Soil Bulk Density

Unit: Soil Science

Objectives

• Define: ammonification, available water holding capacity, nitrification, bulk density, denitrification, respiration, soil porosity, soil water filled pore space, soil water content, gravimetric, volumetric water content
• List and explain factors that affect soil bulk density
• List and describe soil bulk density management processes
• Measure soil bulk density interpret data

Definitions

Ammonification: occurs in the nitrogen cycle when soil organisms decompose organic-nitrogen converting it to ammonia.

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Available Water Holding Capacity: Soil moisture available for crop growth; also defined as the difference between field capacity and wilting point, typically shown in inches/foot.

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Nitrification: Occurs in the nitrogen cycle when soil organisms convert ammonia and ammonium into nitrite and next to nitrate-nitrogen which is available to plants.

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Bulk Density: Weight of dry soil per unit of volume, more compacted soil with less pore space will have a higher bulk density.

Definitions

Denitrification: Conversion and loss of nitrate nitrogen as nitrogen gases when soil becomes saturated with water.

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Respiration: Carbon dioxide release from soil from several sources (decomposition of organic matter by soil microbes and respiration from roots).

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Soil Porosity: Percent of total soil volume made up of pore space.

Soil Water Filled Pore Space: Percent of pore space filled with water.

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Soil Water Content, Gravimetric: Weight of soil water per unit of dry soil weight.

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Volumetric Water Content: Amount (weight or volume) of water in soil core by volume.

Soil Bulk Density

• Bulk density affects the following:
• Ability of water to infiltrate soil
• Rooting depth of plants
• Available water capacity
• Soil porosity
• Plant nutrient availability
• Soil microorganism activity
• Bulk density is a measurement of the weight of dry soil
• 50% solids (soil particles and organic matter)
• 50% pore space (filled with air or water)

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Factors affecting Bulk Density

• The following factors affect soil bulk density:
• Soil organic matter
• Soil texture
• Density of soil mineral
• Packing arrangement of aggregates
• Presence and amount of rock fragment, soil depth and soil texture affect the water capacity of soil
• Loose, well-aggregated, porous, high organic matter soils have a lower bulk density
• Sandy soils have a higher bulk density because of less pore space

Managing Bulk Density

• Increase soil organic matter
• Use no till
• Use cover crops
• Use solid manure or apply compost
• Use high residue crops
• Minimize soil disturbances
• Avoid equipment operation on wet soil
• Use designated roads or rows for equipment transportation
• Decrease compaction
• Reduce the number of trips across the field with equipment
• Reduce the amount of time livestock are permitted to be on the soil
• Use multi-crop systems
• Plant crops with a variety of rooting depths