A practical problem initiated this study. In the Haarlemmermeer, a former lake reclaimed about 1850, several farmers had difficulties with soil structure. Land, plowed in autumn, was very wet in spring. Free water was sometimes present on the soil surface. Planting and seeding were long delayed in spring and yields were unsatisfactory because of irregular crop development.
Some farmers ascribed it to pressure from the back wheel of the tractor in the open furrow during plowing, compacting the subsoil and severely reducing permeability. Plowing practices were therefore changed. Equipment was used that could be moved by driving the tractor with all four wheels over the normal soil surface. This sometimes needed very old horse ploughs, adapted to tractors. Other farmers tried to improve drainage by laying more tile drains alongside existing systems, but usually the drains did not conduct water.
The problems occurred on several soil types, but especially on the somewhat lighter textured sandy loams of the Hoofddorp Complex (Pgb) (Haans, 1954).
The Agricultural extension service helped in the selection of two farms, where a pedon was selected. In one, henceforth called G, no problems occurred. In the other, called B, soil structure had deteriorated. Morphological and physical properties of each pedon were frequently investigated for about 18 months. Soil management was left to each farmer. Profile descriptions, particle size distributions and routine analytical data of each pedon are in Appendix I. Differences in these values between the two profiles are mainly a result of soil management.
Originally the carbonate content of Pedon G was very low throughout. The content of 1%, as found now in the surface soil only, results from the application of lime for 30 years. The subsoil still contains no free carbonates. On Pedon B, decalcification during the formation of the profile, caused by a vegetation (Haans, 1954) was less pronounced. At about 50 cm the profile has much lime deposited as shells and small fragments. The surface soil, however, has been decalcified. It has not been limed as much as Pedon G as is shown by the difference in pH value (Appendix I). Pedon G has been green-manured and dunged for at least 25 years. During the last 10 years every field of the farm has been used for the growth of grass during three years, to be followed by use as arable land for the next three years. When wheat is grown, the soil is always green-manured. The field where Pedon G is found was notoriously bad about 1930. The surface soil had a weak unstable structure. Continued investment in lime and careful management has converted it into a productive soil.
Pedon B, however, is on a field whose ownership has changed several times over the last thirty years. It was never green-manured until 1966. The surface soil therefore contains less organic matter and lime than that of Pedon G.
The particle size distribution of the two pedons is identical. Average yields, as roughly estimated by the farmers from varying data of several years, were different for both farms. For sugar beet a yield of 50 tons, for wheat of 5.0 tons per ha was considered normal for farm G. For farm B this was 40 and 4.5 tons respectively. Such uncontrollable yield values, however, are not satisfying for describing soil quality and suitability. More important therefore was the opinion of the farmers that in general, soil B seemed more susceptible to structure deterioration and offered more problems than soil G. The suitability for agriculture of soil B was therefore considered inferior to that of soil G.
The aim of this study is to investigate this general statement and to describe it in quantitative terms. Before discussing the results of the fieldwork and a series of model experiments, the term suitability, as used when comparing both soils, must be explained.
The soil is used by the farmer to produce agricultural crops. His profit is determined by a difference between crop value and costs. A low yield may not necessarily be unfavourable, at least not when relatively high prices are being paid, as may be expected when generally low yields are found on other soils. On the contrary, in a year when average yields are high, prices tend to be lower. Then, in spite of higher yield, the amount of cash received for the crop may be lower as well, whereas costs of management will be simular. Prices, however,, not only vary as a function of the amount of supply but also because of differences in quality.
Cost figures are governed by the system of management of the farmer. Certain costs are inevitable, like those for seeding or planting and those for fertilization. The procedure followed in practice and its efficiency will determine its magnitude. Others, seem less inevitable. Some tillage practices for example are subject to discussion in this regard.
Each farmer will judge the quality of his land considering these factors. He will call a soil suitable when a regular and high production, both in volume and quality, is possible, especially in years when the general productivity level is low.
Besides, each crop to be grown is one from a cycle of different crops grown in a specific pattern of rota-lion. His final opinion on suitability will therefore be based on a judgment, considering the degree of financial succes by which certain crops can be grown during the years as a part of a rotation, that seems most lucrative.
A farmer will thus be able to give a description of the quality of each of his fields, based on experience of many years and on his own system of management that he considers to be the best.
The question of suitability of certain soil types as shown on a soil map will be posed to the soil surveyer, whether he likes it or not. Access to the experience of farmers is then indispensable. A soil scientist however, will have to realize that his pattern of thinking may differ from that of the farmer. He primarly thinks in terms of well described and classified soil types, as expressed on soil maps, that have been formed in a certain parent material under influence of the soil forming factors. He digs a pit and considers one of its vertical walls. The picture obtained then is considered representative for the type of soil. A farmer thinks more in terms of management and economy. and above that in fields of landuse. A certain field will normally be covered by several soil units of the map. This is certainly true when relatively large fields are present, to enable efficient use of machinery. To get a suitability concept for a certain soil type, occurring on different farms, the soil scientist has to normalize as good as possible all factors not directly related to his object of study, the soil profile itself (Vink & van Zuilen, 1967). These factors are: the capability of the farmer, expressed by his management; landshape and accessibility of the land and the type and size of farm, as well as the usual system of crop rotation.
Except for these factors it should be realized that each judgement is a clear function of the general economic situation, changing with time. Yields obtain on a certain type of soil usually vary considerably among years on the same field and also in the same year on different farms. Differences in weather during the growing season contribute to the first phenomenon, differences in soil management to the second.
Modern agriculture shows many points of agreement with industry, emphasizing an increase in production for each worker. Therefore mechanization developed. In a relatively short period of time concepts of soil productivity and management have changed. Practical problems, often concerning soil structure and soil stability, are submitted ever more frequently to specialists. Their judgment, based on an analysis of farmers experience, can only be given in a rather static agricultural system. When this system changes, as is the case with the present general drive to mechanization, the answers of the past no longer satisfy. Therefore an independant analysis should be made of the soil profile and its soil material only, describing its range of variable properties as a function of soil management. This will be attempted in the next chapters for the two pedons of discussion.