Materials Soil.
Eight sites were selected varying from extremely acid to 159
moderately acid.
They had jarosite horizons at varying depths.
The acidity varies with the depth of the jarosite or catclay horizon, which
is the horizon containing characteristic yellow or pale yellow mottles
of the mineral jarosite.
This horizon is extremely acid.
The Ayutthayaand Maha Phot series have a catclay horizon starting below 100 cm with
pH of surface soils 5.0-6.0, while Rangsit series has catclay below 40
cm but within 100 cm with pH of surface soil 4.0-5.0.
The Rangsit veryacid phase has catclay at the same depth as the main series but pH of
surface soil 3.5-4.5 and there is less calcium than in the Rangsit main series.
Four of these sites were situated in experimental stations and four in farmer's fields.
The location of the sites, the classification of their soils, their suitability of rating for rice production and some
characteristics of soil and sites are given in Table I .
The descriptions of soil series are given by Van der Kevie and Yenmanas (1972).