15.2: Diagnosing Different Types of Compaction

The first step is to decide whether compaction is a problem and which type is affecting your soils. The symptoms, as well as remedies and preventive measures, are summarized in Table 15.1.

Table 15.1 Types of Compaction and Their Remedies

Compaction type	Indications	Remedies/prevention
Surface seal or crust	Breakdown of surface aggregates and sealing of surface Poor seedling emergence Accelerated runoff and erosion	Eliminate tillage or reduce tillage intensity Maximize surface cover: leave residues on surface, grow cover crops Add organic matter
Surface layer	Deep wheel tracks Prolonged saturation or standing water Poor root growth and more disease symptoms Hard to dig and resistant to penetrometer Cloddy after tillage	Use zone builders or strip tillers to break compaction but minimize soil disturbance Use cover crops or rotation crops that can break up compacted soils Add organic matter Use better load distribution with equipment Use controlled traffic Don’t travel on soils that are wet Improve soil drainage
Subsoil	Roots can’t penetrate subsoil Resistant to penetrometer at greater depths	Don’t travel on soils that are wet Improve soil drainage Till deeply with a zone builder or strip tiller Use cover crops or rotation crops that penetrate compact subsoils Use better load distribution Use controlled traffic Don’t use wheels in open furrows

The impact of surface sealing and crusting is most damaging when heavy rains occur between planting and seedling emergence, when the soil is most susceptible to raindrop impact. Keep in mind that this may not happen every year. The hard surface crust may delay seedling emergence and growth until the crust mellows with the next rains. If such follow up showers do not occur, the crop may be set back. Crusting and sealing of the soil surface also reduce water infiltration capacity, which can increase runoff and erosion and lessen the amount of available water for crops.

Surface Sealing and Crusting

This type of compaction occurs at the immediate soil surface when the soil is exposed. It may be seen in the early growing season, especially with clean-tilled soil, and in the fall and spring after a summer crop (Figure 15.1). Certain soil types, such as sandy loams and silt loams, are particularly susceptible. Their aggregates usually aren’t very stable, and once broken down, the small particles fill in the pore space between the larger particles, making very dense crusts.

Figure 15.1. Rainfall energy destroys weak soil aggregates and creates a surface seal that increases runoff potential. This photo is of soil in the wheat-growing Palouse region of Washington state. When it dries, the seal turns into a hard crust that prevents seedling emergence.

Surface Layer Compaction

Figure 15.2. Large soil clods after tillage are indicative of compaction and poor aggregation.

Compaction of the layer immediately below the surface can often be observed in the field through deep wheel tracks, extended periods of saturation, or even standing water following rain or irrigation. Compacted surface layers also tend to be extremely cloddy when tilled (Figure 15.2). A field penetrometer, which we discuss in greater detail in Chapter 23, is an excellent tool to assess soil compaction (you can also push a simple wire flag into the soil). Digging with a shovel allows for direct visual evaluation of soil structure and rooting, as well as of the overall quality of the soil. This is best done when the crop is in an early stage of development but after the rooting system has had a chance to establish. Well-structured soil shows good aggregation, is easy to dig and will fall apart into granules when you throw a shovelful on the ground. If you find a dense rooting system with many fine roots that protrude well into the soil, you probably do not have a compaction problem. Conversely, roots in a compacted surface layer are usually stubby and have few root hairs (Figure 15.3). They often follow crooked paths as they try to find zones of weakness in the soil. Compare the difference between soil and roots in wheel tracks and nearby areas to observe compaction effects on soil structure and plant growth behavior. Note that recently plowed soils may give a false impression of compaction: they are initially loose but will likely compact later in the season. No-tilled soils generally are firmer but have stronger structure and contain large pores from worm activity.

Figure 15.3. Corn roots from a compacted surface layer are thick, show crooked growth patterns, and lack fine laterals and root hairs.

Compaction may also be recognized by observing crop growth. A poorly structured surface layer will settle into a dense mass after heavy rains, leaving few large pores for air exchange. If soil wetness persists, anaerobic conditions may occur, causing reduced growth and high denitrification losses (exhibited by leaf yellowing), especially in areas that have drainage problems. In addition, these soils may “hard set” if heavy rains are followed by a drying period. Crops in their early growth stages are very susceptible to these problems (because roots are still shallow), and the plants may go through a noticeable period of stunted growth on compacted soils.

Reduced growth caused by compaction also affects the crop’s ability to fight or compete with pathogens, insects and weeds. These pest problems may become more apparent, therefore, simply because the crop is weakened. For example, during wet periods, dense, poorly aerated soils are more susceptible to infestations of fungal root diseases such as Phytophthora, Sclerotinia, Fusarium, Pythium, Rhizoctonia and Thieviopsis and plant-parasitic nematodes such as northern root-knot. These problems can be identified by observing washed roots. Healthy roots are light colored, while diseased roots are black or show lesions. In many cases, soil compaction is combined with poor sanitary practices and lack of rotations, creating a dependency on heavy chemical inputs.

Subsoil Compaction

Subsoil compaction is difficult to diagnose because the lower soil layers are not visible from the surface. The easiest way to assess compaction in deeper soil layers is to use a penetrometer, which should be done when the soil is field-moist (not too wet, not too dry). It is surprising how often you find the tool hitting much higher resistance once it reaches the bottom of the plow layer—typically down 6–8 inches—even if it has not actually been tilled for awhile. Rooting behavior below the surface layer is also a good indicator for subsoil compaction, assuming you are willing to expend some effort digging to that depth. Roots are almost completely absent from the subsoil below severe plow pans and often move horizontally above the pan (see Figure 6.8). Keep in mind, however, that naturally shallow-rooted crops, such as spinach and some grasses, may not necessarily experience problems from subsoil compaction.

Some soils are naturally susceptible to the formation of dense subsoils when they become intensively cropped. When soil aggregates become weaker from loss of the organic matter, silt and clay particles can wash down and settle in the subsoil pores, thereby creating a dense layer. This is especially a concern with soils that contain about equal amounts of sand, silt and clay, and where the clay minerals are of the non-swelling 1:1 type. Also, tropical oxisols have naturally high clay contents with very strong aggregates, but when they are limed, the raised pH causes the clay particles to disperse and wash into pores in the lower soil.

Crops That Are Hard On Soils

Some crops are particularly hard on soils:

• Root and tuber crops like potatoes require intensive tillage and a lot of disturbance at harvest. They also return low rates of residue to the soil.
• Silage corn and soybeans return low rates of residue.
• Many vegetable crops require a timely harvest, so field traffic occurs even when the soils are too wet.

Special care is needed to counter the negative effects of such crops. Counter measures may include selecting soil-improving crops to fill out the rotation, using cover crops extensively, using controlled traffic, and adding extra organic materials such as manures and composts. Some potato farmers in New York and Maine are known to rotate fields with dairy farmers who convert them into soil-building alfalfa and grass. In an 11-year experiment in Vermont with continuous corn silage on a clay soil, we found that applications of dairy manure were critical to maintaining good soil structure. Applications of 0, 10, 20 and 30 tons (wet weight) of dairy manure per acre (1 ton per acre equals 2.2 metric tons per hectare) each year of the experiment resulted in pore spaces of 44%, 45%, 47% and 50% of the soil volume, respectively.