Desalting and other soil reconditioning
The agricultural land is restored for cultivation
The agricultural land in the flooded areas has naturally suffered a
great deal owing to;
a. the penetration of salt water;
b. the erosion of the soil by the fast-flowing currents;
c. the fertile topsoil having been washed away by the water;
d. the silting up of the indispensable ditches and water courses with
clay and sand.
The first task of the State Department for Agricultural Reconstruction
therefore was to put the drainage system in order, because not until this
had been done could the water drain from the soil and a start be made
with the desalting.
At the same time the area had to be mapped in order to establish where
erosion had occurred and where clay and sand had been deposited.
The clay deposits are usually not disturbed, as they will turn into fertile
soil after some time. The infertile sand on the other hand must be removed
so as to uncover the original fertile clay.
In this connection it should be pointed out that the Government has
undertaken to restore all the land as far as possible to its original condition.
In some places, however, the land has suffered permanent damage, because
the fertile top soil has been carried away. The cost of restoring the soil
to its original condition is so high that it will usually not be economically
justified. The effects of erosion and sand deposits are more serious in one
place than in another, but the salt problem is one which must be dealt
with throughout the flooded areas.
Salt, crops and desalting
The soil has not been affected by the salt to the same extent everywhere,
as this depended on the time the water had to penetrate into the soil, the
New ditches are dug to promote drainage and desalting.
salinity of the water, the differences in the level of the land and on many
other factors.
In the Netherlands the salinity of the soil is established by determining
the water content of the wet soil and the salt content in the dried soil.
This enables the salt content of the soil moisture to be calculated. The
salt content is expressed in grams of NaCl per litre of soil moisture and the
figure obtained is called the "salt figure".
This method has the advantage that the salt figure permits of a
direct comparison with the salt content of sea water or of other flood
water.
Once the salt figure of the soil is known, it is possible to say whether
a crop can be grown with a reasonable chance of success. A thorough
investigation was carried out into the susceptibility of crops to salt after
the inundations which occurred during the late war, when it was confirmed
that this susceptibility varies considerably. Cereals for example can stand
quite a lot of salt, peas on the other hand very little.
As soon as land fell dry, it was therefore of great importance to
establish the salt figures in these areas as quickly as possible.
On an average one sample was taken for every five or six ha of the
land which had fallen dry. Within the space of a few months some 15,000
soil samples were examined in two laboratories! The majority of these
samples were found to have a salt figure of between 10 and 20, i.e.
between 10 and 20 grams of NaCl per litre of soil moisture.
In this way each farmer was informed of the salt figures of his own
land. The numerous consultants of the agricultural advisory service
were then in a position to give advice at meetings and in personal inter
views with respect to the crops which could be grown.
In this way it was possible in 1953 to bring between 50 and 60 thousand
ha1) of the 90,000 ha of flooded agricultural land back into cultivation. In
most cases the farmers were advised to sow a summer crop of barley, as
this is least susceptible to salt.
Thanks to the cool and wet summer the harvests of this year may be
regarded as satisfactory, although this does not mean that higher yields
were obtained, since in many places the salt content of the soil was still
too high. The graph represented in fig. 1 clearly shows the relationship
between the yield of barley and the salt figures.
In this graph each dot represents an accurately measured yield of a
specific field, of which the salt figure can be read off from the horizontal
axis.
It will be seen that with a specific salt figure the yield varies con
siderably, which is due to the fact that with different plots of land the
yield is also influenced by a number of other factors (the type of soil,
humus content, phosphoric acid and potassium stocks, etc.).
It is clear, however, that on an average these yields gradually decrease
according as the salt figure is higher. It is also evident that barley can
stand a large amount of salt, since with a salt figure of 10 (about one
third of the salt content in sea water!) the yield is still about two thirds
of the normal figure!
As stated before, this result could only be achieved thanks to the wet,
cold summer in 1953. In a dry year the salinity of the top soil rises con
siderably owing to the evaporation of the moisture in the soil and the result
will consequently be much more unfavourable.
Some yields with respect to peas, a crop which is much more susceptible
to salt, are represented in fig. 2. It is clearly evident that even with low
salt figures the yield decreases considerably.
How is the salt problem solved?
The salt is removed in winter by the heavy rains. Practically all the
rain water slowly sinks into the ground and dissolves part of the salt.
Ultimately the salt water is drained off via ditches and canals and it is
1 hectare 2.47 acre.
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