ESTIMATION OF THE AVAILABILITY OF PHOSPHORIC
ing solution was too weak and the acid was mostly neutralised. A number of tests were carried out on
From the above table it can be concluded t h a t when the pH value of the extract remains above 1.7 the extraction of available phosphoric acid is not complete. A ratio of 2 gms. soil to 25 ml. solution may be taken as the most suitable. Soil 4 is an exceptional soil, found alongside a filter cake dump, and not likely to be met with frequently. It is in- teresting to observe that a light sandy soil (soil 5) does not influence the pH of the extract in any of the ratios. Where strongly calcareous soils are sus- pected, for example by the evolution of carbon dioxide when extracting, the pH value of the extract should be taken as a guide.
Available potash was also estimated in the soil extracts detailed above. The results showed t h a t the change of ratio did not have the same marked effect in all cases. In the instance of soil 4, however, the lbs. potash per acre did rise from 1,200 at the ratio 10 : 25 to 2,000 at the ratio of 2 : 25.
The choice of the ratio 2.5 : 25 has finally been made to overcome the well-known difficulty of work- ing with calcareous soils. The majority of methods does not appear to stress that reliable results are only obtainable when the resulting pH of the soil extract is the same for all soils. This applies also to the one per cent, citric acid method. Soil extracting solutions are generally heavily buffered to ensure this, b u t a heavily buffered solution, such as is used in the Hawaiian kit test for extracting potash, was not found desirable by the writer.
The method of extraction can now be stated.
Shake 2.5 gms. of 1 m.m. air-dried soil with 25 ml.
of the extracting solution for one half minute. The soil is measured off in a specially made container and the shaking is best made in a 175 ml. conical flask. Filter immediately through a suitable open grade filter paper, returning a n y turbid extract to the flask, and not making the test until the whole quantity is filtered. The filtrates are clear and water white for almost all types of soils,
four alkaline heavy soils and one acid sandy soil as a control (soil 5 in the table below) :—
A factor which is invariably disregarded is t h a t of temperature. The writer makes all extractions at a temperature in the neighbourhood of 25°C. It was found t h a t the lbs. phosphoric acid per acre of a. soil shaken with extracting solution at 7°C. was 10. At 17°C. this rose to 40 lbs., and at 27°C. it amounted to 48 lbs. Potash likewise was found to increase with increase in temperature. The bottled extracting solution, when too warm or too cold, should be brought to t h e desired temperature before use.
The determination of potash required carefully controlled conditions. One of the most frequent sources of error in the precipitation is the reaction of the extract. To avoid this, titrate the extract with 10 per cent. caustic soda until a precipitate is visible.
Only a few drops will be used. Now add a small square of brilliant yellow paper, and if it turns red, back t i t r a t e with a drop or two of half normal sul- phuric acid, until the paper becomes an orange colour and some precipitate is still visible in the beaker.
This precipitate, largely aluminium, will not be dis- solved at the neutral point, nor will it affect the results. Great care must be taken in securing the neutral point. Brilliant yellow paper has a con- venient range of pH from 6 to 8, and t h e colour of the indicator is not leached from the paper as with brom thymol blue papers.
2.—PREPARATION OF REAGENTS.
(a) Extracting solution.—Prepare 2 litres of N/15 sulphuric acid. To this add 1 gm. pure sodium borate and dissolve. The above solution, first used by the writer, was found too strong, and was diluted to a N/21 solution by making 720 ml. to 1,000 ml. with water. This is the solution now used, and has a pH value of 1.5.
(b) Ammonium molybdate.-—Dissolve 25 gms. of pure ammonium molybdate in 250 ml. water. To this add 750 ml. of 50 per cent. (by volume) sulphuric acid. Keep stored away from direct light.
(c) Stannous chloride. — Dissolve 0.1 gm. tin foil in 2 ml. concentrated hydrochloric acid by warming. Make up to 10 ml. with water. This operation is conveniently done in a test tube with the 2 ml. and 10 ml. marks scratched on the side.
Keep stoppered and prepare fresh each day for use.
Stored suitably under about half an inch of liquid paraffin, the solution in larger quantities was found to keep about three months. The use of the solution is preferred to the dry salt.
(d) Sodium cobalti-nitrite.—Of the numerous ways of preparing this reagent the following has been adopted:—
Weigh out 60 gms. pure sodium nitrite and 15.9 gms. pure cobaltous nitrate. Dissolve these salts separately in distilled water and wash both into a 200 ml. flask. Add 5 ml. glacial acetic acid and make to the mark. Mix and store loosely stoppered in the dark for about two days. Aerate for about 10 min- utes with vacuum, and filter and store in a cool place. The reagent is ready for use. It will keep many months if stood in an ice chest, otherwise it is liable to deteriorate and must be tested after some weeks.
(e) Mercuric chloride—potassium iodide.—Dissolve 50 gms. potassium iodide in 200 ml. water. Set aside 10 ml. of this solution, and carefully add to the re- mainder saturated mercuric chloride (6.2 gms. in 100 ml.), until with constant stirring a final drop leaves a red flush. If too much has been added back t i t r a t e with a few drops of the potassium iodide, set aside until only a red flush remains. Excess potassium iodide makes the reagent less delicate.
3.—DETERMINATION OF AVAILABLE PHOSPHORIC ACID.
Analytical procedure.—Mix the extract and with- draw 2 ml. with a small pipette with a rubber teat on the end. Place in a test tube of internal diameter 3/8 inch and about 6 inches long, with a mark indi- cating the volume 7 ml. scratched on the side. It is advisable to prepare two dozen such tubes. To the contents add three drops of ammonium molybdate reagent, make to 7 ml. mark with water, mix and add three drops of fresh stannous chloride solution.
Invert the test tube twice and compare with the standards (see below) after about four minutes.
Should very high concentrations be found, take 1 ml.
extract, or even less.
Preparation of standards.—In the analytical pro- cedure 2.5 gms. soil are shaken with 25 ml. extract- ing solution, hence 2 ml. extract represent 0.2 gm.
soil.
If we now make a 0.001 per cent. P2O5 solution, by dissolving 0.0192 gms. potassium dihydrogen phosphate in a litre, then 2 ml. contain 0.00002 gms.
P.2O5. If the colour developed in these 2 ml. standard
The procedure for developing the colours is the same as for the ordinary test, making up to 7 ml. in test tube. Artificial colour standards to match the above are now prepared.
For this purpose dissolve 5 gms. nickel ammonium sulphate in 70 ml. distilled water in a 100 ml. flask.
Now add 20 ml. 5N ammonium hydroxide and mix well. Add 10 gms. ammonium sulphate and shake to dissolve. Complete to 100 ml. mark, shake, and filter if necessary.
It was found that ammonium sulphate was neces- sary to prevent hydrolysis taking place when the solution was greatly diluted. Above the range 50 lbs.
P2O3 per acre, the colour of the phosphate solutions was a less pure blue than that produced by the above solution. The addition of a few drops 0.2 per cent. potassium dichromate was found to remedy this. The quantity of ammonia added affects the final colour, a purplish tinge resulting if a larger quantity than that recommended is added.
The following dilutions were found by experi- ment :—
solution matches that in the 2 ml. soil extract (0.2 gms. soil) then the soil contains, by simple calcula- tion, 200 lbs. P2O5 per acre.
From the above the following table can be arrived a t : —
In preparing these artificial standards the water is placed in a small beaker and the blue solution run into it from a burette. The tubes on completion are tightly stoppered or sealed.
There are certain precautions to be observed in developing the blue colour of phosphate solutions.
Sufficient ammonium molybdate must be present or else the solution becomes cloudy on reduction with stannous chloride. Too great an excess of molybdate slows down the rate of development of the blue colour, or m a y even prevent complete development.
For the quantities of phosphoric acid found in these tests the recommended ratio of three drops ammo- nium molybdate to three drops of stannous chloride was found best. Colours developed are not reliable after about eight minutes standing.
4.—DETERMINATION OF AVAILABLE POTASH.
Analytical procedure.—Take 2 ml. of the soil ex- tract (after mixing) and run into a test tube similar to the type used for phosphoric acid determination. Add four drops of 38 per cent. formaldehyde to prevent the precipitation of soil ammonium salts by forming hexamethylenetetramine, which does not interfere with t h e potassium test. In the majority of soils the interference by ammonia, however, is negligible.
Now add three drops of sodium cobalti-nitrite re- agent and mix well by shaking the tube sideways against the palm. With a suitably made pipette (external diameter 5/10 in.) take up 4.6 ml. of 85-86 per cent. (by volume) ethyl alcohol, and insert the tip to the b o t t o m of the test tube. Release the alcohol and at t h e same time commence raising the pipette so t h a t by the time the alcohol has all run out, t h e tip of the pipette has reached the surface.
In this way a thorough contact of the alcohol is made with the contents, a most important stage in the test. After running in the alcohol, invert the tube two or three times, and compare after one minute with turbidity standards. It is advisable always to duplicate the test.
High concentrations of potash can be checked in duplicate by taking 1 ml. extract and carrying out the test as above. For this test complete the remain- ing 1 ml. required to bring the contents to the 7 ml.
mark with more alcohol and mix again. Diluting the extract with extracting solution will not give satis- factory results.
Preparation of standards.—The turbid solutions developed above are either compared with standard colour charts, as in the Spurway method, or, as is more usual, placed over groups of lines of varying intensity to observe when any one group of lines be- comes invisible. The writer knows of no method which compares the turbid solutions with standard
solutions placed in a test tube rack. Believing that comparisons could be made more easily thus, an effort was made to prepare such turbidity standards.
Reasoning along lines similar to those when pre- paring the phosphate standards, we now make a 0.003 per cent. K2O solution, by dissolving 0.0475 gms. potassium chloride in 1 litre, then 2 ml. contain 0.00006 gms. K2O. If the turbidity developed in these 2 ml. matches that in the 2 ml. soil extract (0.2 gms. soil) then the soil contains, by simple cal- culation, 600 lbs. K2O per acre.
Weigh out 0.0238 gms. pure potassium chloride and dissolve in about 490 ml. of extracting solution in a 500 ml. flask. Now carefully neutralise with 10 per cent. caustic soda, using brilliant yellow paper, which must be converted to an orange colour. After neutralisation make the solution to the 500 ml.
mark with distilled water and mix.
Using the above solution the following table can be deduced:—
An amount larger than that required is placed in a small beaker and diluted proportionately. Thus if 200 lbs. per acre is required, take 3.30 ml. and dilute to 10 ml. High dilutions may render the solution again acid, requiring a further adjustment to the neutral point as above. Take the desired quantity and develop the turbidities as for the test, making to 7 ml. mark.
The artificial turbidity standards to match were prepared from a mixture of 9 ml. 1 per cent. potas- sium dichromate and 15 ml. 8 per cent. cobalt sul- phate. To this was added a suspension of very fine silica, obtained by pouring off the suspension which remained in the solution after standing 24 hours. The silica was ground in an agate mortar, mixing a little of the solution in while grinding, then decanting into a cylinder. The cloudy suspension is added drop by drop to the test tube with mixing, until a turbidity matching the standard developed with sodium cobalti-nitrite is obtained. All test tubes used for standards here and elsewhere must naturally be the same type as those used for the test. The potassium dichromate-cobalt sulphate solution pre- pared above may be too dark in colour, and can
always be diluted with water to obtain the colour match, apart from the turbidity.
Turbidities are compared by holding the test tube rack down against diffused daylight, preferably against a dark background. Before using each d a y shake the turbidity standards, as the suspension very slowly settles. Standards prepared thus over a year ago are still in use.
5.—DETERMINATION OF AVAILABLE NITROGEN.
Analytical procedure.—In this procedure the two soluble forms of nitrogen, nitrate and ammoniacal nitrogen, are determined together by first reducing the nitrate with Devarda's alloy. It was found, ex- perimenting with a standard nitrate solution, t h a t the equivalent of 160 lbs. N per acre was reduced by the alloy in 40 minutes, when the tubes were stood in hot water.
Similar tests were made with a standard solution of ammonium chloride to ascertain if a n y ammonia was driven off, but no such loss was found to occur.
Take 2 ml. of the soil extract (after mixing) and place in a test tube similar to those used m all these tests. To save time, the nitrogen estimation is best commenced before the other tests. Now add 3 drops of 10 per cent, caustic soda and mix by tapping side- ways on the palm. Add a little powdered Devarda's alloy, the size of two large pin heads, and t a p to the bottom of the test tube. Support upright in a beaker of hot water just taken off the boil, and allow the action to proceed for 40 minutes. Cool in a beaker of cold water, and add 4 drops 100 per cent. tartaric acid solution, followed after mixing by 1 ml. mer- curic chloride-potassium iodide reagent. Mix again and add 2 ml. or thereabouts of 10 per cent. caustic soda and make to 7 ml. mark with water. Invert twice and compare the colours after standing about 4 minutes. Any continued evolution of hydrogen due to excess alloy does not interfere with the colour, which is very lasting.
Comparison with standards is best made in diffused daylight, using a ground glass screen in t h e com- parator rack. The separate addition of the reagents, as above, is preferred to the usual method in which Nessler's reagent is prepared beforehand. The forma- tion of turbidity is likely to take place on the addi- tion of caustic soda. This m a y be due to excess aluminium from the alloy, and also to too low a ratio of mercuric chloride-potassium iodide reagent to caustic soda. This turbidity, however, does not inter- fere with the colour comparison when a ground glass screen is used.
Preparation of standards.—If a 0.0005 per cent. N solution is prepared by dissolving 0.0191 gms. am¬
In developing the colour from the above, add three drops 10 per cent. caustic soda to render alkaline and proceed as with soil extracts, making to 7 ml. mark.
For the artificial standards to match, prepare a 10 per cent. cobalt sulphate solution and a 0.2 per cent. potassium dichromate solution. The following table gives the amounts of each required for the appropriate colour standards :—
When prepared, these standards should be either corked tightly or preferably sealed.
6.—RESULTS.
Before stating the results, the following table is offered as a provisional guide to interpreting the figures.
Longer experience may require certain modifica- tions to this table :—
Comparison with other methods.—A number of soils were analysed by the present method and two other well-known methods—the 1 per cent. citric monium chloride in 1 litre water, then 2 ml. will contain 0.00001 gm. N. If the colour developed in this quantity matches that in 2 ml. soil extract (0.2 gm. soil), then this will amount to 100 lbs. N per acre of two million pounds.
Using this solution, the following table can be arrived a t : —
The above results do show an approximate rela- tionship between the three methods.
Below are given the results for potash. The first three soils in the table had received dressings of acid method, and the Hawaiian rapid chemical method. In this work available phosphoric acid and potash were determined on citric acid extract ob- tained by shaking 50 gms. soil with 500 ml. 1 per cent. citric acid. Phosphoric acid was determined colorimetrically on the citric acid extracts, using the writer's technique.1 Potash was determined by the well-known sodium cobalti-nitrite method as stand- ardised by Milne,9 with a modified procedure recom- mended by the writer in 1934.2 The following are the results:—
filter cake, which would account for the unusually high potash content:—
Here again a rough similarity of results is obtained.
It must be pointed out, however, that it was not the aim of the present method to obtain any close rela- tionship of results with those of other methods.
Some comparisons were made between the present method and the Hawaiian rapid method for deter- mining total available nitrogen. As, however, an interval of time elapsed between the two series of tests on the bottled soils, the results were not con- sidered strictly comparable. The results obtained, however, showed that the total available nitrogen by both methods was in the same register. In most of the coastal soils examined so far total available nitrogen is highly deficient.
7.—CONCLUSORY REMARKS.
In the introduction to this paper, mention was made of the large number of rapid methods which have been developed during the last ten years. This in itself is an indication of the modern trend, and the necessity for having to find a method whereby a large number of samples can be tested in the shortest possible time, with a moderate degree of accuracy.
With the technique given, about forty tests can be made for each of the three nutrients in a day's work.
Many of the methods in the literature have not been fully examined, and it may be argued that it would have been better to examine one or more of them rather than add another to the list.
The writer, however, contends that his new tech- nique, while following certain features of other methods, has a number of advantages over them.
In no method known to the writer, for instance, are the three principal nutrients determined from a single extraction. The turbid potash standards also m a k e comparison easier, while the estimation of t o t a l soluble nitrogen in a single test is certainly an ad- vantage. Further, the present technique automatic- ally overcomes the problem of calcareous soils.
The final step is to correlate these chemical soil tests with field trials. Work has already been com- menced along these lines. As, however, a consider- able number of samples have to be tested before a n y conclusions can be made, this aspect of the s t u d y goes beyond the scope of the present paper.
8.—SUMMARY.
The subject of rapid chemical tests is briefly described.
A soil extracting solution, consisting of N/21 sul- phuric acid, buffered moderately with sodium borate at pH 1.50, is selected.
The soil extract is prepared by shaking 2.5 gms.
1 m.m. mesh air-dried soil for one half minute with 25 ml. extracting solution and filtering.
A description of the reagents required is given.
Available phosphoric acid is estimated colori¬
metrically by the phospho-molybdate method, a n d compared against a series of colour standards.
Available potash is estimated using the sodium cobalti-nitrite method and a new series of t u r b i d i t y standards.
Total available nitrogen is estimated by first r e - ducing the nitrates and then nesslerising.
Results are given comparing the present m e t h o d with the 1 per cent. citric acid and Hawaiian rapid chemical methods.
literature cited.
1 Beater, B. E. (1933) : "The Determination of Phosphorus by the Coeruleo-molybdate Reaction." Proc. S. Afr. Sugar Tech.
Assoc. 7, 87.
2 Beater, B. E. (1934) : "Proposed Modified Procedure for Soil Potash." J. Soc. Chem. Ind., 53, 712.
3 Bray, R. H. (1929) : "A Field Test for Available Phosphorus on Soils." Illin. Agr. Exp. Stn. Bulletin 337.
4 Bray, R. H. (1936): Mimeographed publication issued by Dept. Agron. University of Illinois.
5 Hance, Francis E. (1936) : "Soil and P l a n t Material Analyses by Rapid Chemical Methods." Hawaiian Planters' Record, 40, 189.
6 Hellige Soil Tester Bulletin 690 and other circulars.
7 Hester, J. B. (1936) : "Soil Testing Methods used in Vege- table Crop Production." Virginia Truck E x p . Stn. Bulletins 82, 83 and 84.
8 La Motte Chemical P r o d u c t s Company circulars.
9 Milne, G. (1929) : " T h e Cobalti-nitrite (Volumetric) Method of Estimating Potassium in Soil E x t r a c t s . " J. Agr. Sci., 19, 541.
10 Morgan, M. F. (1937) : " T h e Universal Soil. Testing Sys- t e m . " Conn. Agr. E x p . Stn. Bulletin 392.
11 Prince, A. L., a n d Blair, A. W. (1934) : " T h e B r a y Method for Available Potassium, applied to Soils of known Potassium Content." N. J. Agr. E x p . Stn. Circular 292.
1 2 P u r d u e University Agr. E x p . Stn., La F a y e t t e , Indiana.
13 Spurway, C. H. (1935) : "Soil Testing." Mich. Agr. E x p . Stn. Technical Bulletin 132 (revised).
14 Thomas, R. P. (1936) : " R a p i d Soil Testing in Maryland."
Mimeographed publication issued by Dept. of Agron., University of Maryland.
15 Thomas, R. P. (1936) : " T h e Use of Rapid Soil Tests in t h e United S t a t e s . " J. Amer. Soc. Agron. 28, 411.
1 6 Truog, E. (1930): " T h e Determination of t h e Readily Available Phosphorus of Soils." J. Amer. Soc. Agron. 22, 874.
E x p e r i m e n t Station,
South African Sugar Association, Mount Edgecombe.
March, 1941.
The P R E S I D E N T said t h a t Mr. Beater h a d pre- sented a very useful paper, and if the results could later be correlated with field trials it would be placed on a firm basis.
Mr. F O S T E R t h a n k e d the author for a v e r y in- structive paper. He agreed t h a t rapid methods h a d come to stay, b u t t h e y would not take the place of the old methods. The fact t h a t one extractor was proposed ought to simplify the tests a lot. He also considered t h e matching of turbidity solutions in determining potash a distinct advance over the Hawaiian method, which he found was affected by a varying source of light. His own experience in determining nitrogen according to the Hawaiian rapid method h a d been rather disappointing, but, like Mr. Beater, he h a d found them low in the majority of soils. He also agreed with the author t h a t there should be different standards of available plant-foods for each soil type.
Mr. DODDS said t h a t the paper was of funda- mental importance and represented a vast amount of work. He asked, in the instance where the author a t t r i b u t e d t h e high available potash in one soil to the addition of filter cake, whether he meant t h a t the filter cake h a d m a d e the soil potash more avail- able, or whether he h a d a direct contribution of potash in mind. It was generally found t h a t filter cake was very low in potash.
Mr. B E A T E R , in reply, said t h a t filter cake ren- dered soil phosphoric acid much more available. He agreed t h a t filter cake h a d very little potash in it, b u t thought it possible t h a t the filter cake might have t h e effect of increasing the available potash in the soil, in the same w a y t h a t it increased the