Soil Fertility EvaluationSoil Fertility Evaluation

Soil Fertility: it is the potential of the earth or inherentinherent capacity of the soilcapacity of the soil to supply plant nutrients in quantity, forms and proportion required for the growth and development of the crop.

Fertility is measured by the amount of chemical elements or compounds required for plant growth

Productivity of a soil is defined as its capacitycapacity to produce plants under specified programme of management.

It is measured by the yield of the crop per unit area of the land

Fertility is one of the factors of soil productivity. Sometimes a soil may be fertile but may not be productive.

Liebig’s Law of Minimum- The growth or yield of a crop is limited by that factor which is present in relatively least amount.

Eg. N P K Requirement 100 50 60 Amount available 40 25 30

40% 50% 50%So, here N is the factor which limits the crop growth.

Justus von Liebig, 1840

Father of modern Agricultural ChemistryFather of modern Agricultural Chemistry

Liebig was probably the first to express the yield as a mathematical function of the given growth factor when all the other factors kept constant

y = Ax - B A,B, = constant

“Just as the capacity of the wooden bucket to hold water is determined by theheight of the shortest stave, crop yields are restricted by the nutrient in shortestsupply!”

von Liebig

1803 -1873

NN PP KK NN PP KK

NN PP KK MgMg SS NN PP KK MgMg SS

Liebig’s law of minimumLiebig’s law of minimum

?

dy/dx = (A-y)C

(by integration) y = A (1-10-Cx)

or, log (A-y) = log A – Cx

“The increase in yield by a unit increment of the deficient factor is proportional to the decrement of that factor from the maximum.”

.

Law of diminishing return :

where increases in yield of a crop per unit of available nutrient decreases as the level of available nutrient approaches sufficiency.

Yield increases (dy) per unit of available nutrient (dx) decrease as the current yield (y) approaches a maximum yield (A) with C being a proportionality constant

Immobile nutrients follow (P, K,and Ca in soil) follow Mitscerlich’s concept

Mitscherlich’s Equation

Cdxy)-A

dy

or, - log (A-y) = Cx +CIf x=0, y=0 , C = - log A

or, - log (A-y) = Cx –log A

or, log (A-y) = log A- Cx

dy/dx = (A-y)C

Cx

AyA

log or,

Cx10

AyA

or,

Cx10 Ay- Aor,

)

Cx10 -A(1y or,

Soil Fertility EvaluationSoil Fertility Evaluation:

Several techniques are commonly employed to assess the fertility status of a soil

(i)(i)Nutrient deficiency symptoms Nutrient deficiency symptoms of plantsof plants

(ii)(ii)Plant analysis and tissue testingPlant analysis and tissue testing

(iii)(iii) Methods involving the Methods involving the growing of higher growing of higher plants and microorganism plants and microorganism

(iv)(iv) Soil Chemical analysisSoil Chemical analysis

(v)(v) Isotopic dilution methodIsotopic dilution method

(i)(i) Nutrient deficiency symptoms(NDSNutrient deficiency symptoms(NDS) of plants; it may be detected as

complete crop failure at seedling stage, severe stunting of plants, specific leaf symptoms, internal abnormalities, abnormal maturity etc. from the visual sysmptoms.

Careful observation of growing plant may help identify specific nutrient stress

Disadvantages:

(i) Visual symptoms could be caused by more than 1 nutrient, or any of several nutrients

(ii) Could be related to toxicity or imbalance of another nutrient.

(iii) It is difficult to distinguish among deficiency symptoms. NDS is sometimes confused with attack of pests and diseases.

Alfalfa: confusion of leaf hopper damage with Boron deficiency

Corn: Sugar accumulation may be due to insufficient supply of P, cool nights and warm days, N-deficiency, transverse creasing of the leaves

1. Many symptoms appear similar. For instance, N and S deficiency symptoms can be very alike, depending upon

plant growth stage and severity of deficiencies. 2. Multiple deficiencies and/or toxicities can occur at the same time.

More than one deficiency or toxicity can produce symptoms, or possibly an abundance of one nutrient can induce the deficiency of another (e.g. excessive P causing Zn deficiency).

3. Crop species, and even some cultivars of the same species, differ in their ability to adapt to nutrient deficiencies and toxicities. For example, corn is typically more sensitive to a Zn deficiency than barley and will show Zn deficiency more clearly (NM 7).

4. Pseudo (false) deficiency symptoms (visual symptoms appearing similar to nutrient deficiency symptoms). Potential factors causing pseudo deficiency include, but are not limited to, disease, drought, excess water, genetic abnormalities, herbicide and pesticide residues, insects, and soil compaction.

5. Hidden hunger. Plants may be nutrient deficient without showing visual clues.

6. Field symptoms appear different than ‘ideal’ symptoms. deficiency/toxicity symptoms observed in the field may or may not appear as they do here. Experience and knowledge of field history are excellent aids in determining causes for nutrient stress.

Hidden hunger is a term used to describe a plant that shows no obvious deficiency symptoms, yet the nutrient content is not sufficient to give the top profitable yield.

Fertilization with sure rate rather than the bare economic optimum for an average leaf helps to obtain the top profitable yield.

Detecting hidden hunger in crops is an increasing problem as yield goals rise and higher profits are sought. In this zone with no symptoms to guide us, we must turn to more diagnostic chemistry to evaluate needs more accurately. Testing of plants and soils is helpful for planning or modifying plant nutrient programmes to avoid this problem in subsequent crops.

(ii) Plant anlysis and tissue testingPlant anlysis and tissue testingThis technique is based on the concept that if the content of a particular nutrient in the plant is greater the higher its availability in the soil (Lundegardh,1945)

Tissue testing:Only unassimilated portion is measured.(i) In one test, the plant parts are chopped up and extracted with reagents. The

intensity of colour developed is compared with standards and used as a measure of the supply of nutrient in question.(ii) Plant is transferred to filter paper by squeezing the plant tissue with pliers.

The tests for N,P,K are made with various reagents.

Plant analysis:Both assimilated and unassimilated element are measured. Plant

grown in the soil is ashed and the different nutrient elements are estimated. Thre is a basic relationship between the content of a plant nutrient and the growth or yield of the plant

In contrast to soil analysis, tissue analysis reflects nutrient uptake conditions of the soil.

(1)Plant Part to be Selected: In general the conductive tissue of the latest mature leaf is

used for testing.

(2) Time of Testing: The most critical stage of growth for tissue testing is at the time of bloom or from bloom to early fruiting stage. Nitrates are usually higher in the morning than in the afternoon if the supply is short.

Test for

Nitrates ……….. DiphenylaminePhosphates ……. Molybdate + Stannous oxalate testPotassium …… Sodium cobalti nitrate

Tissue Tests:

Critical nutrient concentration ranges (sufficiency ranges)

Using Plant Analysis as a Diagnostic Tool

DRIS (Diagnostic & Recommendation Integrated System)

Crop logging:

Tissue Test Interpretation

CNC (Critical Nutrient Concentration):CNC (Critical Nutrient Concentration):Concentration that is just adequate for maximum growth or the level of a nutrient below which crop yield, quality is unsatisfactory.Havlin et al., 1999

Relationship between the nutrient content in the soil solution and the nutrient content of the plant (Mengel and Kirkby,1987Mengel and Kirkby,1987)

The relationship between nutrient content and nutrient availability in the soil generally follows an asymptotic curve. This means that above a critical nutrient level in the plant only small changes in plant nutrient content may occur despite marked increases in nutrient availability in the soil. From this it follows that leaf or tissue analyses are particularly useful in the range of low nutrient availability. In the higher range of availability, however, leaf analysis is not sensitive enough. Here soil analysis is more appropriate.

Provisional DRIS chart for obtaining the qualitative order of requirement for NPK in sugarcane.

Luxury consumption occurs when soil nutrient levels are above optimum and plants take up more of a nutrient than needed for functioning and production. K is commonly taken up in excess.

Crop logging: It is a graphic representation of the progress of

the crop contain a series of chemical and physical measurements.

These measurements indicate the general condition of the plant and suggest changes in the management that are necessary to produce maximum yield.

A critical nutrient concentration approach is used in the crop log system and nutrient concentrations in leaf sheaths 3,4,5 and 6 are utilised for diagnosis of Ca, Mg, S and micronutrient deficiencies. (Sugarcane)

During the growing season plant tissue is sampled every 35 days and analysed for N, sugar, moisture and weight of the young sheath tisue. Analyses are made for P and K at critical times, and adjustments in management practices introduced as needed

(Clements,1960)

Clements Clements (1960)(1960)

Max-Min Temp.

Growth

Nitrogen

Moisture

Total Sugars

K2O

P index

Completed crop log for an irrigated Hawaiian Plantation.

(iii) Methods involving growing of higher plants and Methods involving growing of higher plants and microorganismsmicroorganisms

(a) Plants: (i) Mitscherlich Pot culture method (ii) Neubauer Seedling method

(b) Microorganisms:(i) Azotobacter palque method

(ii) Mehlich’s technique for available K2O by Aspergillus niger method (iii) Mehlich’s Cunninghamella–Plaque method for P

a) (ii) Neubauer Seedling method Neubauer Seedling method (Neubauer and Schneider,1932)

In this technique, 100 seedlings of rye are made to feed exhaustively on 100 g of soil mixed with 50 g sand (Nutrient free quartz) for 17 days in petridishes of (11 cmx7cm). A blank without any soil is also run.

The total P2O5 and K2O uptake is calculated, and the blank value is deducted to obtain the root soulble P2O5 and K2O in 100 g of air dry soil.

These values are designated as the Neubauer numbers expressed as mg/100 g of dry soil. K.....20 mg/100g soil P.....3 mg/100g soils are regarded as satisfactory levels.

It is based on the principle of intensive uptake of nutrient elements by growing a large no. of seedlings on a small quantity of soil

(b) (ii) Mehlich’s technique for available K2O by Aspergillus niger method

Weight of Four pads ( g)

K absorbed by Aspergillus niger per 100 g soils (mg)

Degree of potassium deficiency

<1.4 <12.5 Very deficient

1.4-2.0 12.5-16.6 Moderate –slight deficient

>2.0 >16.6 Not deficient

Critical limits for available K by using Aspergillus niger

To determine Potassium small amounts of soil are incubated for a period of 4 days in flasks containing appropriate solns. The weight of the mycelial pad or the amount of potassium adsorbed by these pads is used as a measure of the nutrient deficiency.

(iii) Mehlich’s Cunninghamella–Plaque method for P The organism Cunninghamella is sensitive to the phosphorus status of the growing medium. The soil (50 g) is mixed with the nutrient soln, a paste is made, spread uniformly in the well of a specially constructed clay dish, inoculated on the surface of the paste and allowed to incubate for 4½ days at 28-29°C.

Normally Cunninghamella olegans is used for the test (22 mm diameter size is adequate).

In Calcareous soil Cunninghamella blakesleana is used (diameter 16 mm is adequate)

4. Soil Chemical analysis4. Soil Chemical analysis

Information gained from soil testing is used in many ways:

To build and /or fertility status of a given field

To predict the probability of obtaining a profitable response to lime and fertilizer

To provide a basis for recommendations on the amount of lime and fertilizer to apply

To evaluate the fertility status of soils on a country, soil area or state-wide basis by the use of soil test summaries.

Objectives of soil testing4. Soil Chemical analysis4. Soil Chemical analysis

Soil testing starts with collecting a good sample

Soil testing is not useful without meaningful samples

Soil Testing basics

Calibration and InterpretationPerhaps the greatest challenge in a soil testing program is calibration of the tests. It is essential that the results of soil tests be calibrated against crop responses from applications of the plant nutrients in question.

Calibration: It is the process of determining the relationship between the crops and soils i.e., the correlation of soil test values with the crop response. From the calibrated soil test values it is possible to predict the extra yield that will be obtained from the addition of extra amount of fertilizer and that the expected yield at that fertility status of the soil. It will also can be predict the amount of fertilizer to be added to obtain an optimum yield. Soil test values should be calibrated in each soil and for each crop.

Lack of calibration of each soil test values is one of the most important reasons as to why soil testing is not so popular.

Two methods of approach in Soil test Calibration. (i)Soil analysis-correlation approach(ii)Critical soil test level approach

Yield response to fertilizer in relation to soil test value (points represent individual soils tested).

Percentage yield = —————————————————————— x 100(Crop yield with adequate nutrients- Yield of control)

Crop yield with adequate nutrients

The most common method is to plot soil test values against percentage yield and to calculate the correlation coefficient between soil test values and percent yield response

However, if the correlation coefficient obtained from a large no. of Experiments is statistically significant, it is acceptable as a guide for the preparation of a fertilizer schedule.

Based on the contents of available nutrients, soil test values(N,P,K), the soils are grouped into classes such as low, medium and high.In general, the greatest response can be obtained from the low class and the least

response from the high class in soil test values.

Interpretation of soil test Values:The interpretation of soil Test values involves determining how much of a particular nutrient will be needed throughout the growing season to provide a sufficient supply of this element to the plant for a predicted yield. The lower the soil test value for a particular nutrient, the higher is the response to the fertilizer nutrient..

1. High - Soil Test Value where probability of response to additional fertilizer is small(10%).

2. Medium - Soil Test Value where probability of response to additional fertilizer is moderate(50%).

3. Low - Soil Test Value where probability of response to additional fertilizer is good (90%).

Max. Profit

Max. Profit

Max. Profit

pHw (1:2.5) Acidic Neutral Alkaline

< 6.5 6.5 - 7.5 > 7.5

EC(dSm-1) Normal Critical Injurious

< 1.0 1.0 - 3.0 > 3.0

ParametersParameters Low Medium HighOrg. Carbon < 0.5 0.5 - 0.75 >0.75Avail N (kg/ha) < 280 280 - 560 > 560 Avail P (kg/ha) < 22 22 - 45 > 45Avail K (kg/ha) < 120 120 - 280 > 280Avail. S (SO4

-2) g g-1 0-10 10-15 >15Critical limit for Micro Nu (g g-1 in soil )(rice) (DTPA extract)

Fe2.0

Mn1.0

Zn0.86

Cu0.20

Boron(g g-1 in soil )(HWS)

Deficiency

< 0.50 Toxicity

> 4.00

Rating Chart for soil test values

Critical Levels developed by Cate and Nelson (1965)

% yield versus soil test level

Two Groups:1. probability of response to added fertilizer is small2. probability of response to added fertilizer is large

Step A.: Calculate Percentage yield values obtained for a wide range in locations

Percentage yield = ——————————————————————--------- x 100

Step B. Soil test values obtained (Check Plot)

Will generate a single % yield and one soil test value for each location

Step C. Scatter diagram, % yield (Y axis) versus soil test level (x axis) should plot

Range in Y = 0 to 100%

Step D. Overlay

(i) overlay moved to the point where data in the +/+ quadrants are at a maximum

(ii) point where vertical line crosses the x = critical soil test level

(Crop yield with adequate nutrients- Yield of control)

Crop yield with adequate nutrients

0

20

40

60

80

100

120

0 20 40 60 80 100 120 140 160

Critical Level

Perc

enta

ge Y

ield

Soil Analysis, ppm P

(i) the soils collected from each field are analysed,(ii) field experiments are conducted with the application of graded dose of fertilizers,(iii) response curves are fitted.(iv) A scattered diagram of percentage yield (y-axis) vs soil test value (x-axis)is then plotted.(v)It is divided into four quadrants. (vi)The point where the vertical line parallel to the y-axis crosses the x-axis is defined as the critical soil test value.

Critical soil test level (Cate and Nelson) is the level of the nutrient below which a reasonably satisfactory economic response should be expected from the application of that particular nutrient and above which the probability of such response is low.

Fried and Dean (1952)

Assuming that plants take up nutrients from two different sources in direct proportion to the amount available, the A-value was developed as the expression

A = B(1-y)/y

where; A = amount of available nutrient in the soilB = amount of fertilizer nutrient (standard) appliedy = proportion of nutrient in the plant derived from the standard

“Lower A values = Higher P Availability”

For specific soil, crop and growing conditions:

A-value is constantindependent of rate of fertilizer applicationindependent of size of test pot and growth rate

A value developed to determine availability of P in soil

(P supplying power of a given soil).

Fried and Dean(1952) used the principles of Isotope dilution to evaluate the experimentally the availability of soil P to the plants. The method was based on the principle that a plant confronted with two source of nutrient would utilise them in direct proportion to their availability. It is to derive an equation for for A-value.

Let the two sources be A and B, where A= soil P, B= fertilizer P, added to the soil as a standard

Let the respective amount of P in the plant from these two source “A” and “B” be “a” and “b” respectively.

Then according to their concept:

A:B = a:b .....................(i) Ab=aB, or A= B. a ......................(ii)

bor, A = B. a/(a+b) .............................(iii) b/(a+b)Let b/(a+b) = y, and it is the fraction of P in plant derived from fertilizer (P contribution from fertilizer source) b/(a+b) = y, or, b= ay+by or, ay= b-by or, a = (b-by)/yNow, a/(a+b) = (b-by)/y = 1-y (b-by)/y +b

From eqn (iii) we get, , A = B. (1-y) .....................(Iv)

y

From IDP, y = Sp / Sf

Isotopic Dilution Principle (IDP):

“ For a given constant amount of radioactivity the specific activity is inversely proportional to the amount of test substance present”

Assumption : After equilibrium mixing, the system is uniform w.r.t. its specific activity of the particular element.

Specific activity: It is defined as the amount of radioactive element per unit mass of the element present. (mCi/g material, Cpm/mg of material)

Suppose , a system contains an unknown amount of A g of test substance. To this system, added known amount of B g of the same susbstance labelled with

initial specific activity, Si , Let, the final specific activity which is measured, be Sf.According to IDP, (A+B)Sf= B.Si (total activity remains constant, irrespective of diln.)

Or, A+B = B. Si/Sf

Or, A= B[(Si/Sf)-1]

f

p

f

p

S

S

S

S

B

1

A

The Hungarian chemist George de Hevesy was awarded the Nobel Prize in Chemistry for development of radiotracer method, which is a forerunner of isotope dilution

Indicator plants: Certain plants are very sensitive to deficiency of a specific plant nutrient and they produce specific symptoms which are different from other deficiency symptoms. Thus the deficiency of that element can easily be detected.The indicator plants are the following

This is a pot-culture study with 10 pots to hold 6 pounds (2.72kg) of soil in each of them. The treatments include:1.No N-1 pot2.No K2O-3 pots(NP)3.No P2O5-3 pots (NK)4.Complete fertilizer- 3 pots (NPK) Oat will be the test crop and grown upto the maturity. The yields of NP and NK treatments are expressed as a percentage of the yield from the complete NPK treatment. From tables prepared by Mitscherlich, the plant nutrient reserve and predictions as to the %age increase in the yield expected from the addition of a given given quantity of fertilizers can be obtained.

(a) (i) Mitscherlich Pot culture method

Sunflower pot culture technique for Boron : In this method

500 g soil is taken in small pot and 5 sunflower seedlings are allowed to grow. The soil is fertilized with a solution containing all the nutrients except B and deficiency of B is noticed and ranked.

(b) (i) Azotobacter palque method (Sackett and Stewart technique,1931)

Winogradsky observed that the growth of Azotobacter serve to indicate the limiting mineral nutrients in soil.

4 petridish was taken and 50 g soil was added in each petridish.The petridish contains this nutrient serially K2SO4 (T1), NaH2PO4(T2) , KH2PO4 (T3), Control(T4) .Soil inoculated with Azotobacter culture and incubated for 72 hrs at 30°C. The soil is rated from very deficient to not deficient in the respective elements, depending on the amount of colony growth.

DRIS (Diagnostic & Recommendation Integrated System)

DRIS is a new approach to interpreting leaf or palnt analysis which was developed by Beaufils at the University of Natal, South Africa.It is a comprehensive systems which identiifes all the nutritional factors limiting crop production and in so doing increases the chnaces of obtaining high crop yields by improving fertilizer recommendations.

To develope a DRIS for a given crop, the following requirements must be met:

(i)All factors suspected of having an effect on crop yield must be defined(ii)The relationship beteen these factors and yield must be described(iii)Calibrated norms must be established(iv)Recommendation suited to particular sets of conditions and based on correct and judicious use of these norms must be continually refined

The way in which this chart is used will be illustrated by means of an example. Assume that the following values are obtained from the analysis of the third leaf blade of sugarcane:

A provisional chart for obtaining qualitatively the NPK requirements of sugarcane is given in Figure 1.

A qualitative reading of this chart can be done by using arrows in the following conventional manner: Horizontal →for values within the inner circles of the chart, Diagonal for values between the two circlesVertical for values found beyond the outer circle.

Because an excess of one plant nutrient corresponds to a shortage of another, by convention only insufficiencies are recorded for the purpose of diagnosis and this is done stepwise for each function. Identical diagnoses are obtained by considering either excesses or insufficiencies or both.

Determination of Relative NPK requirement by using DRIS Chart: The chart is constructed of three axes for N/P, N/K, and K/P, respectively with the mean value for the subpopulation of high yielders located at the point of intersection for each form of expression. This point of intersection of the three axes therefore represents the composition for which one is striving and at which one should achieve the highest yield permitted by limiting factors other than N, P,K. Th concentric circles can be considered as confidence limits, the inner being set at the mean ±15% and the outer at the mean ±30% for each expression.

The value of the function N/P lies in the zone of N insufficiency giving:

while that of N/K lies between the two circles adding a tendency to K insufficiency

and that of K/P lies in the zone of K insufficiency giving

Once the three common functions have been read, the remaining character is assigned a horizontal arrow. The final reading then becomes:

which gives the order of requirements for NPK in terms of limiting importance On yield - viz. :

MobileVariably Mobile

Immobile

Nitrogen Copper Calcium

Phosphorus Zinc Boron

Potassium Sulfur Manganese

Magnesium Molybdenum

Iron

Table 1. Mobility of nutrients within plants.

Plant nutrients which can move from places where they are stored to places where they are needed are called plant mobile. N, P,K are always plant mobile nutrients. Deficiencies are noticeable first on older tissue. Plant immobile element deficiencies are noticeable first on younger tissue. Ca and B are always plant immobile nutrients. S,Cl,Cu, Zn, Mn, Fe and Mo are intermediate in plant mobility. Under certain circumstances the intermediate elements are mobile. Mobility in intermediate elements may be linked to the breakdown under low N conditions of amino acids and proteins in older parts of the plant, and the mobility of these organic compounds to younger parts of the plant in the phloem stream. Under good N availability, these elements are mostly immobile.

Plant Nutrient Deficiency Terminology Burning: severe localized yellowing; scorched appearance. Chlorosis: general yellowing of the plant tissue; lack of

chlorophyll. Generalized: symptoms not limited to one area of a plant, but

rather spread over the entire plant. Immobile nutrient: not able to be moved from one part of the

plant to another. Interveinal Chlorosis: yellowing in between leaf veins, yet veins

remain green. Localized: symptoms limited to one leaf or one section of the leaf or

plant. Mobile nutrient: able to be moved from one plant part to another. Mottling: spotted, irregular, inconsistent pattern. Necrosis: death of plant tissue; tissue browns and dies. Stunting: decreased growth; shorter height of the affected plants.

Interveinal chlorosis. (Fe deficiency)

N deficiency in barley. Top leaves are N deficient, bottom leaf is normal.

P deficiency in alfalfa (L) and normal alfalfa (R). P deficient leaf is dark green and stunted.

P deficiency in corn. Leaves are purplish and tips are brown and necrotic.

K deficiency in corn. Older leaves are chlorotic and leaf edges are burned, but the midrib remains green.

S deficient wheat plant (left) has light green leaves and stunted growth as compared to normal wheat plant (right).

Interveinal chlorosis (Figure 2) occurs when certain nutrients [B, Fe, Mg, Mn, nickel (Ni) and Zn] are deficient. Purplish-red discolorations in plant stems and leaves are due to above normal levels of anthocyanin (a purple colored pigment) that can accumulate when plant functions are disrupted or stressed.

Alfalfa with B deficiency; chlorosis of upper leaves and rosetting of leaves near base.

Cu deficiency in wheat: severely affected (L), moderately affected (Centre), unaffected (R). Deficient wheat shows melanosis with poor grain production and fill

Zn deficiency displaying striped interveinal chlorosis.

Mobile Nutrients

Initial symptoms occur in middle laeves with young & /or oldleaves become chlorotic

Immobile Nutrients

Mineral Deficiency

• The most common deficiencies– Are those of nitrogen, potassium, and phosphorus

Phosphate-deficient

Healthy

Potassium-deficient

Nitrogen-deficient

“Firing”…drying along tips and margins of older leaves

Reddish-purple margins esp. on young leaves

Yellowing that starts at the tip and moves along the center of older leaves

The principle of this method is based on the use of an electric field to separate nutrient fractions from a soil suspension. During separation the voltage is increased from 50 to 400 V, thus increasing the force by which plant nutrients are desorbed from soil particles.

Extraction of nutrients from a soil-water suspension in an electric field and with a ultrafiltration.

1. Fraction (intensity): 30 min, 200 V, < 15 mA, 20o C2. Fraction (quantity): 5 min, 400 V, < 150 mA, 80o C3. Fraction (micronutrients) : 5 min, 400 V, < 150 mA, 80o C with 0.002 M DTPA.

EUF-desorption curve for K+ (NEMETH, 1979)

Soil test correlation: The process of determining the relationship between plant nutrient uptake or yield and the amount of nutrient extracted by a particular soil test method.

Soil test calibration. The process of determining the

crop nutrient requirement at different soil test values.

Soil test interpretation. The process of developing nutrient application recommendations from soil test concentrations, and other soil, crop, economic,

environmental and climatic information .

Yield response to fertilizer in relation to soil test value (points represent individual soils tested).

Percentage yield = —————————————————————— x 100(Crop yield with adequate nutrients- Yield of control)

Crop yield with adequate nutrients