Soil Crust: Formation, Kinds and Control

After reading this article you will learn about:- 1. Formation of Soil Crust 2. Kinds of Soil Crust 3. Influence on Productivity of Soil 4. Control.

Formation of Soil Crust:

A number of binding agents have been identified that are related to soil aggregation and the soil structural stability. These are organic substances, silica, sesquioxides, liming materials, exchangeable cations etc.

The effectiveness of binding agents depends upon whether it is soluble and if so whether the solid phase is in equilibrium with dissolved phase or whether solid phase dries irreversively or rehydrates and re-dissolves on re-wetting or whether dissolved phase ionised or remains as un-dissociated molecule or precipitates as discrete particles or surface coatings, and whether it is crystalline or non-crystalline or can acquire charge.

Water and dissolved solutes determine the spatial arrangement of the soil particles in the soil. Therefore, a saturated or near saturated soil is essential for crust formation. As soil dries out, soil water tension increases, bringing the particles closer together.

Thus the combined effects of increased soil water tension, reduced pore size and increased number of contact points enhance bond formation. The nature of exchangeable cations also affects the particle distribution and packing.

Horizontal stratification causes peeling of the crusts from the underlying soil layer. During drying, cracking of crust takes places. Soil in homogeneities due to uneven packing and flocculation influence the stress distribution in a drying soil which in the turn determines the crust characteristics.

In laterite soils of West Bengal and Odisha and other states of India where the soil particles are held by soft cement, intense desiccation causes irreversible hardening and crusting. It is evident that organic matter (O.M.) prevents crust formation on soils.

In view of the beneficial effects of O.M. on soil aggregation it is necessary to distinguish between soil aggregation and soil crusting. Crusting is also a form of aggregation. The crusts may loose their strength on application of water to soil due to swelling and softening of cementing agents.

Kinds of Soil Crust:

The three principal kinds of soil crust are structural crusts, erosion and depositional crusts and cryptogamic crusts whose brief discussions are presented below:

I. Structural Crusts:

The sub-types of structural crusts are:

(a) Slaking crusts,

(b) Infiltrating crusts,

(c) Coalescing crusts and

(d) Sieving crusts.

A more detailed description of each sub-type is described below:

(a) Slaking Crusts:

Slaking crusts are usually formed due to disintegration (or physical disruption) of a sodic or weakly aggregated soil by the impact of rain drops. It consists of a thin (1 mm to 5 mm thick) dense layer with a sharp boundary with the underlying layer. Textural separation between coarse particles (skeleton) and fine particles (plasma) cannot be noticed.

Size distribution of particles released by break-down of aggregates governs the porosity of sealing crusts. If aggregate-disruption leads to aggregate fragments, porosity is relatively higher but if basic particles are released due to aggregate break-down, porosity is much lower.

(b) Infiltrating Crusts:

Infiltrating crusts are generally formed on medium textured (loam) soils due to the impact of rain drops which erodes the top of surface layer aggregates causing separation of silt particles and sealing of void space to a depth of few millimeters from the surface. They are mainly characterised by a clear textural separation.

They are made by silt sized grains which form net like infillings in the top few millimeters of the soil. In few millimeters depth some clay coating are generally observed on soil aggregates. Porosity is reduced due to clogging of inter-aggregate as well as inter-grain void spaces by infiltrating material.

(c) Coalescing Crusts:

Coalescing crusts are formed in wet soils when the soil material is in viscous state by the drop energy driving force.

They are generally thick (thickness up to 20 mm), more densely packed than the underlying horizon,’ exhibit a gradually transition with the underlying layer for which they are sometimes called as transitional micro horizons. In the top few millimeters of the crust, porosity is low and it gradually increases with depth.

(d) Sieving Crusts:

Sieving crusts are formed by the impact of rain drops on sandy soils where aggregates are extremely fragile, leading to the sorting of soil particles causing concentration of the coarser particles at the top and finer particles in the deeper position.

Thus sieving crusts are made up of two contrasting layers, the upper most of which is 1 to 5 mm thick, consists of loosely packed coarser skeleton grains and the underlying of which in 100 pm to 1 mm thick containing high amount of fine particles resulting very low porosity. The upper and lower boundaries of this plasmic layer are very sharp.

II. Erosion and Depositional Crusts:

The sub-type of erosional and depositional crusts is:

(a) Erosional crusts and

(b) Depositional crust.

A more detailed description of each sub-type is described below:

(a) Erosional Crust:

Erosional crusts are formed from the erosion of sieving crusts when the loose coarse textured upper layer is stripped away by the overland flow, leaving the under laying layer of fine particles at the surface. They are 100 pm to 1 mm thick, plasmic layer which is very dense and coherent.

(b) Depositional Crusts:

Depositional crusts are formed after sealing of the surface by a structural crust and concentration of detached particle from eroded clods or ridges in inter clods micro deposition or furrows through micro-scale run-off. Depositional crusts are thus made up of a 5 to 10 mm or thicker sedimentary layer over a previously developed structural crust.

Textural separation results the development of microbials which are less distinct in the lower part of the sedimentary crust layer. Clear textural variation is noticed between the depositional crust layer and the underlying previously developed structural crust.

III. Cryptogamic Crusts:

Surface crusts made of algae, fungi, lichens, mosses, bacteria over any of the above defined crusts are known as cryptogamic crusts. They are also known as microphytic crusts as they consist of a microbiological layer over one of the above defined crusts. As cryptogams are formed due to multiplication of microbes over a structural or erosion and depositional crust, they are sometimes not considered as soil crust.

Influence of Crusting on Productivity of Soil:

The crust causes a serious barrier for seedling emergence. The effect of soil crusting on seedling emergence depends on crust thickness and strength, as well as on the size of the seeds and vigour of the seedlings. Small sized seeds are more sensitive to soil crusting.

Crust clogs the surface macro-pores and inhibits the infiltration of water into the soil causing more run-off, less water storage in the soil profile and less availability of water to plants. Hence, surface crusts are largely blamed initiating run-off and favouring interrill soil erosion.

Strong crusts with high bulk densities can impede aeration under moist conditions by preventing effective diffusion of oxygen into the soil. Lack of aeration becomes a problem for germination of seeds which would also account for decrease in seedling emergence. Diffusion of oxygen is not restricted in case of dry crust because of cracks. Upon drying hard crust cracks and tears seedling roots.

Control of Soil Crusting:

Soil crusting can be controlled by the following ways:

(a) Surface mulches which protect the soil from the impact of rain drops minimize the formation of soil crust.

(b) Addition of organic matter which promotes stable aggregate formation and resist dispersion, prevent crust formation.

(c) Application of some artificial soil conditioners such as polyanionic soil conditioners like hydrolyzed poly acrylo-nitrile (HPAN) or vinyl acetate maleic acid copolymer (VAMA); nonionic soil conditioners like polyvinyl alcohol (PVA) etc. also reduce soil crust by producing stable aggregation.

(d) In sodic soil, lowering of exchangeable sodium percentage of the soil by application of gypsum, pyrites etc. minimizes the formation of soil crust.

(e) A light tillage while the soil is still moist will break-up the crust before it hardens.