It is the mineral matter which determines the character of the soil, unless the amount of organic matter is high as in Fenland and moorland soils. A large proportion of sand is characteristic of sandy soils: they are easy to cultivate and in this sense are light. A large proportion of very minute mineral particles is characteristic of a clay soil, which, being more difficult to cultivate is called heavy. (Historically heavy horses were required to pull the plough-). Actually, a cubic foot of a light soil weighs more than the same volume of a heavy soil.
The terms descriptive of mineral soils refer to their texture, I.e. the proportions of the different sizes of particles. With experience a satisfactory diagnosis of texture can be made by rubbing a moist sample of soil between finger and thumb – or by walking across the soil under various weather conditions. In the soil laboratory, the mineral matter is separated by sieving and sedimentation into standard grades, the percentages of which can be calculated after drying and weighing; this is the process of mechanical analysis. It has been done with millions of samples in many countries.
Working from coarsest to finest grades the mineral matter is commonly sub-divided into Gravel, Sand, Silt, Clay. The amount of gravel, consisting of particles with a diameter greater than 2 mm (approximately one-twelfth of an inch) is recorded but not included with the other three grades. For these the diameter limits are:-
Sand, 2 mm to .02 mm;
Silt, .02 mm to .002 mm;
Clay, below .002 mm.
These grades are sometimes sub-divided or, for special purposes, the limits are altered. In diameter the largest clay particles are only one thousandth of the largest sand particles.
The effect of this decrease in diameter on total surface is easily calculated. The surfaces of a foot-cube (0.3 m cube) of solid rock add up to 6 sq. ft. (0.54 sq. m). If such a cube were cut into 8 equal cubes, I.e. with each side one half of a foot, the total surface would become:
8 x 6.4 = 12 sq. ft (8 x 0.54 = 1.08 sq. m)
The total surface is doubled when the linear dimensions are halved.
Taking 2 mm as one twelfth of an inch, I.e. 144th of a foot, if the sub-division of the rock were continued until the fragments were just entering the clay grade, the total surface would become 6 x 144 x 1000 sq. ft. which is about 20 acres (8.1 ha.). A soil’s physical properties, such as adsorption, which are connected with surface will be much more marked in a clay, as compared with a sandy soil. The surface per unit weight of soil is called the specific surface: as just shown, it varies inversely with the linear dimensions of the particles.
Gravel consists of stones, flints, rock fragments and sand particles larger than 2 mm in diameter. In highly calcareous soils, the rock fragments will include lumps of limestone or chalk. The gravel constituents behave as individual particles and have an effect on aeration and. One good example are the flints which are such a feature of the clay-with-flints soils in the Chiltern Hills. Sometimes gravel in the form of rounded pebbles makes up such a large a proportion of the soil that crops suffer badly in periods of drought.
Sand as a rule consists mainly of quartz grains, sometimes to the extent of 90 per cent by weight. (Here, also, exceptions to the rule occur, calcareous soils being one example). Sandy soils contain scores of other minerals, usually in small amounts, including felspars, mica, iron oxides. Sandy soils allow rapid percolation of air and water with correlated advantages and disadvantages. The specific heat of water is 6 or 7 times as high as that of minerals, so a sandy soil, with its low water retaining capacity, will tend to warm up more rapidly than a heavy soil with its higher water retention. But the rapid movement of air and water through a sandy soil and its low specific surface will promote speedy decay of organic matter and also permit quicker loss of plant nutrients. Hence, a sandy soil tends to become acid and to be hungry. However, the advantages of sandy soils outweigh their disadvantages for market gardeners who value highly their easy working, their quick drying after rain permitting more working days per year, their earliness and late season cropping capabilities. Moreover, these soils are suitable for overhead irrigation, a practice which has brought into cultivation many.areas formerly regarded as useless for crop production.
Silt, intermediate in size between sand and clay, has physical properties which also are intermediate. Its higher specific surface confers somewhat greater chemical activity and more plasticity and cohesion than are found in the sand grade. Mineralogically the silt particles are closer to
sand than to clay. Very silty soils may prove to be unstable in that their structure is weak when wet. Mole drains on silty soils tend to be short lived.
Clay is the fraction which controls many of the important properties of a soil. The numerous minerals present differ greatly from those found in sand and silt, principally because they are products formed by the weathering of the minerals occuring in the original rock, e.g. granite. In a granite sample the three main constituents are easy to identify: the glass-like guartz, the white or coloured felspar crystals; the white or dark crystals of mica easily split into thin sheets. In the course of long periods of time water containing CO removes the potash and the lime from the felspar, leaving the other two oxides alumina and silica which, joined up with water, form the substance kaolinite. In this, the unchanged guartz and the mica are embedded. The granite of Cornwall and Devon is usually covered with a layer of kaolinite sometimes 100 feet (30.5 m) deep. When washed free from guartz and mica, it becomes kaolin or china clay, a very important commercial product. In soils, many of the clay particles are of the kaolinite type.
The micas, which may be regarded as compounds containing potassium oxide (or magnesium oxide in dark mica), alumina, silica and sometimes iron oxide, are not so easily decomposed as felspars, but eventually they are changed into one or moreof clay minerals called illites, montmorillonites, chlorites and vermiculites. The last, like kaolin, has become an important commercial product used e.g. in the building industry and in horticulture. The five groups mentioned are the most important but there are several others. Usually, the soil clay minerals carry a negative electric charge, the amount of which varies considerably in the different groups and so helps to differentiate them.
Base exchange is one of the important properties of clay. It is similar to the process used in some domestic water softeners which contain a quantity of granular sodium aluminium silicate – a triple partnership of sodium oxide, alumina and silica. When hard water passes through this substance some of the sodium in the surface layer of its granules is replaced by the calcium of the calcium bicarbonate, the chief cause of the hardness. The water which emerges is a dilute solution of sodium bicarbonate which does not form a scum with soap. (Any other metal salts in the water supply which cause hardness are similarly exchanged.) After a time, the silicate is renovated by reversing the process. A fairly strong solution of salt (sodium chloride) is passed through it; the sodium displaces the calcium which runs to waste. If a dilute solution of potassium chloride or ammonium sulphate is passed through a short column of permutit or other base-exchange material,
tests will show the liquid which first emerges to be completely free of potassium or ammonium.
Clay Colloids and Flocculation. The largest of the clay particles will not settle to the bottom of a 3 ½ (9 cm) column of water in 24 hours. As the diameter of the particles decreases the time of settling increases. On the basis of size alone, therefore, the finest clay would take an extremely long time to settle. In addition, the electric charges become increasingly important as the weight (mass) of the particles decreases owing to the repulsion between the charges of the same sign: the settling time increases to infinity, I.e. never. Most of the clay is so finely divided that shaken up with water, it forms a colloidal solution. Colloidal solutions were recognised and studied over a century ago by the botanist Thomas Graham who found that some suspensions which appeared to be solutions would not pass through a parchment filter, unlike solutions of sugar, salt, etc. The substances which he found to behave like this were sticky or glue-like; hence the name colloid derived from the Greek name Kolla for glue. Later it was found that very many substances in no way glue-like, e.g. gold, silver, iron, sulphur – could be obtained in colloid form, but the original name has been retained. They are important in many industries besides agriculture.
Colloids have a number of interesting properties, e.g. most are sensitive to a change in conditions. When a small amount of acid is added to a suspension of clay in water the minute particles begin to join together to form floes or flakes of visible size. These, being so much larger than the initial particles, rapidly settle. The process is called flocculation.
Lime water, like acid, flocculates a suspension of clay. Caustic soda has the opposite effect of keeping the clay particles from forming floes and is said to cause deflocculation. In the past the flocculation of clay by compounds of calcium has been regarded as the main explanation of the improvement brought about by heavy dressings of chalk on clay and clay loam soils in the Eastern counties. Some other difficult soils, however, such as those derived from the Gault formation are not improved by this practice. It may be dependent on the type of clay particles present in the soil, kaolinites, perhaps, being flocculated but not montmorillonites. It is characteristic of a flocculated clay that crumbs formed when it is dried out are stable when re-wetted. The formation of hard clods which disperse when re-wetted is characteristic of a deflocculated clay.
Organic Matter. An enormous amount of work has been done, and millions of words written by soil chemists on the composition of the soil organic matter. It consists of a
series of products ranging from the undecayed plant and animal remains, through short-lived partially decayed substances, to comparatively stable dark coloured material without resemblance to the original matter from which it was formed. This dark-brown or blackish material is called humus. It is not a definite chemical substance with a precise formula; it cannot be completely separated from the non-humified organic matter nor from the mineral constituents of the soil. By the use of reagents it can be subdivided into various fractions but these also are not pure chemical substances. Some of them are in a colloidal condition. These organic colloids tend to resemble those orginally studied by Graham and when flocculated become jelly-like forming what are called gels. Such gels can hold much water.
Soil Pores. The spaces between the soil particles are occupied by air and water in reciprocally varying amounts. Soil air contains much more carbon dioxide (C0Z) than air above the soil, sometimes 100 times as much. Usually it is saturated with moisture. Soil water is a dilute solution of C02 and many other substances including plant foods. In heavy soils, owing to the less dense packing of the minute particles, the total pore space is greater than in sandy soils but, as the average cross section of the spaces is much smaller, movements of air and water are slower. The greater pore space, with its fine structure, also enables more water to be retained by heavy soils – a factor to be taken into account when estimating requirements for overhead irrigation.
Soil Tilth and Structure
Soils composed of very coarse particles may have a single grained structure with each grain acting as an independent unit. However in most soils many of the individual particles are grouped into aggregates becoming crumbs or clods. The size distribution and shape of the aggregates decide the soil structure and tilth.
A soil with a good tilth should have a system of pores wide enough to act as channels for the rapid conduction of surplus water down to the water table or land drains. Between these channels . there should be other pores small enough to hold water against the downward pull of gravity. The surface soil should consist of crumbs large enough not to blow away, small enough to promotegermination and stable enough when slighly moist to carry tractors and implements without being broken into smaller units.
Useful sizes of surface crumbs range between 5 mm and 1 mm l/5th and l/25th of an inch in diameter.
The cements which hold the individual particles together to form crumbs include:
the clay particles themselves;
hydrated oxides of iron, aluminium, silicon;
partially decomposed organic matter, particularly the gum-like products;
the mycelia of some fungi;
products of some bacteria;
Artificial aids to Tilth Formation
Numerous attempts have been made to manufacture special plastics which can be said to improve soil structure. They need to have concentrated into a small weight of material the effective factors of the best natural cements. Krilium and Aerotil are examples of such artificial tilth improvers. They have not completely fulfilled the high hopes once aroused in them and the high cost of manufacture has also been against their wide adoption.
Effect of Frost
On heavy land especially, the beneficial effect of frost is outstanding. The value of leaving soil ploughed over winter (or dug, on a garden scale) is well known. The improvement in tilth, I.e. the workability of the top few inches of soil following the effect of frost (the frost mould) in winter gives ideal conditions for seed, so long as cultivations are done when the soil is dry enough to work.
The introduction of chemical soil conditioners has focused attention on the importance of a good soil structure. Soils possess a highly complex structure which differs greatly, not only from one kind to another, but as a result of its management. Soils which possess structure may have this ruined by mistimed cultivations, the misuse of fertilisers or by heavy machinery.
The average soil is not just a mass of tiny particles of mineral matter, but is a highly complex body, full of living organisms and having as it were its own lines of communication. The clay in soils is the chief determining factor. It swells when wet and shrinks when it dries, leaving cracks in the masses of particles, through which water and air can pass freely. Theoretically lime when applied to soil flocculates the particles, running them together into larger ones and giving considerable air space. But whilst flocculation is easily achieved in the laboratory, in the soil it does not occur very readily, and lime as a flocculating agent is perhaps over-rated. The reason for this is that lime is usually given as calcium carbonate, which is insoluble without the presence of carbon dioxide. This CO2. owes its presence in the soil to the breaking down of manure or other organic matter. This explains the importance of supplying organic matter to soils: not only does it supply carbon dioxide but also natural gums that bind soil particles together. In the intestines of worms there are also bacteria which secrete gums which bind soil particles together. Roots also help a great deal in keeping the crumbs apart. In fact they tend to keep the crumb structure from breaking down. This is one reason why freshly broken up leys are so good for crops. Not only has the fertility been built up but the crumb structure is maintained. Of all the factors that build up soil structure, organic matter is the most important. Lime helps in that it provides a suitable medium for bacteria which dislike acid conditions. A better form of lime to use for flocculation is gypsum (calcium sulphate). This is slightly soluble and does not require carbon dioxide to assist its solubility. Applied and worked into the top 2 3 inches (5 – 7.5 cm), it will form water-stable crumbs of between 1 mm and 4 mm in diameter. Autumn is the best time to apply this material. (Cost prevents its common use).
Organic matter is continually breaking down, and unless it is constantly replenished the soil structure benefits will be lost. Heavy applications of nitrate of soda will deflocculate clay soils, and unnecessary cultivations will also tend to destroy structure. Heavy wheeled tractors, especially when used on wet land, can be very harmful. Chemical soil conditioners are very expensive, and not wholly satisfactory, and it may well be that applications of humus in some form or other coupled with good soil
management, remain the cheapest way of maintaining soil structure. In the future it is likely that much more importance will be given to the formation of a good crumb structure by natural means, I.e. by the use of organic materials such as dung or, straw, residues of organic origin such as spent hops, green manures, short leys, etc. No definite conclusions on soil structure have yet been reached, and students should be careful about being over-optimistic in their views on the use of chemical soil conditioners.
Green Manuring. An old practice now being revived, this helps a great deal, partly by supplying humus of the right kind, but more particularly by providing the soil with a mass of roots. On large areas, short leys ploughed in give excellent results. The clovers, if dug in, improve soils to a marked degree. In fact any plant or crop, if it possesses strong and abundant roots, is beneficial when dug or ploughed in. Contrary to what we were once taught, it is theand not the green top that is most valuable. The only factors against green manuring are the cost of the lost production coupled with the costs of growing the green manure itself.
SOIL STRUCTURE AND SOIL POROSITY (advanced)
Soil structure can be defined as the spatial distribution and total organisation of the soil system as expressed by the degree and type of aggregation and the nature and distribution of pores and pore spaces.
Individual particles are often grouped together into aggregates which have distinctive shapes and sizes. These aggregates are termed peds. The structural properties of the soil depend on the character of the individual particles and the way in which they are held together as peds. The aggregation into peds may be due to the mutual attraction of ions on the clay particles or to organic matter causing a natural cohesion; to the action of chemical cements; or due to plant roots.
Peds can be regarded as permanent aggregations, persisting through wetting and drying, compared to the less permanent aggregates formed at or near the soil surface by cultivation – clods. A concretion occurs when localised accumulations of an insoluble compound permanently cement or enclose the soil particle.
There are four main types of peds:
which are or crumb
be granular non-porous, very porous.
These may relatively
which are very porous. They are uniform in colour and arise from the inter-action of soil particles and soil organisms. They are common in the ‘A’ horizon especially in grasslands and deciduous forests.
These may be angular or subangular. The faces may be coated by organic matter, clay or mineral oxides. They are found mainly in the ‘B horizons and because of their structure there is room forpenetration between the peds.
The faces of platy peds are mostly horizontal and they may be found immediately above an impermeable fB’ layer or in the surface of a soil compacted by heavy machinery. .Their structure impedes root growth and drainage.
These are commonly found structures in the sub-soil of heavy clay soils, where water-logging is frequent. A variation is the columnar type which have rounded caps and are often associated with the B horizon in soils affected by sodium.
Peds give the soil a chemical and physical stability which is greater than that of loose sediments. They help to control the movement of water through the soil, by allowing excess water to drain away but by retaining moisture in the small pore spaces within the aggregates.
The structure of the soil also determines the size of the pores and this together with the sizes and shapes of the peds accounts for differences in porosity Two types of pores can be identified:-
1. textural pores – these occur within the soil structures between the individual grains;
2. structural pores – these occur between soil structures.
The packing of the aggregates will determine the amount and size of pore spaces between structures. For example soils with a crumb structure will be packed loosely, the pore spaces will be larger and there will be free percolation of excess moisture, space for roots and good gaseous movement. Soils with platy or prismatic structure will have little pore space which will restrict water movement and root development.
Porosity can be measured as a ratio:
Pore space ratio (P.S.R.) volume of pores/ total soil volume.
N.B. the P.S.R. depends on the water content of the soil.
Soil structure can be damaged by using machinery on the soil when it is too wet or frosty: by using too heavy machinery on light soil: by application of irrigation water of large droplet size.
Structure can be improved by the incorporation of organic matter, correct cultivation and drainage, weathering, action of soil organisms, use of‘leys’ and the application of lime which assists the flocculation of clay particles.
When the soil pores are not occupied by soil water they will be occupied by air. This soil air has a characteristic composition which is slightly different from atmospheric air.
Soil air % by vol. Atmospheric air % by vol.
Dxygen 20.3 20.
Nitrogen 79.2 78.
Carbon dioxide 0.5 0.
The CO2 level may rise especially when the soil porosity is low as, for example, in a dense clay and in wet weather. The production of C02 shows a marked seasonal cycle being higher in warm wet weather when root and micro-organisms activity is greater.
As aerobic respiration involves the breakdown of carbon molecules with the consumption of O2. and the release of CO., water and energy this can be defined as a respiratory guotient.
volume of Coa released RQ = volume of Oa consumed =
Soil respiration is supplemented by the respiration of living plant roots and micro-organisms. Soil respiration rates can be measured by a respirometer.
Respiration rate depends on:-
a. soil condition, I.e. organic matter content, moisture.
b. cultivation and cropping operations,
c. environmental factors, especially temperature.
This is the process of exchange of Oz and Coj. between the soil air and the atmosphere. There are 3 ways aeration occurs.
I. Dissolved 02 in rainwater – this is only a small amount due to the low solubility of 02 in water.
ii. Mass flow of gases due to pressure changes created by wind turbulence over the soil surface.
iii. Diffusion of gas molecules through the soil pore space. This is the most important method of gaseous movement. Diffusion through the air filled pores is
rapid but diffusion through the thin water layer around the roots may limit the oxygen supply required for root respiration.
Oxidation state of the soil
In plant growth the main function of oxygen is that of electron acceptor. Systems containing oxidising agents and reducing agents are known as redox systems, and by measuring the redox potential the rate of oxygen diffusion in the soil can be determined. – The measurement is made electronically and can be useful on water-logged sites.
Soil Air and Soil Fertility
In the absence of oxygen or when it is only present in small quantities, there may be several consequences.
a. The reduction of nitrate to nitrite, which is in turn
reduced to gaseous nitrogen, resulting in a lowering of
the nutrient level in the soil.
b. Metabolic by-products may result, e.g. hydrogen
sulphide, ferrous iron, methane, ethylene, which can be
injurious to plants.
c. As the soil oxygen content decreases the Coa. increases
and CO2. toxicity may occur.
d. Diffusion through water-filled pores is slow so if
water-filled pockets occur within the soil, areas may
become deficient in oxygen.
Water is important in most of the physical, chemical and biological processes that occur in the soil which it enters either by rainfall or from irrigation. The form the water takes in the soil depends on a variety of factors including the existing moisture content of the soil, the structure and texture of the soil.
There are three main forms of water in the soil:-
a. gravitation water – this is free water that moves through the soil due to the forces of gravity. It generally only occurs in pores with a diameter of more than 0.06 mm. This water is not often used by the plant as it moves out of the soil profile so rapidly. Gravitational water can cause wilting of plants as it occupies air space in the root zone.
capillary water – (matric water). This is water which is held in medium-sized pores or held loosely around the soil particles. Most of this water is available to the plant.
hygroscopic water – this is water held in the very small pores; it forms a fine film around the soil particles and is not usually available to the plant.
CHEMICAL PROPERTIES OF SOIL INCLUDING ASPECTS OF SALINITY
There are several chemical processes which occur in the soil which help towards creating an environment suitable for plant growth.
A soil colloid is a soil particle which is small enough to stay suspended in water. This usually happens when a particle is less than 0.002 mm in diameter. In order for the particle to stay suspended it is usually electrically charged and on soil colloids this charge is negative.
There are three groups of colloids in the soil. The clay colloids, hydrous oxides and organic colloids. The clay colloids we have already looked at (pg. 8 ). Hydrous oxides are important as they are dominant in the soils of the tropics, semi-tropics and S.E. U.S.A. These inorganic colloids are oxides of iron and aluminium and occur as coatings on soil particles. They may be positively or negatively charged depending on the acidity of the soil. It is thought they may have some influence on the absorption of fertilisers.
Organic colloids coat soil particles in the ‘A’ horizon of mineral soils, or are the main colloidal state in organic soils. It should be noted that organic matter is of a colloidal nature when it is decomposing.
Soil colloids are important as they contribute both to the physical properties of the soil and to the chemical properties through cation exchange capacity. The effect on the soil of soil colloids will depend on the amount present, e.g. montmorillonite in large quantities gives a soil which is plastic and sticky with a very high nutrient-holding capacity. A small amount would be beneficial as it would increase the water holding capacity and reduce leaching.
(ii) Cation Exchange
As soil colloids are negatively charged they are able to attract positive ions. Cations are attached to soil colloids and are found in the soil solution. Cations come from fertilisers, the breakdown of soil minerals and from organic matter.
Cation exchange takes place when one of the soil solution cations replaces one of the cations on the soil colloids.- This exchange only takes place when the soil solution is not in equilibrium with the soil colloids.
Cations obtained from soil solution
In all but the most acid or alkaline soils, the major cations are Ca++, Mg++, K+ and Na+ in the rough proportions, Ca – 80%, Mg – 15%, Na and K 5% and variable amounts of NH depending on the microbial nitrification. Trace amounts of other cations such as Cu++, Mn++ and Zn++ are also adsorbed.
Cation exchange capacity is the ability of a soil colloid to hold cations. This capacity is directly dependent on the amount of charge on the soil colloid.
Measurement of cation exchange capacity
C.E.C. is measured in terms of milligram equivalents per 100 grams of soil. ( A millequivalent of any cation is that amount of cation which is required to replace 1 millequivalent of hydrogen.)
The exchangeable acidity is not all due to H+ ions. As the soil pH drops below 5, increasing amounts of A1+++ ions can be displaced by leaching with strong salt solutions. Under acid conditions the weathering
of clay minerals is accelerated with the release of Al, Si02 and small amounts of Mg, K, Fe and Mn.
Examples of C.E.C.
sphagnum peat 100 – 120
N.B. The C.E.C. of humus is about twice that of pure clay.
Factors involving the C.E.C. of soils
1. The soil texture affects the number of colloids present.
2. The type of colloid present, I.e. need to know the type of clay in order to estimate the C.E.C.
3. The amount of organic matter – for every per cent of humus in the soil the C.E.C. is increased by 2 me/100 g.
4. Availability of nutrients for plant growth; if C.E.C. is high it indicates that nutrients are available.
5. The C.E.C. determines the freguency and guantity of lime application.
6. The C.E.C. determines how crop nutrients can be applied. . On soils with a very low C.E.C. potassium may be needed as a side dressing during the growing season but on a high C.E.C. soil it can be broadcast before-hand.
(iii) Anion exchange capacity
The anions H . Poq. , HC03 SCV Cl~, are adsorbed on colloids when these are positively charged.
Anion exchange capacity is affected by the acidity of the soil. If the acidity is high then the PO
combines with Al and Fe to give insoluble aluminium and iron phosphate which are then slowly available to the plant.
It should be noted that peats have very little A.E.C. and contain little Fe and Al.
This term is used to describe the process of aggregation of particles. The clay particles adhere to one another in a suspension depending on the balance of positive and negative charges. It follows then that flocculation and de-flocculation are determined by the concentration and types of ions in the soil solution. In a growing medium de-flocculation is not desirable as it has an adverse effect on pore size and distribution, reducing the rate of flow of water.
(v) Soil Acidity
Soil acidity is centred around the colloidal fraction, (clay humus complex) and depends on the concentration of hydrogen ions in the soil water.
The pH scale is the degree of acidity (or alkalinity) of a solution expressed in terms of the concentration of H+ on a scale 0-14, where pH 7 is the mid point, when H+ balance OH and the solution is neutral.
The pH scale is based on logarithms of the concentration of hydrogen ions and this means that the concentration increases or decreases 10 times in moving from one pH figure to the next.
pH = log (_1_) H+
Percentage base saturation concept
This term, often used in the expression of soil fertility, means the percentage of the cation exchange complex which is saturated with basic cations. (Basic cations are any cations except hydrogen and aluminium). A high per cent base saturation means there are good levels of nutrients and low soil acidity.
Why soils become acidic
a. through leaching – percolating water removes
nutrient elements which are replaced by hydrogen
b. nutrient removal through crops
c. the use of acid-forming fertilisers
d. the breakdown of organic matter releasing
e. acids produced by growing roots.
Harmful effects of soil acidity
a. Some plants do not grow well at low pH.
b. The activity of soil organisms is reduced, e.g.
nitrogen-fixing bacteria and bacteria which
convert ammonium to nitrate.
c. Elements such as aluminium and manganese become
soluble and toxic to plant growth.
d. Phosphorus and molybdenum may become insoluble
and unavailable for plant growth.
e. Low pH may be indicative of low levels of calcium
(vi) Soil salinity
Soils or growing media are described as being saline if they contain excess soluble salts, e.g. sodium chloride, calcium sulphate and sodium, as exchangeable cations in excess of a certain maximum value. Excess soluble salts and sodium in particular may limit plant growth by affecting osmosis. This reduces the rate of water uptake by the plant. The amount of soluble salts can be estimated by conductivity measurements on a soil sample.
For a few species which have evolved under saline conditions, e.g. sugar beet, sodium is an essential element. It may be substituted for potassium for beet crops. Sodium may be beneficial for barley and carrots.
However if there is a large amount of sodium in the soil, e.g. after flooding by sea water, the sodium often reduces the stability of the soil; this is a very serious risk in silty or clay soils. After rain the soil easily caps, resulting in poor seed germination rates.
Soil salinity may be measured electrically as the pC, (similar to the pH and the pF for soil moisture tension.) The pC is of considerable importance to growers of tomatoes, cucumbers and lettuce crops usingsystems like the ‘Grodan’ rockwool culture system. If the pC rises too high, the salinity of the nutrient solution (the crops’ ‘water supply inch) exerts a suction pressure sufficient to make the plants flag from shortage of water. Their root-hairs are unable to compete sufficiently well for the available water. Hence during periods of good light, heat and ventilation it may be desirable to lower the soluble salt concentration of the nutrient solution (its salinity level) so that the plant is able to transpire sufficiently. Higher levels of salinity may be helpful in slowing down excessive rates of vegetative growth during the dull, short winter days. Growers of such crops tend to measure the salinity levels several times per week and adjust as necessary.