Quantifying differences in soil structure induced by farm management

Methodology for defining sustainable land management practices is increasingly needed to overcome environmental problems and to maintain production potentials. From the large amount of definitions for sustainable management the following was used here: "Sustainable land management combines technologies, policies and activities aimed at integrating socio- economic principles with enviromnental concerns so as to simultaneously: (i) maintain or enhance production and services; (ii) reduce the level of production risk; (iii) protect the potential of natural resources and prevent degradation of soil and water quality; (iv) be economically viable and (v) socially acceptable". Indicators to quantify sustainability are used to analyse the effects of different management types on the soil structure within one soil series in the Netherlands, a loamy, mixed, mesic, Typic Fluvaquent. It is logical to concentrate on soil structure when evaluating and comparing the effects of different management practices as they reflect management practices at an integrated level. Comparison was focused on three soil structure types, formed by different management practices. (i) A biodynamic system (Bio) where no chemical crop protection or commercial fertiliser has been applied since 1924. Animal manure and a crop rotation system with clover are intended to supply the required nutrients. (ii) A conventional system (Conv) representing a management system that is most common in the region. (iii) A system which has been permanently meadow since 1947 (Perm).The biodynamic and conventional system were compared by converting "static" soil parameters, like organic matter content, bulk density, hydraulic characteristics, into a "dynamic" assessment by using a simulation model to calculate water-limlited productivity. A thorough soil characterisation was made, including morphological and physical characterisation as well as monitoring of soil water contents and groundwater levels. Results of the comparison between simulated and measured moisture contents were such that the model was considered to be adequately validated. The simulated water-limited productivity for potatoes was significantly higher for the biodynamic system, indicating a favourable effect of the higher organic matter content. (Chapter 1)Modern mechanised agricultural practices require soils to be able to be subjected to tillage and traffic, without adverse effects on soil structure. Threshold values for workability and trafficability were obtained for the three management types. Workability by the Atterberg test, trafficability by penetrometer measurements and an additional field-traffic experiment. Threshold values for Conv were most favourable, i.e. during relatively wet conditions Conv could still be tilled and trafficked. Periods of workability and trafficability were obtained by combining measured threshold values with simulated moisture contents. Conv had the longest workable and trafficable period in a year followed by Perm and Bio, respectively. (Chapter 2)Applying appropriate spatial and temporal scales is essential in quantifying sustainability. The appropriate spatial scale was analysed by comparing the effect of averaging hydraulic characteristics of each management type. The required temporal scale, i.e. how many years should an experiment be continued so as to minimise effects of weather variation, was studied by using 30 years of historical weather data. The ratio between simulated actual and potential transpiration was analysed for these 30 years. Results show that averaging hydraulic characteristics within each management type does not yield significant differences in comparison with using individual point data followed by averaging. The simulated transpiration ratios of the 30 years of weather data indicate that the length of an experiment should be at least seven years to significantly reduce the effect of the variable weather conditions. (Chapter 3)Another temporal scale problem is related to the leaching of pollutants towards the groundwater, i.e. the definition of the time-aggregation level over which critical threshold values should be considered. Simulated nitrogen leaching was compared for the three management types at different time-aggregation levels, ranging from one day till 30 years. Leaching of nitrogen was highest for Conv followed by Bio and Penn, respectively. A clear double-log relationship exists between aggregation level of nitrogen leaching and the number of time elements during which the threshold value was exceeded. The appropriate aggregation level will have to be based on the travel time from the groundwater below the considered area to the drinking well. (Chapter 4)Macro-porosity of the three management types, was quantified by staining technique combined with digital processing. This study was made to investigate occurrence of bypass flow which invalidates simulations assuming soils to be isotropic. Each of the 55 measured staining patterns was quantified by 17 different parameters defined in terms of size, shape and distribution. Continuous macro-pores were mainly restricted to the topsoil for the three management types. Factor analysis shows that the most important factor, the "total pore quantity", was similar for the three fields but had a clear depth effect. Factor two and three, the "individual pore quantity" and the "shape", were influenced by the applied management. (Chapter 5)A key aspect in defining sustainable land management is the capacity of a soil to supply roots with water and nutrients. The concept water "availability" implicitly assumes all soil water to be "accessible" which may not be the case in coarse structured soils such as occur in Bio and Conv. A transpiration experiment with barley, using 15 undisturbed 6 dm 3soil cores, showed that Penn had the highest supply capacity followed by Conv and Bio, respectively. The "rooting pattern characteristic" combined with the "water supply characteristic" was used to quantify the accessibility of soil water. Accessibility was highest for Perin and Conv, and lowest for Bio. (Chapter 6)The genetically defined soil series, a "geno-form", can have various "pheno-forms" as a result of different management practices. Each "geno-form" will therefore have a characteristic range of soil structures, which was quantified by a dynamic simulation model. Four scenarios were quantified for the three "pheno-forms" based on productivity and nitrogen leaching towards the groundwater. Results illustrate that environmental regulation can be based on "acceptable" risks and that soil series (genoforms), cannot be evaluated with a representative soil profile in specific field-level studies. (Chapter 7)An extensive field survey, within one soil series, was performed to relate farm management practices, present and past, with organic matter contents. From the 15 analysed management practices a regression equation was developed which can, in turn, be used to recommend management practices which are likely to lead to a desired organic matter content. This exploratory study shows the combined use of soil survey and management information in defining a soil quality indicator represented here by the organic matter content. (Chapter 8)Soil survey could focus in future on defining sustainable forms of landuse, considering that soils within one soil series are not similar. Farm management information combined with quantifying the associated soil structure types by simulation models, can form the basis for defining sustainable management systems for the soil series being studied. As many soil surveys are completed all over the world, the proposed procedure appears to be a worthwhile continuation of the rich tradition of soil survey research.