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Inappropriate land use has caused a series of environmental disasters such as floods and sand storms, and some of them involved unused land changes with highly localized distributions in arid and semi-arid regions with fragile ecosystems. As the third-rank category of first-level land use/cover in China, unused land (UL) is now playing an increasingly important role in protection of the natural environment and sustainable utilization of land resources. In this article, we assessed the effects on regional eco-environments employing a quantitative EL (ecological effect index) model, which can be used to evaluate and represent the contribution of UL changes to the eco-environmental quality. Results show that UL changes generally contributed to the deterioration of eco-environmental quality during the study period. Some major contributors to improving eco-environmental quality were transformation of sandy land and saline-alkali lands to grasslands, expansion of water bodies in UL areas, and reclamation of farmland in UL areas (except for marsh lands). In contrast, the main contributors to worsening eco-environmental quality were grassland degradation to UL (except marshes), reclamation of marsh areas, and shrinkage of water bodies to leave desert or saline-alkali land. Some suggestions are provided about UL management, utilization, and protection issues.

Ter gelegenheid van het vijftig-jarig bestaan van de Landinrichtingsdienst wordt ingegaan op het cultuurtechnisch onderzoek, dat verricht wordt bij de voorbereiding en uitvoering van landinrichtingsplannen. De nadruk wordt gelegd op het cultuurtechnische onderzoek, met inbegrip van de relaties tot andere voor de landinrichting relevante disciplines

In potential acid sulphate soils acidity may arise from any combination of reclamation and drainage lowering the groundwater table in adjacent areas, and unusually dry seasons affecting the regional groundwater table. In the long run, natural processes of deacidification will finally make these soils productive. But this natural process is very slow and may take decades for any significant improvement. Thus natural reclamation does not provide the solution to the immediate need of increasing food production at a sufficiently high rate. Developing economic means of enhancing the process of reclamation of these soils is therefore considered very important. Feasible management packages for the development of acid sulphate soils are determined by the soils, the climate, the hydrology and the economic opportunities.The causes of low productivity of acid sulphate soils include soil acidity, salinity, aluminium toxicity, iron toxicity and low nutrient tatus. Different reclamation and improvement methods for low land rice re reviewed in Chapter 2. A simple low-cost lysimeter drum (60 cm diameter, 90 cm depth) was developed to collect undisturbed soil cores for studying the chemical changes and the growth of rice in acid sulphate soils. Forty undisturbed soil cores from Aparri and Sinacaban soils in the Philippines were subjected to different water management practices, liming rates and mulch application for two leaching periods and three rice crops. There was a great difference in chemical properties and grain yields of the two soils. In Aparri soil, there was no marked effect of drainage, liming and mulch practices on the chemical properties of the soil solution during the two leaching periods and during three rice crops. Also the grain yields of three crops in this soil were not influenced by different treatments.In Sinacaban soil, the effects of drainage, liming and agronomic practices on chemical properties of soil solutions were very pronounced. Deep drainage of the soil created favourable conditions for pyrite oxidation which resulted in high acidity and high Fe 2+and Al 3+. Shallow drainage did not cause severe oxidation in the subsoil layers and produced less acidity and Fe 2+, Al 3+. Shallow drainage in combination with mulch application (5 cm of rice straw) minimized the water loss from the soil, and hindered oxygen movement to the subsoil. Thisresulted in less acidity development and less Fe 2+, Al 3+. When the soil was continuously submerged, there was no pyrite oxidation, and no Fe 2+and Al 3+production.The influence of different treatments on grain yields was very pronounced. Deep drainage treatments gave the poorest yield in both the second and third crops. Highest yields were obtained in the surface flushing treatments, the next highest yields with shallow drainage. Mulching increased grain yield by 67 and 30 percent for the second and third crops respectively. In deep drainage treatments, only high amounts of lime, 2.5 tons/ha produced a yield exceeding the equivalent of about 1 ton/ha. In this chapter, the concept of average mineral stress index (AMSI) was introduced to correlate grain yield and toxic elements in acid sulphate soils (pH, Al, Fe).During the dry season, oxidation and acidification occur when groundwater falls below the sulphic layers in the subsoil.In Chapter 3, a study on the evaporation and acidification process in an acid sulphate soil was carried out in forteen undisturbed soil columns of 20 cm in diameter and 70 cm length from an acid sulphate soil in Mijdrecht, the Netherlands. These columns were subjected to two groundwater levels: 40 cm and 65 cm below the soil surface, 5 different durations of evaporation and 2 agronomic practices. The average total acidity over 14 layers in the profile did not show much variation among treatments. The presence of a peat layer on the surface reduced the rate of acidification, presumably mainly by reducing. The evaporation rate and perhaps by hampering the oxygen movement in the profile. In treatments with low groundwater table, the average pH decreased sharply at increasing evaporation.When acid sulphate soils develop, dramatic changes in the chemistry of surface waters take place and these are exported from the reclaimed area as drainage waters. Acid sulphate floodwaters generated over large areas of acid sulphate soils may adversely affect crops growing on adjacent, better soils.In Chapter 4, a study on the one-time and cumulative effects of acid sulphate floodwater on the growth of rice and on the chemical changes in three acid sulphate soils were investigated in two successive pot experiments. Artificial acid floodwater were made with pH ranges from 3.5 to 5.8, Al 3+from 0 to 300 mg/l and Fe 2+from 0 to 500 mg/l.In both experiments, pH of acid floodwater alone, as low as 3.5, did not affect the chemical changes of three acid sulphate soils, whereas the presence of Al 3+and Fe 2+in acid sulphate floodwater produced low pH and enhanced the solubility of Fe 2+.pH of acid sulphate floodwater alone as low as 3.5 showed no effect of the growth of rice at on early stage of growth. Dry matter yields were negatively correlated with the applied Fe 2+and Al 3+.A single factor, total acidity, being the sum of H +, Al 3+and Fe 2+in equivalents, was introduced as a factor determining the plant dry matter yield in cases that none of these reached the limiting toxicity values.In acid sulphate soils, rice responded favourably to rock phosphates. Phosphorus is the main limiting macro-nutrient for crop production in these soils, but the use of superphosphate is a luxury in many developing countries because of their limiting foreign exchange. Therefore, the effectiveness of rock phosphates for lowland rice on acid sulphate soils under various management practices was studied in pot experiments on surface horizons of two acid sulphate soils under three application methods and two rates. Details of this experiment is given in Chapter 5. Rice response to phosphate sources varied between soil. In Malinao, the highest rice yield was obtained when rock phosphate was applied and incubated three weeks before transplanting at field capacity.In Chapter 6 a computer simulation model was developed to calculate the time course of acidity production from pyrite oxidation in relation to the changes of groundwater table, evaporation, entrance of oxygen and different chemical reactions. The model is based on a multi compartment model in which the soil profile is divided into a number of layers, is written in CSMP and contains three main parts: INITIAL, DYNAMIC and TERMINAL. For each compartment of a clay loam and a silty clay soil, the compartment thickness, its water content, pyrite content, cation exchange capacity, total cations, adsorbed cations and oxygen concentration are given as inputs. Three evaporative demands of 4, 6 and 9 mm.d -1were used as a boundary condition at the soil surface. These substantial differences in evaporative demand lead to increasing waterloss. Results show that the oxidation process and the associated acidification of the soil are only marginally affected by the different evaporative demands, although the oxidation rate was very much influenced during the early part of the evaporation process.Results from this thesis shows that minimizing the oxidation of pyrite by application of mulch during the dry season and maintaining higher groundwater tables can avoid the acidic hazards. Research along this line should be directed toward the feasible application of this package to the field. In addition, other experiments should be carried out to study the following:- varietal screening for short duration, acid-tolerant, salt-tolerant, iron-tolerant cultivars;- studies on fertilizer application should aim at optimizing the use of phosphate, preferably rock phosphate;- development of quantitative models to predict the effects of drainage at various depths, irrigation and other management practices on the physical and chemical properties of acid sulphate soils.In order to validate these models, basis data on physical and chemical processes under various boundary conditions are needed.

The and and semi-arid regions of the world, totalling about 30% of the land surface of the earth, are predominantly used for extensive grazing, as low and erratic rainfall presents too high a risk for arable farming. The population that can be sustained by the animal products -meat, milk or wool- is largely determined by the availability of primary production for fodder.An increase in primary production in these areas is possible on two levels:- without irrigation: either by an increase in the amount of available water or by a more efficient use of the moisture. The first may be achieved by controlling run-off or deep drainage. The latter through improvement of the botanical composition, mainly by legumes or by application of nitrogen fertilizer;- with irrigation: when water and nutrients are non-limiting, very high production levels may be attained because the environmental conditions, radiation intensity, temperature and length of the growing season are favourable in these regions.This study was done within the framework of the project 'Actual and potential herbage production under and conditions'. This project aimed at summarizing existing knowledge on the functioning of agro-ecosystems in dynamic simulation models, describing plant growth under semi-arid conditions. Such models were developed on three levels of interest:- optimum supply with water and nutrients;- optimum supply of nutrients, but limited moisture;- conditions where either water or nutrients were the limiting factor.Validation data for the models were collected in the field and relevant processes were studied under controlled conditions.This book deals mainly with the different aspects of the relation between water use and plant production under moisture limiting conditions.The validation experiments were carried out at the Tadmor Experimental Farm in the northern Negev desert of Israel (34°25' OL, 31°22' NB. The average rainfall in this area is 250 mm year -1, concentrated in winter (October-March); strong fluctuations between years occur with extremes of 42 mm in '61/'62 and 414 mm in '64/'65. Radiation intensity ranges from 1150 Jm -2day -1in December to 2750 Jm -2day -1in August, while average daytime temperatures range between 12.6°C in January and 26.8 °C in August. The soil consists of a sierozem, developed in a 10-20 m thick mantle of löss. Its physical properties are favourable for plant growth: high water-holding capacity and low resistance to root growth.The vegetation is an abandoned crop land vegetation and consists mainly of herbaceous annuals. The predominant species are the grasses Phalaris minor, Hordeum murinum and Stipa capensis, the crucifers Erucaria boveana and Reboudia pinnata, the compositae Anthemis melaleuca and Centaurea iberica and the legumes Trigonella arabica and Medicago polymorpha. The actual botanical composition shows a great variation both in time and place but this hardly influences the production potential.Experiments were carried out to determine the course of above ground dry matter production and of available soil moisture during the growing season. Dry matter production was determined via a doublesampling technique of visual estimates. In each treatment 200-400 estimations were carried out and each fifth sample was also harvested and weighed. The harvested samples provide a calibration curve, which is used to give an estimate of the average yield of the field.Soil moisture was measured with the neutron moderation technique. In each treatment 30-60 aluminium access tubes were installed and soil moisture was measured over two-weekly periods and after sufficiently heavy showers in 30 cm intervals. The upper 30 cm of the soil were at the same time sampled for gravimetric determination of the moisture content.Meteorological observations were obtained from a standard weather station of the Israeli Meteorological Service, located in Gilat, about 8 km from the experimental site. Rainfall however was recorded in rain-gauges at the experimental site.The results of the experiments, which were carried out in the fields fertilized with P and K as well as in fields that received NPK fertilization show that under the prevailing conditions actual production is more often limited by nitrogen shortage than by moisture availability. Nitrogen application increased the production up to 2-3 fold.Maximum growth rates of the natural vegetation reached values of 170 kg ha -1day -1under optimum conditions. A local wheat variety, used in another experiment attained the same value, which is also equal to the theoretical optimum calculated for the conditions.The efficiency of water use was also the same for the wheat and for the natural vegetation. However the nitrogen deficient fields showed a much lower efficiency of water use, due to either a lower photosynthetic capacity or greater losses through continuous break-down and rebuilding of nitrogenous compounds.Experiments carried out to determine the influence of grazing on primary production showed that under normal grazing pressures, the consumption by animals during the growing season is negligible compared with the growth rates. Only heavy overgrazing may affect the level of primary production.The simulation model 'ARID CROP' developed in the second part of this study, written in the simulation language CSMP/360, describes crop growth under water limiting conditions.A hierarchical approach is applied to incorporate the various processes when their time constant is too small. A detailed model of evaporation from a bare soil is described and the results are compared with both laboratory and field measurements.A simplified procedure to calculate the water use efficiency on a daily basis is worked out, based on the results of a detailed model, described elsewhere. The calculated values are compared with the results of pot trials carried out in the framework of the project. In practically all cases the agreement is within 20% which is considered reasonable in view of the assumptions.Validation of the model shows that reasonable agreement between measured and simulated values is obtained, both for above ground dry matter production and for soil moisture in spite of the limitations of the model. Special difficulties arise from the initialization due to the gross simplifications with regard to germination. Some examples for the application of the model have been described: potential production under moisture limiting conditions is calculated for a 15-year period at the experimental site, based on historical weather data. Comparison with the measured yields shows that mostly actual production was limited by nitrogen availability. Fertilization would not only increase the average yield, but also the fluctuations to about the same range as the rainfall. Management problems will then arise as short- term adaptation of herd size is not possible.These may either be overcome by adaptation of the grazing pressure to some average production level so that food reserves may be build up in good years or by adjustment of the size of the herd to a smaller grazing area. Part of the total available area may be sown with small grain, which will yield seed in favourable years, and may be used as additional grazing area in unfavourable years.Another conclusion derived from this example is, that the distribution of the precipitation over the growing season is of equal importance for the production as the total amount of rainfall. An unfavourable pattern-many small showers-increases losses through direct soil evaporation and limits the amount of moisture available for plant growth.Finally it is shown that spatial inhomogeneity of the soil, which is so troublesome during experimentation counteracts the fluctuations described earlier, mainly through increase of production in unfavourable years.In conclusion it may be stated that the use of dynamic simulation models to establish the production potential of new areas is preferable to the application of correlation models. Simulation models provide a better insight into the relevant processes and have a much greater potential for extrapolation. Moreover they supply a useful tool for the design of relevant experiments when new regions have to be explored.

Afscheid prof. Hellinga

Uitgave 25 jaar ICW

Cultivation and drainage are two major management techniques used to modify soil physical conditions for crop growth. In England and Europe ploughing increased the volume of macropores (pores of diameter >50 μm) but markedly reduced their continuity between topsoil and subsoil compared with direct drilling using a triple disc drill. In Australia, during the first year after pasture, tine cultivation to 100 mm had no greater adverse effect on pore continuity than had direct drilling using a tine drill. The importance of macropores for effective land drainage was evident from the reduced efficiency of mole channels in the presence of a cultivation pan. Root growth and activity were related to the volume of macropores but were impaired when the continuity of those pores was reduced or, as in undrained land, they became water-filled.

Uitgave 25 jaar ICW