Later, Esmarch extended his investigations to a far larger number of samples, 395 in all, of soils of different types from Schleswig-Holstein. He found that blue-green algæ were very widely distributed in soils of certain types, though they occurred rarely in uncultivated soils of low water-content, and he described no less than 45 species of which 34 belonged to the Oscillatoriaceæ and Nostocaceæ. Certain of the commoner species were obtained from soils of widely different types, as shown in [Table X.], while other forms occurred only rarely and with a much more limited distribution.

TABLE X.—FREQUENCY OF OCCURRENCE OF CERTAIN COMMON SPECIES IN ESMARCH’S SOIL SAMPLES.

Species.Percentage of Samples containing
given Alga.
Uncultivated
Damp Sandy Soil.		Cultivated Soils.
Shores
of
Elbe.		Shores
of
Lakes.		Sea-
shore.		Sandy.Clay.Marsh-
land.
Anabæna variabilis46  43   9  10·360  46  
Anabæna torulosa31  14·363·627·634·356·4
Cylindrospermum muscicola23  28·6 0  24  48·659  
Cylindrospermum majus 0  14·3 0  38  40  33·3
Nostoc Sp. III. 7·7 0   0  38  37  48·7

Taking the number of samples containing blue-green algæ as a rough measure of their relative abundance, Esmarch obtained the following interesting figures ([Table XI.]):—

TABLE XI.

Kind of Soil.Percentage
of Samples
Containing
Blue-green
Algæ.		Number
of Samples
Examined.
Cultivated marshland95  40
Cultivated clay soil94·637
Uncultivated moist sandy soils88·635
Cultivated sandy soil64·445
Uncultivated- Woodland12·540
Sandy heathland 9  34
Moorland 0  35

In noting that the soils fell into two groups, those relatively rich and those poor in blue-green algæ, Esmarch concluded that the two chief factors governing the distribution of the Cyanophyceæ on the surface of soils are, (1) the moisture content of the soil, (2) the availability of mineral salts, cultivated soils being especially favoured in both of these respects. He further distinguished between cultivated land of two kinds, viz. arable land and grass land, and found that on all types of soil grassland was richer in species than was arable land.

Esmarch examined, in addition, 129 samples taken from the lower layers of the soil immediately beneath certain of his surface samples, 107 at 10-25 cms. and the rest at 30-50 cms. depth.

In cultivated soils, whether grassland or arable land, he found that blue-green algæ occurred almost invariably in the lower layers in those places bearing algæ on the surface and that, with rare exceptions, the algæ found in the lower layers corresponded exactly to those on the surface, except that with increasing depth there was a progressive reduction in the number of species.

In uncultivated, moist, sandy soils the agreement was far less complete, for though algæ were rarely absent from the lower layers their vertical distribution was frequently disturbed by the action of wind and rain. Other uncultivated soils not subject to periodic disturbance were found to be uniformly lacking in algæ in the lower layers, but as the limited number of samples examined came completely from places where there were no algæ on the surface this means very little.