Soil and Plant Growth

This article throws light upon the ten main physical properties of soil that influence plant growth. The physical properties are: 1. Soil Texture 2. Soil Structure 3. Soil Density 4. Soil Porosity 5. Soil Consistency 6. Soil Compaction, Compression and Soil Crusting 7. Soil Water 8. Soil Air 9. Soil Temperature 10. Soil Colour.

Physical Property # 1. Soil Texture:

It can be characterized by the relative proportions of sand, silt and clay particles in a soil mass. It is an intrinsic soil attribute the most frequently used to characterize its physical make up having bearing on the behaviour of soils. Different soils contain different proportions of sand, silt and clay exhibiting properties of dominant fractions.

As for example, coarse textured sandy soils are well drained with good aeration capacity and low water holding capacity and it is easy to cultivate which are light soils while that of the fine textured clayey soils able to retain more water, very plastic and sticky and very difficult to cultivate.

Although, this type of soil retain more amounts of nutrients on clay particles as it possesses greater specific surface, Energy required for tillage is much less in coarse/light textured sandy soils compared to heavy/fine textured clayey soils.

Sandy soils have low water and nutrients holding capacity with low organic matter content, high leaching, no swelling and shrinkage properties, while silty have medium to high water and nutrient holding capacity with medium organic matter content fairly good supply of nutrients and clayey soils have high water and nutrient holding capacity with high swelling and shrinkage.

These soils have good sealing properties and are easily compacted. Leaching of nutrients is very low in these soils. Loam soil, exhibiting light and heavy properties in equal proportions, is usually considered a good soil for plant growth and nutrition.

The capacity of loam soil to retain water and nutrient is far greater than that of coarse textured sandy soil since the former soil are favourable for drainage, aeration, and easy to till. However, different types of soil texture affect plant growth differently.

Physical Property # 2. Soil Structure:

Soil structure is the important physical properties that can greatly affect a variety of soil physical processes viz. porosity, density, aeration, water holding capacity, infiltration, permeability, movement of water, thermal conductivity etc.

Different soil management practices like, tillage, crop cultivation, application of fertilizers and manures, amendments (lime/gypsum), application of irrigation water have the ability to moderate soil structure which in turn affects root growth, uptake of nutrients and water, plant growth and yield by influencing other soil properties.

Soil structure is not a plant growth factor, in-spite of that, all plant growth factors are affected by soil structure. Soil structure has a pronounced effect on soil erosion. Usually, if the soil aggregation is more than the erodibility will be less. Soil structure also affects the supply of water to plants particularly in soils having spheroidal type of crumb sub-type structure which is especially porous consisting of micro pores paces.

Soil structure influences the availability of plant nutrients by moderating redox potential which is related to soil aeration, supply of water, organic matter content and microbial activity.

But in case of platy type of poorly aggregated structure, the germination of seeds, root penetration and growth, proliferation of micro-organisms, aeration, movement of water etc. are very poor and limited and hence affect the plant growth and crop production.

Therefore, for the optimum growth of plants, the most desirable soil structure is spheroidal type compared to other types. Soil structure has both direct and indirect effects on plant growth.

In case of its direct effects, seedling emergence on crusted soils, restriction of root development particularly of hard surface soils, inhibition of root penetration on compacted soils etc. are found. In addition to the direct effects of soil structure on expansion of plant parts, various indirect effects may be of significance.

Because of roots commonly occurs between structural units and not within them, the size of the structural units may affect the rate of delivery of nutrients and water from interior parts to the roots at the surfaces. Air-water relationships are sensitive to the size and continuity of pores. Structural modifications most easily effected in soils result in changes in volume percentage of large or non-capillary pores.

Physical Property # 3. Soil Density:

Soil density includes both bulk and particle density. Particle density is the mass per unit volume of soil solids and expressed as g/cc or Mg/m³, whereas bulk density is the mass per unit volume of dry soils including pore spaces (bulk volume) and expressed as g/cc or Mg/m³. This bulk volume of soil is the sum of the volume occupied by soil solids and the volume occupied by void spaces.

Dry and wet bulk density can also be measured in case of dry (soil solids and pore spaces) and wet soils (field soil samples including field moisture). Wet bulk density must state with water content of the soil.

Bulk density is the ratio of mass of soil solids and the volume of soil including pore space whereas particle density does not consider pore spaces, only considers soil solids, so the importance of bulk density with respect to soil quality and crop growth is much more than that of particle density.

Bulk density of soil is influenced by soil texture, structure, organic matter content and land management practices. Increase in organic matter content decreases the bulk density of soil. The bulk density is usually higher with the depth of the soil.

In swelling soils bulk density decreases with an increase in moisture content and vice versa. Bulk density is related with the porosity of the soil. With an increase in soil porosity bulk density decreases and vice versa.

Therefore, lower value of bulk density of soil is good for optimum growth of plants because at this point of soil conditions, soils are believed to be provided with proper aeration and drainage resulting in optimum root and microbial proliferation in soils which help to greater absorption of water and nutrients by plants.

Physical Property # 4. Soil Porosity:

Porosity is also an another important soil physical properties which can affect the plant growth both directly and indirectly. Porosity also modifies bulk density as well as water transmission through the soil which have an indirect effect on plant growth.

Porosity of soil decreases if crops like potato, maize etc. are grown because of decrease in organic matter content and destruction of soil aggregates due to excess cultivation of lands.

Incorporation of crop residues and farm yard manures in large amounts to the soil tend to increase the total pore space of the surface soil. However, porosity may be affected by various factors namely, organic matter content, depth of the soil, compaction, crop cultivation, texture and structure etc.

The movement of air and water through fine textured clayey soils is slow and the dominating micro pores in these soils usually remain full of water for a sufficient period creating poor aeration especially in sub-soils for satisfactory root development and desirable microbial activity.

In a stable granular structure in fine textured clayey soils, total pore space is likely to be equally shared by macro- and micro-pore spaces. So, the movement of air and water is optimum for plant growth.

The volume of large or non-capillary pores is important in determining the rate of flow of water through saturated soil and the rate of infiltration of water into the soil. The volume of large pores is important in providing aeration and the volume of both large (non-capillary) and small (capillary) pores affects the capacity of the soil for water retention and transmission.

However, capillary pore spaces are important for the steady supply of water to the plants since water storage capacity is more in these pore spaces.

Physical Property # 5. Soil Consistency:

The behaviour of the soil to an applied stress is expressed as dynamic properties of soils. Soil consistency is the manifestations of the physical forces of cohesion and adhesion acting within the soil at various moisture conditions. It covers soil properties such as resistance to compression, friability, plasticity and stickiness etc.

In most soils, consistency varies from wet to dry soil moisture regimes corresponding to sticky and plastic (wet soil) to hard (dry soil) and friable (moist soil) consistency. Soils containing more than 15% of clay exhibit plasticity. Plasticity has three indices namely, lower plastic limit, upper plastic limit (liquid limit) and plasticity index.

Soils with high plasticity indices are very difficult to till. Soils with expanding lattice clays such as smectites have high liquid limits and plasticity indices. Soil consistency may be described at three moisture conditions such as dry, moist and wet as mentioned. Soils are rated for consistency as a part of describing a soil profile and evaluating suitability for tillage.

Different forms of soil consistency are important for developing tillage system. If soils are tilled at hard consistency, heavy draft is required and clods are formed. At friable consistency, light draft is required and soils are easily tilled.

Plastic and sticky consistency is not suitable for cultivation. Therefore, friable consistency of soils is most important and optimum condition for tillage and most agricultural operations for crop cultivation.

Physical Property # 6. Soil Compaction, Compression and Soil Crusting:

All these soil physical constraints are related with aeration status of the soil as well as water transmission, retention and permeability which ultimately affect the growth of plants by affecting absorption of nutrients and water, reducing use efficiency of different agro-inputs (fertilizers, irrigation water etc.).

Soil compaction may be defined as the process of compression of an unsaturated soil by the applied force or load, whereas soil compression refers to the reduction of volume in dry, wet and saturated soil due to applied force or load. Soil crusting is a phenomenon associated with deterioration of soil structure where natural soil aggregates break and subsequently disperse.

If such dispersion is followed by rapid drying the soil solids re-arrange into crust and as a result of which blocking soil pores takes place and hence affect water and nutrient movement. Besides, the hydraulic conductivity of surface crust layer is low encouraging run-off and loss of soil.

Soil crusting is a major structural feature in soils of arid and semi-arid regions. The problem of soil crusting is acute in silty clay loam soil. Due to such constraints, germination of seeds and seedling emergence are significantly affected. However, germination of seeds and seedling emergence affected by different soil physical conditions are being discussed subsequently.

Physical Property # 7. Soil Water:

Water is essential for plant growth and it is required in large quantities than that of essential plant nutrients whereas a large fraction of any nutrient absorbed by plants is retained. The most interesting and noteworthy characteristics of water is its continuous one way flow from the soil via roots to the stems and leaves where water is lost through evapotranspiration.

The evaporation of water from the leaf stomata is called as transpiration and such transpiration is regulated by the micro-climate outside the leaves, if the plant is grown in a moist soil. The resistance to the upward flow in the vascular system and transfer across of walls of leaf cells only appears to limit the rate of transpiration when plants are taking their water from a deep subsoil.

Soil water plays a significant role in controlling the energy balance of the soil and of its radiation exchanges and thermal properties. Its content in variable amounts changes the volume of the air-filled pore space and regulates the gaseous exchange in the surface soil. Soil water helps plants to carry out basic and fundamental physiological and metabolic processes including uptake of nutrients from the soil.

It dissolves salts and nutrients and also controls their transport process in the soil. Water is also an important input for facilitating microbial activity in soils for carrying out decomposition of organic matter, fixation vis-a­-vis release of soil nutrients.

However, the presence of water in the soil facilitates various beneficial processes like, physical, chemical and biological undergoing in the soil system and as a result of which plant growth is affected by moderating different plant growth factors.

The maximum proportion of the photo synthetically active component of the net energy that can be used in photosynthesis is only 13% and some crops can reach this level for short periods provided they are well supplied with manures and water.

However, the use efficiency of applied nutrients in the form of manures and fertilizers and decomposition of organic resource materials, mineralization process and other beneficial soil processes etc. are significantly affected by the water present in the soil. So, it may be concluded that the plant growth and subsequent crop production is not at all possible.

Physical Property # 8. Soil Air:

Plant roots need oxygen to function and in most agricultural crops once they have passed the seedling stage, the source of oxygen is the soil air. The roots of marsh plants such as rice and many other seedlings have an alternative source where oxygen from the atmosphere diffuses from leaves through the stem (arenchyma tissue) to the root.

The root systems of crops require an aerobic soil if they are sensitive to substances produced by soil bacteria when growing in anaerobic conditions where some are toxic to root cells and some may disturb the balance of plant growth factors such as auxin and abscissic acid within the plant resulting in epinasty of leaves or abscision of the reproductive organs.

When the root cells of a number of plants start to be affected by lack of oxygen because they start to convert glucose into ethyl alcohol which appears in the xylem exudates. This alcohol inhibits the development of plants very badly particularly at the flowering stage resulting loss in crop yield.

However, plants grown under poorly aerated soil conditions like waterlogged conditions, suffer due to root toxicity out of an accumulation of various toxic acids like malic, butyric acid etc. resulting from the incomplete decomposition of organic matter. It is possible that the root system of most crops functions normally if the partial pressure of oxygen outside the root is in between 0.06 and 0.02 bars.

A root subjected to oxygen shortage has its growth or rate of elongation decreased and the rate of movement of water and nutrients from soil to the xylem tissue is also decreased which might be due to reduction in rate of movement from soil into root cells and from cells into the xylem stream. The increase in partial pressure of CO₂ in the root zone may have the similar effect to that of decreased O₂concentration.

Although, the increased CO₂ concentration in the soil may have some favourable effects on nutrient solubility. In the fields, plant roots tend to grow into the larger pores in the soil where soils are poorly aerated or waterlogged or water is standing on the soil surface. Root growth takes place rapidly in soils having good aeration which might be due to the enhancement of the power of roots to penetrate into soils by supplying oxygen to the root tips.

Root elongation is particularly sensitive to aeration conditions. Plant adaptations is an another important mechanism through which the aerobic plans can grow without the supply of oxygen as because the plant itself can develop internal air spaces, morphological adaptations and anaerobic respiration for its survival.

The microbial population of soil has a wider range of adaptation than do higher plants and can carry on respiration in very dry soils to support plant growth as well as in saturated soils. In relatively dry soils, there is sufficient air space for rapid exchange of gases and hence the respiration of micro-organisms does not cause an important competition with plants.

In wet soils, the exchange of gases is slow and respiration by micro-organisms alone may be sufficient to exhaust the concentration of oxygen in the soil.

Plant roots and soil borne disease organisms occupy the same environment and respond in characteristic ways to prevailing conditions such as aeration. Changes in aeration may modify the susceptibility of the plant, the virulence of the organism, or both, so that disease incidence may vary with soil aeration.

Physical Property # 9. Soil Temperature:

It is evident that soil temperature is also an important physical property which indirectly affects seed germination, seedling emergence, root and shoot growth etc. The minimum temperature 1-3°C required for germination of seeds winter crops like rye, mustard, clover, wheat, barley, pea, lentil etc. Other winter crops like sugar beat, sunflower, potato need a minimum temperature of 5-8°C.

Corn, millets and beans do not germinate if the soil temperature falls below 10°C. Many of the important soil processes are affected by soil temperature namely, decomposition of organic matter, movement and absorption of ions, solubility of insoluble compounds, microbial proliferation, water transmission, soil aggregation, soil colour etc. which ultimately affect the plant growth.

The optimum soil temperature for many temperate crops is about 20°C, while for other crops like sorghum, cotton, summer vegetables, sesame, moong etc. the possible temperature is in between 30-35°C.

Soil temperature affects the type of root growth i.e. low temperature encourages white succulent roots that suberise slowly with little branching, while high temperatures encourage a browner or dark brown, fine with more branching which suberise quite rapidly.

Low soil temperature is harmful at seedling time to crops, such as maize and sorghum which require high temperature during active seedling stage. The rate at which roots take up water and nutrients from soil also increases with soil temperature. As the movement of phosphate in soils is very slow, optimum soil temperature can facilitate the its movement through soil.

Soil temperature also affect the microbial proliferation particularly of thermophilic micro-organisms which can carry out various beneficial processes undergoing in the soil system by moderating different physical, chemical and biological properties of soil. The optimum temperature for carrying out most of the micro-organisms is in between 25-35°C.

The rate of microbial activity like respiration becomes more than double for every 10°C rise in temperature. The activity of ammonifying and nitrifying bacteria, spore forming bacteria, cellulolytic bacteria, actinomycetes decreases as the soil temperature decreases from 35 to 15°C.

Optimum microbial growth occurs at constant temperature than fluctuating temperatures. Nodule initiation, nodule growth an fixation of nitrogen decreases at or above 30°C. When the atmospheric temperature is greater than 35°C, soil temperature becomes greater than 50°C and this heating process is known as “Solarisation” which reduces certain fungal plant diseases.

Solarisation is used to control pests and diseases of high valued crops. A low temperature produces toxic substances which are injurious to plants, whereas a high decomposition rate at optimum temperature generally produces beneficial substances for plant growth.

The plant growth usually starts only when the soil is reached a minimum or threshold temperature and the rate of plant growth increases until approaching towards an optimum temperature.

However, the highest and lowest soil temperature varies from crop species and stages of crops. Root growths as well as activity of roots are greatly influenced by soil temperature. The rate of water absorption and uptake of nutrients are significantly affected by soil temperature. Phosphorus deficiency symptoms are usually found in winter season due to increase in soil temperature.

Physical Property # 10. Soil Colour:

Soil colour is usually inherited from its parent’s material. Soil colour may be inherited (lithochromic) and acquired from soil forming processes. It may vary from place to place and horizon to horizon within the profile. Soil colour varies with organic matter and moisture contents of the soil. Moist soils are generally darker than that of dry soils.

Soil colour is also indicative of other factors such as presence of excessive salts (white) erosion etc. It is also evident that the amount of organic matter content, oxides and hydrous oxides of iron and manganese contents and water contents etc. imparts soil colour depending upon the relative amounts of those components present in it.

As for example, if a soil contains higher amounts of organic matter, the possible soil colour is dark brown or black, while the light or yellow colour exhibits with low amounts of organic matter content.

Such variation in colour due to presence of organic matter content might affect other soil physical properties like absorption of soil heat, retention of soil moisture, aeration, drainage, activity of soil micro-organisms etc. and as a result of which the plant growth suffers.