Soil Crusting

Bare soils can suffer from crust formation. The effect seems to be more serious in spring after freezing and thawing cycles cause aggregates to disintegrate. Soil crusting has serious effects on soil productivity and function. It leads to decreased infiltration rate, reduction of soil moisture, increased runoff, and reduced groundwater recharge. The hydraulic conductivity of a crust may be a hundred or even a thousand times lower than that of the underlying soil. Crusting also slows or completely inhibits seedling emergence.

There are three types of crusts: chemical, biological, and physical crusts. Chemical crusts are the result of encrusted salt on soils in arid or semi-arid regions. Biological crusts are primarily formed by cyanobacteria, algae, mosses, and lichens growing on soils when ponded water stands and retreats on soils with low permeability. Physical crusts are the result of structural degradation of the surface soil and may be classified as structural or depositional (see photo). Physical crusts are most common in Pennsylvania and are discussed here. Structural crusts are formed when surface aggregates disintegrate due to the forces of raindrops or irrigation water. They typically consist of two layers: a very thin skin underlain by a thicker (up to one inch thick) washed-in layer. Depositional crusts are formed by transport and deposition of fine particles from eroded surfaces to depositional areas.

Both rainfall characteristics and soil properties influence crust formation. Heavy, intense rain has high kinetic energy primarily because of large drop size and high rainfall rate. Similarly, there are differences between irrigation systems due to drop size distribution reaching the soil surface. Normally, the highest intensity rainfall is experienced in summer in our state. Rainfall characteristics vary across the state, high intensity rainfall increasing from north to southeastern PA. Due to higher rainfall intensities, crust formation can be expected to be a greater problem in southern than in northern PA.

The most important explanatory factor of differences in crusting, however, is the soil. It seems that most of our soils are susceptible to crusting because they have large, medium, and small soil particles on the surface. When the soils are weakly aggregated, the impact of drops causes aggregates to fall apart, and the fine soil particles fill in the pores between the large particles. The result is a very thin layer (only tenths of an inch thick) that, upon drying, is cemented into a crust with low permeability and high tensile strength. This is called a structural crust. A depositional crust is formed when runoff water with fine soil particles in it settles in depositional areas. These crusts can become thicker as several layers of clay and silt may be deposited (see photo). Water will infiltrate very slowly through crusts, leading to runoff on soils that may still be dry under the crust. Another effect is that gas exchange between air and soil atmosphere will be inhibited, and seedlings must exert great strength to push through the crust.

There are several ways farmers can address soil crusting. First, protect the soil from the impact of raindrops by keeping it covered. This means no-tillage and no removal of crop residue from the row. If a crop leaves little crop residue, it may be necessary to plant a cover crop to avoid crust formation. Second, the surface soil structure needs to be improved. This can be done by increasing the organic matter content in the very top of the soil, as is achieved by using no-tillage continuously, and by adding organic matter in the form of manure, compost, cover crops, and crop residues. Dense root systems help make soil less sensitive to crusting. If you do any tillage, limit it as much as possible and leave the soil rough with as much residue cover as possible to limit crust formation. Stimulating soil biological activity is another way to make soil resist crusting. For example, earthworms will bury through crusts when they are moist, deposit casts on the soil surface, and these casts exhibit high resistance against disintegration.

If you have a field that suffers from crusting right now, there are a few things you can do. The crust needs to be broken up, and the best tool to achieve that is probably the rotary hoe. However, not many farmers own that piece of equipment anymore. Another tactic is to plant seeds close together because they will be able to push through a crust by joint force. This tactic could be applied by planting soybeans with a planter at 30-inch row spacings and narrower spacing in the row instead of with a drill with 7.5-inch row spacing and wider spacing in the row. Increasing seeding rates may also help for the same reason. Seeding depth may also be a factor to consider, deep planting should be avoided in a crust-sensitive soil to allow the seedling to emerge before the crust re-forms. It is also advisable to avoid very early planting, so crusts do not have time to form above the seedlings prior to emergence.