Effect of soil tillage system and straw retention on soil aggregation and water capacity

Abstract: Arable land supplies food and it is therefore important to develop the production and land-use in a sustainable way. To grow crops for food should be both economical and environmentally sustainable and the soil structure and quality should be taken in consideration when cultivating our land. We need to find new approaches to maintain good soil structure, and minimized tillage systems have many advantages, including costs for the growing of crops, while leaving straw in the fields can reduce erosion and increase the biological activity and humus content of the soil. The texture has two important physical properties when it comes to indicate the soil quality those are aggregate stability and size distribution. The particle size distribution is the most essential physical property which defines the soil texture, and influences the soil properties the most. These two physical properties mentioned above reflects the resistance of soil erosion, especially in no-tillage system, which is why they are the most important factors when it comes to soil quality. The soil structure defines which different types of particles that are stored in the soil and it exert control over the physical, biological and chemical processes. It also explains how and where the particles are located, which is important for how suitable the soil is for growing crops. If the soil has a poor structure, it can affect the nutrient availability and the nutrient uptake negatively and increase the power requirement for tillage, increase the nutrient loss and the denitrification, which is negative from an environmental point of view. Organic matter, tillage system, biological activity etc. matters for the aggregate structure in a soil. There are natural structure building processes, such as root development and drying, but there is also structure depleting processes, which basically all the human activities are. A non-cultivated soil generally has a better structure due to the generally higher content of organic matter and less compaction than a cultivated soil has. Soil structure is being influenced by soil and crop management inputs and has an impact on soil quality. One of the factors that influence the quality is tillage. This input is an important factor and relevant in the point of sustainability, that is why the quality of the soil is depending on the choices of human activities. A soil with higher proportion of clay and humus usually increases the stability of structure and aggregates. Aggregate stability is characterized by the sensitivity to external influence. The essence of aggregate stability is the organic matter, because large parts of plants and roots acts like a barrier and prevent aggregates to break into smaller units with help from decomposing of microorganisms that provides with an adhesive effect. The factors that influences the soil aggregate stability is soil texture, soil structure, the different types of clay minerals, the content and different types of organic matter, cementing agents and which kind of crops that were grown through the history. Permeability is the property of a material that lets fluids to diffuse through the medium without being affected chemically or physically, that is the soil´s capacity to drain off water. The structure of a soil is influenced in both long and short term of tillage and cultivation measures, which in turn affects the soil physical properties. Vegetation and recycling of organic matter contributes to a better structure and physical environment. Soil cultivation measures do the opposite, even though tillage contributes to structural stabilization and structural-building processes. If the structure should be improved, the structure-building measures needs to be greater than the structure depleting measures. Adding organic matter can preserve soil structure and increase the crop safety. Measures to improve the structure and provide better conditions for the crops, is to return straw and crop residues to the soil, grow cover crops in the autumn and only apply shallow tillage, which could increase the humus content in the top layer. Increased humus content will give a lower bulk density, increased aggregate stability and increased porosity, which in turn give the soil increased water holding capacity and infiltration capacity. The macro pores is responsible for the soils capillary ability, it provides the plants with available moisture. If the moisture is in the narrow pores, micro pores, the plant roots needs to develop an increased suction force to be able to absorb the moisture. The greatest amount of plant available moisture is found in silty loam soils, while the soil with the least amount is sandy soils because of their inability to bind water due to its larger particles. Heavy rains can also damage the aggregates in the topsoil if the soil is uncovered or unfrozen, which is why organic matter and straw incorporation could prevent damage of this type. Ploughless tillage and direct drilling gives favourable structure development in the topsoil, and green manure and cover crops are often suggested as effective methods to increase the organic matter, along with reduced tillage system. Though, the experiment at Aleksandras Stulginskis University in Lithuania shows that no-tillage system has the highest level of compaction of the soil compared to deep ploughing system. On the other hand, another experiment in Sweden, with ploughless tillage and straw incorporation, has showed that ploughless tillage system gives a reduced compaction, though; straw treatment are facing problems, such as “straw stops” while cultivating the soil with different tillage methods. If the straw should be incorporated, it needs to be finely chopped and evenly spread evenly over the field. At Aleksandras Stulginskis University in Lithuania, a long-term field experiment has been running since 1999 in the Experimental Station, Kaunas district. The experiment is made by six different tillage systems: deep ploughing; shallow ploughing; shallow loosening with sweep cultivator and disc harrows; shallow loosening with rotor cultivator; catch crops & green manure incorporation with rotor cultivator; and no tillage. Another factor of this experiment is about straw incorporation and straw removal in the different tillage systems. The soil type of this field is sandy loam. The soil samples have been analysed in the laboratory of Aleksandras Stulginskis University to investigate which impact the different tillage systems and straw incorporation or straw removal have on the soil aggregate stability, soil structure and water capacity. The experiment showed that with straw incorporation in 0-10 cm depth there were less micro aggregates than in the treatment were straw was removed. The aggregate stability was higher in 10-25 cm depth with straw incorporation compared to straw removal. Shallow loosening was the treatment which gave the highest bulk density in both depths, which means that the soil with this treatment was more compacted than with deep ploughing. No-tillage treatment had lower bulk density in the deeper layer, which means that this soil had more porosity. Deep ploughing had a tendency not to be able to hold a high amount of water at 0-10 cm depth, up to -300 hPa, while no-tillage treatment in the deeper layer could hold water the best at lower pressures. In the treatment with shallow loosening, the porosity decreased, while in the no-tillage treatment the porosity increased.