eChapters

Soil texture refers to the relative distribution of sand, silt and clay on a sample of <2 mm size. It is an intrinsic soil property and basically refers to the particle size distribution in a soil. A number of physical characteristics and chemical properties of a soil are influenced by texture or particle size distribution. To estimate the content of sand, silt and clay particles, the soil samples are first brought into a dispersed state by mechanical and chemical treatments so that different binding agents are removed. In the dispersed soil samples, the soil particles settle down at a differential settling rate according to their size. In laboratory, the common methods for analysis for particle size distribution are : Buyoucos hydrometer method and International pipette method. In this chapter, the procedure of Buyoucos hydrometer method is described.

Principle

This method is based on the principle of variable rate of settling of particles as per their size and as described by Stokes’Law. Large particles shall settle fast. The measurement of density of the suspension is done at different times using a hydrometer (ASTM 152H-Type hydrometer is preferred). The measurement in this hydrometer is based on a standard temperature of 20 _oC and a particle density of 2.65 Mg m_-3 and the units are expressed in grams of soil per liter.The principal source of error in this procedure is the incomplete dispersion of soil clays as the clay particles are cemented by agents such as organic matter, CaCO and iron oxides. Incomplete dispersion results in low values for clay and high values for silt and sand. Thus, soils with higher organic matter (>1.0%) should be first treated with 6% hydrogen peroxide (H₂O₂) for removal of organic carbon. The rate of sedimentation is also affected by temperature and the density of the dispersing solution.

2.1. Particle Size Distribution Analysis

Soil is composed of particles of varying size and shape. The expression of the proportion of the different size particles is termed as ‘soil texture’. It is one of the most important basic characters being little changed by cultivation or other practices. Particles are arbitrarily distinguished into following groups on the basis of their diameters.

2.1 Particle Size Distribution Analysis

Soil is composed of inorganic particles and organic molecules of varying size and shape. The expression of the proportion of the different size inorganic particles is termed as ‘soil texture’. It is one of the most important basic characters being little changed by cultivation or other practices. Particles are arbitrarily distinguished into following groups on the basis of their diameters.

Physical properties of a soil greatly influence plant growth. The plant support, root penetration, drainage, aeration, retention of moisture and availability of plant nutrients are all linked with the physical condition ofthe soil. The physical properties of a soil which depend on the amount, size, shape, arrangement and mineral composition of its particles, also influence its chemical and biological behaviour.

Q. 1 If the forward speed of a tractor drawn seed drill is increased twice,
then the seed rate delivered will be
(A) Reduced to half            (B) Double
(C) Increased by four time (D) Remains the same

In modern times, soil tillage has emerged as an integral part of a crop system. Soil tillage or soil surface management has been found important in alleviating soil related constraints encountered in crop production. An important effect of soil tillage on crop productivity is through its influence on soil processes, soil Soil Tillage 39 properties and crop growth. At the same time, soil tillage has its impact on the environment e.g. soil degradation, water quality, emission of green house gases from soil related processes etc.

The plants draw water from the soil for growth and development. The soil acts a reservoir for soil moisture (Fig.5.1). The basic aim of a soil water balance model is to quantify the manner in which the water supplied to the soil either through precipitation and irrigation is disposed off. In the process, the soil water balance models were found to be extremely useful to determine the amount of water utilized by the crop for evapotranspiration.
The concept of water balance was originally developed by Thornthwaite, an American geographer and it formed the basis for development of more scientifi c advances due to its wide range of applicability including
• Choice of crops/ cropping systems based on water availability
• Irrigation scheduling
• Optimisation of limited water resources for higher productivity
• Characterization of droughts and agricultural droughts
• Assessment of crop yield and pre harvest forecasting of crop yields
• Designing irrigation projects
• Development of water storage structures
There are quite a large number of water balance models available in literature nowadays. Several of these models may be applicable for regions where there are developed. There were considerable efforts to develop water balance models can be used by the crop on daily basis in case of different crops as well. The choice of a water balance model to a large extent depends on the availability of data required for running the model. It may not be possible to cover all models available in

5.1 Kinds of Soil Water

The following re the three main classes of soil water:

(i)  Hygroscopic water. Water held tightly to the surface of soil particles by adsorption forces.

(ii) Capillary water. Water held by forces of surface tension and continuous films around soil particles and in the capillary spaces.

(iii) Gravitational water. Water that moves freely in response to gravity and drains out of the soil.

The movement of water through soil is fundamental to irrigation efficiency, drainage, nutrient transport and plant water availability. This chapter examines the principles governing water movement in both saturated and unsaturated soil conditions. It introduces the movement of water through the soil profile and discusses saturated hydraulic conductivity as an important parameter describing the ability of soil to transmit water. Water flow under unsaturated conditions is considered in greater detail, including the relationship between soil-water content and hydraulic conductivity. The chapter presents the equation for unsaturated flow and describes experimental approaches for studying water movement. Unsaturated hydraulic conductivity, represented as K(θ), provides an important basis for understanding the changing ability of soil to transmit water as soil moisture conditions vary. The chapter also considers prediction of soil-water flux, linking theoretical concepts with practical irrigation and soil-water management. Understanding water movement helps in determining irrigation requirements, reducing water losses, improving drainage and maintaining favourable soil-water conditions for crop growth.

Water can move in the liquid and gaseous phases:

1. Liquid water

i) Saturated flow: Most pores are filled with water; water in this condition is tension free

ii) Unsaturated flow: Pores are partially filled with air; the water is under tension.

2. Water vapour

i) Diffusion: Water vapour moves by diffusion as a result of vapour pressure differences (partial pressure)

ii) Mass flow: Water vapour flows in a mass with other gases of the system in response to differences in total pressure.

Plant Relationship with Soil in Climate Change

1. Gravimetric Moisture

Significance of Gravimetric Analysis

Gravimetric moisture analysis, being the foundation of soil water assessment, allows for a meticulous examination of the water content in the soil. The process involves obtaining a representative soil sample, subjecting it to controlled drying conditions, and determining the weight loss after moisture removal. This method not only quantifies the total water content but also helps in understanding the soil’s water-holding capacity and drainage characteristics. Gravimetric analysis is indispensable in precision agriculture, aiding farmers in optimizing irrigation schedules and preventing overwatering or underhydration.

5.1 Soil

Soil and water are the two most important natural resources required for crop production. Without soil, no crop can be produced even if there is availability of plenty of water. It is vital not only for agriculture but also very vital for sustenance of life. No building can stand on any place without soil. Soil gives foundation support for any building. The term ‘soil ’is defined in various ways depending on the general professional field in which it is used. However to an agriculturist, soil is the substance existing on the earth surface, which grows and develops plant life. It contains all vital elements including its inherent fertility contents that support the growth of plants. For an engineer, soil is the unaggregated deposits of mineral and organic particles covering major part on the earth crust. It includes widely different materials like boulders, sand, gravel, clays and silts. Soil is produced by the weathering of solid rocks. The formation of soil is due to geologic cycle continuously taking place on surface of the earth. The geologic cycle comprises weathering or denudation, transportation, deposition and upheaval, again followed by the above mentioned processes. Weathering may be natural or artificial. In natural weathering, physical processes like temperature changes, flow of water, ice, wind and rain play the role. In artificial weathering animals especially human beings interference causes soil erosion and formation. It is obvious that it takes hundreds of years to form one inch of soil.

The phenomena related to the retention and movement of water in soils, its uptake and translocation in plants and its loss to the atmosphere are all energy related. The more strongly water is held in the soil, the greater is its energy. It means that if water is to be removed from a moist soil, work has to be done against the adsorptive forces. The water movement is always from a zone where the free energy of water is high (standing water table) to one where the free energy is low (a dry soil). This is called soil water energy concept.

17.1. Soil Water Potential

The free energy of soil solids for water is influenced by:

i) Matric force: the attraction of the soil solids for water (adsorption) which markedly reduces the free energy (movement) of the adsorbed water molecules.

Water is the elixir of life. It is required for the evapo-transpiration of growing plants. Water acts as a solvent and, together with dissolved nutrients, makes up soil solution from which plant roots derive nutrients. Soil water content controls soil air and soil temperature.

16.1. Forces of Water Retention

The forces responsible for water retention in soils are adhesion and cohesion. Adhesion refers to the attraction of solid surfaces for water. Cohesion refers to the attraction of water molecules for each other.

The force with which the soil particles retain water is defined in terms of pF scale (Schofield, 1935). pF is defined as “the logarithm to the base 10 of the numerical value of the negative pressure of soil moisture expressed in centimetres of water.”

Water is absolute necessary constituents for the survival and growth of all the living substances on earth. It is the most essential ingredients for many of the physiological and metabolic process in plants, like photosynthesis to form sugars, translocation of cell constituents and maintenance of cell turgidity. Soil, being an adsorbent of water received from precipitation or rainfall acts as reservoir to meet requirement by plant during growing season. Water retained in the soil serves as solvent to dissolve salts and nutrients and as a carrier to translocate them for absorption by plants. It plays significant role in weathering reaction, soil forming processes and profile development. Water is also important for growth and activities of microorganisms influencing decomposition of organic matter, transformation of mineral nutrients and other physical, chemical and biological properties in soil. This chapter deals with the retention and movement of water in soils to obtain better understanding of soil-water-plant-atmosphere relationships.

The soil consists of many solid particles of different sizes and is capable of absorbing and retaining water in the soil within certain limits when water is received through either from precipitation or by irrigation. The water that is held in the soil is called soil water (Fig.4.1). It can exist also in the form of water vapour as well. The crop production depends upon the soil water. All the water that is present in the soil may not be available for plants.
The water availability to the plants from the soil depends upon some physical properties of the soil as well as on the physical processes that govern the existence of water in the soil. Therefore, it is necessary to understand about the relationships that govern the water availability in the soil as explained in the following text-
4.1. Energy Concept of Soil Water
Soil water energy concept is all about potential energy, gravitational potential, osmatic potential, pressure potential energies.
4.1.1. Soil Water Energy: The total energy state of soil water is defined by its
equivalent potential energy as determined by various forces acting on the water per unit quantity. Different energies are involved including kinetic and potential energy.

The soil is a heterogeneous, polyphasic viz., solid, liquid and gaseous, particulate, disperse and porous system (Fig. 3.1). The solid phase constitutes the soil matrix, the liquid phase consists of soil water, which always contains dissolved substances so that it should properly be called as soil solution and the gaseous phase is the soil atmosphere.

Increasing susceptibility of vegetables to various biotic and abiotic stresses and very high cost of hybrid seeds has warranted the attention of the vegetable growers to improve the nursery raising technology of vegetables. Now-a-days protected nursery raising of vegetables has become a full flagged industry in several developed countries like Israel, Japan, Spain, Netherlands, USA etc. In countries like Israel no vegetable grower is growing his own nursery, but they are getting the required kind of seedlings from an established nursery owner on pre- order basis. Similar system of nursery raising is also prevalent in several other advance countries. In those situations where vegetables are being grown under protected conditions it becomes pre-requisite to raise the required vegetable seedlings only under protected environment to get virus free healthy seedlings.

Raising nursery from seeds provide an easy and convenient way to nourish tender and young seedlings in a well managed, small and compact area for better germination of small and costly seed. In general, vegetables crops are divided into three groups according to their relative ease for transplanting. Beet root, broccoli, brussels sprouts, cabbage, cauliflower, tomato and lettuce are efficient in water absorption and rapidly form new roots after transplanting. Vegetables crops that are moderately easy for transplanting are brinjal, onion, sweet pepper, chilli and celery which do not absorb water as efficiently as crops that are easy to transplant, but they form new roots relatively quickly. The vegetable crops which are difficult to transplant are: all cucurbits and sweet corn which require special care during nursery raising and transplanting. Seedlings not only reduce the amount of time of the crop but it also increases the uniformity of the crop and harvesting as compared to direct sowing of the crops. Transplanting of seedlings also eliminates the need for thinning and provides very good opportunities for virus free, vigorous and even off-season seedlings if grown under protected conditions in soil-less media. So it becomes necessary to know the protected structures which can be efficiently used for successful raising of vegetable seedlings, suitable types of containers and growing media, method of seeding in containers in soil-less media, growing conditions and water and fertilizer requirement of seedlings along with hardening and transplanting to the field. Protected nursery raising technology is highly suitable and can be established as a small scale industry in major vegetable growing areas of our country by progressive farmers especially in peri-urban areas. By this way the vegetable growers will get virus-free healthy and off season nursery as per their requirement and it generate extra employment in rural areas.

Cation exchange: The ‘soil cations’ essential for plant growth include ammonium, calcium, magnesium, and potassium. There are three additional ‘soil cations,’ which are not essential plant elements but affect soil pH. The additional ‘soil cations’ include sodium, aluminum and hydrogen.

The major distinguishing characteristic of cations is their positive charge. Just like a magnet, a positive charge is strongly attracted to a negative charge. When soil particles have a negative charge, the particles attract and retain cations. These soils are said the have a cation exchange capacity. Although most soils are negatively charged and attract cations.

The ‘soil cations’ are further divided into two categories. Ammonium, calcium, magnesium, potassium, and sodium are known as the ‘base cations,’ while aluminum and hydrogen are known ‘acid cations.’ Unlike the other base cations, sodium is not an essential element for all plants. Soils that contain high levels of sodium can develop salinity and sodicity problems.

The words ‘base’ and ‘acid’ refer to the particular cation’s influence on soil pH. As you might suspect, a soil with a lot of acid cations held by soil particles will have a low pH. In contrast, a highly alkaline soil predominately consists of base cations.

Cations in the soil compete with one another for a spot on the cation exchange capacity (Table 42 and Fig 18). However, some cations are attracted and held more strongly than other cations. In decreasing holding strength, the order with which cations are held by the soil particles follows: aluminum, hydrogen, calcium, potassium and nitrate, and sodium.