University of Bristol Agricultural and Horticulture Research Station, Long Ashton, Bristol
London — Published by His Majesty's Stationary Office — 1943 — Crown Copyright Reserved —
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
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.
Essential Points in the Nutrition of Plants
THE NATURE OF PLANT GROWTH PROCESSES
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
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
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.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:
( 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.)
(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.