Do Rock Fragments Increase Water Holding Capacity?

Chapter 2 - SOIL AND WATER

2.one The soil
two.two Entry of water into the soil
2.3 Soil moisture weather
2.4 Available h2o content
2.5 Groundwater table
2.6 Soil erosion by water

2.ane The soil

2.1.1 Soil limerick
2.ane.2 Soil contour
two.1.3 Soil texture
2.one.4 Soil structure

2.one.ane Soil composition

When dry out soil is crushed in the manus, it can be seen that information technology is composed of all kinds of particles of different sizes.

Most of these particles originate from the degradation of rocks; they are chosen mineral particles. Some originate from residues of plants or animals (rotting leaves, pieces of os, etc.), these are called organic particles (or organic matter). The soil particles seem to touch on each other, merely in reality take spaces in betwixt. These spaces are chosen pores. When the soil is "dry", the pores are mainly filled with air. After irrigation or rainfall, the pores are mainly filled with h2o. Living fabric is constitute in the soil. It tin can be live roots as well as beetles, worms, larvae etc. They help to aerate the soil and thus create favourable growing conditions for the plant roots (Fig. 26).

Fig. 26. The composition of the soil

two.i.ii Soil profile

If a pit is dug in the soil, at least 1 m deep, diverse layers, unlike in colour and limerick can be seen. These layers are chosen horizons. This succession of horizons is called the profile of the soil (Fig. 27).

Fig. 27. The soil profile

A very full general and simplified soil profile tin can exist described as follows:

a. The plough layer (20 to 30 cm thick): is rich in organic thing and contains many live roots. This layer is field of study to land preparation (due east.g. ploughing, harrowing etc.) and often has a dark colour (chocolate-brown to blackness).
b. The deep plough layer: contains much less organic matter and alive roots. This layer is hardly affected by normal land grooming activities. The colour is lighter, oftentimes grayness, and sometimes mottled with yellowish or reddish spots.

c. The subsoil layer: hardly any organic matter or live roots are to be constitute. This layer is non very important for plant growth as simply a few roots will reach it.

d. The parent rock layer: consists of stone, from the deposition of which the soil was formed. This rock is sometimes called parent material.

The depth of the unlike layers varies widely: some layers may be missing altogether.

2.1.3 Soil texture

The mineral particles of the soil differ widely in size and can be classified as follows:

Proper noun of the particles	Size limits in mm	Distinguisable with naked eye
gravel	larger than 1	obviously
sand	one to 0.5	easily
silt	0.5 to 0.002	barely
clay	less than 0.002	impossible

The amount of sand, silt and dirt nowadays in the soil determines the soil texture.

In coarse textured soils: sand is predominant (sandy soils).
In medium textured soils: silt is predominant (loamy soils).
In fine textured soils: clay is predominant (clayey soils).

In the field, soil texture can be determined by rubbing the soil betwixt the fingers (see Fig. 28).

Farmers often talk of low-cal soil and heavy soil. A fibroid-textured soil is light because it is easy to piece of work, while a fine-textured soil is heavy because it is difficult to piece of work.

Expression used by the farmer	Expression used in literature
light	sandy	coarse
medium	loamy	medium
heavy	clayey	fine

The texture of a soil is permanent, the farmer is unable to change or change information technology.

Fig. 28a. Coarse textured soil is gritty. Individual particules are loose and fall apart in the hand, even when moist.

Fig. 28b. Medium textured soil feels very soft (like flour) when dry out. Information technology tin can exist easily be pressed when wet and then feels silky.

Fig. 28c. Fine textured soil sticks to the fingers when wet and can course a brawl when pressed.

2.1.iv Soil structure

Soil construction refers to the grouping of soil particles (sand, silt, dirt, organic matter and fertilizers) into porous compounds. These are called aggregates. Soil structure also refers to the organization of these aggregates separated by pores and cracks (Fig. 29).

The basic types of aggregate arrangements are shown in Fig. thirty, granular, blocky, prismatic, and massive structure.

Fig. 29. The soil structure

When present in the topsoil, a massive structure blocks the archway of water; seed germination is difficult due to poor aeration. On the other hand, if the topsoil is granular, the water enters easily and the seed germination is better.

In a prismatic structure, movement of the water in the soil is predominantly vertical and therefore the supply of water to the plant roots is commonly poor.

Unlike texture, soil construction is not permanent. By means of tillage practices (ploughing, ridging, etc.), the farmer tries to obtain a granular topsoil construction for his fields.

Fig. thirty. Some examples of soil structures

GRANULAR	BLOCKY

PRISMATIC	MASSIVE

2.2 Entry of water into the soil

2.2.1 The infiltration procedure
2.2.2 Infiltration charge per unit
2.2.iii Factors influencing the infiltration rate

2.ii.1 The infiltration process

When rain or irrigation water is supplied to a field, information technology seeps into the soil. This process is chosen infiltration.

Infiltration tin can be visualized past pouring water into a glass filled with dry powdered soil, slightly tamped. The water seeps into the soil; the colour of the soil becomes darker as it is wetted (see Fig. 31).

Fig. 31. Infiltration of water into the soil

ii.2.2 Infiltration rate

Repeat the previous test, this fourth dimension with ii spectacles. I is filled with dry sand and the other is filled with dry dirt (come across Fig. 32a and b).

The infiltration of h2o into the sand is faster than into the clay. The sand is said to accept a college infiltration charge per unit.

Fig. 32a. The same amount of h2o is supplied to each glass

Fig. 32b. After one hour the water has infiltrated in the sand, while some water is still ponding on the clay

The infiltration charge per unit of a soil is the velocity at which water can seep into information technology. It is commonly measured by the depth (in mm) of the water layer that the soil can absorb in an hr.

An infiltration rate of 15 mm/hr means that a water layer of fifteen mm on the surface of the soil, will take one 60 minutes to infiltrate (encounter fig. 33).

Fig. 33. Soil with an infiltration charge per unit of 15 mm/hour

A range of values for infiltration rates is given below:

Low infiltration rate	less than fifteen mm/hour
medium infiltration rate	15 to 50 mm/60 minutes
high infiltration rate	more than 50 mm/hour

2.2.three Factors influencing the infiltration charge per unit

The infiltration rate of a soil depends on factors that are abiding, such as the soil texture. It also depends on factors that vary, such as the soil moisture content.

i. Soil Texture
Coarse textured soils have mainly large particles in between which there are large pores.

On the other hand, fine textured soils take mainly pocket-size particles in between which there are small pores (run into Fig. 34).

Fig. 34. Infiltration rate and soil texture

In coarse soils, the pelting or irrigation water enters and moves more easily into larger pores; it takes less fourth dimension for the water to infiltrate into the soil. In other words, infiltration rate is higher for coarse textured soils than for fine textured soils.

2. The soil wet content

The water infiltrates faster (higher infiltration rate) when the soil is dry, than when it is wet (run into Fig. 35). Equally a upshot, when irrigation h2o is practical to a field, the water at outset infiltrates easily, but every bit the soil becomes moisture, the infiltration rate decreases.

Fig. 35. Infiltration rate and soil moisture content

three. The soil structure

Generally speaking, water infiltrates quickly (high infiltration rate) into granular soils just very slowly (low infiltration rate) into massive and compact soils.

Considering the farmer can influence the soil construction (by means of cultural practices), he tin can also change the infiltration rate of his soil.

ii.3 Soil moisture atmospheric condition

2.iii.1 Soil moisture content
two.3.2 Saturation
2.3.3 Field capacity
2.three.4 Permanent wilting indicate

ii.3.1 Soil moisture content

The soil moisture content indicates the amount of water present in the soil.

Information technology is commonly expressed as the corporeality of water (in mm of water depth) nowadays in a depth of 1 metre of soil. For example: when an amount of water (in mm of water depth) of 150 mm is present in a depth of i metre of soil, the soil moisture content is 150 mm/k (meet Fig. 36).

Fig. 36. A soil moisture content of 150 mm/chiliad

The soil moisture content can too be expressed in percent of volume. In the example above, ane one thousand³ of soil (east.grand. with a depth of 1 m, and a surface surface area of 1 mⁱⁱ) contains 0.150 m³ of water (e.g. with a depth of 150 mm = 0.150 m and a surface area of 1 thousandⁱⁱ). This results in a soil moisture content in book percent of:

Thus, a wet content of 100 mm/1000 corresponds to a moisture content of 10 book pct.

Notation: The corporeality of water stored in the soil is not constant with time, but may vary.

2.3.ii Saturation

During a rain shower or irrigation application, the soil pores will fill with water. If all soil pores are filled with water the soil is said to be saturated. There is no air left in the soil (run into Fig. 37a). It is like shooting fish in a barrel to determine in the field if a soil is saturated. If a scattering of saturated soil is squeezed, some (muddy) water will run betwixt the fingers.

Plants need air and water in the soil. At saturation, no air is present and the constitute will suffer. Many crops cannot withstand saturated soil weather condition for a flow of more than than 2-5 days. Rice is i of the exceptions to this rule. The period of saturation of the topsoil commonly does non concluding long. After the rain or the irrigation has stopped, part of the water present in the larger pores will movement downwards. This procedure is chosen drainage or percolation.

The h2o drained from the pores is replaced by air. In coarse textured (sandy) soils, drainage is completed within a period of a few hours. In fine textured (clayey) soils, drainage may take some (two-3) days.

2.iii.three Field capacity

After the drainage has stopped, the large soil pores are filled with both air and h2o while the smaller pores are nonetheless full of water. At this phase, the soil is said to be at field capacity. At field capacity, the h2o and air contents of the soil are considered to exist platonic for crop growth (see Fig. 37b).

two.3.4 Permanent wilting point

Little by little, the water stored in the soil is taken up by the institute roots or evaporated from the topsoil into the atmosphere. If no additional water is supplied to the soil, information technology gradually dries out.

The dryer the soil becomes, the more tightly the remaining water is retained and the more difficult it is for the found roots to extract it. At a certain phase, the uptake of water is not sufficient to come across the plant's needs. The institute looses freshness and wilts; the leaves change colour from greenish to xanthous. Finally the found dies.

The soil water content at the phase where the plant dies, is called permanent wilting betoken. The soil yet contains some water, merely it is too hard for the roots to suck information technology from the soil (meet Fig. 37c).

Fig. 37. Some soil moisture characteristics

two.four Available water content

The soil can exist compared to a water reservoir for the plants. When the soil is saturated, the reservoir is full. However, some water drains apace below the rootzone before the plant tin use it (see Fig. 38a).

Fig. 38a. Saturation

When this h2o has drained abroad, the soil is at field capacity. The plant roots draw water from what remains in the reservoir (see Fig. 38b).

Fig. 38b. Field capacity

When the soil reaches permanent wilting signal, the remaining water is no longer available to the plant (come across Fig. 38c).

Fig. 38c. Permanent wilting point

The amount of h2o really bachelor to the found is the amount of water stored in the soil at field capacity minus the water that volition remain in the soil at permanent wilting point. This is illustrated in Fig. 39.

Fig. 39. The available soil moisture or water content

Available h2o content = water content at field capacity - water content at permanent wilting bespeak ..... (thirteen)

The bachelor h2o content depends greatly on the soil texture and construction. A range of values for different types of soil is given in the following table.

Soil	Available water content in mm water depth per m soil depth (mm/k)
sand	25 to 100
loam	100 to 175
clay	175 to 250

The field capacity, permanent wilting signal (PWP) and available water content are called the soil moisture characteristics. They are constant for a given soil, but vary widely from one blazon of soil to another.

2.5 Groundwater table

two.v.i Depth of the groundwater tabular array
two.v.ii Perched groundwater table
2.5.iii Capillary rise

Part of the water applied to the soil surface drains below the rootzone and feeds deeper soil layers which are permanently saturated; the top of the saturated layer is called groundwater table or sometimes merely h2o table (encounter Fig. 40).

Fig. 40. The groundwater table

two.5.ane Depth of the groundwater table

The depth of the groundwater table varies greatly from place to place, mainly due to changes in topography of the surface area (come across Fig. 41).

Fig. 41. Variations in depth of the groundwater table

In 1 item place or field, the depth of the groundwater table may vary in time.

Post-obit heavy rainfall or irrigation, the groundwater table rises. It may even attain and saturate the rootzone. If prolonged, this situation tin be disastrous for crops which cannot resist "moisture feet" for a long menstruum. Where the groundwater table appears at the surface, it is chosen an open groundwater tabular array. This is the instance in swampy areas.

The groundwater table can too be very deep and afar from the rootzone, for example following a prolonged dry period. To keep the rootzone moist, irrigation is then necessary.

two.5.2 Perched groundwater tabular array

A perched groundwater layer tin be institute on peak of an impermeable layer rather shut to the surface (20 to 100 cm). It covers usually a limited area. The top of the perched water layer is chosen the perched groundwater tabular array.

The impermeable layer separates the perched groundwater layer from the more than deeply located groundwater table (come across Fig. 42).

Fig. 42. A perched groundwater table

Soil with an impermeable layer not far beneath the rootzone should be irrigated with precaution, because in the example of over irrigation (also much irrigation), the perched water table may rise apace.

2.5.three Capillary ascent

So far, it has been explained that h2o can move downwardly, as well as horizontally (or laterally). In addition, water can motility upwards.

If a piece of tissue is dipped in water (Fig. 43), the water is sucked upward past the tissue.

Fig. 43. Upward movement of water or capillary ascension

The aforementioned process happens with a groundwater tabular array and the soil above it. The groundwater tin exist sucked upward by the soil through very small-scale pores that are called capillars. This process is called capillary rise.

In fine textured soil (dirt), the upward motility of h2o is slow simply covers a long altitude. On the other manus, in fibroid textured soil (sand), the upward movement of the water is quick but covers only a curt distance.

Soil texture	Capillary rise (in cm)
coarse (sand)	20 to 50 cm
medium	50 to eighty cm
fine (dirt)	more than eighty cm upward to several metres

2.6 Soil erosion past water

2.6.1 Sail erosion
two.six.2 Gully erosion

Erosion is the transport of soil from one place to another. Climatic factors such as wind and rain can cause erosion, just also under irrigation information technology may occur.

Over a short period, the procedure of erosion is almost invisible. However, it can be continuous and the whole fertile top layer of a field may disappear within a few years.

Soil erosion by water depends on:

- the slope: steep, sloping fields are more exposed to erosion;
- the soil structure: light soils are more sensitive to erosion;
- the book or rate of menses of surface runoff water: larger or rapid flows induce more erosion.

Erosion is usually heaviest during the early on role of irrigation, especially when irrigating on slopes. The dry out surface soil, sometimes loosened past cultivation, is easily removed by flowing water. After the commencement irrigation, the soil is moist and settles down, so erosion is reduced. Newly irrigated areas are more sensitive to erosion, especially in their early on stages.

There are two master types of erosion caused by h2o: canvass erosion and gully erosion. They are oftentimes combined.

ii.6.1 Sail erosion

Sail erosion is the even removal of a very thin layer or "sheet" of topsoil from sloping country. It occurs over large areas of land and causes most of the soil losses (see Fig. 44).

Fig. 44. Sheet erosion

The signs of sheet erosion are:

- only a thin layer of topsoil; or the subsoil is partly exposed; sometimes even parent rock is exposed;
- quite large amounts of coarse sand, gravel and pebbles in the abundant layer, the finer fabric has been removed;

- exposure of the roots;

- deposit of eroded material at the foot of the slope.

2.half dozen.ii Gully erosion

Gully erosion is defined as the removal of soil by a full-bodied water flow, large plenty to class channels or gullies.

These gullies carry water during heavy rain or irrigation and gradually get wider and deeper (see Fig. 45).

Fig. 45. Gully erosion

The signs of gully erosion on an irrigated field are:

- irregular changes in the shape and length of the furrows;
- aggregating of eroded fabric at the bottom of the furrows;
- exposure of institute roots.