Crop Comments: Hypoxia: Bad for animals, crops & soils
In a video clip filmed in North Carolina’s Outer Banks, a 4WD truck grew mired. Its 60 PSI tires dug deeper and deeper, rendering the vehicle helpless as its operator tried to back away from the mess. The narrator reduced tire pressure to 15 PSI, enabling the truck to easily back out of its sand trap and get repositioned on granular soil, then quickly return to effortless forward motion.
The narrator pointed to the tire track width left by the two different pressures: the 15 PSI track was approximately 40% wider than the 60 PSI track. The greater contact between tire and soil surface markedly lowers soil compaction.
University of Minnesota (UMN) Cooperative Extension published a bulletin with the title “Soil Compaction” (extension.umn.edu/soil-and-water/soil-management-and-health). Soil compaction concerns have been on the rise in that state as both annual precipitation and farm equipment size have increased. Wetter soils are particularly susceptible to compaction, as heavier equipment and tillage implements multiply damage to soil structure, decreasing pore space, further limiting soil and water volume.
This is unlike well-structured soil, which holds and conducts water, nutrients and air needed for healthy plant root activity.
Compaction occurs when soil particles press together, reducing pore space between them. Heavily compacted soils contain few large pores, thus less total pore volume, hence greater density. Such soils suffer from reduced rates of water infiltration and drainage: large pores move water downward more effectively than do smaller pores.
Compacted soils also reduce the exchange of gases, increasing the likelihood of aeration-related problems. Also, roots must exert greater force to penetrate compacted soils; this force takes energy from crops, which could have been used for production.
The UMN researchers dispelled one common soil compaction myth: that temperate climates’ freeze/thaw cycles counteract machinery-induced soil compaction. Although soils in the northern half of the continental U.S. are subject to yearly freeze/thaw cycles (with frost depths sometimes surpassing three feet), only the top two to five inches experiences more than one freeze/ thaw cycle each winter.
The idea of such freeze/thaw cycle benefits developed years ago when compaction was relatively shallow, machinery weighed much less and rotations included more grass and deep-rooted legumes. Now, heavier axle loads and wetter soil conditions increase compaction’s downward reach.
USDA-NRCS research shows that respiration increases along with soil moisture. However, that oxygen was limited when soil pores filled with water, hindering soil organisms’ respiration. Ideal soil moisture is reached when a maximum of 60% fills with water. When water fills more than 80% of pore space, soil respiration drops to a minimum and most aerobic organisms begin to use nitrate (NO3) instead of oxygen. This results in nitrogen loss (as N2 and nitrogen oxide gas), along with potent greenhouse gases kicked loose into the atmosphere. With this situation, we should expect reduced yields and increased nitrogen requirements.
While hypoxia is normally a medical term, the word also deals with oxygen depletion in the soil, restricting essential gas exchange. Usually caused by waterlogging, flooding or soil compaction, hypoxia stifles root respiration, impairs nutrient uptake and promotes the proliferation of toxic, anaerobic microbes. This leads to stunted plant growth, root rot and a rapid decline in overall soil fertility.
Certified Crop Advisor Tom Kilcer explains that a load of 10 tons/axle or more, on soggy soils (increasingly common with modern tractors), extends compaction to depths of at least two feet. Because this is well below normal tillage depth, such compaction is more likely to persist, compared to shallow compaction that can largely be removed by tillage. Raindrops landing hard on bare soil cause compaction naturally, as evidenced by soil crust thickness – usually less than a half-inch thick on the soil surface – but still capable of interfering with seed emergence. Fortunately, rotary hoeing usually eliminates this problem.
There are tests which measure soil oxygen. Most of them do so indirectly by assaying soil carbon dioxide (CO2). Most soil microbes behave more like animals than plants, emitting CO2, not oxygen. Much of the soil CO2 reacts with soil moisture to form carbonic acid, which is quantified (assigned numerical values) to become a measure of soil biology. Higher levels of soil carbonic acid mean that more oxygen got respired, hence more active soil life. More CO2 remaining in soil means that plants have more building blocks with which to perform photosynthesis.
This also means that less carbon escapes into the atmosphere as the greenhouse gases CO2 and methane.
Some weeds actually help growers score soil biology. For example, nutsedge and fall panicum are encouraged by the presence of anaerobic soil environments. These weeds thank crop growers for compacting soil, thus squeezing available oxygen into the atmosphere. Crops don’t share that gratitude, saying “no thanks” by lowering their yield performance.
Note: Anaerobic microbes are ones that thrive in the absence of oxygen (thus often found in compacted or waterlogged soils). Many disease-causing pathogens and root-rotting bacteria fall into this category, greatly enjoying a hypoxic environment.
by Paris Reidhead
