Numerous determinations on leaves of fruit trees have enabled various workers to suggest minimum values for certain of the elements as being necessary for healthy growth.

The author's minimum values for calcium, magnesium and potassium in leaves of terminal shoots of apple trees collected in late July or early August are:

A notable development in the chemical method has been introduced Lagatu and Maume in France, which they have termed Foliar Diagnostics (known also as Foliar Diagnosis). W. Thomas and his colleagues have, also used the foliar diagnosis method, with great success in the U.S.A..

Since this method illustrates well the stage of development reached in the use of chemical methods to attack problems of mineral deficiencies, its essential points are given below. (See also Refs. 13, 23, 24.)

Leaves at a definite point on the plant, e.g., the third leaf from the base the terminal shoots of a vine, are selected as the material to be analyzed on the plants to be compared, one of which is a healthy, high-yielding plant.

Leaves are taken for analysis at the chosen points on three or more occasions during the growing season, and determinations made of three of the mineral nutrients (say N, P, K or Ca, Mg, K). The sum of the three nutrients on a milligram equivalent basis is calculated for each sample and also the percentages of the three individual nutrients forming the total. The sum gives the "quantity" or "intensity" of the nutrition (e.g., N + P + K in the leaf) and the percentages of each in the total, the "quality" of nutrition (e.g., ratios of N: P: K). The latter values for the various sampling times are plotted on a triangular diagram, and when the points are joined for a given plant the course of the seasonal ratio trend is apparent. The diagram for the plant suspected of a deficiency of some element is compared with that for the healthy plant.

The method has been used successfully to show up many instances of faulty nutrition in a variety of crops.

Another line of research in chemical methods has been the search for quick methods of analysis. The ordinary chemical methods are very tedious and slow. The speeding-up process has occurred in two directions: by the use of approximate tests on plant tissues, generally based on color actions, which can be made in the field, and by the application of spectrographic methods of analysis. The tissue tests have been found to be useful indicating deficiencies in various crops in the U.S.A.

The spectrographic methods provide very rapid means of carrying out large numbers of determinations, but the apparatus required is very expensive and determinations can only be made under expert guidance. Instruments such as the spectrograph and the polarograph are likely to be used very largely for this type of work in the future.

Neubatier, in Germany, has evolved a novel method in which he uses seedling rye plants as the extractant for potash and phosphoric acid. The seedlings are grown in pots under standard conditions in a mixture of washed sand with the soil to be examined and the amounts of potash and phosphoric acid extracted during the seedling growth stage are determined.

Mitscherlich and Wiessmann have developed other methods in which plants are used.

During recent years, much attention has been given to quick soil tests, which can be made in the field and in which the results are judged mainly by color reactions. Here, again, use is made of weak solvents as extractants. The best known of these methods is, perhaps, that of M. F. Morgan, of the Connecticut Agricultural Experiment Station, in the U.S.A.

Soil analyses are very useful for giving indications of deficiencies, particularly of such elements as phosphorus and potassium, and for providing information on soil acidity and alkalinity and organic matter content, but the data require expert interpretation. Moreover, the results must often be regarded only as pointers to deficiencies, and must not be used too rigidly or too categorically. The method of soil analysis can be of great value for indicating the possibilities of deficiencies occurring, even before any crop is planted, thus providing valuable forward information. It has been used very successfully in this way during the war period in connection with the breaking up of grassland and derelict areas for arable cropping. The method has the practical disadvantages that it can only be used by the scientifically trained expert, is expensive and requires extensive laboratory facilities and equipment.

It has long been known that chlorosis of the foliage of vines, due to iron deficiency, could be cured by painting the cut surfaces of pruned branches with sulfate of iron, but the modern technique of utilizing injection and spraying methods for the diagnosis and cure of deficiencies is of recent development. The methods in the early stages of development were mainly worked out for deficiencies of trace elements in trees, especially iron. The importance of this element was early recognized in connection with the occurrence of chlorosis in various parts of the world, and it was found that the trouble could not be satisfactorily treated, by applying the necessary iron salts to the soil.

Bennett, in California, worked out a technique for the injection of iron salts in solid form into the stems and branches of chlorotic fruit trees and applied the treatment successfully in commercial orchards. Since the work of Bennett, this method has been widely used both for diagnostic and curative purposes with several elements. The best time to employ solid injections is during the dormant season just prior to bud break. It is important to use suitable dosages, as under-dosages may not be effective and over-dosages may result in serious injury to the trees. Bennett has given a table of dosages of iron salts for use on fruit trees, which in the author's experience is also suitable for manganous sulfate. Smaller rates are necessary for compounds on, zinc and copper.

Roach, at East Malling, has made a thorough investigation of the subject of liquid injections, and has described his results fully in Technical Communication, No. 10, Imperial Bureau of Horticulture and Plantation East Malling. He has developed techniques for diagnosing deficiencies by injections in leaves, shoot tips, petioles and stems. Liquid injections have also been used successfully for diagnosing deficiencies both of trees and agricultural crops. They are best used on crops in the early of growth. Liquid injection methods, especially the more refined leaf, shoot and petiole injections require a fair amount of skill in operation and experience in the interpretation of the results.

Spraying methods have the great merit of simplicity, the main details requiring attention being the correct strength of spray to use, so as to be effective but not damaging to the foliage, and the appropriate time to spray.

They can be applied both to trees and to all sorts of field crops, and in the latter, for diagnostic purposes, nothing more elaborate is needed way of apparatus than a watering can fitted with a fairly fine rose. For certain crops, such as the Brassicae, onions and leeks, it is necessary to add a wetting agent to the spray solution, and for this purpose ester salts at a concentration of 2% by volume ( Suggested by Dr. H. Martin, Long Ashton Research Station. ) has been found satisfactory.

Like liquid injections, sprays are best applied to leaves at an early stage of growth. Concentrations between 0.1 and 1.0% are suitable for compounds of trace elements, and between 1 and 4% for those of major nutrients. Responses from spray treatments usually require a period of one to two weeks to become clearly visible in the foliage.

This knowledge has been gained in the following way. Plants were grown in sand and water cultures under controlled conditions and fed by means of solutions of pure chemicals. Deficiencies of the individual nutrient elements were created in the nutrient medium by omitting each separately from the complete nutrient solution and the symptoms for each deficiency produced in this way were recorded.

The symptoms were subsequently checked by comparison with instances in the field of deficiencies proved to be due to the same causes.

Since details of the visual method and its use are given in the two following chapters, it is only necessary at this stage to refer to one or two general points. In the first place, it is only possible to use the method because the symptoms of the various deficiencies for any given plant are generally quite distinct visually from each other, and where they are not, other special indicator plants can be used to settle the doubtful issue (see Chapter V), or a check can be obtained by one of the other methods described in this chapter.

Like other methods it has its disadvantages. For instance, symptoms of certain deficiencies on cereal crops are not very distinctive and, in fact, cereals, on the whole, are poor subjects on which to use the method. Again, symptoms may be complicated or suppressed by other factors, such as weather conditions and pests or disease organisms, and the inexperienced may be led to form entirely wrong conclusions. Nevertheless, with experience, the observant farmer can learn to use the visual method quickly and with great advantage. For many years fruit growers in this country have used the symptoms of deficiencies of nitrogen and potassium, and to a less extent, of magnesium, on their trees as the basis of the use of these elements in their manurial programs, and some can also recognize the less common deficiencies of phosphorus, iron and manganese.

A further disadvantage of the method is that the symptoms of the deficiencies must develop before they can be recognized, and this may be too late to apply remedial measures to save annual crops, though if an early diagnosis is made, effective action can usually be taken. The great advantages of the method are its speed and the fact that where the symptoms do not require confirmation no apparatus of any kind is necessary.

In using the plant analysis method, the results may indicate that more than one of the trace elements is in short supply, whereas in the field a measurable response may only be obtained from one of the elements.

Even in field trials in which fertilizer elements are added to the soil, a wrong conclusion may be drawn. Thus with deficiencies of manganese and iron, responses may be obtained from applications of sulfur, due to its acidifying action in the soil, when the result of the trial would suggest sulfur deficiency. Or again, in trees suffering from these two deficiencies, no responses may be obtained from applications of manganese or iron salts to the soil, owing to conditions which render them unavailable to the trees. Under such circumstances the conclusion would be reached that neither element was deficient, whereas injection or spraying treatments would show the contrary to be true.

The method of soil analysis cannot predict deficiencies with absolute certainty. Thus it can only fix approximate levels for potash and phosphate below which deficiencies are likely to be serious and cannot foretell accurately the occurrence of less common deficiencies, such as magnesium, manganese and iron.

The injection and spraying methods may give responses in the early stages of growth which will not be reflected in the eventual crop yields.

With the visual method, there are circumstances in which deficiency symptoms may be masked entirely by pests and diseases, such as by eelworm or by viruses in potatoes, or by the rust fungus, pseudopeziza ribis, on black currants, which completely suppresses mineral deficiency symptoms which would appear late in the growing season.

Again, symptoms produced by pests and diseases, mechanical injury or weather conditions may be indistinguishable from certain mineral deficiency symptoms. Thus leaf symptoms on young cereals and young Brassica plants may be identical for wireworm, cutworm, root injury, cold weather and phosphorus deficiency, whilst symptoms indistinguishable from those of magnesium deficiency may be produced by canker infections or mechanical damage to the bark of apple trees. Symptoms of chloride injury are often almost identical with those of potassium deficiency.

( See plate 68 and plate 114 ).

Finally, for certain plants the symptoms of certain deficiencies are very similar and difficult to distinguish. Thus, chlorosis may be the symptom produced by deficiencies of both iron and manganese, as for example on apples, plums, raspberries and Brassica crops, and at certain stages during the season the particular deficiency could not be diagnosed with certainty from symptoms alone, although the presence of suitable indicator plants would usually make diagnosis possible.

The most successful diagnosis of deficiencies is obtained by a combination of the methods. It will often be necessary to use only two of the methods in a complementary way, as, for example, visual diagnosis in conjunction with injection or spraying, or the visual method with a partial examination of the soil conditions. Thus, when there is any doubt about a supposed instance of manganese deficiency, it can usually be confirmed by spraying or by an examination of the soil.

The method (or methods) of diagnosis used in any given case must depend on the circumstances in the field, on the facilities available and on the technical qualifications and experience of the individual making the diagnosis. When simple procedures can be followed they should be used, but in difficult cases the expert, with his specialized equipment and technique, will be needed.

In conclusion, it can be stated that, by the skilful use of the methods described, it should be possible to solve most problems of mineral deficiencies in crops that are likely to occur in Great Britain in either agricultural or horticultural practice.

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