The Hunter System of Soil Analysis
M.P.W. Farina, Summer Grain Sub-Center,
Cedara College of Agriculture & Research Institute
It is my task today to talk to you about our experiences with the analytical procedures developed by Dr. A.H. Hunter - the so-called Hunter System. Before going any further I want to make it quite clear, that I am not trying to "sell" you anything. As far as I am aware, this particular item on our agenda was arranged in order to give you a chance to asses for yourselves whether or not the Hunter System, which my laboratory adopted in 1977, has any place in your own particular situations. All I have been asked to do is to give you my objective opinion now that we have had some practical experience with the procedures and equipment Hunter developed.
During the course of the last few years most of you have heard of or seen reference to the Hunter System and the ISFEI extractant. Possibly some of you have also heard of Custom Laboratories and Agro Services International Incorporated. However, I suspect that few of you are aware of the close mutual association which exists. Indeed, many of you have probably wondered how the term ISFEI originated and puzzled over the connection with Hunter. Perhaps it will be useful if I spend a few minutes clearing up any possible confusion. You are going to hear today about both the Hunter System and the so-called ISFEI method and it is important that they are viewed in the proper perspective.
In the early 1960's Dr. J.W. Fitts, then chairman of the Soil Science Society of America, and subsequently a guest speaker at the Combined Agricultural Congress in Pretoria, embarked on a major AID (Agency for International Development) Project in Latin America (Brazil, Ecuador, Panama, Costa Rica, Nicaragua, Honduras, El Salvador, and Guatemala). This project, known as the International Soil Fertility Evaluation and Improvement programme (I.S.F.E.I.) had as its' objectives "the judicious use of fertilizers, lime, manures, and other soil amendments to build and maintain high crop yields." Perhaps because of Fitt's background, but more probably as a result of enlightened planning, the analytical process was accorded a pivotal role. It was appreciated that sustained and meaningful progress would not be possible without efficient soil analytical back-up. I am certain that those of you who have experience in developmental agriculture know only too well how difficult it is to give the required advice without soil test information.
In order to ensure the necessary analytical back-up the development of an independent laboratory facility received priority treatment and Hunter was appointed as the project's Laboratory Director. His terms of reference included the development of a soil-plant laboratory capable of speed and accuracy - two prime requirements which all of you will readily appreciate. This involved the development and testing of analytical procedures and the design and manufacture of multiple-unit time saving apparatus. There is little doubt that this laboratory proggramme proved to be one of the strengths of the project and today there are numerous laboratories, both in Latin America and elsewhere, making use of the extractants, procedures and equipment developed. The overall system has become associated with Hunter's name, one of the major extractants and a close variant developed, but not used by Hunter is frequently referred to by the acronym ISFEI, and a firm known as Custom Laboratory Equipment Company is established as the major manufacturer of the time saving and rather ingenious pieces of apparatus developed. Agro Services International is the name of the consulting corporation Drs. Fitts and Hunter established in 1977 after completion of the AID contract and it is interesting to note that this private organisation is now employed on a full-time basis by some of the foreign governments originally assisted by the American Agency for International Development.
Strictly speaking, the extractants and chemical procedures developed by Hunter are an integral part of the Hunter System. However, apart from the fact that I have no desire today to become embroiled in a debate on extractant suitability, I prefer to regard the system per se as consisting of the non-chemical procedures - the mechanics of analysis. Although my own laboratory has adopted most of the chemical methodology developed by Hunter, it is the mechanical aspects which, I believe, have revolutionised soil fertility evaluation. The resulting encreases in analytical capacity are truly dramatic and equipment costs are reduced rather than increased - an uncommon consequence of laboratory automation. Moreover, we have found that there has also been an improvement in reproducability over the more conventional procedures we preciously used.
Most of you visited Cedara College yesterday and were able to examine the procedures and equipment involved. However, I am sure overlooked some points. There may be others which require further expansion and clarification. It is, in any event, often difficult to adequately assess a system such as this without some description, which can be digested at leisure. What I intend to do now, is to run through the attached flow-chart dwelling briefly on aspects, which I feel are particularly pertinent. I must emphasise again though, that reference to various elements should be largely ignored. This depends on the specific extractants used.
Batch handling
The flow-chart has been drawn up in a rather unconventional manner in order to highlight the most important single feature of the system - batch handling from start to finish. Each block, therefore, represents a set of samples (fig. 1) or tray of three sets (fig. 2). Most high capacity laboratories make use of batch processing in one form or another, but the Hunter System is , in my experience, the first to treat batches - in our case nine unknowns plus a blank and standard sample - as a single unit from start to finish. In fact, for many operations e,g. dispensing, the unit consists of three sets (27 unknowns, three blanks, and three standard samples). All procedures are designed so that individual containers are not required to be removed from the polystyrene holders. Consequently, many of the chemists' traditional tools such as beakers, pipettes, volumetric flasks, measuring cylinders, stoppers, filter funnels and racks, and cuvettes have been done away with. Quite obviously this batch processing feature has important implications in terms of cost and time as I have indicated in the sections dealing with some of the pieces of equipment. I suspect that the time required to perform various operations compares rather favourable with systems most of you are currently using.
Sample acquisition
As all of you are alreay aware, in the Hunter System samples are scooped rather than weighed. However, this is not a procedure attributable to the I.S.F.E.I. profect and most high capacity American laboratories have used scoops for many yeas. Actually, the Hunter System per se could equally well be used with weighed samples and any decision on the use of scoops is really irrelevant to this discussion. However, since our laboratory is probably the only in the country currently performing determinations on a volumetric basis and the matter is rather contentious, I have been asked to make some brief comment based on our experience. I realise that to most of you the thought of scooping is anatheme. The pure chemists among you probably associate scoops with "bucket chemistry" and have grave reservation about the effects on accuracy. A perfectly natural reaction, which I also shared before putting scoops to the test. We have found, however, that the inherent error is of the order of 1% - an error which is insignificant in soil fertility evaluation at either the service laboratory or research level. I have never conducted the necessary tests, but suspect that in a routine laboratory handling hundreds of samples the error associated with weighing may, due mainly to operator fatique, very well be of similar magnitude. As far as error is concerned, those of you bound to the laboratory side of fertilizer advisory services should also bear in mind that laboratory errors are insignificant in comparison with that gross errors commonly associated with field sampling, interpretation, and remedial action at the farm level. Apart from considerations of accuracy and time (scooping 33 samples takes approximately 250 seconds as against 750 seconds on an electronic mass meter) there are also sound theoretical reasons for basing soil fertility evaluations on volumetric measurements. Mehlick (1972,1980) has dealt with these in some detail. There is insufficient time here to go into any detail, but when we fertilize unit area of land we deal with a specific soil volume the mass of which may vary considerably depending on texture.
Dispensing extractant
The first step in the analysis proper is the addition of extractant. Here the Hunter System makes use of multiple-dispensers such as that shown in fi. 4. In the particular configuration that we use three aliquots are dispensed simultaneously and it take approximately 60 seconds to dispense 33 aliquots. This is very much faster than individual addition by means of pipettes of measuring cylinders and probably compares favourably with other multiple dispensing procedures. Another major advantage of this equipment is its simplicity. Changing volumes is easy and rapid and maintenance is cheap and simple.
Stirring
The major advantages associated with this piece of equipment are ease of handling, constant speed, and absence of the stoppers usually associated with reciprocal shakers.
Filtering
Here the nature of the containers used makes it possible to dispense with filter racks and funnels (fig. 6). A simple system of colour coding facilitates control.
Diluting and making to volume
Diluters and diluter - dispensers will not be considered separately because of their essential similarity. This equipment has a dramatic effect on speed and it is possible to make up solutions from 33 extracts (aliquot + colour reagent + deionized water to volume) in less than 4 minutes. Done in the traditional fashion using a pipette, a Kipps pipette, and a volumetric flask the same number of solutions would take almost an hour to make up. Here again the equipment is very simple , easy to maintain, and in our experience perfectly accurate ( fig. 7).
pH determination
This attachment (fig. 8) was designed to make it possible to read pH's directly in the trays. Simultaneous stirring eliminates problems associated with suspension effects and automatic electrode washing between samples adds appreciably to speed and ease of operation.
Titration
The titrator too, was designed to fit in with the batch concept. A stirrer attachment eliminates the need to swirl and makes it possible to perform titrations in situ (fig. 9)
Reading elemental concentrations
Both standard atomic absorption equipment and fibre optics colorimeters are well suited to the batch handling concept (figs. 10 & 11). More conventional colorimeters need a flow-cell or the cuvette loading attachment developed by Hunter. Automatic samplers on atomic absorption equipment pose a slight problem, but at worst this entail transferring filtrate into free standing containers.
Washing
The multiple washer (fig. 12) is not unique, but with both tap and deionized water lines provides another very useful time saving device. Plastic and P.V.C. components ensure no contamination in trace element analysis.
Scrubbing
We have experienced some trouble with the particularly stubborn stains left in containers by certain of our red soils. To overcome this we had the machine shown below (fig. 13) built locally and have had no further problems. Hunter has not found equipment of this type necessary, but I suspect that most laboratories in this country would. Contamination resulting from poor washing has long been the bane of our existance and is probably a frustrating problem in many of your laboratories as well.
To sum up, may I say that we are delighted with the effectiveness of the system. As I have already said, we have actually experienced an improvement in overall accuracy and there is absolutely no comparison with the more conventional systems as far as speed is concerned. A laboratory such as ours should be able to complete 100 soil samples a day and bigger laboratories in Latin America are currently analysing 500 samples a day with as few as three operators. With regard to speed, it is interesting to note that Hunter sees no place for auto-analysers in his own laboratory for the simple reason that they cannot normally operate fast enough. The simplicity of the whole system is also a major attraction. Very little chemical expertise is required and the more tedious and soul destroying aspects of soil analysis e.g. pipetting, have been completely eliminated. Other considerations, perhaps more important to commercial than to State laboratories, are the appreciably lower establishment and operating costs and the minimal space requirement.
Finally, I think it is worth mentioning that Hunter also uses much of this equipment in plant analysis. We have found the effect on laboratory capacity to be equally gratifying.
REFERENCES
Mehlick, A.,1973. Uniformity of soil test results as influenced by volume
weight. Comm. Soil Sci. Plant Anal. 4, 475-486.
Mehlick, A., 1980. Why not use relevant data in soil-plant relationships?
Soil Sci. Soc. Am. J. 44, 440 - 441.