For no-tillers interested in improving the productivity of their fields, measuring bulk density of soils at various levels can provide an indicator of overall soil health and any potential effects of compaction on soil structure. 

Bulk density of soil is the oven-dry weight of soil per unit of volume usually standardized with samples at field moisture capacity and is expressed in grams per cubic centimeter (g/cm3). Total volume of soils at the surface layer generally consists of roughly 45% soil particles and usually 5% or less organic matter, and about 50% is relegated to the empty areas surrounding the solids, or pore space.

IDEAL HEALTHY SOIL. An approximate 50/50 ratio of soil solids to pore space makes up what experts term an ideal soil structure, one for which bulk density measurements would be near 1.3 grams per cubic centimeter of an oven-dried soil sample. Source: Paul Jasa

“A healthy soil structure maintains the approximate 50/50 pore space-to-solids balance to enable the soil to ‘breathe’ by allowing air and water to move throughout the soil,” says Paul Jasa, the ag engineer who has overseen the University of Nebraska’s no-till and control tillage plots near Lincoln since 1981. “After a rain, those pores may be filled with 90% water, and in a drought, they may contain 90% air, but with open pore space that air and water can move, which is vital to providing an environment for healthy soil biology.”

Tillage vs. No-Till

Jasa says his years comparing no-till plots with adjacent conventional tillage plots have shown him tillage significantly reduces soil structure through increased compaction, which reduces soil capacity to hold moisture. 

“Solids are not compressible, so physically, any compaction removes water- and air-holding capacity of the soil,” Jasa says. “You have less water stored simply because you have less room for storage. A compacted soil gets saturated quicker when it rains, leading to losses of valuable moisture to runoff, and it dries faster when the rain stops.”

EFFECTS OF COMPACTION. Reduction in pore space through compaction results in less volume for air and water percolation, and it is consistent with soil settling. Bulk density measurements for a compacted soil would typically be higher than 1.3. Source: Paul Jasa

Using a rainfall simulator in field conditions, Jasa compared 13 years of wheat and fallow with tillage management vs. 13 years of wheat and fallow without tillage.

“We applied water at the rate of 2½ inches per hour for 20 minutes,” he says. “With 1 inch of water applied, we had runoff on the tilled ground, showing the results of reduced bulk density. At 90 minutes in, we finally saw runoff the area with standing residue (no tillage) after 3¾ inches of water was applied. The idea of having to till the soil to open it up for water infiltration doesn’t hold water.

“Point blank, tillage always destroys soil structure (pore space) through compaction. As pore space is destroyed, soil particles collapse together to fill the void, and that happens even faster if the soils are wet when tillage occurs. The loss of pore space is compaction.”

Jasa recalls a farm tour at no-till and tillage plots at Kansas State University where the surface level of tillage plots was visibly lower than the adjacent no-till plots.

HIGHER GROUND. Continuous till plots at a Kansas State University field day show several inches of settling effects of compaction and degraded soil structure compared with adjacent no-till plots. Source: Paul Jasa

“That was full-width compaction, found across conventionally tilled fields,” he says. “It’s difficult to see because the entire field is affected, but comparisons to a fence row or another constant like the side of a road will show it.”

Ideal Measurement

Jasa says an ideal bulk density reading of silt-loam soils is typically 1.3 g/cm3. Lower numbers indicate better soil structure, and higher numbers indicate more compacted soils. He notes 20 years of records in his plots show positive effects of not disturbing farm soils.

“Bulk density measurements in the 3–6-inch soil depth at the Rogers Memorial Farm ranged from 1.11 in no-till plots to 1.39 in plots which have been disced over the years. At the 6–9-inch depth, no-till measured 1.2 and disked plots measured 1.45.

MEASURING BULK DENSITY. Two decades worth of comparison between tillage plots and no-till plots show improved bulk density measurements and soil structure associated with lack of soil disturbance. Source: Paul Jasa

“Our no-till plots are building soil structure that is less dense than ideal soils, but it didn’t happen in 1 year. These samples were taken after two decades of continuous no-till, so the takeaway is to stick with it if you’re no-tilling. It’s worth it.”

Click here for more Industry News.