You know that something’s wrong when your field doesn’t soak up the rain like it used to, but instead puddles after a light shower; or roots that should reach three feet stop at eight inches. Or yields just slide a bit from year to year for no apparent reason: no sign of drought or any other obvious culprit. In all of these scenarios, underlying cause may be simply this: the bulk density of the soil has crept up, meaning there’s more dry soil packed into a certain volume of space.
That shuts off the spaces where water and air are meant to reside, and the soil fail to behave the way soil should. What does that all mean in practical terms for growing plants? Here’s a chart (above) of what various bulk density measurements actualy mean. It takes a number from the lab and turns it into a decision: Is your soil still within bounds that support healthy root growth… Or has it gone past that point?
What Is Soil Bulk Density?
That sort of translation is important, since most farmers don’t think in terms of grams per cubic centimeter when they are walking their fields. They see if the earth is easy or difficult to work; they see if water is pooling on top. Now we know a little more about what that means.
There are several common causes of increased bulk density. The most obvious culprit is heavy equipment running across wet ground. Pounding rains on bare soil also creates a surface crust that seals off the top few inches. Particles settle over time, adding tiny increments of compaction pressure.
Not all types of soils responds similarly to these compaction pressures. Because of their different particle arrangement, a heavy clay will restrict root growth at lower weights different than a sandy loam. When compaction begins to increase, however, various crops respond differently, as this chart illustrates. Surprisingly, carrots and potatoes is sensitive souls. Storage roots must have looser soil in order to swell up without splitting or forking.
Wheat and corn fare better with modest compaction; they also experience some loss of yield if roots can’t get down deep enough to reach nutrients and moisture further along. Long taproots like those of alfalfa and sugar beet fare worst from a hard layer around the thirty-centimeter mark; they cannot pierce it. Understanding your crop mix will help you gauge just how urgent the issue may be.
To test or not to test? Equipment doesn’t have to be fancy. An oven plus a simple core sampler will yield consistent results if used correctly. This means collecting undisturbed samples and then drying them properly. A penetrometer (a quick in-the-field check) is popular with many growers, too. Generally speaking: If the rod reaches 300-plus pounds per square inch of resistance before poking through, that’s typically a sign of sufficient density to restrict roots.
Consistency is key. Repeat tests at same locations every couple of years and see if what you’re doing is improving things (or making it harder).
To fix compaction… well, there’s no one thing. Sometimes just deep ripping breaks apart a hardpan (though not if the soil is too wet to shatter instead of smear). Aggressive-root cover crops like tillage radish makes channels for water and subsequent crops to use. Building up soil organic matter, whether with manures or compost (slowly reduces its density); organic particles are lightweight compared to mineral ones, and help hold them together in stable aggregates. Keeping machines’ wheels on fixed paths called controlled traffic prevents repeat passes that otherwise damage all but the path. These measures don’t work alone, but together restore the balance toward more pore space for plant use.
With good practice the most noticeable results are on the surface: you’ll find quicker infiltration and easier root growth in the upper part of the soil within a few seasons. Lower down it will improve more gradually; that’s where keeping track over multiple years is important, rather than hoping for instant gratification. You don’t want everything uniformly perfect all along (according to some bookish standard), just loose enough up and down the profile to suit whatever you’re growing.
But ultimately, what are some of these quiet variables that pay off if you think about them before they cost you big? Bulk density is one such variable. A well-draining, aerated root zone allow for good plant growth by holding more water when it’s needed and providing space for roots to function effectivey. The advantage builds over time; it’s one reason why this variable should of its place in anyone’s serious soil management plan.