The Effect of the Soils Bulk Density

A soils bulk density plays a huge role in determining the effectiveness of the soil and the future productivity of a farming operation. Bulk density reflects the soil’s ability to function for structural support, water and nutrient and microbial life movement, and soil aeration. Bulk density depends on soil make-up for example, course textured sandy soils have less pore spaces than fine textured soils such as a loam which has high porosity. However management practices can also have a dramatic effect on Soil bulk density. In NZ many farms have seen there soils bulk densities increase over time decreasing soil productivity and forcing farmers to reach for more and more ag-chemicals and fertiliser to chase crop yields. These practices which can lead to poor bulk density include:

  • Heavy and consistent tillage
  • Large machinery use, especially on wet soil,
  • Using a limited crop rotation without variability in root structure or rooting depth,
  • Incorporating, burning, or removing crop residues,
  • Overgrazing forage plants
  • Overstocking land

Problems associated with a High Soil Bulk Density

A high soil bulk density can negatively affect soil physical properties, and therefore can limit microbial activity and biochemical processes, which are crucial for nutrient availability. Bulk density of the soil is also important from the point of view of plant growth especially root penetration. A shallow plant root and poor plant growth resulting from compacted and high bulk density soils will influence crop yield and reduce vegetative cover available to protect soil from erosion. It also affects soil aeration, which influence uptake of water and nutrients. By reducing water infiltration into the soil, this can lead to increased runoff and erosion from sloping land or waterlogged soils in flatter areas. 

Reducing Soil Bulk Density and Improving Soil Structure

As a farmer, management plays a huge role in determining the bulk density of your soil. Any management practices that proactively improve soil structure will decrease bulk density. There is a caveat to this as in some cases these improvements may be temporary. An example of this is heavy tillage at the beginning of the season will temporarily decrease bulk density but heavy machinery climatic events, stock, and other disturbance activities can re-compact soil.

Long-term solutions to bulk density and soil compaction problems revolve around decreasing soil disturbance and increasing soil organic matter. This is where biological inputs can help. Applying a biological input like EM, accelerates the soil building process, helps to breakdown soil organic matter and build humus and promote beneficial microbial communities. EM will help the decomposition process of organic materials, and during fermentation will produce normally unavailable organic acids, such as lactic acid, acetic acid, amino acid, malic acid and bioactive substances and vitamins. The soil building process revolves around soil aggregate formation. Soil aggregates form as mineral and decomposed organic matter particles are bound together by microbial secretions that act as glues. Stable and well-formed soil aggregate is the key to overall soil function.

In addition to inputs like EM incorporating cover crops, crop residues, compost, and less ground disturbance e.g. reduced tillage, will lead to an increase in soil organic matter, less structural damage and reduced bulk density. In addition diverse cropping and pastoral species, specifically targeted at the use of plants with different rooting depths can help break up compacted soil layers.

To reduce the likelihood of high bulk density and compaction:

  • Minimise soil disturbance and production activities when soils are wet,
  • Reduce impact of heavy machinery on soils
  • Subsoil to disrupt existing compacted layers, and
  • Use practices that maintain or increase soil organic matter including microbial inoculants like EM.

The Effect of EM on Bulk Density - Trial Data

In this trial published in the European Journal of Agronomy (vol. 46, April 2013), they looked at Long-term effective microorganism’s application to promote growth and increase the yield of rice. This trial, conducted over 11 Years, found that the long-term application of EM caused significant changes in soil physical-chemical properties. Soil bulk density and pH were significantly (p < 0.05) lower in the two compost plots than in the control plot. Moreover, soil pH was significantly (p < 0.05) lower in the EM compost plot than in the traditional compost plot. Soil organic matter, total N, alkaline-hydrolysable nitrogen, and available K content was significantly (p < 0.05) higher in the two compost plots than in the control plot.

In this next trial published in the African Journal of Microbiology Research, they looked at the influence of EM and green manure on soil properties and productivity of pearl millet and alfalfa grown on sandy loam. This trial found that Soil Bulk density was decreased by using green manure and EM application. In addition application of green manure and EM1 in both fields increased the retained water in treated soils. In fact, the obtained results of SOM, Bulk density and saturated hydraulic conductivity may explain the results of soil water retention. Increasing the soil moisture retention capacity might be due to the enhancement in physical parameters of soil including the increasing of SOM, deceasing of bulk density and saturated hydraulic conductivity. Graphs below