642 
Supphmmt to the " Tropical Agriculturist." [MabcH 1, 1900. 
Hocks are composed uf Tarious materials, some of 
which iii decomposiiig or weatheiiiig yield large 
quiintities of solubla mnfier, while others yield 
little or no reiidily soluble matter; so tliat the rock 
is not dissolved as a whole, but the chemical action 
of weathering so changes some niinerals that parts 
of them are made soluble. Tims feld.-par, & 
common constituent of many of our rocks, is not 
soluble in water, but as weathering goes on under 
the atmospheric conditions, the mineral whitens, 
bi'eaks down, and the insoluble kaolin (clay) and 
■ilica (sund) are left, while potassium carbonate is 
washed out in solution. On the other hand, 
quartz, the most common of all minerals, does not 
change during the weatheiiug, but simply breaks 
up into finer particles of sand, very little of which 
is removed by solucion. 
With the formation of the soil this process of 
solution and removal does not cease, but as long as 
water is in contact with the soil grains there are 
chemical changes taking place and salts are being 
dissolved. Pure water will dissolve as much as 
one per cent of some of our soils, but the average 
amount dissolved will be much less than this, 
probably nearer one-tenth of one per cent. 
The soils, like the rocks, are composed of various 
materials, and the water dissolves these just as it 
did the minerals of the granite- Some of the 
minerals contain plant food, some do not, and 
when all of the minerals containing plant food are 
dissolved and washed away, the soils become 
sterile and will not support plant growth. Plants 
need only a very small amount of mineral food at 
one time, but this small amount must be supplied 
throughout the growing season. 
A number of chemists have made chemical 
analyses of the matter dissolved by water iu 
various soils. They have shown that all of the 
minerals aie more or le^s dissolved by the water, 
and that the solutions contain all of the mineial 
ingredients of a plant. The important plant foods, 
phosphortis, nitrogen, pnd pnta.>h, are present in 
small quantities iu the solution, while lime is 
present in a much larger quaniity in most soils. 
Should the plant foods be present in solution in 
large quantiii'^s, th« first liea.vy rain would wash 
all out that were dis-olved, and their value would 
be lost. The examination of large numbers of 
samples of drai'.Kige water has shown that phos- 
phoric acid and ]iot;:sh are only washed out in 
small quaiitiiies, while nitrogen is removed nearly 
as fast as formed within the s iil. 
It was noted many years ago that a soil pos- 
sessed the property of decolorizing S' luiions and 
of rendering foul water pure. This is well illus- 
trated in our common custom of digging wells ; 
the deeper the vvell the purer the water 
and the less lialile to contamiiiation from surf i<'e 
impuriii(S. This timpief ct of the power of .-.loi 
or soil to take I lie coloiirig matter out of mnnuie 
extracts was noted l)y chemists, wiio found, on 
further eNaminaiion, that the phenomenon was 
much r/icjre general and of a far larger interest 
than \v;is first sup])oso(l. It has been found that 
all soils jjo se-s tlii-; property to a greater or less 
extent. They rs move soluble matters from solu- 
tion and giadn.'iil}' release them ngain, so that at 
no time is any large quantity of matter present in 
solution. When a solution of potassium nitrate 
is poured through a soil the potassium is ia a 
large measure absorbed and retained, while lime 
takes the place of potash iu solution and is 
washed out as the nitrate of lime. Soils, in other 
words, decompose the salts and retain the alka- 
line portion (potash, soda) with tlie phosphoric 
acid, while the acid portions (nitric, muriatic, 
sulphuric, etc.) are removed with part of the 
alkalies and lime. 
Tile great importance of this fact to the agri- 
culturist can readily be seen. When a mineral 
fertilizer is applied to a soil, instead of being all 
dissolved and remair.ing in solution to be washed 
down into the subsoil by the first heavy rain, the 
plant foods are absorbed by the soil and gradually 
supplied for the use of the plant roots. 
During a study of the soluble matter of .soils ia 
the Division of Soils several new methods and 
new apparatus have been u.sed, and the results 
obtained have been of much interest to those 
engaged iu agricultural research as well as to the 
practical agriculturist. 
The soluble matter of soils may be classed 
under two general heads, (1) soluble mineral 
rnatter and (2) .soluble organic matter. Under the 
first head are classified all salts and .soluble com- 
pounds derived ;from the decomposition of rock 
or mineral parts of t,he soil. Under the second 
head are classified all soluble matter derived from 
plant and animal remains. The study has been 
confined almost entirely to soluble mineral matter, 
and' only this class of soluble matter will be con- 
sidered here, 
There are three conditions in which a salt can 
exist within a soil, (1) insoluble ; (2) soluble; (3) 
absorbed — phy.sical and chemical ab.^orption. Of 
the>e three conditions the first, in which the salts 
are insoluble, is of importance as a reserve, for 
there exists in this condition all of the undecom- 
posed minerals with their stores of plant food. 
The term insoluble is used in a relative sense, for 
all the minerals are soluble in water to a slight 
degree. These minerals are constantly being 
decomposed, thus yielding their store of plant 
food in a more available condition for plant 
growth and furnishing supplies for continued 
fertility for a long or short time, accordingly as 
the soil contains much or little plant food locked 
up in insoluble form. In the second condition, 
in which the salts are actually in solution, they 
are, of course, in a condition in which the plant 
can most readily use them. 
The absorption of salts has been briefly dU- 
ci-issed in an earlier part of this paper. The con- 
dition of absorbed salts is one which has received 
much attention in recent years. There are two 
kinds of absorption, chemical and physical. The 
chemical absorpiion has alre dy been treated iu 
ano'l er paragraph, and the desc iption which 
1:!' .vs rer^rs to tlie piiysical absorption, A.iy 
Solid body placed within a solution has the power 
of absorbing or conden.sing upon its surface 
matter whieli is in the solution. The<e salts 
when abso-bed or condensed seem to have lil;tle 
effect, upon the growth of the plant. They are 
very slowly given up by the solids to tbs solution 
again as the soil is leached, Sueli phy.sical ab- 
sorption or condensing action varies ' with the 
condition of the surface, that is to saj-, whether 
it is rough or smooth, compact or porous, aud it 
aslo varies with the extent of the surface. 
