogen fast-growing legumes can use if the soil is also well endowed with other mineral nutrients rhizobia need, especially calcium and phosphorus.  Most other plant families are entirely dependent on nitrate supplies presented to them by the soil.  Consequently, those regions or locations with soils deficient in mineral nutrients tend to grow coniferous forests while richer soils support forests with more protein in their leaves.  There may also be climatic conditions that favor conifers over deciduous trees, regardless of soil fertility.

It is generally true that organic matter with a high ratio of carbon to nitrogen also will have a high ratio of carbon to other minerals.  And low C/N materials will contain much larger amounts of other vital mineral nutrients.  When we make compost from a wide variety of materials there are probably enough quantity and variety of nutrients in the plant residues to form large populations of humus-forming soil animals and microorganisms.  However, when making compost primarily with high C/N stuff we need to blend in other substances containing sufficient fixed nitrogen and other vital nutrient minerals.  Otherwise, the decomposition process will take a very long time because large numbers of decomposing organisms will not be able to develop.

C/N of Compostable Materials

+/-6:1 +/-12:1 +/-25:1 +/-50:1 +/-100:1 Bone Meal Vegetables Summer grass cornstalks (dry) Sawdust Meat scraps Garden weeds Seaweed Straw (grain) Paper Fish waste Alfalfa hay Legume hulls Hay (low quality) Tree bark Rabbit manure Horse manure Fruit waste Bagasse Chicken manure Sewage sludge Hay (top quality) Grain chaff Pig manure Silage Corn cobs Seed meal Cow manure Cotton mill
          
                                                     waste

The lists in this table of carbon/nitrogen ratios are broken out as general ranges of C/N.  It has long been an unintelligent practice of garden-level books to state “precise” C/N ratios for materials.  One substance will be “23:1” while another will be “25:1.”  Such pseudoscience is not only inaccurate but it leads readers into similar misunderstandings about other such lists, like nitrogen contents, or composition breakdowns of organic manures, or other organic soil amendments.  Especially misleading are those tables in the back of many health and nutrition books spelling out the “exact” nutrient contents of foods.  There is an old saying about this:  ’There are lies, then there are damned lies, and then, there are statistics.  The worse lies of all can be statistics.’