The Electric Force holds atoms together and also plays a key role in the growth of plants. There is a Strong Electric Field surounding the Earth and it will need to be understood if we are to grow plants in space.
— Tommy Cichanowski —

The Diagnosis of Mineral Deficiencies in Plants by Visual Symptoms

by Thomas Wallace, M.C., D.Sc., A.I.C.
University of Bristol Agricultural and Horticulture Research Station, Long Ashton, Bristol

London — Published by His Majesty's Stationary Office — 1943

— Crown Copyright Reserved —


Preface

 Cauliflower Plant --- Magnesium Deficiency 
Older leaves highly tinted in intervenal areas;
cream chlorotic marbling and orange, red and purple tints.

During the past two years, the Agricultural Research Council has been concerned in coordinated investigations at a number of Agricultural Research Institutes, University Departments, and Advisory Centers, designed to increase our knowledge of the frequency and importance of abnormalities in crop development caused by deficiencies of those minerals, particularly trace elements, that are essential for normal plant growth, and of the methods by which such deficiencies may most effectively be remedied.

In these investigations, the diagnosis of specific deficiencies by changes in the appearance of the leaves has played an important part. Dr. Wallace, whose studies in this field are widely known, has collected a valuable series of color photographs, showing the appearances characteristic of different deficiencies in a wide range of horticultural and agricultural crops commonly grown in this country. It seemed to the Council that they would be performing a useful service, not only to research workers, but also to agricultural advisory officers, to practical farmers, to fruit growers and to gardeners, by making this collection easily available to all who might be interested in a subject that has gained additional importance during the war, as a result of bringing into cultivation large areas of land on which no crops had previously been grown for many years. Attention may be called to the method, devised by Dr. Wallace, of diagnosing particular deficiencies from the changes produced in a selected series of indicator plants.

The Council would wish to express to Dr. Wallace their thanks for placing at their disposal the collection of photographs from which these illustrations have been prepared, and for writing the explanatory text.

Agricultural Research Council,
6a, Dean's Yard,
London, S.W. 1
March, 1943

Introduction

This Book has been written primarily for the use of technical officers and advisers concerned with problems of crop production, and for progressive farmers, vegetable growers and fruit growers, but it is thought that it will not be without interest and of use to the large body of the general public who are interested in gardening in peace time and to whom the growing of food crops has become of vital importance during the war.

It is hoped that the book will meet an important war-time need which has been felt by many technical officers who have had to deal with new and difficult problems of crop failures since the outbreak of war, for which quick solutions have been required.

The war has brought many new cropping problems to the agricultural community, which is only to be expected, considering that several million acres of grassland, embracing a great variety of soils, have been brought under the plough during the course of four seasons and that crops have been introduced into districts with little or no previous experience of their suitability to local conditions of soil and climate.

Manurial deficiencies, new both to technical officers and farmers, have been revealed: lime deficiency in potatoes; magnesium deficiency in cereals, potatoes and Brassica crops; manganese deficiency in oats, wheat, barley, potatoes, sugar beet, mangolds, swedes and turnips; boron deficiency in sugar beet, mangolds and Brassica crops; and iron deficiency in cereals. All these deficiencies were known to plant nutrition experts before the war, but their occurrence in this country was generally regarded as only of local importance or merely of academic interest. The ploughing-up program and the intensity of the present crop production drive have greatly increased the importance of these little-known deficiencies, and also the need for recognizing quickly deficiencies of the more familiar nutrients, nitrogen, potash and phosphate.

The present book describes a method of recognizing by sight deficiency symptoms of the various plant nutrients in commonly grown agricultural and horticultural crops. Where the method can be used it provides the quickest means of determining the causes of failures due to mineral deficiencies, and it will often enable a full crop to be harvested with little expenditure of time, materials and labor, where otherwise complete failures might result. This is especially true where trace elements, such as manganese, boron and iron, are concerned and where the deficiencies are recognized at an early stage.

The most important feature of the book is the production in color of the various deficiency symptoms shown by important crops and it is hoped that it will serve as a color atlas for their recognition. The photographs have all been collected during the war and for this reason the series in the present edition is in some respects incomplete. Attention, however, has been specially given to the most urgent and difficult problems of war-time production and the omissions can easily be added if a future edition is called for. The illustrations which are omitted concern nitrogen deficiency, the symptoms of which are familiar to farmers, and deficiencies of sulfur, copper and zinc, which are not known as practical problems in crop production in Great Britain. Flax is not included in the illustrations, tie to lack of opportunity of studying this important war-time crop.

In using the book, it is suggested that it may often be most profitable for farmers and others engaged in the actual growing of crops to give most attention to the color plates. The main purpose of the text is to provide a suitable basis for those who wish to study the subject beyond the point of the mere recognition of the deficiency symptoms in the individual plants, and to help in this a bibliography of scientific papers etc. relating to the subjects discussed is appended to each chapter. Chapter V, describing the use of the visual method in the field, including the laying out of field trials, should be of special use for technical officers and advisers.

The production of the book would not have been possible without the help of many colleagues. In particular, grateful acknowledgement is made to Messrs. L. Ogilvie, J. 0. Jones, H. E. Croxall, D. A. Osmond, W. Plant, E. H. Hobbis and W. H. Neild, of Long Ashton Research Station, who have assisted in the collection of material for the photographs; to various Advisory Chemists, among whom should be mentioned Mr. W, Morley Davies, Harper Adams Agricultural College, who have been specially interested in deficiencies of trace elements; and to numerous county officers who have brought to the notice of the writer many instances of unusual mineral deficiencies.

Mr. G. H. Jones, photographer, Long Ashton Research Station, has been responsible for all the original photographs used in the illustrations. Warmest thanks are due to him for the long hours he has spent on the work and for the skill and energy he has shown in carrying out the important and onerous tasks assigned to him.

Thanks are also due to Professor E. J. Salisbury F.R.S. for valuable suggestions during the preparation of the text, and for checking the original MS. and proofs.


CHAPTER I

Essential Points in the Nutrition of Plants

THE NATURE OF PLANT GROWTH PROCESSES

The processes concerned in the growth of plants are the subjects of study by plant physiologists and plant biochemists. A comprehensive account of these processes is outside the scope of the present work, the special object of which is to deal with the outward and visible signs of imperfections in the plant's activities caused by faulty mineral nutrition. Nevertheless it is useful to have before us the general features of the main processes involved and to realize that the symptoms we shall be discussing later have a physiological basis, and are not direct and unchangeable signs of the specific deficiencies but result from the derangement of the complicated mechanism of the plant's vital activities.

The main processes involved in plant development may be summarized as follows:

Absorption: Intake of water and mineral elements by the root system.

Carbon assimilation or photosynthesis: Intake of carbon dioxide from the air by the leaves, and reaction of the gas with water in the leaf in the presence of the green chlorophyll to form sugar and free oxygen.

Formation of protoplasm: Protoplasm is the living material of the plant, consisting mainly of proteins, complex compounds of nitrogen built up by the plant from more simple compounds of this element.

Respiration: The combination of oxygen with various food substances synthesized by the plant, especially sugars, whereby energy is produced.

Transpiration: Loss of water from the plant, mainly from the leaves.

Translocation: The movement of materials within the plant.

Storage: Storage of reserve products in various organs and tissues.

During growth there are continuous processes of building up of complex compounds of carbon and nitrogen and breaking down of these into more simple substances, in which water and oxygen are intimately concerned. These processes together comprise plant metabolism.

In the course of the metabolic processes innumerable substances are formed, such as sugars, starch, cellulose, acids, lignin, tannins, amino acids, proteins, amides etc., and many plants also produce special products, as for instance nicotine in the tobacco plant.

For the normal functioning of the above processes there must be an adequate intake of water by the plant to maintain the plant cells in a more or less turgid condition and, since water is being continuously lost at a varying rate from the plant, intake and movement within the plant tissues must be capable of ready adjustment to these changes.

As a result of metabolic activities plants develop special organs of growth and reproduction, each of which has its special characters and makes particular demands on the nutrient supplies of the plant.

With all plants there are well defined seasonal growth cycles. Thus annuals, such as cereals, begin from the seed, give rise to seedlings, which later flower, form grain and ripen off, whilst perennial deciduous trees, such as apples, pears, etc., begin growth in the spring, using stored reserves of food, form leaves, make shoots, blossom and form fruits and subsequently shed their leaves, but meanwhile pass on reserve foods to various storage organs in preparation for the next season's growth. Coincident with these growth cycles there are well defined chemical cycles of nutrient elements and elaborated products in the leaves, stems and roots, etc. It will be shown later that these cycles are of great importance in considering p-deficiency effects and in diagnosing their causes.

THE PLANT ENVIRONMENT

Nutritional problems must be considered in relation to all the conditions in which plants live, and not merely in terms of the amounts of plant nutrients contained in or added to the soil. For example, those who are accustomed to growing plants know that the temperature must not be too low or no growth may result or that if too high the plants may be injured. An optimum temperature is usually recognized and this may vary according as to whether the plant is young or old. Similarly the importance of light is well known and plants may be put in special positions to obtain a maximum supply of light at one stage and may be shaded at another.

The actual duration of the daily period of illumination also affect growth and there are plants which are classified as requiring "long day" conditions to complete their growth cycles and others as needing "short day" conditions. If the special "long" or "short" day periods are not forthcoming for the respective classes of plants requiring these, their growth cycles are abnormal and they may fail entirely to produce flowers, grain or fruit.

The humidity of the atmosphere, as distinct from the water supply in the soil, is of importance in determining the water conditions within the plant, as these are dependent on both water intake by the roots and water loss from the leaves, the latter being largely influenced by the air humidity.

Even the presence of adequate quantities of plant nutrients in the soil is no guarantee that they will be absorbed by the plant roots. It will be shown later how these may be present in forms which are not available to the plants, but even when they would be considered as being present in suitable forms for absorption, other factors may prevent this taking place. An example of this latter condition is afforded in poorly aerated soils where lack of oxygen near the roots may prevent them from actively absorbing mineral nutrients

The problems of such influences in the plant environment as those just mentioned are complicated by the fact that they do not act independently, but their effects are modified by one another. Thus the effects of light intensity or period of daylight may vary with different temperature conditions.

The requirements of plants for different nutrients may be affected by conditions of light, temperature and water supply, and by other factors of the general environment. Thus the need for nitrogen may be less under conditions of relatively low light intensity whereas the need for potash in these circumstances may be greater, these facts being of importance in growing tomatoes under glass. The effect of nitrogen in relation to light may be shown by growing a plant under normal light conditions with insufficient nitrogen, when the leaves will show the well known symptoms of nitrogen deficiency-pale green, yellow, orange and red tints. If such a plant be then shaded, the leaves will turn a darker green and growth may be visibly increased. It can be shown that the lowered light conditions result in an increase of "soluble" a breaking down of proteins, thereby rendering the nitrogen of these available for growth processes.

This interrelationship of environmental factors is well illustrated by an experiment on apple trees at Long Ashton.

Bramley's Seedling trees were grown in compost in large pots and given a small dressing of a nitrogenous fertilizer. Some of the trees were grown in a specially constructed glass house and an equal number in an adjoining wire enclosure. The trees in the enclosure showed severe symptoms of nitrogen deficiency-pale green and yellow leaves, reddish brown barks and highly colored, red fruits. The condition was corrected by further dressings of nitrogen. In contrast, the trees under glass, where the light was of less intensity and the temperature higher, made vigorous growth, carried large, green leaves and bore large, green fruits.

Iron and zinc deficiency symptoms may be less severe under conditions of low light intensity, whilst boron deficiency effects are less severe and magnesium deficiency effects are more pronounced in wet seasons than in dry ones.

The rate of water absorption is less at lower temperatures than at higher ones and efficient intake is also dependent on good aeration. These facts may result in a water deficit within plants growing in cold, wet soils when the air temperature is high.

Soil conditions greatly complicate the problems of nutrient supplies to crops and are discussed in some detail in Chapter II.

The raw materials needed for plant growth consist of carbon dioxide, which is obtained from the atmosphere through the stomata of the leaves, and water and the so-called mineral nutrients, which normally enter the plant through the medium of the roots.

The importance of water and carbon dioxide in the nutrition of plants will be apparent from the facts that water often comprises 80 to 90 % of the total weight of growing plants, and carbon and oxygen together may account for over 80% of their dry matter, i.e., the solid matter remaining after water is removed. As against these large amounts, the mineral nutrients, as measured by the ash content of the plants, i.e., the mineral residue obtained when the organic matter is destroyed by heat, often contribute from 5 to 15% of the dry matter.

It has been shown in recent years that certain organic compounds, known as "growth promoting substances" or "hormones", which occur in plants, and some of which are also present in soils and natural manures, are capable of producing marked growth responses, such as increased root growth, shoot and leaf curvatures, stimulation or suppression of buds, increased fruit setting, prevention of fruit abscission etc. They appear to perform important functions in the growth of plants. Examples of substances of this kind which can produce growth responses are 13 indole-acetic acid, 13 indole-butyric acid, phenyl acetic acid, A, naphthalene-acetamide, vitamin B1

It is not at present clear to what extent growth substances are absorbed by plants from soils, although it has been shown that vitamin B1, which occurs naturally in soils, can be obtained in this way.

THE MINERAL NUTRIENTS

It has been shown in numerous researches that certain elements are necessary for the healthy growth of plants. They are sometimes spoken of as essential elements and, since some are needed in relatively large quantities and others in very small amounts, the former are referred to as "major" elements and the latter as "minor" or "trace" elements, or as micro nutrients.

The terms "major" and "minor" do not refer to the relative importance of the functions of the elements in plant growth, and for this reason the term "trace" element is preferable for the latter class.

Major elements: Nitrogen, phosphorus, calcium, magnesium, potassium, sulfur.

Trace elements: Iron, manganese, boron, copper, zinc and molybdenum.
( Iron occupies an intermediate position and is usually included in the major elements group. In dealing with field problems it is more convenient to group it with the trace elements.)

In addition, there are, other elements, such as sodium, chlorine and silicon, which produce beneficial effects on the growth of certain plants but which have not so far been shown to be absolutely essential to growth. The element aluminum is of general occurrence in plants, but seems to be without direct nutritional value, although aluminum sulfate is used, because of its acidifying properties, to change the color of hydrangeas growing on alkaline soils from pink to blue, and aluminum may also exert indirect influences on nutritional processes.

Other elements often occur in plants but they are not known to serve any useful function and frequently they act as plant poisons or toxins.

The nutrient elements can only be absorbed by plants when present in certain forms: nitrogen from nitrates and ammonium salts; phosphorus from phosphates; calcium, magnesium and potassium from their salts (e.g., as sulfates or chlorides, etc.); sulfur from sulfates; iron from ferrous or ferric salts (more readily from ferrous salts); manganese from manganous salts; boron from borates; copper and zinc from their salts, and molybdenum from molybdates.

There may appear to be certain exceptions to this statement in practice. For instance, nitrogen may be applied to a soil as "organic" nitrogen, as in hoof meal or urea, and sulfur may be added as the element itself, as in flowers of sulfur, ground sulfur, etc. In such conditions the added materials are, however, converted into the nitrate and sulfate forms respectively by soil organisms before being absorbed by the plants.

Further points of importance in connection with the absorption of the mineral nutrients by plants are as follows:

(a) They must be absorbed from relatively dilute solutions or the plants will be! injured or even killed.

(b) Certain of the elements slow down the absorption of others into the plant, e.g., calcium slows down potassium and vice versa. The phenomenon is known as "antagonism".

(c) Healthy plants result when the nutrients are absorbed in certain relative proportions. When the proportions are suitable the nutrient medium is said to be "balanced". When ratios between nutrients are too wide, deficiency conditions are created. Thus if a high proportion of nitrogen to potassium is absorbed, the plant will suffer from potassium deficiency.

(d) Nutrients, even though present in the nutrient solution in satisfactory amounts and proportions, may not be absorbed by the plant unless the "reaction" of the solution as regards acidity and alkalinity is satisfactory. The reaction is measured in terms of the pH scale, which is merely a convenient notation for stating the conditions of acidity in the solution (strength or intensity of acidity, not total amount). The neutral point (i.e., when acidity and alkalinity are equal and neutralize the effect of each other) is represented by pH 7.0; below this value the solution is acid and above it is alkaline. Many crop plants prefer a reaction slightly on the acid side-pH 6.0 to 6.5 and extreme values are in the neighborhood of 4.0 on the acid side and 9.0 on the alkaline side.

(e) The nutrient medium must contain an adequate supply of oxygen, i.e., aeration must be satisfactory.

THE FUNCTIONS OF THE MINERAL NUTRIENTS

Knowledge of the main functions of the mineral nutrients is useful in helping us to understand the effects produced by deficiencies of any one of them.

A. ESSENTIAL MAJOR ELEMENTS

Nitrogen. Nitrogen is a major constituent of several of the most important substances, which occur in plants. It is of outstanding importance among the essential elements in that nitrogen compounds comprise from 40 to 50% of the dry matter of protoplasm, the living substance of plant cells. For this reason nitrogen is required in relatively large quantities in connection with all growth processes in plants. It follows directly from this that without an adequate supply of nitrogen appreciable growth cannot take place and that plants must remain stunted and relatively undeveloped when nitrogen is deficient.

Proteins, which are of great importance in many plant organs, e.g., seeds, are compounds of nitrogen whilst chlorophyll, the green coloring matter of the leaves, also contains the element.

From this latter fact it will be apparent that when nitrogen is deficient leaves will contain relatively little chlorophyll, and will thus tend to be pale green in color.

In addition to the above substances, numerous other organic compounds of importance in plants, such as amino acids, amides, and alkaloids, are compounds of nitrogen.

Certain compounds of nitrogen are very mobile in plants, and this enables them readily to mobilize supplies of the element at vital growing points and to transfer stored supplies to points where they are most required. Such transference is common from old tissues to young growing points when supplies of the element are short. This mobility and re-utilization of nitrogen explains why deficiency symptoms of the element always appear first in the older parts of plants and why growing points are the last to be affected.

Phosphorus: This element, like nitrogen, is closely concerned with the vital growth processes in plants as it is a constituent of nucleic acid, and nuclei in which this occurs are essential parts of all living cells. Hence a deficiency of this element will also be expected to result in greatly restricted growth. Phosphorus is also of importance in seeds and in connection with the metabolism of fats. Compounds of phosphorus are concerned with the processes of respiration and with the efficient functioning and utilization of nitrogen. This relationship to nitrogen probably accounts for the fact that several of the symptoms of phosphorus deficiency are identical or similar to those which result from a deficiency of nitrogen, Phosphorus is also of special importance in the processes concerned in root development and the ripening of seeds and fruits.

Calcium: Calcium occurs in plants chiefly in the leaves and the amounts present in seeds and fruits are relatively low. One of its main functions is as a constituent of the cell wall, the middle lamella of which consists largely of calcium pectate. This function appears to be of fundamental importance since, if calcium is replaced by any other of the essential elements, such as magnesium or potassium, the organic materials and mineral salts in the cells are readily leached through the walls.

Other functions attributed to calcium are as follows:

It provides a base for the neutralization of organic acids; it is concerned with activities of growing points (meristems), especially with root tips; it may be of importance in nitrogen absorption.

Although a large proportion of the calcium contained in the plant may be soluble in water-as much as 60% in cabbage calcium does not appear to move freely from the older to the younger parts of plants, and hence young tissues contain lower proportions of calcium than older ones. This may explain why calcium deficiency effects begin at the tips of shoots.

Magnesium: The outstanding fact about magnesium is that it is a constituent of chlorophyll, and is essential to the formation of this pigment. As a result, when magnesium is deficient, one of the symptoms commonly shown by plants is chlorosis. Magnesium is also regarded as a carrier of phosphorus in the plant, particularly in connection with the formation of seeds of high oil content, which contain the compound lecithin.

The element seems to be very mobile within the plant, and when deficient is apparently transferred from older to younger tissues where it can be re-utilized in the growth processes. This agrees with the observation that signs of magnesium deficiency invariably make their appearance first on the oldest leaves and progress systematically from them towards the youngest ones.

Potassium: Unlike all the other major elements, potassium does not enter into the composition of any of the important plant constituents, such as proteins, chlorophyll, fats and carbohydrates, concerned in plant metabolism. For this reason its role is more difficult to determine, and in spite of much study it cannot be said that the functions of potassium are clearly understood.

The element is present in all parts of plants in large or fairly large proportions. It seems to be of special importance in leaves and at growing points, as these are especially rich in potassium. Probably the whole of the potassium in plants is present in soluble form, and most of it seems to be contained in the cell sap and cytoplasm.

It is outstanding among the nutrient elements for its mobility and solubility within the plant tissues, and these properties no doubt account for the ready way in which potassium can be re-utilized by young tissues when the element is in short supply.

Among the functions which have been attributed to potassium and the processes with which it may be concerned, the following may be mentioned: The formation of carbohydrates and proteins; the regulation of water conditions within the plant cell and of water loss by transpiration; as a catalyst and condensing agent of complex substances; as an accelerator of enzyme action (e.g., for diastase); as contributing to photosynthesis through its radioactive properties.

It has been shown in many instances that the potassium content of plants is frequently much higher than is necessary for healthy growth, and it is generally considered that luxury (i.e., unnecessary) absorption of potassium often takes place.

The great mobility of potassium in plants, its special importance for and its reutilization by young tissues, and its apparent functions as a regulator of plant processes on a large scale are in harmony with the observations that, when potassium is moderately deficient, the effects are seen first in the older tissues and progress from these towards the growing points, but, when the deficiency is acute, growing points are severely affected, and die-back and general collapse of the plants commonly occur.

Sulfur: Sulfur occurs in plants as a constituent of proteins (e.g., cystine), and of certain volatile compounds such as mustard oil. It seems to be connected with chlorophyll formation, although it is not a constituent of this substance. Its functions in connection with proteins and chlorophyll doubtless account for the similarity of its deficiency effects to those due to deficiency of nitrogen.

B. ESSENTIAL TRACE ELEMENTS

Iron: Iron is closely concerned with chlorophyll formation but is not a constituent of it. Its role appears in this connection to be that of a catalyst. As a result of this function of iron, chlorosis is invariably an outstanding symptom when the element is deficient. Iron may also act as a catalyst, in the role of an oxygen carrier, in respiration.

A point of great importance in connection with iron is its relative immobility in plant tissues. Its mobility seems to be affected by several factors, such as the presence of manganese, potassium deficiency and high light intensity. There is evidence that the amount of chlorophyll is related to "active" (i.e., readily soluble) iron in plants.

It will thus be seen that so-called iron deficiency in the plant may in fact usually mean iron immobility. Lack of mobility may also account for the fact that iron deficiency is first shown in the younger tissues.

Manganese: The functions of manganese are regarded as being closely associated with those of iron and as being concerned with chlorophyll formation. Hence, when manganese is deficient, chlorosis is a common symptom. Manganese may decrease the solubility of iron by oxidation and hence, an abundance of manganese within the plant may lead to iron deficiency and chlorosis.

Manganese is regarded as having the functions of a catalyst; its activities being specially concerned with oxidation and reduction reactions within the plant tissues.

Boron: The exact role of boron is not known, but again the evidence as for the other trace elements, suggests that, its functions are those of a catalyst or reaction regulator. Particular effects attributed to boron are as follows:

It can delay the onset of calcium deficiency effects but cannot replace calcium; it tends to keep calcium soluble; it may act as a regulator of potassium/calcium ratios, and of the absorption of nitrogen; it may be concerned with the oxidation-reduction equilibrium in cells.

Such functions as the above accord with the results which follow from a deficiency of the element, when growth processes show sudden collapse and drastic derangements of metabolism occur.

Zinc and Copper: Although specific functions have not been determined for these elements, here again the evidence points to their roles as catalysts and regulators. Deficiencies of both are associated with chlorosis and a serious general collapse of vital growth processes.

Since catalysts are not used up in the chemical reactions which they promote, we can understand how it comes about that quite small or even minute quantities of the "trace elements", iron, manganese, boron, zinc and copper, may nevertheless be essential to the plant's health and growth.

C. ELEMENTS SOMETIMES BENEFICIAL

Sodium: As sodium is not strictly an essential element it cannot be expected to have a specific role in the metabolic activities of plants.

Where sodium produces significant effects it is often regarded as a conserver of potassium and as being able partly to replace that element in its role. In no instance, however, has it been shown that sodium can wholly replace potassium where the latter is acutely deficient. In such circumstances, sodium is ineffective as a substitute for potassium, even for sodium-loving plants, such as sugar beet, mangold and barley. Sodium seems to affect the water relations of plants and often enables sugar beet and other crops to withstand drought conditions which would otherwise produce severe adverse effects.

Chlorine: The evidence of the role of chlorine in plants is somewhat contradictory, and no general statement can be made. In tobacco it has been shown to increase the water content of the tissues and to affect carbohydrate metabolism, leading to an accumulation of starch in the leaves. The element is present in plants as chloride, and is wholly soluble.

TOXIC EFFECTS OF MINERAL ELEMENTS

Toxic effects in plants may be produced both by essential nutrient elements and non-essential elements.

In the first class, the major nutrients are much less toxic than the trace elements. Indeed, for the major nutrients, there exists a fair safety margin for excess or "luxury" consumption, but for the trace elements the margin is very narrow. Similar conditions exist in relation to non-essential elements thus some plants will tolerate fairly large amounts of elements such as sodium or chlorine, but are injured by relatively small amounts of elements like arsenic or chromium.

Two types of injury may occur: ( 1 ) An excess of one element may lead to a deficiency of another which ultimately results in a deranged metabolism, e.g., excess nitrogen or excess phosphorus may result in insufficient potassium and excess potassium may lead to deficiency of magnesium or calcium. This type of injury applies particularly to essential nutrients. ( 2 ) The presence of an element may directly injure the protoplasm and bring about the speedy death of the plant.

SIGNS OF FAULTY NUTRITION IN PLANTS

When plants are grown in unsuitable environments, including conditions of faulty mineral nutrition, they react to the particular defects in more or less specific ways. Thus, if light is insufficient, the green coloring matter of the leaf will be lacking and the leaves may be almost white (chlorotic), and the plant may be very spindly and "drawn" in appearance; if the temperature is too high, the growth may be rank and soft; if water is insufficient, growth may be restricted, the tissues woody and the green of the leaves show a bluish tint. Again, deficiencies and excesses of the individual elements produce characteristic effects on various organs of plants: foliage characters, including color, density, size and shape of leaves; stem characters, such as thickness, color and length of inter-nodes; root characters, such as color, amount of fiber, abnormal thickening; blossom characters, including amount and time of opening of the flowers; fruit characters, such as size, color, hardness and flavor.

Ability to recognize these particular effects forms the basis of the visual method of diagnosing plant deficiencies. Many of them can be readily learned and applied by practical farmers. Indeed, for many years progressive fruit growers in this country have used the leaf symptoms of deficiencies of nitrogen and potassium, and more lately of magnesium, as the main guide to manuring their fruit trees and bushes with these elements.

Detailed descriptions of deficiency symptoms and of the methods of using them in the field for diagnosing the manurial needs of crops are given in Chapters IV and V.

Bibliography

(1) Bewley, W.F., and White, H.L (1926). Some Nutritional Disorders of the Tomato. Ann. Appl. Biol. 13, 323.

(2) Blackman, G. E., and Templeman, W. G. (1938, 1940). The Interaction of Light Intensity and Nitrogen Supply in the Growth and Metabolism of Grasses and Clover (Trifolium repens) — I, II, IV. Ann. Bot. New Series 2, 257, 765; 4, 533.

(3) Burkholder, P. R. (1936). The Role of Light in the Life of Plants. I and II. Bot. Rev. 2, 1, 97.

(4) Garner, W. W. (1937). Recent Work on Photoperiodism. Bot. Rev. 3, 359.

(5) Gregory, F. G. (1936). The Effect of Length of Day on the Flowering of Plants. Scient. Hort., 4, 143.

(6) Hoagland, D. R. (1937). Some Aspects of the Salt Nutrition of Higher Plants. Bot. Rev. 3, 307.

(7) Hoagland, D. R., and Arnon, D. 1. (1941). Physiological Aspects of Availability of Nutrients for Plant Growth, Soil Sc. 51, 431.

(8) Hoffer, G. N. (1938). Potash in Plant Metabolism. Deficiency Symptoms as Indicators of the Role of Potassium. Indust. and Engin. Chem., 30, 885.

(9) Lundegirth, H. (1935). The Influence of the Soil upon the Growth of the Plant. Soil Sc. 40, 89.

(10) Murneek, A. E. (1941). Vitamin B, versus Organic Matter for Plant Growth. Plant Amer.S.Hort. Sc. 38, 715.

(11) Nicol, H. (1938). Plant Growth Substances. Published by Leonard Hill, Ltd., London.

(12) Salisbury, E. J. (1937). The Plant and its Water Supply. I, II. J. R. Hort. Soc., 62, 425, 473.

(13) Thomas, M. (1937). Plant Hormones and their Possible Importance in Horticulture. Scient. Hort., 5, 141.

(14) Thompson, H. C. (1940, 1941). Effect of Temperature and Length of Day on Growth of Vegetables. Minnesota Hort. 68, 163. (Also Hort. Abs., 11, 22).

(15) Tincker, M. A. H. (1938). Photoperiodism and Horticultural Practice. Scient. Hort., 6, 133.

Chapter II — Soils in Relation to the Supply of Mineral Nutrients

Color Pictures of Mineral Defeciencies in Plants - 1943


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