Part of the ten essentials is extra food. Many Scouts and other explorers have had their adventures extended. Those that packed extra food faired better than those who didn't.
It is far easier to learn how to identify poisonous plants than it is to correctly identify all the edible plants in an area.
WSU lists the following as poisonous (mostly for livestock):
Pigweed (Amaranthus species)
Lambsquarter (Chenopodium album)
Greasewood (Sarcobatus vermiculatus)
Hemp dogbane (Apocynum cannabinum)
Horsebrushes (Tetradymia glabrata and T. canescens)
Kochia (Kochia scoparia) aka fireweed or burning bush
Tall fescue (Festuca arundinacea) – toxic fungus grows on this
Sneezeweeds (Helenium hoopesii or H. microcephalum)
Bracken fern (Pteridium aquilinum) - destroys thiamin (Vitamin B1)
Horsetail (Equisetum arvense) - destroys thiamin (Vitamin B1)
Clovers (Trifolium species)
Poison hemlock (Conium maculatum)
Water hemlock (Cicuta douglasii)
Arrowgrass (Triglochin maritima and/or T. palustris)
Lupines (Lupinus sericeus and other species)
Locoweeds and milkvetches (Oxytropis species and Astragalus species)
Ponderosa pine (Pinus ponderosa) – Don’t make tea from this
False hellebore (Veratrum californicum)
Halogeton (Halogeton glomeratus)
Black locust (Robinia pseudoacacia)
Fiddleneck or tarweed (Amsinckia intermedia)
Tansy ragwort (Senecio jacobaea)
Nightshades (Solanum species)
Brassica species and other members of the mustard family (kale, cabbage, rape, turnips, etc.)
Prickly lettuce (Lactuca scariola)
Yellowstar thistle (Centaurea solstitialis)
Russian knapweed (Centaurea repens)
Larkspurs (Delphinium species)
Sweet clover and white sweet clover (Melilotus officinalis and M. alba)
St. Johnswort (Hypericum perforatum)
Buttercups (Ranunculus species)
Black walnut (Juglans nigra)
Oaks (Quercus species) - young leaves, acorn buds and green acorns
Foxglove (Digitalis purpurea)
Elderberry (Sambucus canadensis) - berries are harmless once cooked
Jimsonweed (Datura stramonium)
Dock or sorrel species (Rumex species) - consumed in great quantity can cause oxalate poisoning
Milkweeds (Asclepias species)
Universal Edibility Tests can be found in military manuals. They sound like a neat tool to have in your survival repertoire, but the true of the mater is that there is NO Reliable Universal Edibility Test. Using this tool can get you killed, even with access to advanced medical care.
The Universal Edibility Test is something to consider as a last resort when a military pilot is stranded behind enemy lines long term (30 plus days) and is starving to death. Most lost Hikers and Scouts are found within 72 Hours. 72 Hours is not enough time for you to starve, but is enough time for you to die a horrible death from poisoning.