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. 64

Krantenbank Zeeland

Watersnood documentatie 1953 - brochures | 1954 | | pagina 66