Uptake of Principal Nutrients
Sucrose, which is the product of the sugar-cane upon which the whole economy of the industry rests, is a chemical substance con- sisting of carbon, hydrogen and oxygen in combination, none of which is derived from the soil. If the return of all plant residues to the land after the sugar has been extracted from the cane were possible, there would be no depletion of the soil reserves of essential nutrients. This ideal condition does not, however, characterize the conduct of operations. Even where a sugar-cane estate is in close association with a sugar factory and under the control of the same organization, the practical difficulties and expense of returning the factory residues to the point from which they originally derived are insurmountable. Moreover many of the plant constituents undergo changes in the course of the extraction of the sugar, while additions are made in the form of lime and other materials, and a large part disappears in the furnaces in which the fibrous residue, the bagasse, is burnt. Most soils are capable of natural regeneration in so far as all essential plant nutrient require- ments are concerned, with the exceptions of nitrogen, phosphorus and potash and of these the two latter are often present in amounts adequate for the support of good crops. A cane crop of 50 tons will remove from the soil the following approximate quantities of replaceable nutrients:
75 lb. nitrogen (N)
50 lb. phosphates (as P205) 150 lb. potash (as K20).
These must already be present or be replaced in assimilable form in adequate quantity within the root zone of the cane if succeeding crops are to be fully supplied. Where the sugar-cane planter is unable to obtain and make use of factory residues he will find his land suffers from the continuing drain of plant nutrients to a greater extent than if he were able to return some of the by- product material to the land.
154
Sugar Factory Residues
Filter mud is the principal convenient and useful sugar factory residue for returning to the fields. It contains most of the phos- phoric acid and some of the nitrogen in the cane. Potassium, or potash to use the generally understood term, mainly leaves the sugar factory in the molasses which is processed elsewhere so that the distillery waste in which most of the potash is finally present is not available in any form for return to the land. Occasional variations occur when, for example, a distillery is operated in association with a sugar factory, but the dunder even then is in most cases wasted. The comparatively rare process of potash recovery from distillery waste is conducted away from the sugar estates which originally supply the molasses, and the potash-rich ash containing about 37.5% K20 obtained by burning the con- centrated waste liquor is disposed of as a fertilizer through trade channels. The small amount of nitrogen in molasses is completely lost. The ash of bagasse accounts for part of the phosphate and potash originally present in the cane, but the high temperatures attained in the factory boiler furnaces bring about their combina- tion with silica and the production of insoluble vitreous material so that they become of no practical value when returned to the land. Surplus unburnt bagasse is a useful soil improver for light impoverished land of poor water retaining capacity. It is bulky material which decomposes slowly, and is costly to handle, trans- port and distribute. It is occasionally used for the preparation of compost for application to cane fields.
C o n d i t i o n s E s s e n t i a l t o E f f e c t i v e U s e o f F e r t i l i z e r s Even where useful factory residues are available, the addition to the soil of plant nutrients or fertilizers, particularly those con- taining nitrogen, is essential for the maintenance of fertility and for the production of good crops.
In addition to nutrient requirements, the plant needs water, which may be supplied by annual rainfall or by irrigation. If the plant is inadequately provided with water it cannot take up nutrients already present in the soil, nor can it derive full advan- tage from those which are applied in the form of fertilizers. The two essentials, adequate water and adequate nutrients, are com-
i56
plementary. Both are related to the use of agricultural systems which are strictly suited to the type of soil in which they are conducted, so that a third factor is introduced. The work done on the land in the course of preparation for cultivation not only prepares the soil for the physical support and growth of the plant, but assists in liberating and making available the essential nutrient materials which dissolve in the soil water and are ab- sorbed by the plant. In considering, therefore, the subject of fertilizers and their use in sugar-cane agriculture, it must be assumed that correct methods of land management are practised and that sufficient moisture for the needs of the plant is supplied, any excess being removed by drainage.
Determination of Nutrient Requirements
Soils differ remarkably, as has already been shown, in their fertility status, capacity for regeneration, and capability of stimula- ting and supporting plant growth. Various methods have been used from time to time to determine the nature and extent of additions which should be made to the soil to replace plant nutrients removed by the harvested crops, and the methods of supplying them for the benefit of succeeding crops. The use of the cane itself as an indicator of its own requirements in these directions has been extensively studied, and as a result new methods of deducing nutrient deficiencies and their extent have been evolved. It has long been known that the appearance of the cane varies considerably with its nutrient status, and that extreme cases of deficiency of one or other of the essentials to its health and vigour are displayed in the leaves and by the habit of the whole plant.
Methods which are coming into extended application for the control of the fertilization of sugar-cane involve the chemical, examination of leaf material in regard to its content of nitrogen, potash and phosphorus. Extensively studied in Mauritius, the method has been further examined in Hawaii and Jamaica. It forms part of the Clement's Crop Log System, and gives results which can be used in determining the nature and rate of fertilizer application. It therefore possesses a very great advantage over methods which depend upon ascertaining the comparative crop yields resulting from various applications of fertilizer to small
plots of sugar-cane, though these are of great value and form an essential part of field investigations as will be seen.
Fertilizer Control by Foliar Analysis
The modem method of determining the nutrient requirements of sugar-cane is by analysis of leaf tissue for nitrogen, phosphoric acid and potash, and comparing the result with standards which have been previously established for the optimum values in comparative field plot trials which show no response to fertilizers and result in the best and most vigorous growth of cane. By this means the results show whether supplies of each of the three fertilizer elements are deficient, normal or in excess of the amounts needed for satisfactory growth and yield, and applications can be adjusted in accordance with the indications thus obtained. The plant tissue used varies in the different countries in which the foliar diagnosis method is applied. In Mauritius the third leaf blade is taken, the partially unfolded one being regarded as the first; in Jamaica, the third fully opened leaf; and in Hawaii, the third, fourth and fifth. It has been found that leaf sampling errors may affect the potash indication. Nitrogen and phosphorus determinations on the second, fourth and fifth leaves give results closely agreeing with those of the third leaf. T h e crop is sampled under normal growth conditions. If affected by drought, strong wind, unusual rainfall, or other abnormal influences, the value of the results of analysis of the leaf tissue for the determination of the nutrient status of the plant will be impaired, and the indications possibly be quite misleading. In Mauritius, where long ratooning is practised, leaf samples are taken from ratoons only in full vegetative growth at 5 to 7 months of age ; that is midway in the growth cycle. The whole leaf samples, or punch samples taken from the centre of each side of the blade are used for analysis.
P. Halais has described in detail the technique used in Mauritius for sampling and analysis in a paper presented to the 7th Congress of the I.S.S.C.T., 1950, with a list of references to publications on the various aspects of foliar diagnosis.
T h e Method in Practice
T h e application of this proved method which conserves fertilizers and controls the quantities applied in strict relation to
158 AGRICULTURE OF THE SUGAR-CANE
the actual needs of the crop is extending rapidly. The basic nutrient standards have first to be established for the principal varieties grown in each ecological area. Sampling technique, in regard to the age and condition of the cane, the actual leaf material taken, methods of collecting, preserving and sub-sampling the leaf tissue, and preparation for analysis, must be specified. The chemical examination requires the use of standard methods under highly competent supervision in properly equipped laboratories, and staff capable of conducting large numbers of analyses with speed and accuracy. Many large plantations are carrying out this work in their own laboratories as a part of the scientific routine control of fertilizer applications. Smaller estates and farmers need central laboratories to conduct the work on their behalf, such as the one established by the sugar industry in Mauritius in 1948.
Relation to Field Plot Trials
It does not replace the regular types of plot trials which are designed to determine the response of the cane to different levels of fertilizer dosage. Foliar analysis carried out on leaf material from cane in such trials is used to establish the standards of N.P. &
K. content of the leaf tissue on which advice regarding the rates of application of fertilizer on a field scale can be based in relation to the results of the leaf sampling technique. Experimental plots thus provide amongst other information the references necessary to interpret and apply the foliar diagnosis method. T h e approxi- mate amount of increase or reduction in fertilizer application can be determined by comparisons made with the indications of the field plot trials, and in that way bring about a speedier and more extensive correction of fertilizer practice than is possible by the field plot method alone.
The standards for composition of leaf material on a dry basis found to apply in Mauritius are quoted :
PzOs K20
o-55% i-75%
o.45%-o-55% I . 2 6 % - I . 7 5 %
o.35%-o.45% o.75%-i.26%
o-35% °-75%
Farmyard M a n u r e and Artificial Fertilizers
Farmyard manure still finds a place in the cane fertilizing pro- gramme of estates and cultivations where animals form the major source of power for work on the land, and for transport of cane, but it is no longer regarded as an essential for the maintenance of soil structure, fertility, and the production of good crops of cane.
The availability of artificial fertilizers, the ease with which they can be applied in regulated quantities suited to the needs of the soil and the crop, and the proof by experiment and experience that their use in the proper kinds and quantities does all and more than farmyard manure ever did has in recent times completely altered plantation practice. It has been shown that artificials can match farmyard manure in cane and sugar yields in good soils, and give better results on poor soils. The effect of mechanization, which has displaced animals completely in some areas, and to a great extent in others, has not, as was thought by some authorities, been accompanied by a disastrous lowering of fertility levels.
Indeed, the contrary has occurred ; yields have increased ; and there is no evidence of any decline in the qualities of the soils.
There are no sound reasons for the retention of livestock for use on cane plantations because of any contribution they may make to the land in the form of manure. It may be convenient or economic to use livestock, in which case full use should be made of the manure, which is undoubtedly valuable when properly conserved and effectively applied. The essential point is that just as the large scale use of farm animals has proved uneconomic in extensive cane production areas, so has the cost of farmyard manure increased far beyond its actual value as a fertilizer. It is by no means intended to deprecate the value of organic residues for the amelioration of soil and the increase of nutrients available to plants. The use of farmyard manure on the larger estates is expensive and it suffers from the disadvantage of being an unsuit- able medium for the application of regulated amounts of the fertilizers it contains in relation to the actual requirements of the cane. When available it should therefore be used in ample quantities on limited areas where transport to the field, and dis- tribution on the land are relatively low in cost. On small cultiva- tions where animals are used everything possible should be done to conserve the manure and apply it effectively to the cane land.
Composition of F a r m y a r d M a n u r e .
The composition of farmyard manure varies with the type of animal, the nature of the food ingested, the litter used, the method of making and conserving the material and its age. The average results of a large number of analyses conducted at Rothamsted are quoted:
Water 76%
Nitrogen 0.64% (15 lb. per ton) Phosphate (as P2055) 0.23% ( 5 lb. per ton) Potash (as K20) 0.32% ( 7 lb. per ton)
The composition of the cattle manure produced on cane farms is closely similar, but losses of nitrogen occur more rapidly under the influence of the higher temperatures, while as a general rule the methods of making and conserving the manure are less carefully conducted. The wastage is therefore usually high, especially when no special arrangements are made to prevent loss of the urine and decomposition of the urea.
Loss of Nitrogen from F a r m y a r d M a n u r e .
The two most highly regarded constituents of farmyard manure are nitrogen and humus, and its utility when applied to the soil is mainly related to the increase of humus in the soil and the im- provement of the water retaining capacity of the land when ade- quate amounts are applied. The nitrogen is mainly present in the form of urea, which is rapidly decomposed into free ammonia and carbonic acid, both of which almost entirely escape into the atmosphere in the form of gases. In the process of this change, ammonium carbonate is formed as an intermediate product which breaks down when the liquid containing it evaporates. It is also subject to decomposition by the action of bacteria which first convert it into nitrate and then to free gaseous nitrogen, a process called denitrification, which is affected by the condition of the manure. These are but two of the complex series of changes which occur, but they are sufficient to indicate that farmyard manure as ordinarily prepared is by no means an effective way of conserving combined nitrogen.
Organic Dressings
The question of whether or not to use farmyard manure in cane land is decided therefore by factors largely unrelated to the fertilizing value of the material. Where dressings of organic material are desirable they can be provided in various ways, among which are the composting of organic residues, the use of factory waste, particularly filter mud and bagasse, and to some extent under favourable circumstances by the practice of green manuring and trash conservation. In no sense is it intended to convey that the application of organic matter in suitable decom- posed form should be discontinued; rather should it be en- couraged and developed, the essential point being that it must be economic, and only those methods can be economically used which provide the requirements of the cane cheaply and effectively.
Fly-Penning
The folding of cattle in temporary enclosures in the cane fields, called " fly-penning " is a method used in parts of the West Indies, particularly in Jamaica with the object of improving the fertility of small and usually comparatively poor sections of the area to be replanted. Each pen is bounded by a post and wire fence and the animals are confined during their rest periods, being watered before and after as it is not always possible to supply water to them in the fold. Litter consists of cane trash, bagasse, and the residues of the feed which is usually cane tops and grass supplied to the animals in the pen. The theory is that the excre- ment is deposited on the land in the place where it is needed, and that the decomposition of the litter and unconsumed food under its influence adds substantially to the humus content of the soil.
In practice the method is costly, wasteful and less effective than the use of properly prepared f.y.m. carted out and applied to the fields. Decomposition of the urea, and denitrification are very rapid; very little humification of the litter can occur; and a high proportion of the food is trampled and spoiled. The soil is temporarily improved by the other nutrient salts voided in the excreta and by the litter which is subsequently turned in when the area is ploughed. The expense of erecting and removing the fences, transporting Utter and feed, and attendance (the control of move- ment and care of the animals) far outweigh the benefits derived.
Application to Plant Cane
The practice in regard to the times and methods of applying fertilizers varies considerably. Studies have been made of the effects of supplying the indicated requirements of the cane in two or three divided doses as compared with a single application. In general little if any advantage has been shown to attach to the use of the former method. Apart from the nutrient effect on the cane, the damage to soil tilth brought about by additional movements over the land for the supply of the full amount of fertilizer in two or three parts is a definite drawback. For plant cane the dropping of the fertilizer into the open furrow at the same time that the setts are planted has been found entirely satisfactory. To be most effective phosphates and potash should be placed in the root zone, while nitrogen may be applied to the surface, though again there is no evidence that nitrogen is less useful when buried. It is desirable that the germinating buds should not be in direct con- tact with concentrated fertilizer, and this may be avoided by the placing of the fertilizer in relation to the sett. With mechanical planting the full dose of fertilizer is often dropped at the same time as the sett, a practice which does not appear to affect germination adversely.
F e r t i l i z i n g R a t o o n s
The fertilizing of ratoons is more troublesome when the cane is not burnt prior to reaping unless the material is scattered on the trash over the cane rows, which is a method occasionally used.
When cane is burnt surface cultivation of the ratoons often takes the form of a light chiselling or ploughing on each side of the cane row, and it is a comparatively simple matter to apply fertilizer mechanically at the same time. When trash is left on the field it has been found best to move it from immediately above the cane row, though if not too dense it may be left in position and the fertilizer scattered over it where the cane is grown under conditions of natural rainfall. Where surface irrigation is practised, it is necessary in any case to cut furrows either after removing the trash from the cane row, or by cutting through it with a specially designed implement which makes the furrow at the same time.
It is a simple matter to combine the application of fertilizer with these operations.