Soil fertility evaluation P K MANI


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Soil Fertility evaluation -5 methods are involved. A value, Crititical concept by Cate and Nelson etc.

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  • An excellent overall reference on the practice and theory of soil sampling and analysis is the book:
    Soil Testing and Plant Analysis. 1990. R.L. Westerman (ed.) Soil Sci. Soc. Amer. Book Series 3. Madison, WI.
  • Soil fertility evaluation P K MANI

    1. 1. Soil Fertility Evaluation
    2. 2. Soil Fertility: it is the potential of the earth or inherent capacity 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 capacity 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.
    3. 3. 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 Requirement 100 Amount available 40 40% Justus von Liebig, 1840 P K 50 25 50% 60 30 50% So, here N is the factor which limits the crop growth. “Just as the capacity of the wooden bucket to hold water is determined by the height of the shortest stave, crop yields are restricted by the nutrient in shortest supply!” Liebig was probably the first to express the yield as mathematical function of the given growth factor when a the other factors kept constant y = Ax - B A,B, = constant Father of modern Agricultural Chemistry
    4. 4. Liebig’s law of minimum von Liebig 1803 -1873 N N P P K Mg S N K N P P K K Mg S ?
    5. 5. 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. “The increase in yield by a unit increment of the deficient factor is proportional to the decrement of that factor from the maximum.” Mitscherlich’ s Equation dy/dx = (A-y)C (by integration) y = A (1-10-Cx) or, log (A-y) = log A – Cx . Immobile nutrients follow (P, K,and Ca in soil) follow Mitscerlich’s concept 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
    6. 6. dy/dx = (A-y)C ∫ dy = ( A - y) ) ∫ Cdx or, - log (A-y) = Cx +C If x=0, y=0 , C = - log A or, - log (A-y) = Cx –log A or, log (A-y) = log A- Cx or, log or, ( A − y) A ( A − y) A = −Cx = 10 −Cx or, A - y = A 10 −Cx or, y = A(1 - 10 −Cx )
    7. 7. Soil Fertility Evaluation: Evaluation Several techniques are commonly employed to assess the fertility status of a soil (i)Nutrient deficiency symptoms of plants (ii)Plant analysis and tissue testing (iii) Methods involving the growing of higher plants and microorganism (iv) Soil Chemical analysis (v) Isotopic dilution method
    8. 8. (i) Nutrient 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
    9. 9. 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.
    10. 10. 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.
    11. 11. 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.
    12. 12. (ii) Plant anlysis and tissue testing This 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.
    13. 13. Tissue Tests: (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 ……….. Diphenylamine Phosphates ……. Molybdate + Stannous oxalate test Potassium …… Sodium cobalti nitrate
    14. 14. Tissue Test Interpretation  Critical nutrient concentration ranges (sufficiency ranges)  Using Plant Analysis as a Diagnostic Tool  DRIS (Diagnostic & Recommendation Integrated System) Crop logging:
    15. 15. 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
    16. 16. 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. Relationship between the nutrient content in the soil solution and the nutrient content of the plant (Mengel and Kirkby,1987)
    17. 17. Provisional DRIS chart for obtaining the qualitative order of requirement for NPK in sugarcane.
    18. 18. 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.
    19. 19. Crop logging:  (Clements,1960) 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
    20. 20. Max-Min Temp. Growth Nitrogen Moisture Total Sugars K2O Completed crop log for an irrigated Hawaiian Plantation. P index Clements (1960)
    21. 21. (iii) Methods involving growing of higher plants and microorganisms (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     
    22. 22. a) (ii) Neubauer Seedling method (Neubauer and Schneider,1932) 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 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.
    23. 23. (b) (ii) Mehlich’s technique for available K2O by Aspergillus niger method 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. Weight of Four pads ( g) K absorbed by Aspergillus niger per 100 g soils (mg) <1.4 <12.5 1.4-2.0 12.5-16.6 >2.0 >16.6 Degree of potassium deficiency Very deficient Moderate –slight deficient Not deficient
    24. 24. (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)
    25. 25. 4. Soil Chemical analysis
    26. 26. 4. Soil Chemical analysis Objectives of soil testing 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.
    27. 27. Soil Testing basics Soil testing starts with collecting a good sample Soil testing is not useful without meaningful samples
    28. 28. Calibration and Interpretation Perhaps 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.
    29. 29.  Two methods of approach in Soil test Calibration.   (i)Soil analysis-correlation approach (ii)Critical soil test level approach (Crop yield with adequate nutrients- Yield of control) Percentage yield = —————————————————————— x 100 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. Yield response to fertilizer in relation to soil test value (points represent individual soils tested).
    30. 30. 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.
    31. 31. 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.. Max. Profit Max. Profit 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%). Max. Profit 3. Low - Soil Test Value where probability of response to additional fertilizer is good (90%).
    32. 32. Rating Chart for soil test values pHw (1:2.5) Acidic < 6.5 EC(dSm-1) Neutral Alkaline 6.5 - 7.5 > 7.5 Normal Critical Injurious < 1.0 1.0 - 3.0 > 3.0 Parameters Low Medium High Org. Carbon < 0.5 0.5 - 0.75 >0.75 Avail N (kg/ha) < 280 280 - 560 > 560 Avail P (kg/ha) < 22 22 - 45 > 45 Avail K (kg/ha) < 120 120 - 280 > 280 Avail. S (SO4-2) µg g-1 0-10 10-15 >15 Critical limit for Micro Nu (µg g-1 in soil )(rice) (DTPA extract) Fe 2.0 Boron (µg g-1 in soil )(HWS) Mn 1.0 Deficiency < 0.50 Zn 0.86 Cu 0.20 Toxicity > 4.00
    33. 33. Critical Levels developed by Cate and Nelson (1965) % yield versus soil test level Two Groups: 1. probability of response to added fertilizer is small 2. probability of response to added fertilizer is large Step A.: Calculate Percentage yield values obtained for a wide range in locations (Crop yield with adequate nutrients- Yield of control) Percentage yield = ——————————————————————--------- x Crop yield with adequate nutrients 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
    34. 34. 120 100 Percentage Yield 80 60 40 20 Critical Level 0 0 20 40 60 80 Soil Analysis, ppm P 100 120 140 160 (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.
    35. 35. 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 soil B = amount of fertilizer nutrient (standard) applied y = 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 constant independent of rate of fertilizer application independent 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).
    36. 36. 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 Ab=aB, or .....................(i) ......................(ii) A= B. a b or, 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) From IDP, b/(a+b) = y, or, b= ay+by or, ay= b-by or, a = (b-by)/y Now, a/(a+b) = (b-by)/y = 1-y (b-by)/y +b From eqn (iii) we get, , A = B. (1-y) .....................(Iv) y y = Sp / Sf
    37. 37. 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, Or, A+B = B. Si/Sf A= B[(S i /S f )-1]
    38. 38. 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 S  1 − p Sf  A = B Sp   S f        
    39. 39. 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
    40. 40. (a) (i) Mitscherlich Pot culture method 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 pot 2.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.
    41. 41. 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.
    42. 42. (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.
    43. 43. 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
    44. 44. 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 circles Vertical for values found beyond the outer circle. 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: 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.
    45. 45. 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.
    46. 46. 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. :
    47. 47. Table 1. Mobility of nutrients within plants. Variably Mobile Immobile Mobile Nitrogen Copper Calcium Phosphorus Zinc Boron Potassium Sulfur Manganese Magnesium Molybdenum Iron 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.
    48. 48. 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.
    49. 49. N deficiency in barley. Top leaves are N deficient, bottom leaf is normal. Interveinal chlorosis. (Fe deficiency) 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.
    50. 50. 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. 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).
    51. 51. Cu deficiency in wheat: severely affected Alfalfa with B deficiency; chlorosis of (L), moderately affected (Centre), upper leaves and rosetting of leaves near unaffected (R). Deficient wheat shows base. melanosis with poor grain production and fill Zn deficiency displaying striped interveinal chlorosis.
    52. 52. Mobile Nutrients
    53. 53. Immobile Nutrients Initial symptoms occur in middle laeves with young & /or old leaves become chlorotic
    54. 54. Mineral Deficiency • The most common deficiencies – Are those of nitrogen, potassium, and phosphorus Healthy Phosphate-deficient Reddish-purple margins esp. on young leaves Potassium-deficient Nitrogen-deficient “Firing”…drying along tips and margins of older leaves Yellowing that starts at the tip and moves along the center of older leaves
    55. 55. Extraction of nutrients from a soil-water suspension in an electric field and with a ultrafiltration. 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. 1. Fraction (intensity): 30 min, 200 V, < 15 mA, 20o C 2. Fraction (quantity): 5 min, 400 V, < 150 mA, 80o C 3. Fraction (micronutrients) : 5 min, 400 V, < 150 mA, 80o C with 0.002 M DTPA. EUF-desorption curve for K+ (NEMETH, 1979)
    56. 56. 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. Yield response to fertilizer in relation to soil test value (points represent individual soils tested). Soil test interpretation. The process of developing nutrient application recommendations from soil test concentrations, and other soil, crop, economic, environmental and climatic information . (Crop yield with adequate nutrients- Yield of control) Percentage yield = —————————————————————— x 100 Crop yield with adequate nutrients