Absorption of Potash by Soils of known Composition. 345 
filtered through the same soil. This sandy soil contains a good 
deal of hydrated oxide of iron, a constituent well known to 
possess considerable chemical affinity for caustic potash, and 
none for sulphate of potash. In consequence of this special 
display of chemical affinity which manifests itself when a solution 
of caustic potash is passed through a soil containing hydrated 
oxide of iron, more potash was fixed than when a solution of 
sulphate of potash was brought into contact with the same soil. 
1, It is worthy of special notice that the solution of sulphate 
of potash after filtration through the sandy soil contained a 
weighable quantity of sulphate of ammonia, which was not the 
case with the marly soil in the preceding experiment. 
The power of soils to retain ammonia is generally assumed to 
be greater than their power of retaining potash. Here, however, 
an instance is presented to us in which a salt of potash, by acting 
on the ammoniacal combination in a soil, overcomes the supposed 
superior affinity of ammonia. Contrary to all expectation, 
ammonia, in combination with sulphuric acid evidently supplied 
by the sulphate of potash, passed into the solution, whilst potash 
took its place and was retained in the soil. 
3. On comparing the analysis of the sterile sand with that of 
the marly soil, it will be seen that, whilst the former is very poor 
in lime, the latter contains it in a large proportion. In the 
sandy soil but little sulphate of lime was produced, on account 
of the deficiency of lime in the soil ; and the consequence was, 
that sulphate of ammonia, as well as sulphate of lime, passed 
through the soil. In the case of the marly soil, there was 
sufficient lime present to lay hold of the sulphuric acid sepa- 
rated from sulphate of potash on the fixation of potash by the 
soil, and no sulphate of ammonia — at least not any appreciable 
quantity, passed through it. In other words, in these experi- 
ments the deficiency of lime in the sandy soil caused the 
elimihation and loss of valuable ammoniacal compounds, which 
were retained in the marly soil. Under favourable circum- 
stances, lime thus becomes, as in the case before us, a preserver 
of ammoniacal compounds in the soil. 
4. The preceding experiments throw new light on the uses of 
lime in agriculture. We know practically how essential the 
presence of lime is for the healthy growth of every kind of 
cultivated produce. On soils very deficient in lime, most crops, 
especially green crops, are subject to all kinds of disease ; and, 
consequently, roots fail altogether on such land, even if it 
has been liberally manured with good yard-dung or guano. Up 
to a certain stage, corn and roots grown under such conditions 
appear to thrive well, but as the season advances they sustain a 
check, and at harvest-time yield a miserable return. The 
remedy for such failures, which are not at all uncommon in 
