Left: Normal clay soil at the
time of the floods.
Right: The same soil after
salt-water flooding.
therefore of the utmost importance that the drainage facilities should be
freed from silt and sand. With favourable conditions even the saltest land
in the Netherlands will be almost free from salt after only two winters.
Soil structure
Even after the salt has been removed this does not mean that all
difficulties have been overcome. In addition to the high salt content, the
structure of the inundated soil presents serious problems. The theoretical
explanation must be sought in the chemical binding of metal ions to the
fine (colloidal) clay particles. In normal soil these fine particles together
with sand and humus particles form small pieces of porous soil, the crumbs.
The firmness of these crumbs must be largely ascribed to the presence of
adsorbed calcium round the fine particles. As a result of the flooding with
sea water the calcium lias been largely replaced by sodium and it is found
that the structure of fine clay particles, sand and humus has suddenly
lost its strength. The crumbs can no longer withstand pressure and during
the heavy rains they simply disintegrate into their component parts.
The separate particles of clay, sand and humus can conglomerate to
a much denser mass than the soil crumbs and the result is a closed soil
surface impervious to water. On drying, the fine particles still adhere
together and the soil cracks, so that hard lumps are formed when it is
worked. Naturally this condition is not conducive to sound development
of the plant roots, which require a nice loose layer of soil crumbs.
This unfavourable structure of the soil can be improved by replacing
the sodium in the fine clay colloids by calcium and this can best be done
with the aid of gypsum (CaS04 2aq). Experiments have shown that the
application of gypsum gives the best results after the soil has been worked
in autumn. The gypsum is slowly dissolved by the winter rains and sheds
its calcium, while the sodium sulphate (Na2S04) formed is washed away.
To improve the structure of the soil in the areas which have suffered
from the floods large quantities of gypsum are required. Heavy soils
(containing a high percentage of colloids), which were inundated with salt
water of high concentration, are treated with up to 18 tons of gypsum per
ha, but elsewhere the dosage can be reduced to one ton per ha. It has
been calculated from the salt figures and the types of soil that some 450,000
tons of gypsum will be required in all. This product is imported from about
seven different countries. The gypsum is placed at the disposal of the
farmers free of charge.
There is every reason to believe that the disastrous and lasting after
effects of this structural deterioration of the soil, which formerly lasted
more than ten years in the case of heavy soil, can now be neutralized in
from three to four years by the treatment with gypsum. Gypsum is of the
greatest importance to bring the harvests back to normal.
In the case of these inundations agricultural research and information
have clearly demonstrated their great significance and their direct econom
ic effect by limiting the losses of the farmers and thus of the entire
Netherlands community.
Seed production
in quintals/ha
50
40
Fig. 1Yield of spring barley on salty
land in 1953
30
20-
10
10 15 20
Salinity Figure 5-20cm layer
Fig. 2. Yield of peas on salty land in
1953.
Seed product ion
in quintals/ha
Salinity figure 5-20cm layer
65
