This is one of the best distributed and most universal fertilizers. Marl
proper contains nearly equal proportions of clay and lime. Sand-marl is
spoken of, in which sand and lime are the main ingredients. Clay-marls
are to be applied to sandy and gravelly soils, and sand-marls to clayey
soils. Shell-marls are very valuable, and seldom contain clay. Marls may
easily be known, even by those not at all acquainted with chemistry.
Apply any mineral acid, or even very strong vinegar, and if it be a
marl, an effervescence will at once be observed: this effect is
produced by acid upon lime.