Since plants obtain their supplies of mineral nutrients from the soil, it is necessary to have some knowledge of soil conditions in order to understand the main problems of the supply of nutrient elements.

Soils are very complex bodies but, although showing such great variation, they all consist of five main components, mineral matter, organic matter, soil water, the soil atmosphere and a population of microorganisms. The last, however, need not concern us in detail here.

Mineral Matter: The mineral matter furnishes the skeleton of the soil. It consists of materials which range in size from rock fragments and large pebbles to minute particles of clay which can be suspended in water for considerable periods. The mineral portion, which largely determines the texture of the soil, can be separated into its component parts or "fractions" by a combination of sieving and sedimentation in water. The British standard method of grading is as follows:

FRACTION	DIAMETER LIMITS
Stones	> 2.0 m.m.
Coarse Sand	2.0—0.2 m.m.
Fine Sand	0.2—0.02 m.m.
Silt	0.02—0.0002 m.m.
Clay	< 0.002 m.m.

A preponderance of the coarser fractions tends to make soils light, free working, freely drained and hungry, whilst where the finer fractions predominate (especially clay) the soils are heavy, sticky, difficult to work and retentive of water and manures.

The clay fraction is of special importance as it possesses colloidal properties, which give to soils many of their characteristic properties, such as swelling and shrinkage, holding up of water and absorption of mineral nutrients from manures. The clay colloid, together with the organic colloid (humus), acts as the soil storehouse for available plant nutrients.

The mineral matter of soils provides the main natural source of mineral nutrients, which become available to plants through the weathering of rock minerals and the complex processes of soil formation.

Organic Matter: The content of organic matter in soils shows great variation. Organic matter may comprise almost the whole of the solid matter of peats, while in mineral soils it may account for only 1 or 2% of the soil. It is usual in mineral soils in this country for the organic matter to range from 3 to 10% in the soil, the lower values occurring in arable soils and the higher ones in old gardens and under grass covers in pastures and meadows.

Plant residues, either naturally or artificially added, provide the source of organic matter in soils. These residues vary greatly in character and composition, depending on the plants from which they are derived, whether from trees, grasses, clovers, cereals, root and vegetable crops, etc., and on the nutrient supplying powers of the soils on which they are grown. In addition to plant residues, soil organisms and animal remains contribute to the organic matter.

In the soil the fresh material undergoes chemical change, especially by the action of soil organisms, and when this has proceeded to the stage where the original cellular structure is no longer recognizable we speak of the brownish product as humus. The organic matter in the soil at any time thus consists of fresh and partly decomposed residues and humus.

The main points in the formation of humus are as follows:

The raw residues contain a variety of compounds, such as proteins and other nitrogenous materials, and sugars, starch, cellulose, tannins, lignin and nutrient salts. The soil organisms use compounds like sugars (carbohydrates) as sources of energy, ultimately oxidizing them to carbon dioxide. To do this they require a supply of nitrogen which they way take either from the nitrogenous compounds in the residues, or if this supply is insufficient they may use other nitrogen present in the soil. The most resistant to their attack of the compounds contained in the residues appears to be lignin, and it seems probable that the end-point of the more rapid decomposition processes humus is a complex mixture in which lignin derivatives preponderate.

During these processes the ratio of carbon to nitrogen (C/N) in the residues alters from about 40/1 to 10/1 so that the process entails a large reduction in carbon.

It will thus be seen that the organic matter of the soil supports the population of soil organisms, and of this population the bacteria alone may be of the order of 20 to 40 millions per gram of soil.

These organisms play a very great part in determining the availability of mineral nutrients in soils, by breaking down plant residues and also by providing carbon dioxide which, in combination with water, is of great importance for the weathering of the soil minerals. Certain of the soil organisms, especially bacteria, are also able to fix free nitrogen from the air and thus enrich the soil with this element. Other bacteria are present which cause the soil to lose nitrogen, and there are multitudes of protozoa which feed on the bacteria, and if too abundant render the soil unhealthy. A properly balanced soil population is requisite for good crops and disturbances of this balance either in a beneficial or deleterious direction may be brought about by soil treatment.

It is important to realize, however, that bacteria and other soil organisms do not work for the special benefit of crops, and that the changes they bring about are to meet their own particular needs, which are not always in harmony with the requirements of the crops. This point can be illustrated by digging into soil fresh straw or sugar and growing a crop on it immediately, when the crop will most probably show symptoms of acute nitrogen deficiency due to the fact that, in order to consume the large amount of carbohydrate contained in the straw or sugar, the organisms deplete the soil of its readily available nitrogen.

Organic matter produces both chemical and physical effects on soils. Like clay, the humified material possesses colloidal properties, depending on the minute size of the constituent particles and the enormous surface they collectively present. It can swell and shrink and absorb nutrient salts. Together with clay, humus coats the mineral particles and can bind these together forming "crumbs", thus giving "structure" to soils. Humus acts as a binding agent for the particles of coarse sandy soils and exerts an "opening" effect on close-textured clay soils. The binding properties are not associated with stickiness to the same extent as those of clay. Humus is superior to clay colloid as a storehouse of nutrient bases.

Soil Water: The solid particles of the soil, consisting of the mineral particles and the organic matter, being of varying dimensions, have spaces of varying sizes between them which are occupied by the soil water and the soil atmosphere. The total volume of these spaces in any soil is known as the pore space. It varies for different soils and even in the same soil under influences such as rainfall, drought, frost and management, but usually occupies from 30 to 60% of the soil volume. It will be clear that the amounts of water and air in the pore space are complementary and that when the soil is wet there will be correspondingly little air present and vice versa. A high water content in the soil thus means a condition of relatively poor aeration.

The manner in which water is held in soils and the way in which it moves through the soil mass provide difficult scientific problems. There is general agreement that in a wet soil a certain proportion of the water can be readily removed, and that some is held very strongly by forces in the soil. Moreover, it can be shown that only a proportion of the water in soils is available to plants. The movement of water through soils is also limited, and water tables at considerable depths below the surface cannot be relied upon to supply water to the surface layers of the soil by the well known capillary action which occurs in narrow tubes. Similarly, if a dry soil is wetted from the surface, the whole of the soil to a considerable depth does not become wetted through uniformly, but the top layers only are wetted, practically to full capacity, and the layers below remain dry.

This strictly limited movement of water through soils is of great practical importance, since it means that for practical purposes water does not move appreciably to the plant roots but the roots must go after the water.

The soil water contains the soluble products of the soil, and is the main nutrient medium for the plant roots. As such it is commonly called the soil solution. There is evidence that plants may also feed directly on nutrients contained in the soil colloids.

The soil solution is very dilute. Measurements made on displaced solutions show concentrations of soluble materials between 0.1 and 1.0%, whilst if drainage waters are analyzed the corresponding values are of the order of 0.02 to 0.5%. In both, the proportions of calcium, nitrate, sulfate, carbon dioxide, silica and organic matter are relatively high, and there may be large proportions of chloride, sodium and magnesium, but potassium is always low and phosphate and ammonia, are present only in traces.

The soil organisms play an important part in providing soluble products for the soil solution. They liberate into the solution nitrate, sulfate and carbon dioxide and these, acting as acids, dissolve corresponding or equivalent amounts of base-forming elements, particularly calcium. Further changes then occur, the calcium in particular bringing into solution other "bases" such as magnesium and potassium by "base" exchange reactions with the colloidal clay and humus materials.

Under normal conditions, the whole of the nitrate and chloride is present in the soil solution, and they pass into the drainage water if not quickly absorbed by plants. Sodium is also mainly present in soluble form. Calcium, magnesium and potassium are held in considerable amounts in the soil colloids as "exchangeable bases", and in this form are less easily washed out of the soil.

Phosphorus is mostly present as insoluble compounds, as calcium phosphates in neutral and alkaline soils, and as iron phosphate in acid soils. Both potassium and phosphorus are only present in the soil solution in very small amounts, but apparently supplies of these elements in the solution are quickly replenished when removed by plants.

The amounts of iron, aluminum and manganese dissolved are largely dependent on the soil reaction (pH) and in strongly acid soils (i.e., low pH values) they may assume toxic concentrations to plants.

The conditions governing boron solubility are not known though, under drought conditions and at high pH values, availability to plants is reduced.

Copper and zinc (perhaps also manganese) appear to present complicated problems in which organic matter and the soil organisms are concerned.

Molybdenum may be more readily soluble in soils under alkaline than acid conditions, as it has been shown to be more readily available to pasture plants under the former conditions.

The Soil Atmosphere: The soil atmosphere or soil air, near the surface, has a composition similar to that of the ordinary air and diffusion between the two is rapid. Except near the surface the soil air is saturated with moisture.

The carbon dioxide content may fluctuate considerably, being increased by organic matter, by cropping, by high temperatures, which speed up the activities of soil organisms, and by the respiration of plant roots.

In poorly drained soils the concentration of carbon dioxide may be very high, with a correspondingly low oxygen content, and in extreme condition this may actually lead to inefficient absorption and even to root injury due to lack of oxygen. The fact that carbon dioxide accumulates and oxygen supply is depleted under conditions of poor drainage is important in considering deficiency problems in wet soils where practical results often appear contrary to expectation.

The mineral elements available to plants in soils may be present as the result of natural soil processes or may come from additions of natural manures and fertilizers.

Nitrogen: Nitrogen availability is intimately connected with the activities of soil organisms. Nitrate is the normal form in which nitrogen is absorbed by plants from soils, although ammonia can also be utilized. Nitrogen in organic compounds is converted into nitrate by a chain of reactions brought about by organisms. For protein nitrogen this may be represented as follows:

Protein —> intermediate organic products, such as amino acids —> ammonia —> nitrite —> nitrate.

The effect of excess carbohydrate on these reactions has been mentioned.

Ammonia may be held in the colloid complex as an "exchangeable base" in the same way as calcium, magnesium and potassium (see below).

Phosphorus: This element exists in the soil in many forms, both as organic and inorganic compounds, and is also added to the soil in manures and fertilizers in a variety of materials, such as in meat meal, bone manures, basic slag, ground phosphate rock (mineral phosphate) and water-soluble superphosphates. Evidence indicates that water-soluble forms are generally most readily available to plants even though they are rendered insoluble almost immediately after application to the soil. Organic forms of phosphorus are usually less readily available than the inorganic compounds. Phosphates undergo many changes in soils both by organisms and by purely chemical reactions and, even though very heavy dressings are applied, the amount of water-soluble phosphate in the soil at any time is very small.

The movement of phosphates in soils is very limited, and soils are said to have high "fixing powers" for phosphates. Heavy soils, as a rule, show higher fixing powers than light ones, and soils with high iron contents possess specially strong fixing properties. The two elements mainly responsible for the fixation of phosphates are calcium, in neutral and alkaline soils, and iron in acid soils.

Important practical points in connection with the fixation of phosphates are that a large proportion of the phosphates added to many soils never become available to the crops, and that phosphates should always be placed as near as possible to the roots of the plants for which they are intended. In some soils where fruit trees suffer from phosphate deficiency, the method of injecting the phosphatic materials into the soils through high-pressure lances has been suggested to obtain local concentrations of fertilizer near portions of the roots. For cereal crops the difficulty has been met by the introduction of combined seed and fertilizer drills, which deliver the seed and phosphate into the soil in close proximity, and by the use of granular fertilizers, of which only a small proportion aof the material comes into contact with the soil.

Calcium: Calcium occurs in soils in a large variety of minerals, and carbonate of lime (calcium carbonate) may comprise a large percentage of the mineral matter where the soil is derived from limestone or chalk rocks. The element is readily leached from the soil and, on sandy soils where it is not abundant, frequent replenishment by means of lime or limestone dressings is necessary.

Carbonate of lime is readily brought into solution in soils by means of carbon dioxide dissolved in the soil water and thus, if the soil contains lime, a supply of calcium in soluble form is readily assured. Calcium also comprises the major proportion of the elements held as exchangeable bases where soils are not strongly acid and it is readily brought into the soil solution from this state.

Magnesium: The mode of occurrence of magnesium in soils is fairly similar to that of calcium. It occurs as carbonate and in a variety of minerals. Like calcium it is readily brought into the soil solution from the carbonate, and is held in soils as an exchangeable base. It is fairly easily leached, and for this reason may become deficient in sandy soils during wet periods.

Potassium: Potassium is widely distributed in soil minerals, such as potash-felspar, mica and glauconite, from which it is slowly converted into soluble forms by weathering processes. Heavy soils contain higher amounts of potassium than light soils. Potassium is fairly strongly "fixed" in soils, possibly largely as an exchangeable base. Very small amounts of potassium are present in the soil solution at any given time, but exchangeable potassium appears to be readily available to plants.

Sodium: Sodium is brought into solution from minerals in a similar manner to potassium, but it remains mainly in the soil solution, being only weakly absorbed by the colloids, and is very readily leached from the soil. In salt marshes recently reclaimed from the sea and in alkali soils, where sodium is the principal base-forming element, it is the main exchangeable base. Under these conditions the soils are often unfit for cultivation, due to bad physical condition, until the sodium has been replaced by calcium.

Soluble sodium compounds are readily available to plants.

Sulfur: Sulfur is present in soils both in organic and inorganic forms. Inorganic sulfur is mainly present as sulfate, but sulfide may accumulate where conditions are favorable for "reducing" reactions to take place.

Under water-logging conditions, calcium sulfate may form crystalline deposits in the subsoil, and this substance may also be seen as a white deposit on the surface of soils when manuring is high, as in market gardens.

Sulfur compounds are changed from one form to another in the soil by special bacteria, the end product of whose reactions is the sulfate form where conditions favor oxidation. Thus, if the element sulfur is added as a dressing to soil, it may be quickly oxidized to sulfuric acid, which reaction is used in treating alkaline soils to lower the pH value. (See under Iron and Manganese, pages 17, 18.)

Sulfate: Sulfur is an important constituent of the soil solution, and in this form sulfur moves readily through the soil.

The widespread occurrence of sulfur in rocks and organic matter and its mobility in the soil doubtless account for the rarity of a deficiency of the element in crops.

Chlorine: This element is of common occurrence in soils in the form of chloride, and as such moves freely through the soil in the soil solution, from which it is available to plants.

Common salt or sodium chloride is. also present in considerable quantity in the atmosphere near the sea, and a proportion of this reaches the soil, adding both chloride and sodium.

Iron: Iron is present in most soils, mainly in the form of its oxides, which are largely responsible for the red and brown colors in soils. Where drainage and aeration are good, ferric compounds predominate, and where water-logging and bad aeration occur ferrous compounds are formed. The insoluble forms of iron are brought into solution by the action of acids. Organic compounds of iron, such as the humate, appear to be mobile in the soil. The availability of iron to plants increases with acidity and is depressed by phosphates.

Manganese: The mode of occurrence of manganese in the soil resembles that of iron in that the oxides are important forms. Both iron and manganese are intimately concerned in the oxidizing and reducing reactions which take place in soils, and compounds of both in the form of concretions appear in imperfectly drained soils. The more highly oxidized compounds of manganese, such as manganese dioxide, are of very low availability to plants. Like iron, the solubility of the soil manganese increases with increasing acidity, and in many soils it is largely unavailable to plants above pH 6.5. The availability of manganese seems also to be greatly affected by organic matter and drainage conditions, deficiency of the element being common on calcareous peats and other soils with high organic matter content, and on soils with high water tables.

Boron: Little is known of the sources of boron available to plants in soils. The element is a constituent of the mineral tourmaline, which occurs in many rocks, but this is very resistant to weathering. When applied to the soil in the form of a borate, boron appears to move fairly readily through the soil. The availability of boron to the plant is decreased by liming and by dry soil conditions. In practice over-liming is a common cause of boron deficiency.

Zinc and Copper: In the neighborhood of zinc or copper mineral deposits the soils are generally toxic to crops. The contents of these elements in normal soils are very small, and maybe are largely the result of concentrations and additions from growing plants and added residues.

Little is known of the factors affecting their availability to plants, but organic matter and soil organisms may be important factors.

Nitrogen: Nitrogen deficiency occurs on all classes of soil, although, of course, some soils are less prone to the deficiency than others. Light sands lacking organic matter are perhaps the poorest of all as regards nitrogen. The nitrogen supplies of soils are greatly affected by cropping and management. Soils under grass and leguminous covers become enriched with nitrogen, and when these are ploughed under they generally yield a flush of nitrogen to the subsequent crops. The continuous cropping of arable soils with non-leguminous crops greatly depletes the nitrogen supply, and even the continuous clean cultivation of a soil without cropping impoverishes the soil of nitrogen by stimulating nitrification processes and destroying the organic matter. Under these latter conditions the nitrate which is formed is lost from the soil by leaching. Instances are known where fruit trees, after growing under clean-cultivated conditions for a number of years, have eventually suffered severely from nitrogen starvation due to destruction of the organic matter.

The manner in which a temporary nitrogen deficiency may arise from the ploughing-in of organic matter of low nitrogen content has previously been described.

Nitrogen deficiency may also occur on soils with ample supplies of nitrogen, such as peats, where drainage is defective or the reaction too acid for nitrification to proceed.

Phosphorus: In Great Britain, phosphorus deficiency is most prevalent in the western and northern districts of high rainfall. This may be due to two causes: the great preponderance in these areas of acid soils, in which phosphates are quickly rendered unavailable, and the actual leaching of phosphates under the influence of high rainfall. Loss through leaching has actually been shown to be substantial in North Wales. Apart from the rainfall effect, great differences are found in the natural supplies of phosphates and in many clay soils the availability of the soil phosphorus is low. Poor clay lands are especially deficient in the element. Lastly, soils derived from ferruginous rocks (the ironstone soils), and many soils of the poor Chalk Downlands and the Fen peats show marked phosphorus deficiencies.

Calcium: This deficiency is confined to acid soils. Like phosphorus deficiency, it is most prevalent in the districts of high rainfall where leaching is excessive.

It is, of course, more likely to occur on soils derived from rocks of low calcium content, such as siliceous sandstones, and light sandy soils are frequently acid and deficient in calcium due to an initial low content of calcium and ease of leaching. Since clays are not readily leached, they are not usually strongly acid, though certain clays, such as the Coal Measures Clay and London Clay, are often acid. Peats, other than Fen peats, are also poorly supplied with calcium.

Under field conditions it is generally difficult to ascribe a crop failure on an acid soil entirely to a deficiency of calcium, since under such conditions toxic effects may arise from other causes and confuse the issue. It will be shown later, however, how in sand cultures symptoms of calcium deficiency in plants can be separated from the effects produced by the other factors concerned in acid soils, which enables calcium deficiency effects to be distinguished in the field. ( See Plate 4 )

Magnesium: Deficiencies of this element generally occur under conditions where calcium deficiency is also a problem, and in many instances it is necessary to correct the latter deficiency in order to bring the magnesium deficiency effects into full prominence. The deficiency is most prevalent on light, acid sands, and is generally accentuated in wet seasons.

Magnesium deficiency can be readily induced on many soils by excessive dressings of potassic fertilizers, especially sulfate of potash. Many examples of this have arisen in fruit plantations, especially with apples on poorer soils, and there is evidence to show that magnesium deficiency is also being produced in commercial glasshouse crops of tomatoes by high potash manuring, where the heavy waterings which must be given may also be of importance.

In the past, magnesium has been neglected as a fertilizer element in this country, and doubtless the need for it did not arise when supplies of farm-yard manure were more plentiful, but, under modern conditions, where artificial manures (which contain no magnesium compounds) are so largely used, and the tendency is towards the use of highly concentrated materials, magnesium deficiency may soon become a serious problem unless the element is added to the soil. The remedy is simple. The need for magnesium can be met fully by the occasional use of magnesium limestone as a liming material.

Potassium: Potassium deficiency occurs more frequently on light soils than on heavy ones, since the potassium supplies in soils are generally highest in the clay fraction. In addition to sandy soils, peats and Chalk soils are often seriously potash deficient. For potash-loving crops, such as potatoes and beans, it is not unusual, however, to find instances of potassium deficiency on clay soils.

Although potassium deficient soils may be found in all parts of Great Britain, the deficiency is most prevalent in the South, south east and east of England, in the area approximating to the Chalk belt, stretching from Dorset to Yorkshire.

One of the most certain methods of bringing about a condition of potash deficiency in agricultural soils is to take hay crops from fields year after year without returning any dung to the fields. This practice was, unfortunately, very prevalent during the period of agricultural depression prior to 1939, and has resulted in many crop failures on ploughed-out meadows during the war. It is well to remember that the removal of any leafy crop from land means the removal of considerable quantities of potassium.

Sodium: Low sodium conditions are most likely to occur on sandy soils with free drainage not situated near the sea. In the sodium-loving crops, such as sugar beet and mangolds, the element is mostly contained in the leaves, so that the non-removal of "tops" from the soil ensures a minimum loss of the element. Sodium is often not appreciably absorbed by plants when supplies of potassium are plentiful. (See Table IV footnote).

Sulfur: No instance of sulfur deficiency has occurred in Great Britain, and in the best known example of the deficiency in the field-in tea in Nyasaland the soil is abnormally poor, having been subjected to severe leaching and erosion following reclamation from bush conditions.

Chlorine: The position is similar to that of sodium as regards conditions for deficiency.

Iron: Iron can be leached from the surface layers of acid soils and carried down into the subsoil, where it is deposited under less acid conditions. A deficiency of the element under such conditions has not so far been reported. Deficiencies of iron have been associated with soils of high pH, usually containing appreciable amounts of carbonate of lime and generally on chalk or limestone formations. A high pH is probably the most important factor in lowering availability of iron to plants, but a high content of phosphorus is also conducive to the deficiency. In the author's experience, wherever iron deficiency has occurred, drainage has been free. The presence of actively decomposing organic matter in the soil or of a grass cover decreases the possibility of iron deficiency, probably by in, creasing the carbon dioxide content of the soil and thereby decreasing the pH in the vicinity of plant roots.

Iron deficiency does not appear to result from a total deficiency of the element in the soil but from its unavailability due to other soil factors, mainly high pH. For this reason, the application of iron salts to the soil to correct the deficiency is usually ineffective, and the most economical method of correcting it is generally by means of foliage sprays (using from 0.2 to 1% ferrous sulfate), or by injection of solid compounds (such as ferric citrate or tartrate) into stems, a method used for fruit trees.

Sometimes the pH may be lowered sufficiently by dressings of sulfur to ensure adequate supplies of iron to plants suffering from iron deficiency. The deficiency may often be cured in trees by growing a grass cover over the roots.

Manganese: This deficiency is prevalent in certain areas in England notably in the Fens, Romney Marsh and Yorkshire, and it occurs sporadically in other parts of Great Britain, as in Somerset, Wilts., and the West Midland counties of Shropshire and Staffordshire, and in North Wales. The soils are generally sands, shallow Fen peats or alluvial silts and clays, but they may be calcareous clays derived from solid geological formations. The sands are usually black in color and drainage may be free or impeded. The alluvial silts and clays and calcareous clays on solid formations in-variably show impeded drainage, and in the alluvial soils the water table is usually near the surface. In all instances the soils show high pH values (above 6.5), and have high contents of organic matter. From field observations it seems that the combination of a high pH and abundant organic matter is of importance in immobilizing soil manganese, whilst soil wetness also appears to act in this way. The latter point is rather complicated in view of the fact that waterlogging conditions have been shown to increase the availability of manganese by producing conditions favoring reduction of the manganese compounds to the manganous state, and this waterlogging effect actually occurs in the field.

The main soil conditions under which the deficiency occurs in Great Britain may be grouped as follows:

1. Thin peaty Fen soils overlying calcareous subsoils, e.g., the so-called "skirt" soils of the Fens.

2. Alluvial soils and marsh soils derived from calcareous materials such as calcareous silts and clays or shelly marine sands and muds.

3. Poorly drained, calcareous soils with high contents of organic matter, e.g., in wet areas overlying the Chalk or Carboniferous Limestone formations.

4. Calcareous black sands and reclaimed heath soils subsequently limed too generously.

5. Calcareous soils freshly broken up from old grassland.

6. Old black garden soils where stable manure and lime have been applied regularly for many years.

Manganese deficiency, like iron deficiency, is often difficult to cure by the application of manganese salts to the soil, though in some soils this method is satisfactory. Spraying methods are much more economical of material and are very effective, more so than for iron, as the risk of spray damage is negligible. Sulfur treatment of the soil is also efficacious as with iron deficiency.

Boron: Apart from the fact that boron availability is decreased by liming and by dry conditions, deficiency of this element is probably mainly determined by the natural supplies in the parent rocks. The deficiency occurs most frequently on sandy soils. Borates move freely in soils and dressings are thus quickly effective but not lasting.

Zinc and Copper: The factors governing the occurrence of these deficiencies are not understood, but low availability of zinc appears to be related in some cases to the activities of soil organisms and to high phosphorus conditions. Copper deficiency has generally occurred on peaty soils, but it has also been noted on light sandy soils. On the peaty soils, copper is believed to act as a catalyst in effecting oxidation. Both zinc and copper are most effectively applied as sprays. Neither deficiency has been recorded in Great Britain.

Molybdenum: Nothing is known of the availability of this element in soils apart from the fact that in the so called "teart" pastures the molybdenum content of herbage is higher under alkaline than acid conditions.

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(2) Hendrickson, A. H., and Veihmeyer, F. J. (1936). The Maintenance of predetermined Soil Moisture Conditions in Irrigation Experiments. P. Amer. Soc. Hort. Sc., 30, p. 421.

(3) jenny, H., and Overstreet, R. (1939). Cation Exchange between Plant Roots and Soil Colloid. Soil Sc., 47, 257.

(4) Keen, B. A. (1927). The Limited Role of Capillary in Supplying Water to Plant Roots. Proc. 1st Internat. Congress. Soil Sc., Comiss, I, 504.

(5) Kramer, P. J., and Coile, T. S. (1940). An Estimation of the Volume of Water made Available by Root Extension. Plant Physiol., 15, 743.

(6) Robinson, G. W. (1936). Soils, Their Origin, Constitution and Classification (2nd Edition). Thomas Murby & Co., London.

(7) Russell, E. J. (1937). Soil Conditions and Plant Growth (7th Edition). Longmans, Green & Co., London.

(8) Veihmeyer, F. J., and Hendrickson, A. H. (1927). Soil Moisture Conditions in Relation to Plant Growth. Plant Physiol., 2, 71,

(9) Waksman, S. A. (1936). Humus. Origin, Chemical Composition and Importance in Nature. Williams & Wilkins Co., Baltimore.