526 CLAT AND LOAM. 



contains the roots of herbaceous and fibrous-rooted plants in 

 greater or less abundance. 



The term " loam" (Angl. Sax. Lam) comes from an ancient word in 

 different langfuagea, signifying a fat, unctuous, tenacious earth. XJnder 

 the name of "loam" there is comprehended a class of compound or 

 mixed earths, composed of dissimilar particles — ^hard, stiff, dense, harsh, 

 and rough to the touch — not easily plastic while moist, readily diffusible 

 in water, and usually composed of sand and a tough viscid clay. Loams 

 are very concisely divided into two kinds — ^the friable and crumbly 

 sorts, composed of sand and a less viscid clay — and the tough and viscid 

 in texture, that are composed of sand and a more adhesive clay. The 

 colours have also been used to distinguish loams — ^the black and white, 

 which are not acted upon by acids ; yellow loams, some of which are 

 affected by acids ; the alkaline brown loams, that are acted upon by 

 acids ; and the green loams, that suffer no disturbance. 



According to Woodward, loam consists of clay mixed with fine sand ; 

 or of clay with a superabundance of sand ; and Bergman found a good 

 loam to contain 87 per cent, of a reddish grey sand, as fine as meal, and 

 13 per cent, of alumina. Supposing clay to contain, as it most frequently 

 does, 70 per cent, of fine sand and 30 of alumina, we shall find, as Mr. 

 Kirwan observes, that loam of the best kind contains an excess of sand 

 amounting to 17 per cent. ; if the excess of sand be greater it wiU form 

 a "sandy loam," if smaller, a "clayey loam." When anything 

 calcareous is found in the loam, it inclines to the nature of marl, or a 

 "marly loam," which may be either sandy or clayey, according as the 

 proportion above indicated is exceeded on either side. But loams most 

 frequently contain a portion of oxide of iron, which produces a con- 

 siderable variety in the colour, and probably in the vegetative powers 

 of the loamy earth, if its proportion be considerable, viz., 4 or 5 per 

 cent. ; they often contain, also, some proportion of sulphuric acid. 

 The sandy part of the loam often has much effect in giving the colour. 

 When gravels and pebbles are mixed with loams, the distinctions arise 

 of " gravelly, stony, siliceous, and limestone loams," according as the 

 substances predominate. 



Loams are generally understood to consist of clay, siliceous sand, and 

 some carbonate of lime. The quantity of iron, magnesia, and various 

 salts, is so inconsiderable, as never to alter materially the texture of 

 the loam. Decayed vegetable and animal matters in the form of humus 

 are often found in loams in very considerable quantities, and the soU is 

 fertile in proportion. Loams vary in quality ; those composed of loose 

 sand with little humus, and impregnated with iron, are very \inpro- 

 ductive ; those which contain too much clay, and lie upon an impervious 

 subsoil, are difloult to cultivate. Between these two extremes there 

 are soila that form the very best that are found on the face of the globe. 



