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Soil Fertility and PlantNutrition Research UnderControlled Conditions: BasicPrinciples and MethodologyN. K. Fageria aa National Rice and Bean Research Center ofEMBRAPA, Santo Antônio de Goiás , GO, BrazilPublished online: 14 Feb 2007.

To cite this article: N. K. Fageria (2005) Soil Fertility and Plant Nutrition ResearchUnder Controlled Conditions: Basic Principles and Methodology, Journal of PlantNutrition, 28:11, 1975-1999, DOI: 10.1080/01904160500311037

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Journal of Plant Nutrition, 28: 1975–1999, 2005

Copyright © Taylor & Francis Inc.

ISSN: 0190-4167 print / 1532-4087 online

DOI: 10.1080/01904160500311037

Soil Fertility and Plant Nutrition ResearchUnder Controlled Conditions: Basic Principles

and Methodology

N. K. Fageria

National Rice and Bean Research Center of EMBRAPA, Santo Antoniode Goias, GO, Brazil

ABSTRACT

In modern agriculture, use of essential plant nutrients in adequate amounts and properbalance is one of the key components in increasing crop yields. Further, in developingcrop production technologies, research work under field and controlled conditions is nec-essary to generate basic and applied information. In addition, research is very dynamicand complex due to variation in climatic, soil, and plant factors and their interactions.This demands that basic research information can only be obtained under controlledconditions to avoid or reduce effects of environmental factors on treatments. Hence,the objective of this article is to discuss basic principles and methodologies of researchin soil fertility and mineral nutrition under controlled conditions. Topics discussed aresoil and solution culture experimental techniques including, fertilizer application andplanting, liming acid soils, experimental duration and observations, composition of nu-trient solutions, preparation and sources of iron (Fe) in nutrient solutions, pH of nutrientsolutions, and stable supply of nutrients in the solution culture.

Keywords: field crops, mineral stress, root growth, soil and solution culture

INTRODUCTION

Research is the foundation of technological improvements. The standard of liv-ing of a country is correlating the use of technology. In agriculture science, soilfertility and plant nutrition is an important area and its contribution in increasing

Received 19 October 2004; accepted 20 July 2005.Address correspondence to N. K. Fageria National Rice and Bean Research Center of

EMBRAPA, Caixa Postal 179, CEP 75375-000, Santo Antonio de Goias, GO, Brazil.E-mail: fageria@cnpaf.embrapa.br

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crop yields is well known. Borlaug and Dowswell (1994) reported that as muchas 50% of the increase in crop yields worldwide during 20th century was dueto use of chemical fertilizers. In the 21st century, importance of chemical fer-tilizers in improving crop yields will continue and expected to be still higherdue to necessity of increase in yields per unit land area rather than increas-ing land areas. Further, judicious use of chemical fertilizers along with othercomplimentary methods such as use of organic manures and exploiting geneticpotential of crop species and cultivars within species in nutrients utilization willbe extremely useful and necessary.

The low yields of crops in some parts of the world or countries are the resultof actions and interactions of many factors and that there are no simple, easilyimplemental solutions. Better understanding of biological, climatic, edaphicand management factors through research and the development of productiontechnologies that in the appropriate socio-political-economic climate can helpin increasing crop production in such regions. In the 21st century research, oneof the key factors guiding the research will be the need to develop low-costtechnology components, which do not require heavy application of purchaseinputs with minimum degradation of natural resources. The objective of thisreview article is to present basic principles and research methodologies forsoil fertility and plant nutrition under controlled conditions. This informationmay be very helpful for scientific community for the research needs to meetthe challenges of soil fertility and plant nutrition problems in the twenty firstcentury to improve crop production and reducing environmental degradation.

EXPERIMENTAL PROCEDURE AND TECHNIQUES

In addition to field experimentation in agriculture science experimental work isalso necessary in the greenhouses and growth chambers. The main objectivesof controlled conditions experiments are to understand basic principles. In thecase of soil fertility and plant nutrition, such experiments are mainly conductedto understand nutrients movements, absorption and utilization processes in soilplant systems. In addition, nutrient/elementally deficiency/toxicity symptomsand adequate and toxic concentrations in plant tissue are also determined undercontrolled conditions. For example, pot experiments with different types of soilscan show the degree of response that may be anticipated at different soil-testlevels and serve as excellent checks on ratings being used. Since such testsprovide no measure of the cumulative effects of treatments on yield or soilbuildup or depletion, they have limited value in determining rates of fertilizerthat should be recommended for sustained productivity. Greenhouse pot studies,in which plants are used for estimating the relative availability of nutrients, alsocan provide useful indices of the relative availability of a standard fertilizersource in different soils and fertilizer sources. In this section, the methodologyaspects of controlled conditions experimental condition will be discussed. This

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information will help those who are involved in soil fertility and plant nutritionresearch to improve and/or better understand the principles of experimentationunder controlled conditions.

Experimental Techniques

The experimental plan and procedure are crucial to its success. In a researchproject, well-formulated hypothesis and clearly defined objectives are essentialpart of the experimental techniques. Most of the controlled conditions exper-iments are conducted in pots using soil, solution culture or sand as a growthmedium. Today plastic pots are commonly used in all the controlled condi-tions experiment-they are most suitable for all soil fertility and plant nutritionexperiments. A wide variety of sizes and colors are available in the markets.Most suppliers offer pots with drainage holes as well as and plastic saucers forbottom watering or collection of leachate in case of over watering. Pots withoutdrainage holes are also available on order. In the opinion of the authors, potswith holes are not necessary, if irrigation water is applied carefully. In the soilfertility and plant nutrition experiments, porous pots may leak some nutrientsand it may affect the treatments adversely. Soils containing montmorilloniteclays shrink upon drying, thus permitting loss of water and nutrients duringroutine watering. Pots without holes solve the problems, however, they requirecareful attention to over watering. In addition, some crops are very sensible toover watering such as common bean. Unglazed or glazed earthen pots are nolonger used in greenhouse experiments because of excess weight, water loss,and possible absorption of salts. However, they may be satisfactory for someexperiments when plastic liners are used.

Most of the controlled condition experimental design is completely ran-domized or randomized complete block design with three or four replications.It is convenient on the part of the researcher to group pots of each replication to-gether on a table and treatments randomized within each replicate. Pots shouldbe rotated twice a week to eliminate any environmental effects, especially thesolar radiation. In a plant nutrition study, under controlled conditions, it is betterto have separate small experiments with various levels of a nutrient rather thanfactorial experiments with two or three levels of each nutrient.

Soil Culture

It should be noted that soil selected for greenhouse experiments in the areaof soil fertility and plant nutrition should be low in fertility. This is so it ispossible to obtain a yield response to applied nutrients. With yield response,effectiveness of fertilizers may be interpreted in terms of crop yield and uptakeof nutrients. Pot experiment can be used as a reference whether a given site inthe field experiment is appropriate for a fertility trial or not.

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On occasion, it may happen that many soils, which show yield response toa given nutrient in the field, fail to show response under greenhouse conditions.The probable explanation is that the high temperatures and moisture levelsusually prevailing in the green house result in more rapid decomposition oforganic materials and greater release of nutrients than occurred in the field(Allen et al., 1976). Cultivated soils having a history of nonfertilization with agiven nutrient for several years and are preferred over virgin or fertilized soils forobtaining yield responses. These results may be useful for a particular soil typeand may be an important consideration if studies are being made of fertilizerproblems related largely to that soil type. However, if brooder principles arebeing studied, then physical and chemical properties of a nutrient-responsivesoil may be more important then employing a particular soil type (Allen et al.,1976). Distance from greenhouse to soil collecting site is also an importantconsideration in order to minimize the cost of transportation. Selected soilshould be as free as possible from soil born diseases, nematodes, insects andweed seeds. Fumigation by dry heat or steam tends to change a number of soilproperties and may render a soil unfit for some soil fertility studies. Methylbromide or some other organic fumigant may be more satisfactory. Soils forproblem solving must be selected from the site where a specific problem isknown to exist, regardless of their suitability by other standard. After selectinga site, soil should be transported near the greenhouse and should be taken ofsoil depth from which it is collected. Generally, it is recommended that soilsfor greenhouse studies should represent the arable soil depth that is 0-20 cm.However, in practice it is always higher soil depth is covered in collecting thesoil for greenhouse experiments. After drying the soil, it should be screened topass through a 0.5–1.0 to 1 cm screen. A screen lower than 0.5 cm mesh canchange the soil physical properties too much and such soil may create problemduring experimentation, especially compaction in the pots. Soil prepared in thisway if not used immediately for experimentation can be stored in plastic bagsor plastic drums.

Fertilizer Application and Planting

After soil preparation, the next step in experimental technique is applicationof fertilizer treatments and sowing the crop seeds under investigation. Eachpot should be filled with prepared soil and weight should be recorded on aportable balance. To determine optimum levels of a nutrient in greenhouse for aparticular crop, a simple experiment with several rates of a single nutrient, andnon limiting levels of other nutrients usually supplied the desired information.There should be minimum five nutrient rates (low to high) with four replications.

As far as quantity of fertilizer application is concerned, generally re-searchers use an equivalent quantity that is used under field conditions. How-ever, with experiments conducted by the author at the National Rice andBean Research Center, Goiania, Goias, Brazil showed that under greenhouse

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Table 1Dry matter and grain yield of upland and irrigated rice under different soilfertility and plant density under greenhouse conditions

Dry matter yield (g pot−1) Grain yield (g pot−1)Fertility

Treatment1 Upland Lowland Upland Lowland

F0 3.36 10.95 2.10 12.55F1 16.82 20.04 12.24 23.27F4 30.80 36.41 26.11 40.75F8 44.42 42.45 34.73 51.87F-Test ∗∗ ∗∗ ∗∗ ∗∗

D1 22.00 29.10 17.18 31.15D2 24.59 25.84 19.24 31.19D3 24.70 29.64 19.46 34.87D4 24.11 25.29 19.30 31.23F-Test ∗ ∗ ∗ ∗

∗,∗∗Significant at 5 and 1% probability levels, respectively. 1F0 = zerofertility level; F1 35 kg N ha−1, 50 kg P2O5 ha−1, 40 kg K2O ha−1, 5 kgZn ha−1 and 2 Mg ha−1 dolomitic lime for upland rice and 60 kg N ha−1,80 kg P2O5 ha−1, 60 kg K2O ha−1, 5 kg Zn ha−1 and 2 Mg ha−1 dolomiticlime for irrigated rice. These levels correspond to recommended under fieldconditions in Brazil during 1980 to 1990 for upland and lowland rice. TheF4 and F8 are 4 and 8 times nutrients levels those recommended under fieldconditions. D1, D2, D3 and D4 correspond to 1, 2, 3 and 4 plants per pot.

Source: Fageria et al. (1982)

conditions the quantity of fertilizer required is much higher for rice, commonbean, corn, cowpea, and wheat crops on an Oxisol (Fageria, 1989a, 1991, 1999;Fageria et al., 1982).

Table 1 shows results of a greenhouse experiment in which two uplandrice cultivars grain yield and dry matter production under different levels ofsoil fertility and plant density were evaluated. The nutrient levels were zero,normal levels recommended under field conditions, and four and eight timesthe normal levels. These levels were tested at four plant densities i.e., one,two, three, and four plants per pot of 6 kg soil. Regression analysis indicatedthat grain yield and dry matter production were affected by nutrient level plantdensity and by the interaction between these two factors. The results showedthat adequate nutrient levels for upland and irrigated rice cultivated in 6 kgpots were approximately eight times those recommended for field conditions.Optimum plant density was obtained with 2 to 3 plants per pot. Terman andMortvedt (1978) and Mortvedt and Terman (1978) also reported that nutrientrates adequate for small greenhouse pots (4.5 kg soil/pot) are much higherthan rates equivalent to normal rates recommended for crops grown under fieldconditions.

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Length of growth period and other growth limiting factors are equallyimportant. Adequate plant density determined by author for common bean twoplants per pot of 6 kg soil (Fageria, 1989a) for wheat 4 plants per pot of 6 kg(Fageria, 1999), and for cowpea 2 plants per 6 kg soil (Fageria, 1991) untilmaturity. However, if plants are grown for short duration higher plant densitycan be used. But in authors, opinion this density should not be more than doublein any case to get meaningful results related to soil fertility and plant nutritionproblem.

There is no problem in using granular or powder fertilizers in greenhousestudies. Analytical grade reagents should not be used for soil fertility experimentin the greenhouse when soil is used as a growth medium. Due to difference inquality, water solubility, and composition, reactions with soil will be differentas compared to commercial fertilizers. Results obtained in this way will befar away from the reality. Due to small quantity, it is sometimes difficult tomix with soil of each pot. This problem can be solved by weighing fertilizersfor each treatment for all replications together. For example, if there are fourreplications of defined treatments, quantity required for all the four replicationscan be weighed together and mixed with soil of four replications and thenseparated into different pots. When plastic bags are used as pot liners, weighthe soil into bags and fit a bag into each pot. If there are holes in the bottomof a pot, put a filter paper in the bottom before filling each pot. All the potsfor an experiment should be filled at the same time to reduce errors in drysoil weighing caused by drying of the stock soil supply. After applying thefertilizer treatments through mixing the soil is very important. Mixing can bedone through a simple soil mixer or by hand-stirring or by rolling on a heavyplastic sheet.

Liming Acid Soils

Liming acid soils is one essential practice either in field or greenhouse exper-imentation. A satisfactory comparison among fertilizer treatments cannot bemade if soil acidity is a limiting factor. To solve the problem, dolomitic limeshould be applied and soil under investigation should be incubated for sev-eral weeks before the experiment is planted. The quantity of lime required fora given soil should be determined through a lime calibration curve. Fageria(1984) developed a lime requirement curve for Oxisol of Central Brazil and hefound that after 30 days of incubation period soil pH was almost stable (raisedfrom 5 to 7 in water, 1 soil: 2.5 water) and Ca+Mg levels increased from 0.5cmol kg−1 to more than 5cmol kg−1. This means in Oxisols about 4 weeks in-cubation period is sufficient to bring soil pH and Ca+Mg level at desired levels.All the chemical reactions in the soils are dynamic processes and never reach inperfect equilibrium. Only semi-equilibrium is reached. After determining thelime requirement through calibration curves, lime may be added to individualpots or bulk lots of soil may be limed to the desired pH prior to beginning an

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experiment. Most field crops grow well in a soil pH range of 6.0 to 6.5 (Fageriaand Baligar, 2003). In addition to pH, base saturation is another effective soilacidity index for liming. Base saturation can be calculated by the followingformula:

Base saturation = (� exchangeable Ca, Mg, K, Na/CEC at pH 7 or 8.2) × 100

Optimum base saturation for maximum yield varied from crop species to cropspecies, soil type and genotypes of the same species. However, most legumesproduce well at base saturation ranging from 60% to 70% and for cerealsoptimum base saturation varied from 50% to 60%.

Soil Culture Experimental Care, Duration, and Observations

During the experimentation, care should be taken of watering, control of insectsand diseases and rotation of pots to minimize environmental effects amongreplications. As far as watering is concerned, it should be used to field ca-pacity of a soil. The weighing method is the most widely used in wateringexperimental pots. Plastic pots currently available are very uniform in weightand tarring is not necessary. If facilities are available use of deionized wa-ter in irrigating the pots is preferred. However, in many developing countries,these facilities are either not available or very expensive; in such situation useof tap water is the only solution. If the laboratory is not equipped to deter-mine soil moisture retention curves, an alternative simple method is to addvarying amounts of water to the surface of soil in pots lined with polyethy-lene bags and to let these stand overnight. A satisfactory level of water is theamount, which just wets the entire soil volume. This may be observed visuallyby carefully lifting the bag of soil from the pot. During initial watering, pro-tect the soil surface from washing by a filter paper. Most of the water shouldbe added along the rim of the pot. Depending on the climatic conditions, inthe beginning of the experiment, generally watering twice a week is sufficient.On later stages depending on growth the crop plants, watering is necessaryeveryday.

Authors conducted several fertilizer experiments in the greenhouse andsome of our observations are discussed here in this respect. Fageria and Gheyi(1999) reported results of rice crop response to applied P in two greenhouseexperiments conditions on an Oxisol. These two experiments were conductedat the same time. In one experiment, harvesting was done at 60 days aftersowing and in another until maturity. From results of these experiments, it canbe concluded that when relative dry matter and grain yields are plotted againstP levels, results are almost identical. This means, dry matter can be used a criteriain greenhouse experiment to evaluate crop responses to fertilization provided allother factors are at optimum level. Three important factors have been observed,

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which can alter greenhouse experimental results and with dry matter and grainyields may show different responses. These factors are infestation of insects anddiseases and low temperature. If these adverse factors occur during reproductiveand grain filling period, there is a great variation can exist between dry matterand grain yield parameters. It can be concluded that in soil fertility experimentunder greenhouse conditions plants can be harvested during flowering legumecrops and during initial reproductive growth stage in cereals to evaluate soilfertility treatments.

In the soil fertility and plant nutrition experiments yield and yield com-ponents should be determined at harvest to understand influence of nutrienttreatments on plant growth and yield parameters and their influence on yield.In addition, if objective of the experiment is to determine critical nutrient con-centration in the plant tissue at different growth stages, plant sampling shouldbe done at defined plant growth stages. Some of the physiological and nutrientuptake parameters, which are important for mineral nutrition studies can becalculated using following formulas:

Specific Leaf Area (SLA, cm2/g)

= [Total leaf area, cm2/Total leaf dry wt, g]

Leaf Area Ratio (LAR, cm2/g)

= [Total leaf area, cm2/Shoot + Root dry wt. g]

Leaf Mass/Unit Leaf Area (LMA, g/cm2)

= [1/SLA]

Root/Shoot Ratio(R/S) = [Wr/Ws],

where Wr is root weight and Ws is shoot weight

Relative Growth Rate (RGR) = [ln (Wt2/Wt1) − (T2 − T1)],

where Wt is total weight (shoots + root), T is time interval in days, and 1 and2 refers to initial and final harvest

Net assimilation Rate (NAR) = [RGR/LAR]

Nutrient Influx (IN) = [(U2 − U1)/(T2 − T1)] × [(lnWr2 − lnWr1)/

(Wr2 − Wr1)],

where U refers to elemental content of shoot (m moles/plant) and T is time inseconds, subscripts 1 and 2 refers to initial and final harvest time.

Nutrient Transport (TR) = [(U2 − U1)/(T2 − T1)]

× [(ln Ws2 − ln Ws1)/(Ws2 − Ws1)]

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In soil fertility and plant nutrition experiments nutrient use efficiency (NUE)is an important parameter to know applied nutrient absorption and utilizationby the plants. The nutrient use efficiency (NUE) can be defined as the max-imum economic yield produced per unit of N applied, absorbed, or utilizedby the plant to produce grain and straw. However, nutrient use efficiency hasbeen defined in several ways in the literature, although most of them denote theability of a system to convert inputs into outputs. Definitions of nutrient useefficiencies have been grouped or classified as agronomic efficiency, physio-logical efficiency, agro-physiological efficiency, apparent recovery efficiency,and utilization efficiency (Bates, 1971). The determination of N use efficiencyin crop plants is an important approach to evaluate the fate of applied chemi-cal fertilizers and their role in improving crop yields. The NUE in greenhouseor controlled conditions experiments can be calculated by using the followingformulas (Bates, 1971):

Agronomic efficiency (AE) = (Gf − Gu/Na) = mg mg−1

Where Gf is the grain yield of the fertilized pot (mg), Gu is the grain yield inthe unfertilized pot (mg), and Na is the quantity of nutrient applied (mg).

Physiological efficiency (PE) = (Yf − Yu/Nf − Nu) = mg mg−1

Where Yf is the total biological yield (grain plus straw) of the fertilized pot(mg), Yu is the total biological yield in the unfertilized pot (mg), Nf is thenutrient accumulation in the fertilized plot in grain and straw (mg), and Nu isthe nutrient accumulation in the unfertilized plot in grain and straw (mg).

Agrophysiological efficiency (APE) = (Gf − Gu/Nf − Nu) = mg mg−1

Where Gf is the grain yield in the fertilized pot (mg), Gu is the grain yield inthe unfertilized pot (mg), Nf is the nutrient accumulation by straw and grain inthe fertilized pot (mg), and Nu is the nutrient accumulation by straw and grainsin the unfertilized pot (mg).

Apparent recovery efficiency (ARE) = (Nf − Nu/Na) × 100 = %

Where Nf is the nutrient accumulation by the total biological yield (straw plusgrain) in the fertilized pot (mg), Nu is the nutrient accumulation by the totalbiological yield (straw plus grain) in the unfertilized pot (mg), and Na is thequantity of nutrient applied (mg).

Utilization efficiency (EU) = PE × ARE = mg mg−1

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Table 2Nutrient solution composition used in the solution culture studies

Hoagland & Johanson et Andrew et al. Yoshida et al.Nutrient Arnon (1950) al. (1957) (1973) Clark (1982) (1976)

NO−3 (mM) 14.0 14.0 2.00 7.26 2.21

NH+4 (mM) 1.0 2.0 — 0.90 0.64

P (mM) 1.0 2.0 0.07 0.07 0.29K (mM) 6.0 6.0 1.10 1.80 1.02Ca (mM) 4.0 4.0 1.00 2.60 1.00Mg (mM) 2.0 1.0 0.50 0.60 1.64S (mM) 2.0 1.0 1.50 0.50 —Mn (µM) 9.1 5.0 4.60 7.00 9.00Zn (µM) 0.8 2.0 0.80 2.00 0.15Cu (µM) 0.3 0.5 0.30 0.50 0.16B (µM) 46.3 25.0 46.30 19.00 18.50Mo (µM) 0.1 0.1 0.10 0.60 0.50Fe 32.0 40.0 17.90 38.00 36.00Cl — 50.0 — —

In the earlier literature nutrient concentration.

Nutrient use efficiency ratio (NUER) = (mg of dry weight shoot or grain/mgof element in shoot or grain)

Solution Culture

Growing plants in solution culture is an important and very traditional techniquein mineral nutrition studies. Some important discoveries in mineral nutritionhave been made using solution culture techniques such as discovery of essen-tiality of nutrients. Solution culture studies are useful for developing deficiencysymptoms of nutrients essential for plant growth. These symptoms can be usedas a guide to identify nutritional disorders in crop plants under field condi-tions. In addition to deficiency symptoms, it is also possible to develop toxicitysymptoms of some element and thus get help in their identification and possiblecorrection measures can be adopted. An example in this respect is Al toxicityin acidic soils, iron toxicity in flooded rice and soil salinity problems in salinesoils.

Critical tissue concentrations for the diagnosis of nutrient deficiencies andtoxicities are frequently established from water culture or sand culture exper-iments. Although many plant and environmental factors have been shown toaffect measured critical concentrations (Johnson et al., 1957), it has been widelyassumed that critical tissue concentrations are comparatively stable plant char-acteristics unlikely to be affected by temporal variation in the external supply

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Table 3Commonly used reagent grade chemicals for nutrient solution

Nutrient Reagent 1 Molar Solution (g liter−1)

Nitrogen (NH4)2SO4 132.0Nitrogen NH4NO3 80.0Phosphorus NaH2PO4·2H2O 156.0Phosphorus KH2PO4 136.0Potassium KCl 74.6Potassium K2SO4 174.2Potassium KNO3 101.1Calcium CaCl2·2H2O 147.0Calcium Ca(NO3)2·4H2O 236.2Calcium CaSO4·2H2O 172.1Magnesium MgSO4·7H2O 246.3Manganese MnCl2·4H2O 197.9Manganese MnSO4·4H2O 169.0Zinc ZnSO4·7H2O 287.4Zinc ZnCl2 136.3Copper CuSO4·5H2O 249.5Copper CuCl2·2H2O 170.4Boron H3BO3 61.8Molybdenum (NH4)6 Mo7O24·4H2O 1235.6Molybdenum Na2MoO4·2H2O 241.9Iron FeCl3·6H2O 270.0Iron FeSO4·7H2O 278.0Iron Fe(NO3)·9H2O 404.0

Iron is generally chelated with EDDHA, HEDTA and EDTA.

of the element concerned (Johnson et al., 1957). However, care should be takenwhen such results are extrapolated to field conditions because, under field con-ditions, variability in environmental factors is quite great, which may influencenutrient concentrations in plant tissues. The composition of nutrient solutionscommonly used in hydrophonic techniques is given in Table 2. In preparingnutrient solutions, all the chemicals should be reagent grade. Table 3 presentscommonly used chemicals for preparing nutrient solutions. The Fe is generallychelated with EDDHA, HEDTA and EDTA. Therefore, the procedure of thischelating is given here.

According to Chaney and Bell (1987), the Fe-chelate of choice for solutionculture of dicots is FeEDDHA and for graminae the recommended chelate isFeHEDTA. Commercial FeEDDHA or FeHEDTA is not pure enough for use incontrolled solution experiments, as well as it is not easily available especially indeveloping countries. Therefore, one should purchase pure chelates and maketheir own solutions. The solutions of these chelates are difficult to prepare.Therefore, methodology of their preparation is given here.

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Preparation of FeEDDHA Solution

To prepare FeEDDHA solution for solution culture experiments requiredEDDHA salt, ferrous sulfate or ferric nitrate and KOH or NaOH salts. Allthese salts should be reagent grades.

1) Prepare 8 mM solution of EDDHA by dissolving 2.972 g L−1 in deion-ized water. The currently available EDDHA salt is about 97% pure withformula weight of 360.37 g mol−1.

2) Prepare 8 mM solution of ferrous sulfate (FeSO4·7H2O) or Ferric ni-trate (Fe (NO3)2·9H2O). In case of FeSO4·7H2O (FW 278.02 g mol−1,99% pure) 2.25 g L−1 reagent is required. In case of Fe(NO3)2·9H2o (FW404 g mol−1, 100% pure) 3.23 g L−1 of salt is required.

3) Prepare 32 mM solution of KOH or NaOH. In case of KOH formula weight66 g mol−1 and 100% pure, 2.11 g L−1 salt is required. In case of NaOHwith formula weight 40 g mol−1, 1.28 g L−1 of reagent is required for asolution of 32 mM.

4) Add 32 mM KOH or NaOH solution to 8 mM EDDHA solution and stir for30 to 60 minutes.

5) Add ferrous or ferric 8 mM solution to mixed EDDHA and KOH or NaOHsolution.

6) Adjust pH 6–7 by KOH or HCl. Stir overnight at >50◦C. Then filterwith Whatman filter paper No. 42 to remove Fe(OH)3.

7) Store in brown bottle in refrigerator.

The two liters of this solution contains 4 mM or 224 ppm of Fe3+. Oneuse a Fe2+ salt to prepare the Fe3+ chelates, but one should recognize that theligand will catalyze oxidation of the Fe2+ if oxygen is present.

Preparation of HEDTA Solution

To prepare one liter of Fe-chelate solution requires 30 mM solution of HEDTAand, 30 mM of ferrous sulfate or ferric nitrate solutions. The process is asfollows:

1) Prepare 30 mM solution of HEDTA by dissolving 11.52 g L−1 in deionizedwater. The commercially available HEDTA has formula weight of about380.24 g mol−1 with 99% purity.

2) Ferrous sulfate solution (FeSO4·7H2O, FW 278.02 g mol−1 with 99% pu-rity), add 8.42 g L−1 to get 30 mM solution. If ferric nitrate is used, it is gen-erally have 404 g mol−1 formula weight, required 12.12 g L−1of deionizedwater to give 30 mM solution.

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3) Add ferrous sulfate or ferric nitrate solutions to HEDTA solution. Stir for 30to 60 minutes at >50◦C. Then filter through Whatman filter paper no. 42.Store in brown bottle and keep in a refrigerator. This 2-liter solution has15 mM or 840 ppm Fe solution.

Preparation of FeEDTA Solution

To prepare FeEDTA solution, sodium salt of EDTA and ferrous sulfate alongwith NaOH solution is used. Along the same lines, 1 liter of FeEDTA solutionfollowing composition and procedure should be adopted and is shown here:

1) Dissolve 33.2 g NaEDTA salt (C10H14N2Na2O8·2H2O FW 372.24 g mol−1)in about 200 mL of deionized water.

2) Make a 89.2 mL solution of 1 N NaOH.3) Dissolve 29.4 g of FeSO4·7H2O in about 100 mL of deionized water.4) Mixed NaEDTA and NaOH solution slowly and stir. After mix this solution

with FeSO4·7H2O solution. Leave this solution for overnight in a dark am-bient. Next day complete the volume to one liter. This solution will contain106 mM or 5936 ppm Fe.

According to Novais et al. (1991), FeEDTA solution can also be preparedby mixing 14.1 g of Na2 EDTA and 10.3 g of FeCl3·6H2O, diluted separatelyin about 300 to 400 mL of deionized water, and then mixed together to make avolume of 1 liter. This solution contains 38 mM Fe. Care should be taken whilechelating iron used in nutrient solution. Chelators added at sufficiently highconcentrations to hydroponic solution can induce micronutrient deficiencies bychelating copper (Cu), zinc (Zn), manganese (Mn), and Fe, making the metalsunavailable to plants (Parker and Norvell, 1999). This loss in metal bioavail-ability can be counteracted by increasing the amount of metal in solution.Hydrophonic solutions that have a chelator concentration greater than the sumof the concentrations of Fe, Cu, Zn, Mn, cobalt (Co), and nickel (Ni) are calledchelator buffered solutions (these solutions are usually designed to also preventFe precipitation) (Chaney et al., 1989). Chelator-buffered solutions offer moreprecise control of micronutrients phytoavailability than do conventional hydro-ponic solutions because, in the buffered solutions, i) there is a greater range inthe total-metal concentration from deficiency to toxicity and ii) the free metalion activity decreases only a small amount during plant uptake of metals becausemetal activities are buffered (Parker and Norvell, 1999; Bell et al., 1991).

pH of Solution Culture

In solution culture experiments two points should be given special importance.One is the control of pH and second maintains stable supply of nutrients. In

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general, pH shifts in nutrient solutions are likely to be greater than is soilsbecause the lack of an exchange complex to adsorb or desorbs hydrogen ions.The principal factor, which leads to change in nutrient solution, is unequalabsorption of cations and anions. Nitrogen (N) is absorbed in large quantityand the form in which this element is supplied exerts a great influence on pHchange. The adsorption of more anion such as NO−

3 can liberate the OH ions inthe rhizosphere by growing plant roots and pH is generally increased. If morecations such as NH+

4 are absorbed pH is decreased due to liberation of H+ ionsin the growth medium. Trelease and Trelease (1935) showed in water cultureexperiments with wheat that by varying the NO−

3 /NH+4 ratio they could cause

the pH to increase, decrease, or remain about constant. These authors also sug-gested that for pH stability in culture solution, approximately 80 to 90% of theN should be supplied in the nitrate form. Crop species are also important inchanging nutrient solution pH due to their different nutrients absorption capac-ities. Even cultivars within crop specie are important in modifying rhizospherepH in solution culture. If has been consistently observed with non-nodulatedjackbeans (Canavalia ensiformis) that the pH of the nutrient solution decreasesmarkedly with time even when all the nitrogen is supplied in the NO−

3 form(Asher and Edwards, 1983). The appropriate pH in nutrient solution is certainlydifferent than in soil. The range reported in literature for conducting solutionculture experiments varied from 5 to 7. For example, Yoshida et al. (1976)reported that rice growth in nutrient solution pH should be maintained around5.0. Rice plants can grow well in nutrient solution even at pH 4.0 provided allessential nutrients are maintained at adequate levels (Fageria, 1989b). Romeroet al. (1981) using a modified Hoagland’s solution with a pH of 6.0 to 6.4 tocompare sand, soil and solution culture systems as methods of assessing K fer-tilization effects on alfalfa yield and K uptake. Bell et al. (1991) maintained thenutrient solution pH 5.9 with the addition of 1 M HCl in a study to determinecopper (Cu2+) activity required by maize using chelator buffered nutrient solu-tions. Ben-Asher et al. (1982) working with tomato plants in nutrition solutionto study nutrient uptake reported that solution pH in their study was maintainedbetween 5.5 and 6.5, respectively. Hohenberg and Munns (1984) studied theeffect of pH on nodulation of cowpea in nutrient solution and concluded thatpH 5.3 ± 0.3 was superior in modulation as compared to pH of 4.4 ± 0.2.Although there were differences among cultivars in relation to pH tolerance.McElhannon and Mills (1978) studied the influence of various percentages ofNO−

3 to NH+4 and N concentration on N absorption, assimilation, growth and

yield of lima beans in nutrient solution. These authors adjusted initial solutionpH to 6.6 and change in pH due to nutrient absorption was not adjusted duringexperimentation so the influence of pH on N uptake at weekly intervals couldbe evaluated. Peaslee et al. (1981) studied absorption and accumulation of Znby two corn cultivars in nutrient solution and they maintained the culture so-lution pH about 6.0. Teyker and Hobbs (1992) studied the effects of N formson growth and root morphology of corn in hydroponic study. They maintained

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the pH 5.5 in the NH+4 treatment and pH 5.0 in NO−

3 treatments during theexperimentation. Elliot and Lauchli (1985) compared rates of phosphorus (P)absorption, P accumulation, and P utilization in inbred maize genotypes undervarying conditions of P supply in a solution culture under controlled conditions.For the first 6 days following transplanting, pH adjusted to between 5.0 and 5.5and thereafter to 4.5 to 5.0, respectively. Brown and Jones (1976) studied theiron uptake efficiency in corn, sorghum, tomato and soybeans plants in nutri-ent solution. The initial pH of the solution was adjusted to 4.5 and during theexperimentation pH was not adjusted. Itoh and Barber (1983) in a controlledclimate chamber studied P uptake by six plant species in solution culture. Thespecies tested were onion, corn, wheat, lettuce, carrot and Russian thistle. Solu-tion pH in this study was adjusted to 5.5 during the experimentation. Miyasakaand Grunnes (1990) in a study to determine the effects of an increase in roottemperature and Ca levels on the shoot and root growth of winter wheat grownin solution culture, the pH of the solutions was maintained to 6.0 by addingHCl or NaOH as required.

To maintain a desired pH, adjust the pH of the culture solution at the desiredlevel every second day either by 1 N NaOH or 1 N HCl. Islam et al. (1980)using flowing nutrient solutions found greatest growth of six species: ginger,cassava, tomato, common bean, wheat, and maize at about 5.5. Yields of gingerand tomato were not significantly changed at higher pH values whereas yieldsof the other four species were depressed. Breeze et al. (1987) found that theincrease of dry weight of white clover over a 20-day period, whether fixingatmospheric nitrogen or dependent on NO−

3 in solution, was not significantlylower at pH 4.0 than at pH 5.0, 6.0 or 7.0. Edmeades et al. (1991) reported thatincreasing the nutrient solution pH from 4.7 to 6.0 had no significant effect onthe yield of the temperate grasses examined but significantly decreased yieldsof paspalum and veld grass.

From these reviews, it can be concluded that there is no consistency re-garding pH of the nutrient solutions in the experimental studies. In the author’sopinion, with a pH value of around 5.5, all crops can be grown in nutrientsolution satisfactorily. At pH values higher than 5.5, there are always possi-bility of precipitation of many nutrients, especially micronutrients and therebyaffecting their availability. Only in some, studies like aluminum (Al) toxic-ity, solution culture pH should not be more than 4.0 to avoid precipitationof Al. To maintain a desired pH, adjust the pH of the culture solution at thedesired level every second day either by 1 N NaOH or 1 N HCl. However,two methods of pH control that have been shown to have promise are ion ex-change resins (Checkai et al., 1987) and the organic buffer, 2-(N-morpholino)ethanesulfonic acid (MES) (Wehr et al., 1986). Miyasaka et al. (1988) com-pared an unbuffered nutrient solution titrated one or twice a day, with solutionbuffered either by the organic buffer, MES, or by an ion exchange system usinga weakly acidic cation exchange resin loaded with Ca, Mg, K, and hydrogen(H). These authors recommended that among the pH buffer method studied,

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1 mM MES method is recommended as a pH buffer for the hydroponic cul-ture for winter wheat. Five millimolar MES gave the most consistent controlof solution pH. However, it also inhibited Zn accumulation by wheat. Imsandeand Ralston (1981) found that 1 to 2 mM MES had excellent buffering capac-ity, did not interfere nutritionally with soybean growth, and did not impedeN2-fixation.

On the contrary, Rys and Phung (1985) found that MES at 9 and 12 mMconcentrations resulted in reduced growth of Trifolium repens L., which isdependent on symbiosis to provide N, and they suggested that the N-fixingability of nodules was impaired by high levels of MES. Wehr et al. (1986)considered MES to be the most useful buffer in the pH range of 5.0 to 6.5for growth of algae because of its biological inertness, high buffering capacity,and minimal metal-complexion ability. However, Clark (1982) stated, “bufferedsolutions often induce more complications than original solutions.” This meansmore research work is needed to clearly understand the effects of bufferingreagents interactions with nutrients uptake and plant growth.

Stable Supply of Nutrients

In soil culture mineralization of organic matter, weathering of primary minerals,biological activities and chemical reaction provides replenishment of mineralnutrients. In addition to this as roots elongates, they came in contacts with moresoil volume and more nutrients are available for absorption. This means insoil environment depletion of nutrients takes place over a longer time and soilprovides a buffering capacity. In water culture, the composition of the nutrientsolution is essentially unbuffered and large changes in nutrients concentrationscan take place within a relatively short space of time. This may affect nutrientsabsorption pattern of a crop and consequently growth and yield. The depletionof nutrients by plants nutrient solution depend on original concentration, cropspecie or cultivar’s rate of absorption, temperature of root rhizosphere andvolume of solution in which plants are growing. Looking into these factors somemeasures can be adapted by the researcher to minimize the rapid depletion ofthe nutrients in a nutrient culture experiment. These measures are use of highconcentration, planting in large containers, maintaining adequate temperatureand frequent renewal of culture solution. As far as solution renewal is concerned,Yoshida et al. (1976) suggested that change the culture solution once a week atearly growth stages and twice a week from active tillering until flowering. Asherand Edwards (1983) suggested a mathematical equation for the calculation oftime interval between solution renewals:

T = DVC/100RU,

where: T = time interval in hours, D = maximum acceptable depletion (%), V= volume of solution per pot or per plant (liters), C = initial concentration of an

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ion in the solution (µ M), R = root weight per pot or per plant (g fresh wt) andU = uptake rate per unit root weight (µ mol g−1 fresh wt. h−1 at concentrationC).

The draw back of this equation is that one should know the maximumacceptable depletion, which may depend on yield reduction of a crop due toparticular depletion value and second parameter is rate of nutrient uptake. Some-times, if a researcher use published information, it may not be available for aparticular crop species or a cultivar of specie.

Use of continuous flow technique is another way to maintain stable nutri-ents concentration in solution culture studies. A consideration of maintainingstable concentrations of nutrients, together with the amount of labor involved inrenewing a large series of solutions, leads obviously to the suggestion that thesolution might be made to flow continuously through the culture vessel, the in-flow being of known composition and the outflow being discarded or reutilized.The continuous flow system has many advantages:

1) Concentration of the dilute solution can be kept constant at a given value.2) It is suitable for experiments where pH is to be maintained at a given value.3) It keeps a constant flow rate of the nutrient solution at a given temperature

and humidity.4) The technique is well suited for comparative studies in the nutrition of plant

species.5) There is no risk of injury to plant material on renewal or replenishment of

the solution during the experiment.6) It is ideally suited for studies of nutrient interactions, because the con-

centration of all the nutrients can be controlled throughout the period ofexperimentation.

7) It can also be one of the important techniques in screening crop genotypesfor nutrients use efficiency.

The basic principle of the continuous flow system is that the rate of nu-trient uptake (U) is equal to the product of the flow rate (F) and the dif-ferences between the concentration of the solution entering the system (Co)and of the outgoing solution (Cs). A mathematical equation can be written asfollows:

U = F(Co − Cs)

The rate of ion uptake expressed in µg h−1 g−1 root weight (may befresh or dry) is calculated from the following formula (Hai and Laudelout,1966):

Rate of ion uptake = [(1 − Cs/Co) × (F × C)/(root weight)],

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Table 4Composition of flowing culture solution

Nutrient Islam et al. (1980) Fageria (1976)

Macronutrients (µM)Nitrogen (NO−

3 ) 250 193Nitrogen (NO+

4 ) — 227Phosphorus 15 31Potassium 250 250Calcium 250 125Magnesium 10 41Sulfur 261 26.8

Micronutrients (µM)Manganese 0.25 2.0Zinc 0.50 0.17Copper 0.10 0.2Boron 3.0 9.7Iron 20.0 9.5Molybdenum 0.02 0.004Chlorine 5.0 46

where Cs is the concentration of the outgoing solution, Co the concentrationof the ingoing solution, and C the concentration of the stable ion in nutrientsolution (ppm or µmol L−1).

In the continuous-flow culture technique, flow rate through the system isone of the most important parameters to be considered in ion uptake studies.The technical and methodological aspects of this technique can be found inmany publications (Hai and Laudelout, 1966; Fageria, 1974, 1976; Callahanand Engel, 1986; Wild et al., 1987). Composition of flowing culture solution ismuch lower than stable or non-flowing culture solution (Table 4).

Duration of Solution Culture Experiment and Observations

Solution culture experiments are generally short duration experiments. Theobjective of the solution culture experiments is to gain a better understandingof fundamental factors, which govern plant growth and nutrients uptake. Insolution culture experiments seeds are generally germinated in quartz sandor moistened paper towels. Paper towels generally soaked in dilute solutions(1/10 of strength) or simple in 0.1 mM CaCl2 solution during germination.Seeds should be surface sterilized for few minutes in an appropriate solution toavoid fungus development during experimentation. For rice seed germination,Yoshida et al. (1976) recommended that seeds should be sterilized for 4 minute

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with 0.1% mercuric chloride solution or soaks them in a formalin solution for15 minutes.

Then, wash thoroughly with several changes of demineralized water. Grainlegumes seedlings should be transferred in nutrient solution treatments after 2 to3 days of germination and cereals seedlings may be transferred 4 to 5 days aftergermination. This timetable is arbitrary and it may change according to needs orobjective of an experiment. The duration of growing plants in treated solutiondepends on objective of the experiments but 3 to 4 weeks growth duration issufficient for mineral nutrition studies. Some researchers use only a few hoursor few days in nutrient uptake studies.

It is my opinion that such short duration experiments cannot produce anymeaningful results of the subject under investigation. Plants should grow at leastfew weeks to show their response to applied treatments as well as interactionswith environments in which they are growing. Some researchers argue thatlonger duration experiment may change the nutrients uptake behavior of theplants and exact mechanisms are not understood. However, one should notforget that plant would not produce meaningful dry matter in few hours or evendays.

As far as data collection is concerned, change in pH, depletion of nutrientsconcentration, plant tops and roots fresh as well as dry weight and root lengthsmay be recorded in solution culture experiments. These parameters can provideadequate information for analysis and interpretation of experimental results.Further, some data collection or observations discussed in the soil culture sectionabove may also be used in solution culture studies. In Table 5 are presented someplant parameters and their unit commonly used in plant nutrition research.Similarly, in Tables 6 and 7 are presented some conversion factors for non-metric units to metric unit or one concentration to another concentration, whichare useful in transferring results of soil fertility and plant nutrition researchfrom one unit to another.

RESULTS AND DISCUSSION

Research in agriculture is a complex process and demands constant efforts andexperimentation due to change in weather conditions, difference in soil prop-erties, difference in adaptation of crop species and different socio-economicalconditions of the farmers. Soil fertility and plant nutrition research like anyagricultural research involves laboratory, greenhouse or growth chamber, andfield experimentation. Laboratory and greenhouse experiments are generallyshort duration experiments conducted to develop and understand some basicprinciples of subject under investigation. For example, pot experiment withdifferent types of soils can show the degree of response that may be antic-ipated at different soil-test levels and serve as excellent checks on ratingsbeing used. Since such tests provide no measure of the cumulative effects

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Table 5Soil and plant parameters and their unit commonly used in soil fertility and plant nutritionresearch

Symbol orParameter Unit preferred SI unit

Land area Square meter, Hectare m2, haGrain or Dry Matter

YieldGram per square meter,

kilogram per hectare,megagram per hectare, tonper hectare

gm−2, kg ha−1, Mg ha−1, t ha−1

Ion Uptake Mole per kilogram persecond dry plant tissue,mole of charge perkilogram per second dryplant tissue

Mol kg−1 S−1, Molc S−1

Nutrient Conc. inPlant Tissue

Millimole per kilogram,gram per kilogram,milligram per kilogram

mmol kg−1, g kg−1, mg kg−1

Nutrient Conc. inSolution

Milligram per litter, Centimolperlitter

mg L−1, cmol L−1

Soil Extractable ion(mass basis)

Centimol per kilogram,milligram per kilogram

cmol kg−1, mg kg−1

Fertilizer ApplicationRate to Soil

Gram per squiremeter,kilogram per hectare

g m−2, kg ha−1

Lime or GypsumApplication Rateto Soil

Ton per hectare, megagramper hectare

t ha−1, Mg ha−

Soil Bulk Density Megagram per cubic meter,gram per cubic centimeter

Mg m−3, g cm−3

ElectricalConductivity

Siemen per meter,decisiemen per meter

S m−1, dS m−1

Cation ExchangeCapacity

Cation exchange capacity perkilogram

Cmol kg−1

Absolute Growth Rate Milligram per day mg d−1

Crop Growth rate Milligram per square meterper day

mg m−2 d−1

Relative Growth rate Milligram per gram per day mg g−1 d−1

Leaf Area Index Square meter per squaremeter m2 m−2

Leaf Area ratio Square meter per kilogram m2 kg−1

Leaf Weight Ratio Gram per gram g g−1

Net Assimilation rate Gram per square meterper day

g m−2 d−1

Specific Leaf Area Square meter per kilogram m−2 kg−1

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Table 6Conversion factors for non-SI units to SI-units most commonly used in soil fertility andplant nutrition research

Conversion unit Multiplied by Converted unit

Acre to Hectare 0.405 HectareAcre to Squire Kilometer 4.05 × 10−3 Squire kilometerSquire Mile to Squire kilometer 2.590 Squire kilometerSquire Foot to Squire Meter 9.29 × 10−2 Squire meterSquire Inch to Squire Millimeter 645 Squire millimeterPound per Acre to Kilogram

per hectare1.12 Kilogram per hectare

Pound per Acre to Tons per Hectare 1.12 × 10−3 Tons per hectarePound per Acre to Megagram

per Hectare1.12 × 10−3 Megagram per hectare

Millimhos per Centimeter toSiemen per meter

0.10 Siemen per meter

Millimhos per Centimeter toDecisiemen per Meter

1 Decisiemen per meter

Milliequivalent per 100 Grams toCentimol per kilogram

1 Centimol per kilogram

Percent to Gram per Kilogram 10 Gram per kilogramParts per Million to Milligram

per Kilogram1 Milligram per kilogram

Milliequivalent per liter toMilligram per Liter

Equivalent Weight Milligram per liter

Millimole to Mol per Cubic Meter 1 Mol m−3

P2O5 to P 0.4365 PK2O to K 0.8301 KCaO to Ca 0.7147 CaMgO to Mg 0.6032 MgSO4 to S 0.3339 SNH3 to N 0.8225 NNO3-N to N 0.23 N

To convert converted unit into conversion unit divide by multiple factor.

of treatments on yield or soil buildup or depletion, they have limited valuein determining rates of fertilizer that should be recommended for sustainedproductivity.

Due to large variation in environmental factors, results of controlled condi-tions experiments can hardly be extrapolated to field conditions and vice-versa.However, these two types of experiments should serve as complementary com-ponents in developing a crop production technology. In the controlled con-ditions experiments soil and solution culture are generally used as medium ofplant growth to test treatment effects. Although, use of nutrient solutions allows

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Table 7Conversion factors for plant nutrients from ppm to mmol and mmol to ppm

Nutrient ppm to mmol (multiply by) mmol to ppm (multiply by)

N 0.071 14.01P 0.032 30.97K 0.026 39.10Ca 0.025 40.08Mg 0.041 24.31S 0.031 32.07Zn 0.015 65.38Cu 0.016 63.55Mn 0.018 54.94Fe 0.018 55.85B 0.093 10.81Mo 0.010 95.94Cl 0.028 35.45

precise control of experimental variables, it eliminates entirely the soil-root as-pect, an important part of soil-plant system. The pattern of exploration and ac-tivity in root systems subjected to zonal salinization as well as the significanceof ionic motilities in determining quantities of a given element absorbed fromsoil suggest the importance of testing hypothesis in a soil system, especially asystem similar to that found in the field.

Many of the successful conditions and details involved for successfulgrowth of plants in soil and solution cultures are not explained in publica-tions where these methods have been used. Much of the information aboutconducting controlled-condition experiments is taken for granted and left tothe ingenuity and experience of investigators. Many helpful ideas and prac-tices come only from experience. Some of the concerns, problems, and carerequired to conduct controlled-condition experiments have been discussed. It ishoped that the comments and ideas given will be helpful to others who conductcontrolled condition experiments in the field of soil fertility and plant nutrition.

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