What Is Soil Crusting? How a Hard Surface Forms
As an Amazon Associate we earn from qualifying purchases.
What is soil crusting? It is the formation of a hard, dense layer on the soil surface, usually after rain or irrigation. The University of Illinois Soil Quality program explains the basic sequence: “Water droplets striking soil aggregates and water flowing across soil breaks aggregates into individual soil particles,” and those fine particles then “wash, settle into and block surface pores.” Once the pores are sealed, water struggles to enter the soil and seedlings struggle to come up. Penn State Extension says crusting “leads to reduced water infiltration and inhibits seedling emergence.” This explainer covers how crusts form, the three types the sources describe, and why they matter for a garden bed.
Quick answer
- A crust forms when raindrops or irrigation water break soil aggregates, and the loose fine particles seal the surface.
- The sources describe three types: chemical crusts from salt, biological crusts from living organisms, and physical crusts from surface structure breakdown.
- Crusts reduce infiltration, increase runoff and can slow or stop seedling emergence.
- Crusts are most often linked to rain and irrigation, and the sources also note freeze-thaw effects in spring.
How we chose
The sources are two university extension pages, both opened and checked on 2026-10-27. The Penn State Extension page covers crust types, causes and effects in a Pennsylvania context, and the University of Illinois Soil Quality page explains structural crust formation and its effect on water and air. Retail pages and product listings were not used, and no product is recommended in this explainer.
How a physical crust forms
Penn State defines physical crusts this way: “Physical crusts are the result of structural degradation of the surface soil and may be classified as structural or depositional.” The structural type is the common one. The source explains that “Structural crusts are formed when surface aggregates disintegrate due to the forces of raindrops or irrigation water.”
The Illinois source describes the same process step by step. First, the impact of water breaks the soil apart: “Water droplets striking soil aggregates and water flowing across soil breaks aggregates into individual soil particles.” Second, those particles move into the gaps: “Fine soil particles wash, settle into and block surface pores causing the soil surface to seal over and preventing water from soaking into the soil.” The surface becomes a seal.
Drying finishes the job. The Illinois source says that “As the muddy soil surface dries out, it crusts over,” and it adds that “Structural crusts develop when a sealed-over soil surface dries out after rainfall or irrigation.” The crust is therefore formed in two stages, sealing while wet and hardening as it dries.
Penn State describes the other physical type: “Depositional crusts are formed by transport and deposition of fine particles from eroded surfaces to depositional areas.” In this case the crust is made from soil that has been moved from elsewhere and laid down, rather than from the surface breaking apart in place.
What makes a crust hard and thick
A crust is not just a thin film. The Illinois source says that “Structural crusts range from a few tenths to as thick as two inches.” It also describes the material: “A surface crust is much more compact, hard and brittle when dry than the soil immediately beneath it, which may be loose and friable.” The difference between the crust and the soil below is one of the signs a gardener can see.
Penn State explains that both weather and soil play a role. It says that “Both rainfall characteristics and soil properties influence crust formation.” On the rainfall side, the source notes that “Heavy, intense rain has high kinetic energy primarily because of large drop size and high rainfall rate.”
Season can also change the picture. The Penn State source states that “The effect seems to be more serious in spring after freezing and thawing cycles cause aggregates to disintegrate.”
The three crust types
Penn State lists three types: “There are three types of crusts: chemical, biological, and physical crusts.” Each one forms in a different way, and the sources treat them separately. The physical crust is the one described above. The other two are worth knowing, because a gardener may see them in different soils.
A chemical crust comes from salt. Penn State says that “Chemical crusts are the result of encrusted salt on soils in arid or semi-arid regions.” The Illinois source adds that “A precipitated, chemical crust can develop on soils with high salt content.”
A biological crust is made of living organisms. Penn State says that “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.” The Illinois source describes the crust as a community: “A biological crust is a living community of lichen, cyanobacteria, algae, and moss growing on the soil surface that bind the soil together.” This type binds the surface rather than sealing it, which is a different effect from the physical crust.
Why crusting matters for plants
The main concern is water. Penn State states that crusting “leads to decreased infiltration rate, reduction of soil moisture, increased runoff, and reduced groundwater recharge.” The effect on the crust itself is also large. The source says that “The hydraulic conductivity of a crust may be a hundred or even a thousand times lower than that of the underlying soil.” Water that cannot enter the surface cannot reach the roots below.
The Illinois source explains the link to plant growth. It says that “Increased runoff results in less water available in soil for plant growth.” It also states that “Surface sealing and crusts greatly reduce infiltration, and increase runoff and erosion.”
Seedlings are the most visible casualty. Penn State says that “Crusting also slows or completely inhibits seedling emergence.” The Illinois source describes the same result after planting: “Crust development soon after a crop is planted can result in such poor emergence that the crop might have to be replanted.”
Air exchange is affected too. The Illinois source says that “Crusts can also reduce oxygen diffusion into the soil profile by as much as 50% if the soil crust is wet.” It also says that “A surface crust indicates poor infiltration, a problematical seedbed, and reduced air exchange between the soil and atmosphere.” The crust therefore affects the soil’s air as well as its water.
Temperature and soil sodium
A crust changes the surface temperature too. The Illinois source explains that “The sunlight (and energy) reflectance of a surface crust is higher than that of a non-crusted soil, so soil temperature may be lower and surface evaporation reduced where a crust exists.” It adds that “This could negatively affect germination and development of healthy seedlings in cooler climates.” The source ties that risk to cooler climates.
The Illinois source also links crusting to sodium. It says that a surface crust “can also indicate that a soil has a high sodium content that increases soil dispersion when it is wetted by rainfall or irrigation.” A crust is therefore sometimes a clue about the soil chemistry, not only about its structure. A gardener who sees repeated crusting may want to consider both the surface and the soil underneath.
What helps a crust form less often
The sources describe the causes, and the causes point to the controls. The Penn State source says that “Both rainfall characteristics and soil properties influence crust formation,” so both the storm and the soil are part of the picture. For ways to build soil structure, see how to keep garden soil healthy and how to improve sandy soil.
The sources do not give a list of treatments for crusted beds, and this explainer does not add one. A gardener who wants a specific fix should check the local extension guidance for the soil type involved.
Frequently asked questions
What causes soil to crust?
The sources point to water hitting the surface and the fine particles that follow. The Illinois source says that “Water droplets striking soil aggregates and water flowing across soil breaks aggregates into individual soil particles,” and the Penn State source notes that “Both rainfall characteristics and soil properties influence crust formation.”
Is a soil crust always caused by rain?
No. Penn State lists chemical crusts from salt and biological crusts from living organisms as separate types. The Illinois source says that “A precipitated, chemical crust can develop on soils with high salt content.”
How thick can a soil crust get?
The Illinois source states that “Structural crusts range from a few tenths to as thick as two inches.” The thickness of other crust types is not given in the sources.
Does crusting affect seedlings?
Yes. Penn State says that “Crusting also slows or completely inhibits seedling emergence,” and the Illinois source notes that poor emergence can be serious enough that “the crop might have to be replanted.”
Can a crust reduce water entering the soil?
The sources say it can. Penn State states that a crust’s “hydraulic conductivity” may be “a hundred or even a thousand times lower than that of the underlying soil,” and it lists “reduced water infiltration” as one effect of crusting.
What we could not verify
The two sources explain crust formation and its effects well, but they do not cover every soil or climate. The gaps below remain open after checking both pages.
What the sources do not say
- Neither source gives a measured crust thickness or strength for a typical home garden bed.
- The sources do not give a time frame for how quickly a crust forms after a particular rainfall.
- The Penn State source notes that physical crusts are common in Pennsylvania. The sources do not say how common they are in other regions.
- The sources do not give a specific treatment for a crusted bed.
