The supply and accumulation of silica in the marine environment

This review reveals that crop residues of common cultivated crops are an important resource not only as a source of significant quantities of nutrients for crop production but also affecting soil physical, chemical, and biological functions and properties and water and soil quality. When crop residues are returned to the soils, their decomposition can have both positive and negative effects on crop production and the environment. Our aim as agricultural scientists is to increase the positive effects. This can only be achieved with the better understanding of residue, soil, and management factors and their interactions, which affect the decomposition and nutrient release processes. Data on nitrogen benefits and nitrogen recoveries from residues show that a considerable potential exists from residues, especially leguminous residues, not only in meeting the N demands of the succeeding crops, but also in increasing the long-term fertility of the soils. In addition, crop residues and their proper management affects the soil quality either directly or indirectly. Intensive cropping systems are very diverse and complex, so no one residue management system is superior under all situations. Ideally, crop residue management practices should be selected to enhance crop yields with a minimum adverse effect on the environment. It is suggested that in each cropping system, the constraints to production and sustainability should be identified and conceptualized to guide toward the best option. Multidisciplinary and integrated efforts by soil scientists, agronomists, ecologists, environmentalists, and economists are needed to design a system approach for the best choice of crop residue management system to enhance both agricultural productivity and sustainability.

Crop Residues and Management Practices: Effects on Soil Quality, Soil Nitrogen Dynamics, Crop Yield, and Nitrogen Recovery

Although soil compaction in the field may benefit or inhibit the growth of plants, the harmful effects are much more common. This paper emphasizes the deleterious effects of predominantly high levels of soil compaction on plant growth and yield. High levels of soil compaction are common in heavily used recreation areas, construction sites, urban areas, timber harvesting sites, fruit orchards, agroforestry systems and tree nurseries. Compaction can occur naturally by settling or slumping of soil or may be induced by tillage tools, heavy machinery, pedestrian traffic, trampling by animals and fire. Compaction typically alters soil structure and hydrology by increasing soil bulk density; breaking down soil aggregates; decreasing soil porosity, aeration and infiltration capacity; and by increasing soil strength, water runoff and soil erosion. Appreciable compaction of soil leads to physiological dysfunctions in plants. Often, but not always, reduced water absorption and leaf water deficits develop. S...

Soil Compaction and Growth of Woody Plants

The structure of soil can determine both the effectiveness and the impact of farming practices. Soil structure influences the extent to which crop breeding and the management of weeds, insects, disease, soil fertility, and water are manifested in increased crop yields. Soil structure also influences the loss of agricultural chemicals through erosion and leaching and can, therefore, have a significant bearing on the environmental impact of some agricultural practices.

Rates of Change of Soil Structure Under Different Cropping Systems

Oxides, Hydroxides, and Aluminosilicates

The cutting of soil by narrow blades

Very little research has been done to investigate soil loosening as a function of the geometry of the tillage tool and of the original soil properties and moisture content. A field experiment was conducted to observe the effects of the geometric parameters of flat tillage tools on their draft, cutting efficiency and loosening of a moist clay soil. The test tool variables included rake angles to the horizontal of 30, 60 and 90°, widths of 75 and 150 mm and depths of operation of 100, 150 and 200 mm. Measurements were taken of draft, disturbed soil cross sectional profiles and the initial area of soil disturbed by the tools. The resulting draft requirement increased with width, depth and rake angle of the tool. The cross sectional area of soil disturbed did not change appreciably with rake angle, but the significant increase in draft with angle resulted in markedly diminished soil cutting efficiency (area divided by draft). The degree of soil loosening was generally smaller at a rake angle of 60° than at 30 or 90°, and tended to be higher at greater depths of operation. In addition, a larger depth to width ratio generally increased the degree of loosening. Results for the soil studied indicate that the best implement design for low draft, high cutting efficiency and superior soil loosening should have a rake angle of about 30° and should be fairly narrow with a depth to width ratio of 2 or more.

Effect of design parameters of flat tillage tools on loosening of a clay soil

T HE shear strength of compacted soils is affected by many factors including soil density, overburden pressure, moisture content, energy applied for compaction and soil type. Measurements were made of the shear strength of three soil textures, varying in organic matter contents and subjected to three levels of compaction energy. Soil compaction increased the shear strength of the soils irrespective of moisture content, while organic matter incorporation decreased their shear strength. Based on the liquid limit, the pressure applied for compaction and the water content, a method for predicting the shear strength of soils was developed. The estimated shear strength using the model was in good agreement with the measured values.

Shear Strength Prediction of Compacted Soils with Varying Added Organic Matter Contents

Post-glacial Champlain Sea sediments in Quebec and Ontario are often extremely sensitive. Suggestions of considerable amorphous material in these soils come from X-ray diffraction results, as well as electron micrographs. A method for selective dissolution and analysis of amorphous material is used to study two Champlain Sea deposits. The amorphous material removed amounts to 11–12 per cent of the soils and consists mainly of silica and iron oxide. The treated samples have enhanced X-ray diffraction peaks and much cleaner particles as seen by electron microscopy.

Amorphous Coatings on Particles of Sensitive Clay Soils

Field experiments were conducted in clay and sandy clay loam soils using flat tillage blades of varying width, depth and rake angle. Measured quantities of draft force and disturbed soil areas for the different test conditions were compared to those predicted by a model of soil wedge failure in front of narrow blades, with reasonable agreement. Both the specific draft force per unit soil area and the degree of soil loosening were observed to increase with the relative narrowness of the tillage blades and with the rake angle, as predicted by the model.

E. McKyes

Papers

The cutting of soil by narrow blades

Effect of design parameters of flat tillage tools on loosening of a clay soil

Shear Strength Prediction of Compacted Soils with Varying Added Organic Matter Contents

Amorphous Coatings on Particles of Sensitive Clay Soils

Prediction and field measurements of tillage tool draft forces and efficiency in cohesive soils