MARL (OF. erode, merle, Fr. marne, OHG. mergiE Ger. Jlergel, from ML. diminu tive of Lat. inurgu. marl. from Gall. tourga, Bret. mum marl, Gk. drytXos, argilos, white clay). A somewhat indefinite term applied in different localities to widely different materials. In a general sense it means essentially a natu rally occurring mixture of calcium earbonate and clay with more or less sand, which usually falls to pieces on exposure to the air. Although prob ably the greater number of the marls of Sias United States conform to this definition, and depend for their agricultural value on their lime content. there are quite extensive deposits of the Cretaceous marls, known as greensand (espe cially in New Jersey), which contain variable hut usually small amounts of lime and con siderable amounts of potash (mainly silicate) and phosphoric acid. The name is also some applied to friable Plays, or mixtures of clay and sand, in winch there is almost no trace of lime. Marl beds are widely distributed in the United Slates and have been exploited to a con siderable extent in New Jersey. Maryland, Vir ginia. Kentueky. North Carolina. and South Carolina. The marls of these deposits generally belong to three classes and oecur in geological formations which are found. as a rule, one above the other in immediate succession. The upper layer. blue or shale marl (Neocene). generally found at or near the surface. consists (-Melly of sea umd with partially decomposed shells and hones. lts value depends mainly upon its con of carbonate of lime (40-50 per cent.). al though it contains in addition small percentages of potash (.25 to 4.75 per cent-) and phosphoric acid (trace to 1.75 per (Tilt.). This class pre &mina if'A in Ilaryland. Virginia, and North Carolina. and has been used to a considerable extent with good resnIts on worn-out or nat urally infertile soils. The second class, Wayne or chalky marl. is commonly a coarse:friable chalk, consisting of comminuted shells and corals, of a light yellowish or grayish color to white, sometimes compacted into a solid limestone. its content of lime is greater (50-95 per cent.) than that of the shell marl and the percentage of pot ash and phosphoric acid is smaller. In the lower layer occur the Cretaceous marls (green sand), which vary considerably in chemical com position and agricultural calm.. Their fertilizing
value is determined largely by their content of potash (3.5 to 7. per cent.) and phosphoric acid (I to 4 per cent.), although many are calcareous (1.25 to 9 per cent. of lime). These marls have long been used with beneficial results by New Jer sey farmers, although the benefit is more marked in ease of marls rich in phosphoric acid and lime than in case of pure greensand containing a high percentage of potash, probably because the potash is in the form of an insoluble silicate (glau conite) and is very slowly available to plants.
Marl is both a direct and an indirect fertilizer, improving both the chemical and physical condi tions of soils, correcting acidity, unlocking in soluble plant food, and promoting nitrification. It is very lasting in effect and has been used from ancient times for restoring worn-out lands to fertility or for improving naturally infertile soils. But because lime (q.v.) is quicker in action and of greater efficiency it has been used in many cases instead of marl, although some kinds of marl are extremely useful on certain soils. On account of its bulkiness and the large amounts which must be applied in order to secure beneficial results, marl can lie used profitably only in close proximity to the deposits. Booth, in a report of the State geologist of Delaware, recommends 60 to 100 bushels per acre as the proper amount to be applied on poor light soils, TOO to 200 bushels on clay soils, while 200 to 500 bushels may be used with advantage on soils of good quality abundantly supplied with humus. The addition of quicklime to marl (30 to 40 bush els of lime to 300 to 400 bushels of marl) has heen found to quicken the action of the marl. It is generally advisable to let marl lie exposed to the air sonic time before it is incorporated with the soil, thus destroying any poisonous compounds which may be present.
Consult: Ruffin, Calcareous Manures; Ullmann, Kalk nod Mergel; State Geological reports of Delaware. Kentucky, Maryland. New .Jersey, North Carolina, and Smith Carolina ; Maryland Agricultural Experiment Station Report, 1889.