Plants Are Energy Converters

Plants absorb carbon dioxide from the air and through the action of light, directed by chlorophyll, liberate oxygen to its free molecular state and it to the environment. The freed carbon is elevated to a higher energy by the absorption of light energy and reacts with water to from formaldehyde, the primary building block of organic compounds. The plant condenses the formaldehyde under the direction of, and in combination with, various elements and vitamins to from sugars other vitamins, starches, proteins and finally cellulose, which goes into the makeup of plant cell walls and tissue.

The whole operation is guided primarily by the extremely complex RNA molecule. RNA acts as a template in the production of life-related organic compounds. Unless acted upon by outside forces, the plant faithfully reproduces in fact what the RNA molecule outlines in an abstract form. Conversely, the fact of the existence of the plant cell allows for the existence of the RNA molecule.

Improper environmental conditions are capable of interfering with the cell's ability to comply with the basic pattern laid down by the RNA molecule, resulting in a reduction of sugar production and thus the reduction of vitamin production.

Thus, it is to our advantage to know the optimum environment for plant life in order that we may enhance the existing environment of our region and thus produce plants that are large, healthy and, far more important, contain the hundreds of delicate vitamin compounds that encourage vigorous, healthy animal life.

The most easily controlled aspect of plant life is its growing medium. Perhaps the most important aspect of the growing medium is its mineral content.

ABSORPTION – Intake of water and mineral elements by the root system.

PHOTOSYNTHESIS or CARBON ASSIMILATION – 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, 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, from 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, makes 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 deficiency effects and in diagnosing their causes.

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 affects 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 are 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 lowered light condition results in an increase of "soluble" at the expense of "insoluble" nitrogen within the plant, due to 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 fruit.

Magnesium, iron and zinc deficiency symptoms may be less severe under condition 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 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 and fruit swelling, 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 indolylacetic acid, indolylbutyric acid, phenylacetic acid, napthylacetamide, napthoxyacetic acid, 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.

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 other 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" elements, or as "micro nutrients" are 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.

Growth responses with various crops have also been obtained both in laboratory and field experiments with other elements, such as nickel, chromium and gallium, but it is not clear whether these responses are direct effects or whether they have resulted from interactions of the elements with others.

There are also several elements which commonly occur in plants, but which are not known to serve a useful function. Moreover, elements in these latter two categories, when present in plants, frequently act as plant poisons or toxins.

It should, however, be pointed out that the present list of elements regarded as essential to the growth of crop plants is not regarded as a final one, and may be added to as methods for proving the essential nature of trace elements become more refined and exact.

The nutrient elements can only be suitably 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 sulphates or chlorides, etc.); sulphur from sulphates; 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 sulphur may be added as the element itself, as in flowers of sulphur, ground sulphur, etc.

Toxic effects in plants may be produced 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 may be 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 fairly 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 in 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, though other elements may also produce or accentuate deficiencies of essential elements. Thus a high sodium content may result in effects which appear identical with those resulting from calcium deficiency and which are accentuated by low calcium status (e.g. in celery); again, high concentrations of chromium, cobalt, copper, manganese, nickel or zinc may each induce a deficiency of iron in addition to producing other toxic effects. (2) The presence of an element may directly injure the protoplasm and bring about the speedy death of the plant.

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 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 shows 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 internodes; 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.

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.

One of the first points which is brought out in an examination of a number of soils is their great diversity. In practical terms, soils may be good or bad; light or heavy; wet or dry; fertile or hungry; deep or shallow; black, red or brown; peaty, sandy, silty or clay; all terms denoting points of importance in relation to fundamental characteristics and to their practical utilization.

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 micro–organisms.

The mineral matter furnishes the skeleton of the soil. It consists of material which ranges 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:

Fractions Diameter Limits

• stones 2.0 mm.
• coarse sand 2.0–0.2 mm.
• fine sand 0.2–0.02 mm.
• silt 0.02–0.002 mm.
• clay 0.002 mm.

A preponderance of the coarser fractions tends to make soils light, free working, freely drained and hungry; 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.

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, 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 main 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 residues 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 state where the original cellular structure is no longer recognizable, we speak of the brownish product as humus. The organic matter of the soil at any time thus consists of fresh and partially 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 may 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 in weathering of the soil minerals. Certain of the soil organisms, especially rhizobia 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 for their own particular needs, which are not always in harmony with the requirements of the crops. This point can be illustrated by digging fresh straw or sugar into the soil, and growing a crop on it immediately. 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 organism depleted the soil of its readily available nitrogen.

Organic matter produces both chemical and physical effects on soils. Like clay, the humidified 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 mixtures of clay 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 clays are.

The solid particles of the soil, consisting of the mineral particles and the organic matter, being of various 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, frost and management, but usually occupies from 30 to 60% of the soil volume. It will be clear that the amounts of air and water 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 provides 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 present in the soil solution in very small amounts, but supplies of these elements in the solution are apparently 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 in plants, especially manganese and aluminum.

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

Copper, zinc and manganese seem 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, 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 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.