Soil Testing, Plant Analysis and Fertilizer Recommendations

After reading this article you will learn about soil testing, plant analysis and fertilizer recommendations.

Soil testing and plant analysis are useful tools for making recommendations for the application of fertilizers to crops. Whereas soil testing gives a measure of the availability of nutrients to crops, plant analysis indicates the actual removal of the nutrients from the soil.

In soil and plant analysis, the lower and higher critical limits become significant in relation to agricultural development of a country. At the lower production level, the lower critical limit becomes useful.

Soil testing is multipurpose in nature.

Soil testing aims at:

(i) Grouping soils into classes relative to the levels of nutrients for suggesting fertilizer practices,

(ii) Predicting the probability of getting profitable responses,

(iii) Helping to evaluate soil productivity, and

(iv) Determining specific soil conditions, like alkali, salinity and acidity which limits the crop yields.

Plant Analysis aims at:

(i) Diagnosing or confirming the diagnosis of visible symptoms,

(ii) Identifying hidden hunger,

(iii) Locating areas of incipient deficiencies,

(iv) Indicating whether the applied nutrients have entered the plant and,

(v) Indicating interactions or antagonisms among nutrient elements.

Principles of Soil Testing and Pre-Requisite for Successful Soil Testing Programme:

Soil testing started in one form or the other, as soon as man became interested in how plants grow. Soil testing may be defined in more restricted as well as in a broader sense. In a restricted sense soil testing may be defined as a rapid chemical analysis to assess the available nutrient status and reaction of a soil.

In a broader sense soil testing may be defined as inclusions of interpretations and evaluations of the soil test values and fertilizer recommendations based on the results of the chemical analysis and on other considerations.

There are 20 elements known to be essential for crop growth. Three of these elements— nitrogen, phosphorus and potassium are widely deficient in the soil. Soil pH also is a common limitation to plant growth. Soil testing, therefore, chiefly involves nitrogen, phosphorus, potassium and pH. Secondary and micro-nutrient analysis may be done on a regional basis.

Farmer’s acceptance of soil testing is strongly dependent on the extent and severity to which nitrogen, phosphorus, potassium and pH are problems for crop production in the area, and the accuracy with which the tests can be used to predict crop responses and fertilizer needs.

Since the late 1940’s soil testing has been widely accepted as an essential tool in formulating a sound fertilizer programme. But many misconceptions still seem to prevail as to just what soil tests can or cannot do. Some consider “soil testing” as little more than a “gimmick” and is used to promote fertilizer sales, provide personal prestige or influence administrative or legislative bodies.

Whereas, other consider it as an accurate and indispendable tool for the assessment of the fertility status of soils to form basis on which fertilizer needs are determined. The divergent views on this important aspect are, probably very often due to absence of a sound soil testing programme.

Soil testing is a programme that may be divided into four phases:

(i) Collecting the soil samples,

(ii) Extraction and determining the available nutrients,

(iii) Calibrating and interpreting the analytical results, and

(iv) Making the fertilizer recommendations.

The success of a soil testing programme depends upon the operative precision in all the four stages. Unfortunately, when errors are made at any stage of the programme the chemical analysis are the popular scapegoats. In India introduction of high yielding varieties, chemical fertilizers and soil testing service were contemporary.

The former two gained tremendous popularity, but soil testing is simple limping behind. One of the reasons of un-success of the soil testing service in India is lacking of adequate background research. A sound soil testing programme requires an enormous amount of background research.

The background research should determine:

(i) The significant chemical forms of the available nutrients in the soils of the area,

(ii) The extractants most suitable for accurately measuring the available form of nutrients,

(iii) The relative productive capacity of the soils for the various crops, the differential response of the various rates and methods of fertilizer application for the different crops,

(iv) Field sampling techniques, methodology etc.

The second most important reason for poor progress of soil testing programme is too much emphasis on the number of soil samples collected and analysed and very little attention to calibration and interpretation of the soil test values.

The chemical methods and the sampling techniques can be transferred easily from state to state or country to country, but the research background required for valid interpretations and sound judgements.

It is said that the success of a soil testing programme is directly proportional to its research backing basing which interpretations of the test values is responsible for the failure of soil testing service. Too often, especially in under developed countries, soil testing programme are started without an adequate research background.

As a result, soil testing programme often flounder. Another factor which is very important for the success of a soil testing programme is to consider the socio-economic and other local factors while making the fertilizer recommendations.

Thus, the following may be considered as pre-requisites for successful soil testing service:

1. Effective educational activities.

2. Uniform reporting system.

3. Improved sampling procedures.

4. Improved analytical procedures.

5. Better correlation and calibration of data.

6. Consistent interpretation and recommendations of fertilizers.

7. Timely delivery of the test results.

Principles and Methods of Soil Sampling and Processing:

It includes collection of soil sample from the fields and preparation of the sample in the laboratory for its employment in chemical analysis.

Collection of Soil Sample from the Field:

Suppose, there is a vast land. Would the soil sample be collected from the entire land? Or would it be collected from a part of the land (soil unit)? Soil is a heterogeneous body. The vast area of land generally shows heterogenity.

It is not possible to collect a soil sample which would be representative of the heterogeneous land. So, first of all the heterogenity of the land is minimised by dividing the land. By making division of the vast land, soil unit is determined. The heterogenity of the soil unit is minimum.

Determination of Soil Unit:

The soil unit is determined based on the following:

(i) Topography:

If the land is undulating the land is divided into high land, medium land or low land based on the level of the land. Because, the properties of these three categories would be variable.

(ii) Colour:

Sometimes, the land may show two distinct colour is e.g. light and dark colour. Dark colour of the land usually indicates high content of organic matter than that of light coloured portion of the land. There might be other differences in the properties of these two coloured land. Therefore, the land should be divided into light coloured land and dark coloured land.

(iii) Texture:

If the land shows various textures like clayey, loamy and sandy. The land should be divided into those textural classes. Because, the properties of these textural classes would differ.

(iv) Fertility status:

This could be indicated by the previous crop. Suppose, crop management practices of the previous crop (say rice) were similar throughout the land. Still, the crop yield of a portion of the land was less while that of other portion was more.

It indicates that the fertility status (status of the nutrient elements) of these two portions of the land were not similar. Under this circumstance, the land should be divided into two: one part which yielded more and the other part which yielded less.

(v) Management unit:

Suppose for cultivation of the previous crop, more fertilizers were applied to a portion of the land while to other portion of the land less fertilizer was applied. But, other cultural practices like, irrigation, weeding etc. followed was same.

In this case, the field should be divided into two portions; one fertilized more and the other fertilized less. Because, the residual fertility left by the fertilizers in two portions of the land would vary.

Collection of Composite Soil Sample:

After the soil unit is determined, the soil samples are to be collected from the soil unit. This soil sample would be representative of that soil unit. Soil is a heterogeneous body. So, if sample is collected from only one spot of the soil unit, it would not represent it. Suppose, available phosphorus (P) is to be determined.

If that P is determined in sample collected from only one spot that would not be the representative value of the soil unit. To have the representative value what can be done? The samples should be determined in each spot. Then, mean (average) value of these values should be calculated. The mean value of P would, then, represent the soil unit. But this process is most time consuming as well as then, represent the soil unit.

But this process is most time consuming as well as analysed separately. These samples should be well-mixed. The P value of that mixed sample should, then be determined. The P value of this mixed sample would be equivalent to the mean P value of the individual sample. This mixed sample includes the process of collection, drying, grinding, sieving, mixing and partitioning of the sample.

For collecting a composite soil sample, the following steps are required:

(i) Cores (when auger is used) or furrow slice (when spade is used) should have the same volume,

(ii) Cores or furrow slice should be taken at random if the previous crops were grown broadcast or in a zigzag way if the previous crops were grown in rows. The cores or furrow-slice should not be positional regularly, on the rows or on the crop hills,

(iii) Enough cores should be taken.

(iv) There should be no chemical interactions of soil material composited.

(v) In practices, usually 8-10 cores are taken but this number may increase to 20-30. Increasing the number of cores decreases variations in soil characteristics.

Samples from unusual or abnormal spots of the soil unit should not be collected.

These spots may be:

1. Near gates, buildings, highways etc. and along field boundaries, margins, etc. In these sites, normal crop management practices cannot be followed.

2. Under stacks of manure. If the fertilizers or manures were stacked on a site of the field for their application to the crops, this would enrich that site with nutrient elements which would be higher than the rest of the field.

3. Shaded area.

4. Crop hills and rows.

5. Local abnormal sites (e.g. acidic or alkaline pocket etc.).

Preparation of the Soil Sample in the Laboratory:

It involves the following steps:

(i) Drying,

(ii) Grinding,

(iii) Sieving,

(iv) Mixing,

(v) Partitioning,

(vi) Weighing and

(vii) Storing.

(i) Drying:

Soil samples are dried for the soil chemical reactions when dried samples are more nearly at equilibrium. The soil samples are air dried in shade at room temperature.

Due to large and rapid changes that take place in the status of some ionic species on drying, many types of analysis must be carried out on moist samples immediately after collection. Examples are determinations of exchangeable ferrous iron, pH, exchangeable K, etc.

(ii) Grinding:

By grinding the soil aggregates are broken up roller, rubbed pestle in an agate morter, motorized grinder, wooden morter etc. may be used for grinding. Wooden morter is the best for avoiding contamination of other elements from the grinder itself. Crushing primary sand and gravel particles is avoided.

(iii) Sieving:

There are generally two types of sieves (20 and 80 mesh) which are used for sieving. The sieve should be made of brass or nylon. For micro-nutrient analysis nylon sieve is preferred. The sieve should possess round hold. For organic carbon determination and elemental analysis fine sieve is used. For determination of pH, exchangeable cations etc. coarse sieve is used.

Stones or gravels (coarser that 2 mm) remaining on the screen are ignored and discarded. The “fine earth” sieved is analysed. But for stony field (containing 2 per cent of stones or more), the “fine earth” is analysed. Necessary correlation may then be made to express the results to refer to plough-layer volume. Though granular secondary particles are disaggregated and passed through sieve.

This may be done by trituration in water. Entire partitioned sample is passed through the sieve. For silty and clayey soils sieving a portion may be justified assuming that un-sifted aggregated material is the same as that which has passed the sieve.

For other soils, this practice should not be done. Because, sieving only a portion of the gross sample and discarding the remainder increases the concentration of elements in the sample.

(iv) Mixing:

Soil samples are spreaded over a cloth or paper. Opposite corner of the

cloth or paper are grasped and one is pulled diagonally across the sample slowly so that the soil rolls over but not slides) toward the opposite corner. Then the opposite corner of the cloth or paper is pulled back over the soil to roll it back. The process is repeated by grasping the other two opposite corners. Rolling on opposite diagonals is repeated at least 5-10 times.

(v) Partition of sample:

The soil samples may be partitioned by the following processes:

(a) Riffle Technique:

Riffle has a series of narrow slots. Alternate slots deliver soil material to opposite sides.

(b) Quartering:

The soil sample is coned in the centre of the mixing sheet (polythene sheet or paper etc.) Care should be taken for making symmetry of fine and coarse soil material. The cone is flattened and divided through the centre with a flat wooden sheet. One- half is moved to the side quantitatively.

Then each half is further divided into half, the four quarters being separated into separate “quarters”. Two diagonally “opposite quarters” are discarded quantitatively. The other two are mixed by rolling. This process is repeated, until 250-500 g soil material is obtained.

(c) Paper Quartering Technique:

For small samples this technique may be employed. Four strips of paper are woven together. Soil sample is coned in the centre. Pulling the strips apart results in accurate quartering.

(vi) Weighing:

Find textured soil (e.g. clayey) which does not tend to segregate is weighed as usual i.e. a portion of the sample is taken with a spatula or clean fresh paper strip or spoon and weighted on a torsion of analytical balance having sensitivity to 0.1-0.5 per cent of the sample weight.

Camel’s hair brush is used for complete transfer of the weighed soil material. Coarse textured soil samples which tend to segregate should be taken by partitioning to approximately the desired weight and then the entire portion should be weighted accurately.

(vii) Storage:

Storage Soil samples may be stored in series of cardboard cups in a tray. Placing the samples in screw-cap jars is most satisfactory.

Sampling for Field Crops:

Samples are taken as follows:

(i) Make a traverse over the soil unit,

(ii) Take core or furrow silca of the polugh layer (0-6″ or 9″) at intervals of 15 to 20 steps cores the obtained inserting auger or sampling tube, when spade or khurpi is used, furrow slice is obtained. In this case, dig a ‘V-shaped’ hole and cut 1.5 cm thick slice of soil from top to bottom of the exposed face.

Sampling for Local Problem Spots:

Samples are taken as follows:

(a) Areas of high calcium carbonate content,

(b) Exposed sub-soil,

(c) Extremely acid soil,

(d) Saline area,

(e) Alkaline or sodic area,

(f) Areas underlying hard pan.

Soil samples from these local problems spots are collected as follows: Local problem spots should be treated as separate soil unit and separate composite sample should be collocated. Both surface soil sample and sub-soil sample are collected.

1. For collecting surface soil samples take 10 to 30 cores two metres or more apart. Each core should extend through A₁ horizon.

2. For collecting sub-soil samples, dig a pit of one metre depth. Take samples beneath at horizon from different depth e.g. 0-15 cm, 15-30 cm, 30-60 cm, 60-100 cm. Take the soil slice as described earlier.

Basic Principles of Soil Test Recommendations:

Recommendation of Fertilizers:

For preparing the fertilizer recommendation norms for each class of soil test values, (low, medium, high) crop response curves are to be prepared. This may be prepared for a particular situation or may be the average of a number of experiments in which climate, soil and the operator varied.

Soil testing interpretation involves economics because it is used to make a fertilizer recommendation for an economic goal i.e. usually for maximum profit per hectare of land. This involves the cost of the fertilizer and price of the produce. From the response curve the actual amount of fertilizers can be calculated to give the maximum profit per hectare.

In the absence of crop response curve, the fertilizer needed for maximum yield can be evaluated with maximum field experiments. For the soil with maximum soil test values and soil in which practically no fertilizer response is obtained should be included.

The fertilizer need for maximum yield at different fertility index is determined. This will give a linear relationship. The curve may be used as a base line for recommendation of fertilizer at different soil test values or fertility index. As a thumb rule two-third of the amount needed for maximum yield is considered as the most economic rate. (Fig. 25.2).

Including the soil test values the most economic rate of fertilizer will depend upon many other factors whose interpretation mainly depend upon personal judgements.

The factors are appended below:

1. For recommending fertilizer dose, the following points should be considered:

(a) Initial fertility status of soil

(b) Additional produce and the price of the additional produce

(c) Cost of fertilizer

(d) Farm management quality

(e) Productive potentiality of the land

(f) Possibility of risk involved

(g) Availability of the fertilizer

(h) Crops to be grown

2. For selection of the fertilizer, the following points should be taken into consideration:

(i) Soil characteristics

(ii) Crops to be grown

3. For methods of fertilizer application, the points to be considered are:

(i) Soil characteristics

(ii) Other factors, like crops, type of fertilizers etc.

4. For time of fertilizer application, the following points are to be considered:

(i) Soil texture.

(ii) Other factor, like crops, kind of fertilizers.

Calibration and Interpretation of Soil Test Values for Fertilizers Recommendations:

A method for determining the nutrient supplying power of soils is considered to be suitable when the amount of nutrients extracted by the reagent is proportional to the amount absorbed by a crop. Chemical values obtained by extraction have no absolute meaning.

It neither furnishes any information about the nutrient supply available to the root system of plants, nor does it indicate anything about the yield expected at that fertility level. But it can be meaningful when the soil test values can speak in the language of the crops. Thus they can be given meaning by calibration of the test results against crop response.

The process of determining the crop soil relationship is commonly referred to as calibration of chemical soil test values. A calibrated soil test value for a particular nutrient will indicate the degree of deficiency of that nutrient, expected per cent increase on fertilizer to correct the deficiency.

The present system of calibrating the soil values as low, medium and high has a handicap for varying interpretation of such terms by different research workers. For its use cooperative efforts among the workers of different parts of the country is necessary. To calibrate test values they should be expressed in terms of relative yield or percentage at any soil test value may be termed a fertility index.

Where to conduct Experiments?

For calibrating soil test values by crop response, the plant should be grown in the same manner as they would grow in the field. Therefore, studies based on growth of seedlings, plants, plant growth under artificial conditions, as in pots, are not valid basis for calibration of soil test values. Because this will not count the full life cycle of the plant, the sub-soil nutrition etc.

How to do it?

The percentage yield or the percentage sufficiency of a particular soil test value with respect to a nutrient can be determined by growing crop in the field with different levels of the nutrient.

The maximum yield beyond which no response to nutrient application is obtained is taken as 100 and the yield of unfertilized plot expressed as percentage of the maximum yield is the relative yield percentage or percentage sufficiency of the nutrient.

This will be the basic principle of calibrating the soil test values in terms of percentage sufficiency. As the crop responses to soil test values are influenced by many other factors like soil type, crop species, climate, farm management etc. these differences should be taken into consideration.

But unfortunately these are not taken into consideration in our country. One easier way of accounting these effects is to categories the calibration. Any factor that changes the response relationship can be the basis for grouping data into calibration category. Usually soil characteristics and crop species are considered for such calibration categories.

Because of the wide range in soil characteristics and many crops for which recommendations are to be made, exhaustive research under all conditions is impractical. Therefore both soils and crops must be combined into manageable groups based on data available.

By field experiments the various soils test values may be calibrated in terms of percentage sufficiency level or fertility index in different soil types for various crops for which fertiliser recommendations are needed. The data available through these experimental will not only calibrate the test values, but also help in grouping the soils and the crops. This is possible by inserting the values in scattered diagram.

It there are four soil types, A, B, C, D and the calibrated soil test values show the following pattern:

According to the distribution it is clear that the soils A, B and C, D may be grouped into two categories for all purposes in calibrating the chemical test values and fertilizer recommendations. In the same way the crops also may be grouped for all practical purposes.

As for example, wheat, paddy, oat may be put in one group and maize, jowar, sugarcane may be put in another group on the basis of their behaviour towards Soil test values. Three categories with respect to soil may show following types of responses to soil test values (Fig. 25.4).

Once the test values are rated in terms of fertility index, the values of each category are put to a number of cells like low, medium, high or very low, medium, high, very high for operational convenience. The basis of such partitioning may vary from state to state and country to country.

Soil Test Crop Response and Targeted Yield Concept:

Due to adoption of multiple cropping and introducing of high yielding varieties of principal crops in our country, soils are depleted in nutrients at a much faster rate than in the case of old cropping system. As a result, crop production has become highly fertilizer oriented.

As costs of fertilizers are very high in our country, judicious uses of fertilizers are required. Soil testing is one of the accepted methods for the economic use of fertilizer but there are many problems in making fertilizer recommendations based only on soil test values.

Recently in our country systems of soil test ratings are being modified incorporating crop response data available from systematic field experiments.

It has been accepted by the scientists that any soil test method intended for use in advisory work needs to be correlated with actual crop response obtained under field conditions, and the success of the fertilizer recommendations programme will depend on the accuracy of the calibrations obtained this way. Modern approaches of soil fertility evaluation are mainly focused towards increasing fertilizer use efficiency.

The approaches may be as follows:

1. Soil analysis and correlation,

2. Critical soil test level approach,

3. Agronomic approach,

4. Soil fertility cum soil survey,

5. Inductive approach based on soil test and crop response correlation,

6. Deductive approach based on soil test crop response correlation, and

7. Targeted yield concept approach.

The purpose of different approaches is to utilize soil and fertilizer nutrients judiciously and effectively in a manner best suited to different agro-climatic concept approach is being discussed.

From the soil test, crop response field experiments, it has been possible to derive three basic parameters like:

(i) Nutrient requirement in kg per quintal of the produce,

(ii) Percentage contribution from soil available nutrients and

(iii) Percentage contribution from added fertilizers towards making effective fertilizer prescriptions for specific yields.

The parameters have been calculated as follows:

(i) Nutrient requirement in kg for producing one quintal of grain

= Total uptake of nutrient (Grain + Straw) in kg/ha/Yield of grain (q/ha)

(ii) Contribution from soil (in per cent) (cs)

= Total uptake of nutrient in control plots (kg/ha)/Available soil test values of control plots (kg/ha) × 100

(iii) Contribution from fertilizer (cf) (in per cent)

= Total uptake of nutrient in treated plot – (available soil test value of nutrient in treated plot ×cs)

(iv) Contribution percentage from fertilizer = cf/Fertilizer dose applied in kg/ha × 100

Basic data have been derived from the field experiment conducted under the project of Soil Test Crop Response Correlation in the district of Burdwan, West Bengal on Vindhya alluvium soil using paddy (Cv. Ratna) during kharif season which are as follows:

From the above basic data soil test calibrations are given in the form a ready reckoner to recommend the fertilizer doses for obtaining specific yield targets.

Based on such scientific information, soil test results can be made useful for making fertilizer recommendations for different agro-climatic regions.

Plant Analysis:

Soil tests, although useful in predicting fertilizer and soil amendment needs, are not the final measure of what nutrients a plant will absorb. Because temperature, moisture regimes, soil acidity and other soil conditions may modify the uptake of different nutrients by plants, it is sometimes necessary to determine nutrient contents in the plants to evaluate the actual soil nutrient availability status.

Total plant analysis and green tissue analysis the most important diagnostic techniques for determining deficient, sufficient or excessive amounts of essential elements in plant tissue. A combination of methods that involve soil tests, plant tissue tests, and plant nutrient deficiency symptoms may be necessary to accurately diagnose a plant growth problem.

Green tissue tests differ from the total plant analyses in that green tissue tests determine semi-quantitatively the concentration of soluble nutrient elements in the plant sap. Green tissue tests can be performed rapidly and therefore, it is designated as quick tissue tests.

Plant analysis, therefore, constitutes two aspects—quick tissue tests and total plant analysis. They are best to check whether a fertilizer schedule recommended proved to be all right. Plant analysis in soil fertility evaluation is limited to perennials and plantation crops and not to quick growing annual crops.

Coupling soil testing with plant analysis, however may lead to a correct diagnosis and treatment of nutrient requirements of crops. Plant analysis is still qualitative in approach. The present trend for the evaluation of soil fertility is to go in for the quantitative approach rather than being qualitative one.

Interpreting Plant Analysis:

The success of plant analyses as a diagnostic technique depends upon the accurate and practical interpretation of test results. Plant analysis nutrient evaluation forms can be obtained from a testing laboratory that performs such analyses or from a fertilizer dealer.

These forms should be completed and submitted with the plant samples to ensure accurate interpretation of the test results. The forms include items such as variety of crops, density of plant, geographic location, date of planting etc.

The following five categories are generally used for interpreting plant tissue analysis:

1. Deficient:

Plants clearly show visible symptoms of nutritional deficiency.

2. Low:

Plants are normal in appearance but will likely respond to an application of the low testing elements.

3. Sufficient:

Plants are normal in appearance and have sufficient concentrations of the elements for good growth and maximum yield.

4. High:

Plants appear normal with anticipated optimum yields, but concentration of the particular element is higher than normal.

5. Excessive:

Plants may appear normal or manifest symptoms of nutritional disorder. Yields may be reduced. The critical value only classifies the plant analysis into two groups, i.e. sufficiency (above the critical value) and deficiency (below the critical value). The concentration versus growth per unit of time as percentage is plotted to obtain the critical value as shown in Fig. 25.5.

Critical limits for rice (based on the analysis of the third leaf) are given below:

N – 1.40%

P – 0.05%

K – 1.08%

Cu – 5.0 ppm

Mn – 80.0 ppm

Zn – 10.0 ppm

Fe – 63.0 ppm

Modern Analytical Method:

The electron microprobe X-ray analyzer is used to study the nutrient contents of plants. The microprobe is essentially a combination of an electron microscope and spectrophotometer and is capable of detecting all elements excepting hydrogen, helium, lithium and beryllium at spectacularly small concentrations.

Electron “light waves” from the microprobe hit and excite the K, Ca and Mg molecules which give of energy in the forms of X-rays. These X-rays have characteristic wavelengths for each element. By detecting only those X-rays of a given wavelength, such as those from K, the presence as well as amount of the K can be determined.

Green Tissue Analysis:

Plants growing under sufficiently fertile conditions absorb more nutrients than they can assimilate at any-one time, thus holding a surplus of nutrients in the sap of the tissue. Hidden hunger can be better recognised by the total plant analysis than green tissue analysis.

Sampling:

Three major nutrients N, P, K move from old to new tissue where deficiency symptoms are found. So plant sampling should be done both the old and the new tissue, but a test of the old tissue is sufficient. For the evaluation of the availability of plant nutrients, it is best to sample the tissue at regular intervals. If only one test is to be made, it is best to make the test when the plant is under severe nutrient stress.

Analytical Methods:

Two main methods are usually employed for the analysis of green plants tissues for N, P and K. Paper test and Glass vial test. The materials and equipment required for making tests for N, P and K by the Paper Test method are K-test papers (papers containing three spots of different amounts of dipicrylamine); nitrate powder, a sharp knife and needle nosed pliers.

The NO₃—N test is made by placing the cut portion of green plant tissue (stem or petiole) on a clear portion of the folded test paper, adding NO₃-powder to the tissue, and squeezing the paper and tissue together with pliers.

The colour of the powder turns into shades of red if NO₃-N is present, whereas a faint pink colour indicates a low or deficient level of NO₃-N. A complete red colour of the powder indicates a high test or sufficient nitrogen.

The phosphate-phosphorus test can be made readily by squeezing sap from freshly cut plant tissue on the paper strips. An adequate supply of P is indicated by a medium blue to dark blue colour. A light blue colour denotes P deficiency in the plant sap.

For the K-tests, the plant sap is squeezed on the three test spots containing dipicrylamine and Regent No. 1 is added to give an orange colour present. The intensity in the brightness of colour determines the amount of K present.

The disappearance of orange colour from all three spots on the test paper indicates a very low test, orange in the middle and bottom spots indicates a medium level and when the orange colour persists in all three spots, a high level of K is indicated.

Interpretation:

As with soil analysis and total plant analysis, an exact interpretation of green tissue tests depends upon sufficient soil cropping information.

The various factors like soil pH, amount of P and K, drainage, moisture regimes, fertilizer management etc. of which anyone is limiting for plant growth, the tissue test perhaps will not reflect the nutrient status. However, the methods of interpretation have not been developing so fast.