2. HYDROLOGY, SOIL ARCHITECTURE AND WATER MOVEMENT

The hydrological cycle

An understanding of the hydrological cycle is essential for the effective management of rainwater and soil water. Water occurs not only as a liquid, but also as a solid (e.g. hail, snow) and as a gas - water vapour. The total amount of water in the world is constant, but water is continuously changing from one form to another and is continuously moving at different speeds. These interrelationships are shown in a simplified form at a regional scale in Figure 2.

FIGURE 2. Simplified diagram of the hydrological cycle (Adapted from Ward, 1975)

Heat from the sun causes water at the surface of oceans, lakes and rivers to change into water vapour in a process called evaporation. Transpiration in plants is a similar process, in which water is absorbed from the soil by plant roots and transported up the stem to the leaves, from where it is released (transpired) as water vapour into the atmosphere.

As the water vapour produced by evaporation and transpiration rises into the atmosphere, so the temperature decreases and water vapour changes into water droplets (condensation), which accumulate as clouds. Depending on their size, these may be released as rainfall.

Once rainfall reaches the land surface it can infiltrate into the soil, run off over the surface as overland flow, or accumulate on plant leaves or in puddles from where it evaporates back to the atmosphere. A combination of these processes is commonly the case.

The rainfall that infiltrates into the soil forms part of the soil water, of which some may be used by plants for transpiration, some may return to the atmosphere through evaporation from the soil surface, and some - if sufficient infiltration occurs - may move beyond the rooting zone to the groundwater. Annex 7 deals with soil moisture use under different land uses and vegetation.

The groundwater moves laterally and slowly towards the sea to complete the hydrological cycle, but part of it will seep into springs, streams, rivers and lakes on the way. In this way the groundwater maintains the water level in wells, and the continuity of river and streamflow during dry periods (referred to as base flow).

Rainwater that runs off the land moves rapidly downhill towards river courses, contributing to peak flows, and is of great concern. Runoff is not only a waste of rainfall that could have contributed to crop production and groundwater supplies, but it frequently causes floods or damage to roads and farmland, and erodes soil that is redeposited in river courses and reservoirs downstream.

Groundwater is derived from rainwater that has infiltrated into the soil and drained beyond the rooting zone in excess of both the quantity needed for the crop or the vegetation and the water-storage capacity of the soil (FAO, 1995a and FAO, 2002).

Groundwater moves very slowly through subsoil materials in the direction of the dominant drainage course. If its upper surface, the water table, does not sink below the level of the streambed, water is released to springs that feed streams and tributaries. This occurs throughout the year and in this way groundwater acts as a buffer in maintaining stream base flows and water levels in wells during dry periods.

In soils with relatively impermeable subsoil layers beneath more highly permeable layers, perched water tables may develop above the groundwater, due to water being held up by the impermeable layers. The water in a perched water table, sometimes referred to as interflow, will slowly move laterally and may emerge into stream courses or springs at lower elevations. It does not contribute directly to the groundwater. The presence of groundwater or a perched water table is indicated by saturated soils, and usually by a dominance of light grey, bluish-grey, bluish or greenish colours. These colours are typical of certain iron compounds that only form in waterlogged soils where oxygen is lacking.

The amount of rainfall that percolates beyond the lower limit of the rooting zone towards the groundwater will depend on the amount of water used for transpiration by the crops or vegetation. For a particular climate and soil type, forest transpires more water than grassland, which generally uses more water than crops. The high water use by forest is due to its generally greater transpiration rate, the longer period of transpiration compared with crops, and the deeper roots enabling it to absorb water from greater depths. Changes in land use can therefore affect the quantity of water transpired and hence the quantity reaching the groundwater. Replacing forest vegetation with grassland or annual crops may increase deep drainage and so provide higher base flows in streams and rivers. Changes in soil management can also affect the quantity of deep drainage replenishing groundwater. The introduction of poor management practices that increase the proportion of rainfall lost as runoff will reduce base flows and increase peak flows and the incidence of flooding. Conversely, an improvement in soil and nutrient management will lead to higher grain and foliage production, higher transpiration rates, and hence less recharge.

In order to consider rainwater for plants and for groundwater as parts of a sequence, it is important to have a mental picture of its journey. After passing through the atmosphere in response to gravity, water from rain or irrigation travels to some or all of the following destinations (Figure 3).

FIGURE 3. The sequence of destinations of rainwater (Shaxson, 2001 after FAO, 1995b)

Legend:

1. Direct evaporation from wetted leaf surfaces.
2. Surface runoff/stormflow.
3. Direct evaporation from the soil surface.
4, 5, 6. Plant-available soil moisture within root-range of existing weeds, crops, trees.
7. Soil moisture within root-range of existing plants but held at tensions unavailable to them.
8. Soil moistures held at all tensions, but below root-depth of existing plants.
9. Water not captured by roots and small pores, moving to groundwater and streamflow.
10. Leakage to deep groundwater beneath catchment floor.

Management of the soil can significantly affect runoff; direct evaporation from the soil surface; the amount of soil moisture available to plants within range of their roots; and the depth to which roots can penetrate. How much water reaches each of these destinations over a given period depends on the physical condition of the soil and its influence on infiltration and runoff, and on the atmospheric conditions as they affect evaporation and transpiration.

Catchments and watersheds

Water caught by a catchment will flow towards the lowest point at the outlet, where it may join water emerging from other catchments. The outer boundaries of a catchment are defined by ridgelines along the crests of the surrounding uplands. From the sides of a valley surface runoff tends to flow perpendicularly to the slope from crest to streamline.

A watershed is the area of land dividing two streamlines. Water moves away from the crest line towards the streamlines on either side. Thus a hill slope can be considered as either the inner slope of a catchment or the outer slope of the watershed. Catchments and watersheds are indicated on maps by the contour lines and by the course of drainage lines (Figure 4).

FIGURE 4. Catchment vs. watershed as distinct but interrelated features of the landscape

Underlying geological formations, together with weathering and uplift processes, affect the form of landscapes. They influence the steepness or shallowness of slopes, whether the streamlines are of relatively sinuous shape or with abrupt changes of direction. The flow of water along the streamlines tends to cut the heads of streamlines back into the underlying materials (Plate 10).

For the purposes of enabling rainwater to soak into the soil and controlling the rate of flow of any excess runoff, we can subdivide a given catchment into a more detailed hierarchy of catchments, in which the smallest subdivisions may be measured in square centimetres, the larger in hectares, within catchments of square kilometres. Rainfall entry into the soil depends on the porosity of the soil at any scale, while management of runoff and erosion across the surface also depends on any physical works that may be constructed when rainfall rates exceed even the best infiltration rate (Figure 5).

FIGURE 5. A nested hierarchy of interrelated constructed and natural catchments

Plate 10. Repeated flows of water cut streambeds downwards and back into an ancient plateau landform; the crests between them divide one catchment from another. Paracatú, Brazil

[T.F. Shaxson]

Overlapping pairs of vertical aerial photographs viewed with a stereoscope provide a three-dimensional view of the landscape and surface features (Shaxson et al., 1977; Carver, 1981). Plate 11 is a stereogram that shows a layout of roads along topographic crests and of conservation banks close to contours that has been designed in conformity with the natural catchments of the landscape. This pattern provides a framework within which planting rows will have been contour-aligned relative to the conservation banks. The maintenance of soil porosity by mulch cover will allow the highest proportion of rainfall to be available as soil moisture for the crops and groundwater for the streams.

PLATE 11. Stereogram of a landscape. Malawi

[Government of Malawi]

A stream catchment may be large or small, of steep or shallow slope and composed of natural subcatchments and then of field catchments. Plate 12 shows two of these field catchments, which also form the left-hand side of a watershed whose crest runs along the ridge seen at top right.

PLATE 12. Two field catchments - parts of the stream catchment whose drainage line runs along the left of the photo. Santa Catarina, Brazil

[T.F. Shaxson]

In cropping agriculture the next smaller subdivision is the bund catchment, between any pair of physical conservation banks with its ridges and furrows (formal or informal) along planting rows (Plate 13). The physical conservation banks, vegetated with a fodder grass, subdivide the field catchment as well as separating the bund catchments. The aim of these structures is to conserve water and soil, though their effects on yield are disappointing. Their most significant function is to provide guidelines for contour planting of crop rows.

PLATE 13. Two bund catchments interlined with "row catchments" along the planting lines. (Caxambú, Brazil)

[T.F. Shaxson]

The smallest subdivision to trap rainwater and give it time to soak in is the microcatchment with its mulch (Plate 14).

In forestry, because the young trees are more widely spaced, the same effect can be obtained by a set of half moon shaped microcatchments, one at each planting position (Plate 15).

Within this overall framework, the key to infiltration is to keep the soil porous with a cover of crop residues, which prevents damaging raindrop impact and provides a substrate for soil organisms (Plate 16).

PLATE 14. A row of microcatchments: one furrow cross tied for water retention, the other covered with a mulch to facilitate rapid infiltration of rainwater; the unmulched furrow also provides a dry-season firebreak between the rows of young tea. Mulanje, Malawi

[T.F. Shaxson]

PLATE 15. Half moons around newly planted Acacia seedlings catch and detain rainwater, in similar manner to the cross tied furrows in the previous photo. Dungarpur, India

[T.F. Shaxson]

The conservation effects of forests are due not so much to the presence of the trees themselves but to the litter of fallen leaves, twigs and branches, plus any low-growing vegetation. If the soil surface has not been damaged by trampling, less rainwater will run off and more will infiltrate into the soil (Plates 17 and 18).

How much plant-available soil moisture remains at a given time depends on the texture and porosity of the soil, the previous volume of soil moisture, the volume removed by direct evaporation, evapotranspiration and deep drainage. Irrigation (if available) is normally required when about two thirds of the available water - between field capacity (FC) and permanent wilting point (PWP) - has been depleted. If irrigation is not an option, it makes sense to manage the soil to develop and retain a maximum amount of soil pores of a wide range of sizes. This will maximize the capacity for water retention and enable plants to withstand drought for longer periods. Loam textures generally have the largest available water capacity, while sand on the one extreme has a small available water capacity, as does clay at the other (Figure 6).

FIGURE 6 Typical available water capacities of different textured soils (after Smith and Ruhe, 1955)

Available water capacity coupled with soil depth determines the volume of water usable by plants at a particular site. This is illustrated by comparing relevant characteristics and consequent amounts of available water for two soils on which tea is grown, one at Timbilil in Kenya, the other at Marikitanda in Tanzania (Table 5).

TABLE 5
Differences in available water capacities between two East African soils (Tea Research Institute of East Africa, 1973)

Flood flows in streams and rivers, which rise quickly after heavy rainfall derive mostly from rapid overland flow of water. Flood flows are often muddy with eroded materials. Clear streamflow originates from rainwater, which has infiltrated the soil and percolated through pores of a range of sizes at different slower speeds (Figure 7).

FIGURE 7. Runoff and percolation: two routes for rainwater

The Plates 19 - 22 show streamflow during the rains from a cultivated catchment without any effective conservation measures (Plate 19) and a nearby forested catchment (Plate 20), both on the slopes of the same mountain, within 1 km of each other. The clear water, which runs throughout the year, more voluminous in the rainy than in the dry season, has percolated through the litter of fallen leaves and branches on the forest floor, which both protects the surface from rainfall impact and nourishes the soil organisms that maintain soil porosity. This water has travelled the slow route down through the soil to the groundwater, which moves into the stream via springs and seepages along the streambanks.

PLATE 16. Mulching within a microcatchment. Mauá, Brazil

[T.F. Shaxson]

PLATE 17. The soil's water absorbing capacity in this Eucalyptus plantation is protected by the litter of leaves and twigs, and little runoff is likely to occur. Tupanssi, Brazil

[T.F. Shaxson]

PLATE 18. Much rainfall is likely to runoff this unprotected and probably compacted, bare surface beneath trees of the same genus (Eucalyptus) as in the previous photo; little rainwater will be able to infiltrate and soil moisture will be scarce. Potosí, Bolivia

[T.F. Shaxson]

PLATE 19. Runoff and soil loss immediately after a rainstorm, Naisi catchment. Zomba Mountain, Malawi

[T.F. Shaxson]

PLATE 20. Clear streamflow from the Mlunguzi catchment. Zomba Mountain, Malawi

[T.F. Shaxson]

PLATE 21. Detention of sediment-laden runoff by a dam wall has been used to provide limited areas of flat cropland (here under rice); this may have been one of the purposes of the dam. Sharam, India

[T.F. Shaxson]

PLATE 22. The catchment for a dam is in good condition: Clean runoff can be stored for many purposes. Sharam, India

[T.F. Shaxson]

As dug wells provide direct access to shallow groundwater, on which many rural communities rely, it is important that enough rainwater penetrates and pass through the soil to replenish groundwater (Plates 23 and 24).

Exceptional rainfall such as during typhoons or hurricanes on already saturated soil can result in floods and erosion that change the landscape, no matter how well the land and the crops, grassland or forest are managed (Hamilton, 1986).

Soil architecture and the importance of pore spaces in soils

Although we generally think of soil in terms of its solid parts, i.e. the sand, silt, clay and organic matter, it is the spaces between these solid particles that are as important as the solid particles. This is because it is the spaces where all the action takes place, just as in a house, where all the important activities occur in the rooms rather than in the walls and floors. It is therefore the architecture of the soil that is important. The pore spaces in a soil vary in abundance according to the type of soil and how it has been managed. Soils under natural vegetation generally exhibit high porosity because of high biological activity and lack of interference by man. Consequently they have superior physical qualities compared with most soils used for crops or grazing. Plate 25 illustrates the contrasting porosity in forest and cultivated soils.

PLATE 23. Dug wells such as this one may go dry because the water table falls below the depth to which the well was dug. Palampur, India

[T.F. Shaxson]

PLATE 24. Shortage of water puts extra burdens of time and effort on people. Palampur, India

[T.F. Shaxson]

PLATE 25. Contrasting porosity, compaction and organic matter content between the topsoil (0-20 cm) of a forest soil (on the right) and from the same soil type, immediately adjoing the forest site, after 4 years cultivation (on the left). Saaverda, Bolivia

[R.G. Barber]

Pore spaces in soils vary in size, and both the size and continuity of pores have an important influence on the types of activities that occur in soil pores. Table 6 shows the functions of pores of different size ranges, and their names, together with the size of crop roots.

TABLE 6
Functions and sizes of soil pores (Hamblin, 1985)

Pore sizes from 0.0002 to 0.05 mm diameter retain water that can be absorbed by crops and are referred to as storage pores, whereas smaller pores (the residual pores) hold water too tightly for plants to be able to extract it. Pores larger than about 0.05 mm diameter, referred to as transmission pores, allow water to drain through the soil and enable air to enter the pores as the water drains out.

Pore spaces are also needed for roots to freely penetrate soils in order to take up nutrients and water. The sizes of roots vary with the type of crop, but the smallest roots, apart from root hairs, have diameters of 0.1 to 0.3 mm and so soils must have pore spaces of at least this size if the smaller roots are to penetrate freely. In most soils roots grow partly through existing pores, the transmission pores, and partly by moving aside soil particles. Roots can only force their way into smaller pores if the soils are sufficiently compressible; the compressibility of soils increases with increasing water content, since water provides a form of lubrication between soil particles.

Soil water movement

The amount of water present in a soil, which is available for crop production, will depend on how much of the rainwater remains in the soil after the losses by runoff, evaporation, and deep drainage. The amount of rainfall that reaches the groundwater and thus contributes to water security, will depend on the extent to which the rainfall infiltrating the soil is in excess of that needed to replenish the soil's water holding capacity and satisfy the transpiration needs of the crops. Good rainwater management aims to maximize the amount of rainwater that enters the soil, and to make best use of it while it is there for use by crops and for recharging the groundwater. Any truly unavoidable surface runoff is conducted away safely in such a manner that it does not cause erosion problems.

When a well drained soil is saturated to the limit of its rooting zone, the rainwater that does not drain out of the root zone within 48 hours will be retained in soil pores smaller than about 0.05 mm diameter (the critical pore size may vary from 0.03 to 0.1 mm diameter). The quantity of water retained after 48 hours corresponds to the soil's field capacity (FC). The forces (or suctions) with which this water is held will vary according to pore size. The largest pores still to retain water will hold the water at about a tenth to a third of the pressure of the atmosphere (or 0.1 to 0.33 bar^[1]), depending on what suction corresponds to the soil's FC; this will vary with soil type and depth of the water table.

The maximum suction most crops can exert to withdraw water from soil varies with the crop, but the generally accepted value is equivalent to about 15 times the pressure of the atmosphere (i.e. 1.5 Mpa). This is approximately equivalent to the pressure that would be experienced when supporting a tonne weight on the palm of the hand. When soil water has been exhausted down to 15 bars, the water remaining in the soil will be that stored in pores smaller than 0.0002 mm diameter, and will correspond to the soil's PWP. Water held at suctions greater than the PWP is not available for plant growth. Consequently, it is water held between FC and PWP which can be used by crops for transpiration, and is termed the soil's Available Water Capacity (AWC). However, after a heavy rainstorm some of the water in excess of the soil's FC may be used by a crop while this excess water is percolating through the rooting zone.

The available water held within the range FC to PWP is retained with different strength, and about a third of it is not easily or rapidly available to crops, especially if the crops are transpiring strongly. The higher the transpiration demand, the more available (i.e. the less strongly held) the soil water must be to avoid crop water stress. In contrast, for a slowly transpiring crop even water held at higher suctions can be used without causing stress.

The maximum amount of available water that a soil can retain (i.e. the available water capacity) will vary with the soil's texture, organic matter content, rooting depth and structure. Soil organic matter is particularly important in that it can retain about 20 times its weight of water. Organic soils and medium textured loamy soils with high contents of very fine sand and silt generally have the highest AWCs, clayey soils intermediate values, and soils with high contents of coarse sand the lowest AWCs. The stone content of soils can also be very important depending on the nature and abundance of the stones. Some ironstone gravel > 2 mm diameter can contain more than 20 percent water (m³/m³) at FC and porous limestone and chalk can also make significant contributions to the AWC of a soil. In contrast, a high content of non-porous stones will greatly diminish the AWC of a soil.

For any given soil, the greater the rooting depth, the larger will be the quantity of soil water available to the crop. This is particularly important for annual crops as they have less time to develop deep and extensive rooting systems than perennial crops. The available water capacity may influence the length of growing period for crops grown on that soil. Soils of high available water capacity will permit longer growing periods because of their ability to provide greater quantities of stored water during dry periods than soils of low available water capacity (FAO, 1995a). Shallow soils have little available water, and even in wet years they will be unable to benefit by storing any more water.

Infiltration of rainwater into soil

In most areas where water shortages occur, maximizing the infiltration of rainfall into soil is indispensable to achieving food and water security. Land management should encourage infiltration as opposed to runoff. Exceptions are where rainwater harvesting is necessary for crop production and where high infiltration can lead to risks of landslides or other forms of mass movement.

The amount of rainfall that infiltrates will be governed by the intensity of the rainstorm in relation to the soil's infiltration rate. Excessive tillage and loss of soil organic mater often result in reduced infiltration rate due to loss of surface porosity. When storm intensity is greater than soil infiltration rate, runoff will occur, resulting in a waste of water that should have been used for crop production and for recharging the groundwater. The rate at which rainfall infiltrates into soil is influenced by the abundance, stability and size of the pores at the soil surface, their water content and by the continuity of the transmission pores into the rooting zone. In many soils the number of surface pores is rapidly reduced by the impact of raindrops, which break surface soil aggregates into small particles that clog surface pores and form surface seals or crusts with very few pores. The destructive raindrop action is avoided where there is a protective cover of crop foliage, residues, mulches or even weeds at or over the soil surface.

Other factors that can reduce the number, proportion and continuity of transmission pores are traffic by machinery, humans and animals, which destroys large pores by compaction, and tillage which disrupts the continuity of transmission pores through the smearing and compression of pores during plough pan formation in the subsoil. Infiltration rates are also affected by (1) the quantity of water present in the soil at the time of the rainstorm, which will depend on when the last rainstorm occurred and the permeability of the soil, and (2) the soil's capacity to retain water, which will vary with soil depth, stoniness, and texture.

Percolation of rainwater through soil

When a heavy rainstorm falls on a well-structured soil, rainwater percolates down through the dry soil as a wetting front, temporarily saturating the soil and displacing air. This is accompanied by the rapid drainage of water from the larger pores (greater than 0.05 mm) through gravity and the pressure of the mass of rainwater above. These larger pores exert only small forces of attraction on soil water. After about two days of drainage field capacity will have been attained and air will have re-entered the larger pores.

In poorly structured soils, rainwater will drain much more slowly. Drainage often continues for several weeks depending on the depth to the slowest horizon and the continuity of the larger pores with depth. In fine textured soils with cracks drainage water will flow down through the cracks in heavy rainstorms before the soil is saturated and while parts of the soil profile may still be dry. If the drainage water subsequently enters a smaller pore while passing through the soil, it will be retained, otherwise it will continue until it reaches the water table and contributes to the recharge of groundwater.

Once the drainage water has been lost from the rooting zone, further water movement within the root zone is slow and is referred to as capillary movement. This movement is caused by forces of attraction, known as surface tension forces, which are exerted by soil particles on water. This movement can occur in any direction and includes the upward movement of water from water tables. Surface tension forces pull water into pores within the soil and the smaller the pores the more strongly the water is attracted and held.

Loss of water vapour from soils

Water is also able to move through soils as water vapour. The most important example of this is the loss of water vapour by evaporation from soil surfaces. This occurs when the concentration of water vapour in the soil close to the surface is higher than that in the atmosphere immediately above. Water vapour will then move from the soil into the atmosphere. The drier and hotter the atmosphere compared with the surface soil, the greater will be the rate of evaporation from the soil, provided sufficient water can be supplied to the surface by capillary movement from below. Fine textured soils have an abundance of small pores and so more capillary movement of water to the surface will generally occur in fine textured than in coarse textured soils.

Water movements into and through a plant

Crops use large quantities of water, which under rainfed conditions come entirely from water in the soil, which in turn is derived from rainfall that has infiltrated the soil. A maize crop may use 400 to 750 mm of water depending on the rainfall and evaporation conditions. This corresponds to 4 000 to 7 500 cubic metres of water per hectare over the growing season.

Almost all the water absorbed from the soil by crop roots passes up through the stem into the leaves, where it evaporates and passes into the atmosphere in a process known as transpiration. This process accounts for almost all of the water absorbed by plant roots (about 99 percent, the remaining 1 percent being used directly in cell processes). Transpiration is essentially the same as the process of evaporation. Evaporation is what happens when a bowl of water is left in the sun. The liquid water disappears as it becomes converted into water vapour, and the higher the temperature, the drier the air, and the greater the wind speed, the greater will be the rate of evaporation. Evaporation occurs whenever water is exposed to the atmosphere, i.e. from lakes, rivers and puddles, and from the raindrops that accumulate on a leaf after a rainstorm.

To ensure an efficient uptake of sufficient water by crops it is important that the crop roots are well distributed and able to penetrate deeply into the soil. As a soil dries out from the surface downwards, so roots in the deeper layers tend to increase in number to compensate. When soil water reaches the surface of a root or root hair, it moves across the root into the xylem, which contains narrow tubes running through the root and extending up through the stems into the leaves. On reaching the leaves, water passes from the xylem into leaf cells where it evaporates into air spaces within the leaf. These air spaces are saturated with water vapour, and are connected to the normally drier outside air by very small openings in the leaves called stomata. During the day the stomata open, which allows carbon dioxide to enter the leaf. Sunlight is used to make sugars within the plant: a process known as photosynthesis. Part of the sugars are used to produce energy by a process called respiration, part are converted into substances forming the various plant organs.

Photosynthesis occurs only during daylight, whereas respiration occurs all the time. When the stomata open to allow carbon dioxide to enter, water vapour escapes into the drier air outside. For transpiration to occur there must be a continuous supply and movement of water from the soil to the plant to the atmosphere. The driving force responsible for this movement is the same as for evaporation, and can be simply described as the tendency for water to move, either as a liquid or a vapour, from where it is more abundant to where it is less abundant. In transpiration, water vapour moves from the very humid (i.e. high water vapour content) air spaces within the leaf into the drier atmosphere outside the leaf where the water vapour concentration is lower.

The movement of water vapour out of the leaf creates a suction (or "pull") on the water in the leaf cells, the xylem, the roots and the soil, so water moves into the root, up the xylem and into the leaves, to replace that which has been lost from the leaves. In addition to the transpirational suction, which causes water to move from the soil into the root, there is another force attracting water into the root known as osmosis. In osmosis, water moves from where it is more pure to where it is less pure across a semi-permeable membrane. A semi-permeable membrane is a very thin skin, which has pores large enough for water to pass through into the root but not large enough for dissolved salts to pass out of the root.

Water therefore passes from the soil where the water is more pure (i.e. contains few dissolved salts) across the root surface (a semi-permeable membrane) into the root where the water is less pure (i.e. contains more dissolved salts).

Water stress - nutrient interactions

Many areas with low and erratic rainfall where crop water stress is common are also deficient in nutrients, and the lack of nutrients is frequently the second most limiting soil factor. An interaction often occurs between soil water and nutrients, which means that soil water can influence the availability of nutrients, and the availability of nutrients can influence the uptake of soil water and a crop's resistance to drought. Thus, both factors can influence each other.

Plants contain a certain amount of water within them, which acts as a buffer against times of water shortage, but the amount is too small to last long. In contrast, plants store sufficient quantities of nutrients within their tissues to provide a buffer for longer periods when nutrients are not being absorbed. Consequently, water deficiencies become more quickly apparent and damaging than nutrient shortages. This suggests that conserving water may often be of prior and quicker benefit than attempting to conserve soil particles per se.

In addition, a lack of water also reduces the uptake of nutrients by a crop. This is largely because nutrients can only move to roots through water films within the soil, and so there must be continuous water films connecting the nutrients with the roots. A lack of soil water continuity, due to drought for example, will severely reduce the rate of nutrient uptake by crops.

A lack of soil water will also diminish nutrient availability by reducing microbial activity, which is responsible for the liberation of nitrogen, phosphorus and sulphur from soil organic matter.

When there is a drought it is the surface soil, (which generally contains the bulk of the plants' roots and the soil's nutrients) which dries out first, and so while a crop may still be able to absorb water from the subsoil, it may suffer from a lack of nutrients.

A lack of available nutrients in the soil can restrict crop water uptake, especially when nutrients are limiting root development. This occurs most often in soils that are deficient in phosphorus. Applying P fertilizer to P-deficient soils will often promote root development, and as a result crop water uptake. Consequently the beneficial effects of applying P fertilizers are often relatively greater in seasons of lower rainfall than in those of higher rainfall.

The effects of drought and nutrient availability on crop yields are difficult to predict, because the effect will depend on when the water or nutrient shortage occurs in relation to the crop's stage of growth, and its needs and sensitivity to a lack of water or nutrients at that time. It is therefore often difficult to assess which factor, e.g. water or nutrients, is the more limiting to yield. The most limiting factor can vary from season to season depending, for example, on when water shortages occur, and even during a season there will probably be periods when water is the main limiting factor, and other periods when nutrients are most limiting.

Water shortages often affect whether or not there is a response to fertilizers, and how much fertilizer should be applied. This is particularly common with N fertilizer, where the optimum response is frequently higher in good seasons than in poor seasons. For example, when there is no shortage of soil water an application of 40 kg/ha may prove to be the optimum application rate, but when water is lacking only 20 kg/ha may be the optimum amount to apply.

This creates difficulties in rainfed agriculture: since it is not possible to reliably predict the distribution and amount of rainfall, farmers cannot know how much fertilizer to apply. One approach that can help to overcome this problem is to apply a modest amount of N fertilizer at the beginning of the season assuming low rainfall, and then to apply additional quantities N later if the season appears promising.

Causes of restricted rooting

The most common cause of restricted rooting is physical restriction due to soil compaction, which results in the collapse or diminution of pore spaces and a localized increase in bulk density. Once pores have been compacted to less than about 0.2-0.3 mm diameter, it is difficult for crop roots to freely penetrate the soil. Although the strength of compacted layers decreases as soil water content increases, a high water content can quickly limit the supply of oxygen to roots, so that roots then become restricted by a lack of oxygen. Certain crops, such as cotton and sunflower, appear to be more susceptible to restricted rooting from compacted layers than others. Compaction often reduces pore sizes sufficiently to inhibit root penetration but not sufficiently to affect the drainage of water through the soil. Pores of 0.2-0.3 mm diameter can restrict roots but water can drain under gravity through pores as small as 0.01 mm diameter (Russell, 1973).

In mechanized cropping systems the continual use of tillage implements, especially disc ploughs, disc harrows, mould-board ploughs and rotovators, over long periods of time frequently results in the formation of dense plough pans containing few pores large enough to be penetrated by crop roots (Plate 26). The plough pans develop just below the depth to which the soil is tilled and often have smooth upper surfaces with sealed pores, caused by the smearing action of mould-board ploughs. The degree of compaction depends on the pressure exerted by the implements on the soil.

PLATE 26. The beginnings of a plough pan formed on a sandy clay loam after 8 years of disc ploughing and harrowing. Ibamirapinta, Camiri, Bolivia

[R.G. Barber]

Land preparation when soils are wetter than the optimum moisture content for tillage promotes soil compaction, because the soils are then much more compressible. This is particularly likely to occur on soils that have deficient drainage, or are difficult to till in a dry state without pulverizing because of their very hard consistence (e.g. hardsetting soils). Compaction is also more likely when farmers use many passes to prepare the seed bed, or when they have only limited tractor power available and are unable to use wide sets of equipment and therefore produce compacted wheel ruts at closer spacing across the field surface. Compaction can also develop in the subsoil from the passage of heavy machinery such as combine harvesters and lorries loaded with grain, especially in wet conditions. The degree of compaction will depend on the total axle load of the machinery.

Soil compaction can also develop from hand tillage. Thin hoe pans just 2-3 cm thick can develop just below the depth of hoe penetration and thus restrict root penetration (Figure 8). When mounds or ridges are formed every year, the combination of hoeing at the same depth and the traffic of people within the furrows during wet conditions may accentuate the compaction.

FIGURE 8. Sideways development of tap-root of a wild okra weed growing in a maize field; the change in growth habit of the root is caused by a compacted hoe pan at the base of the ridges formed by hoeing to the same depth (and the passage of feet during the rains) over many years (Adapted from a picture of G. Evers)

A similar effect to that of compaction can occur when structurally unstable soils, known as hardsetting soils, slump on becoming saturated by intense rainstorms to form dense layers. On drying the dense soil layer becomes very hard and restricts root penetration (Plate 27).

PLATE 27. Heavy rain two weeks after sowing caused this unstable sandy soil to slump and develop a dense layer, which inhibited the growth of young soybean roots. Las Brechas, Santa Cruz, Bolivia.

[R.G. Barber]

Restricted rooting may also be caused by naturally occurring dense horizons containing few pores large enough for roots to penetrate. These horizons may be found in soils formed from river, lake or volcanic sediments and in semiarid and arid areas where chemically cemented calcrete and gypsic horizons are formed.

In some situations root restriction can be caused by a seasonally fluctuating water table. During the rainy season the crop roots are confined to a shallow zone immediately above the high water table. If, when the water table falls during the dry season the crop roots have already completed their development, the roots will remain where they were, close to the surface, and without access to available water in the deeper subsoil. Roots may also be restricted to shallow depths by chemical factors, such as the presence of toxic aluminium or manganese, or by severe nutrient deficiencies in the subsoil. Problems of water stress may be the result of restricted rooting combined with various other factors as illustrated in Box 1 (Barber, 1995).

Indicators of restricted rooting

The most obvious indicator of restricted rooting when a crop is present is the distribution of the crop roots. When roots are physically restricted by a dense layer containing few pores suitable for root penetration, individual roots often develop characteristic growth patterns immediately above the restricting layer as shown in Figure 9. The most common of these is the abrupt change in the direction of growth from vertical to horizontal, and a thickening of roots that do manage to penetrate the restricting layer just above the upper boundary of that layer.

FIGURE 9. Examples of rooting pattern and growth habit when roots are physically impeded or prevented from penetrating a root-restricting soil layer (R.G. Barber)

In mechanized agriculture, plough pans are usually formed at 12-30 cm depth, depending on the implement used and its normal working depth. Naturally occurring dense layers may occur at any depth. The optimum time to observe roots is after flowering when most of the roots will have largely completed their growth.

When no crop is present, it is much more difficult to identify the existence of potentially root-restricting layers in a soil. However, the rooting pattern of mature weeds, either rooted or uprooted, that remain in the field after the crop has been harvested can be used to reveal the existence of a root-restricting layer.

When neither a crop nor weeds are present, the presence of a dense soil layer of high strength and containing very few visible pores will often be a useful indicator. The presence of dense layers is often revealed when digging by the abrupt increase in resistance to the spade or hoe when the restricting layer is reached. However, sudden increases in soil resistance can also be experienced when the soil changes from moist to dry. To avoid this problem, it is advisable to wet the soil to 30 cm depth two days prior to carrying out the field examination.

Physically restricting layers can be identified by the scarcity of visible pores. The smallest pore visible to the naked eye (0.1 mm diameter) coincides reasonably well with the smallest pores into which the seminal roots of cereals (0.1 to 1 mm) and the tap-roots of dicotyledons (0.3 to 10 mm) can penetrate. When the density of visible pores observed in fragments of the dense layer from a soil pit is less than about six in an area of 10 cm × 10 cm, root restriction is likely to be severe, and responses to breaking up the restricting layer are likely. Other indicators of potentially root-restricting layers that can be used in the field in the absence of a crop are soil strength determined with a penetrometer, and soil bulk density determined from undisturbed soil samples of a known volume. Critical penetrometer resistance and bulk density values at which the roots of most annual crops are restricted have been established for soils of different textures.