4.2 Nutrientdemandandfertilizerrequirements
P.M. Driessen
InSection 4.1 theeffects of themainplant nutrients nitrogen, phosphorus
and potassium on crop yield and the relations between nutrient supply and
nutrient uptake were discussed using a three quadrant presentation. If these
principles are applied to actual cropping situations, four practical questions
mustbeanswered:
- howcananutrientshortageberecognizedinacertainsituation?
- howmuchofthatnutrientdoestheunfertilizedsoilsupply?
- whatistherecoveryof acertain fertilizer- nutrient if applied inaspecific
way?
- how much fertilizer must be applied to realize Production Situation 2,
where crop yield is solely determined by weather conditions and water
availability?
As is evident from the discussion of the principles of nutrient demand and
supply, suchquestions pertaining toaspecific cropping situation canonlybe
answered withtheaidof information collected infield experiments and from
chemical analyses.Thepurposeof thissectionistoshowhowthesequestions
can beanswered on thebasis of minimal information. Forpractical reasons,
thediscussionwillbelimitedtosituationsinwhichtheavailabilityof nitrogen
and/or phosphorus determinescropperformance; situations whereyieldsare
limitedbypotassiumshortagearelesscommon.
4.2.1 Recognition ofnutrientlimitation
The easiest way to recognize nutrient stress in a crop is through chemical
analysisof suchbiomasscomponentsasstrawandseed,or - ifthedeficiency
isserious enough to manifest itself inthephysical appearance of thecrop throughidentification of specific deficiency symptoms suchasdiscolouration
ornecrosisof plantorgans.Theinformation needed forcorrectdiagnosiscan
onlybecollected withthebackingof anadequately equipped plant analytical
laboratory. This is the major reason that such data arescarce. Soil chemical
data aremorecommonly available asthey areroutinely collected insoilsurveys. Unfortunately, there is not a generally valid quantitative correlation
between soil analytical data and crop performance. Soil analysis data canat
best give an indication of likely element deficiency or of the occurrence of
unfavourable soil conditions which would conceivably obstruct the normal
functioningof plants.ThissubjectwillbefurtherelaboratedinSection5.3.
A convenient way to recognize nutrient limitation is by comparing actual
182
plant production in the field with the calculated production in Production
Situation2. Obviously, suchcalculations mustbedoneforacropspeciesand
variety with similar properties asthe one grown inthe field and for weather
conditions and a water regime the same as that in the field. Likewise, the
actual field production must havebeenobtained underconditions thatapply
to Production Situation 2, viz. in aweed-free environment, with adequate
control of pestsanddiseasesandwithoptimum harvesting methods. Inaddition, chemical and physical soil conditions must be such that they have no
adverseeffect onplant performance. Theoutcome of suchacomparisoncan
beeitheroftwopossibilities:
- one is that the actual production isclose to thecalculated production. In
that case growth is not limited by a defiency of nitrogen or mineral elements.
- the other is that the actual production is clearly lower than the one
calculated for Production Situation 2. For identification of the element
thatisinshortsupplyitistheninevitabletoperformachemicalanalysisof
cropcomponents.
If, forexample, thenitrogenconcentrations of theanalysed plant partsare
distinctly higher than the minimum values as given in Section 4.1, it maybe
concluded that nitrogen availability isnotthelimiting factor andtheconcentrations of other nutrients must be checked. Phosphorus shortage is then a
likely candidate. If, on the other hand, the nitrogen concentrations of the
analysedplantpartsapproachtheirminimumvalues,itmaybeconcludedthat
nitrogen shortage limits crop production. It is then worthwhile to consider
applicationof anitrogen fertilizer.Theimprovedplantgrowthresultingfrom
this N application will also increase the demand for other elements such as
phosphorus and potassium. If this additional demand cannot be met bythe
soil, i.e. if the effect of N application remains below expectations, mineral
elementsmustbeappliedinadditiontonitrogen.
In practical plant nutrition research, nutrient demands are not identified
sequentially, butsimultaneously. Thisisdoneinfertilizer experiments,which
involve cropping anumber of identical fields, arranged ina randomized designandeachplantedtothesamevarietyandfertilizedinasimilarfashionbut
withdifferent combinations of N, PandKfertilizers. Theadvantage of such
experimentsisthatpossibleeffects ofnitrogenapplication,mineralelement(s)
application, or combinations thereof, become apparent after only onecropping season with one weather and one water regime which facilitates their
interpretation. Obviously, thenumberof plotsincludedinaparticularexperiment depends on the number of types/combinations and doses of fertilizers
tested;itisoften considerable. Moreover, theexperiments mustbedonewith
a numberof replications to ruleout misinterpretation of thesituation dueto
local anomalies or human failure. Each experiment includes at least oneunfertilized plot, the 'control plot'. The function of this control plot deserves
183
particular attention, as it is instrumental in answering the second practical
questionposed atthebeginningof thissection:Howmuchof anutrientdoes
theunfertilized soilsupply?
Exercise57
InSection 2.3 (discussing Production Situation 1,wherewater, nitrogenand
mineral elements are optimally supplied) anexample was given in which the
production of the high yielding ricevariety IR8 was calculated for abunded
experimental site near Paramaribo, Suriname. Suppose that on this same
stationanunfertilized field plantedtoIR8(onthesamedate)producedatotal
above-ground dryweight of 2000 kgha"1 with agrain-straw ratio of 1.0.
The total quantity of nitrogen contained in the above-ground production
was14kgha"1.
- Identify the total above-ground biomass production and the grainyield
calculated for Production Situation 1 inSection2.3;calculatetheproductionofstrawinthissituation.
- What would the above-ground biomass production be at Production
Situation2?(Remember:bundedricefields)
- Do you think that fertilizers must be used for high production on that
location?
- If so, is nitrogen the first element that has to be supplied? (Recall that
minimum nitrogen concentrations inricegrainandstrawarearound 0.01
and0.004kgkg"1,respectively)
4.2.2 Nutrientuptakefrom unfertilizedsoils
As unfertilized soils cannormally still support acrop, theycannot beentirelydevoidof nutrients.Influx ofelementswithwind,rainorirrigationwater
isapossibility, but morecommonly thesenutrients originate from sourcesin
the soil itself. Nitrogen is predominantly supplied through microbial breakdown of soil organic matter, which - depending on its botanical originand
genetic history - normally has a nitrogen concentration between 0.01 and
0.05 kgkg"1. Mineral nutrientsarealsosupplied inthisprocessbut, withthe
possible exception of phosphorus, only in rather negligible quantities. The
bulkofthemineralelementsoriginatesfromweatheringrockfragments.
The rate at which indigenous nutrients are supplied depends partly on soil
characteristics such as organic matter content and composition, soil mineral
composition and soil pH, but also on a score of exogenous factors that are
often highlyvariable(Section4.1). Anexampleissoiltemperature, subjectto
184
daily and seasonal fluctuations, and also influenced by weather conditions,
vegetationcoverandtheaction of man.Tocomplicatethingsevenmore,not
all thenutrients released areavailable for uptake byplant roots, astheymay
leachoutoftherootzone,precipitateaslow- solubilitycompounds,etc.
This complexity makes it virtually impossible to predict the uptake of nutrientsfromanunfertilized soilonthebasisoftheoreticalconsiderationsonly.
Forthetimebeing, thequantityof thegrowthlimitingelement takenupby a
certaincropfromtheunfertilized soilcanbestbeestablished bysimplydividing the control yield by the slope of the yield-uptake curve for that element.The growth limiting element can be identified, as explained, by
determining element concentrations intheplant tissue. Theslope of theyield- uptake curveis established on thebasis of theratio of economic product
andcropresiduesandtheirminimumelementconcentrations.
If fertilizer trials arecontinued at anexperiment station over anumberof
years,itisnotuncommonthatthe'baseuptake',i.e. thequantityof nutrients
takenupfrom unfertilized plots, increasesinsuccessiveyears.Tounderstand
this,itmustberealizedthatthelocationofthecontrolplotchangeseachyear,
because the individual fields of the experiment are laid out in a randomized
design. Nutrients applied, but not completely taken up in one year may to
some extent remain in the soil and increase the level of soil- supplied nutrients in subsequent experiments. The same happens in practical farming
where fertilizers are used: natural soil fertility improves inthe course of the
years and base uptake reaches a new (higher) level. The magnitude of this
improvement dependsonbothcultivationpracticesandenvironmentalconditions.Thismaybeillustratedbytwoextremeexamples:inthecaseofnitrogen
application tobunded rice,carry-over effects areoften lowbecausemostof
thenitrogenthatisnotdirectlytakenupislostthroughdenitrification and/or
leaching. A completely different situation exists in natural pastures in semi-arid regions, where almost no nitrogen is lost from the soil except by
plantuptake.
Phosphorus is less mobile in the soil- plant- atmosphere system than nitrogen. Therefore, losses of phosphorus from the system arenormally lower
than losses of nitrogen and, consequently, the carry-over of phosphorus is
higher. Inpractical farming, phosphorus issometimes applied inhighdoses,
which promise asatisfactory Psupply overanumberof years. Howeverthis
effect should not be overestimated, asalarge part of the fertilizer not taken
upinthefirstyearistransferredtoformsthatarefarlessavailabletoplants.
Exercise58
Table 44, from 'A Summary Report on Yield Response of some Thai Rice
Varieties to Varying N and P205 Combinations' (Thai Rice Department,
1956), presentsaverageyieldsof someThailowland ricevarietiesobtainedin
185
Table44. Averageyields(kgha"l)ofThailowlandricevarietiesatdifferent levelsof N
and PcombinationsatBangkhenExperimentStation.
Ratesofnitrogen Ratesofphosphorusapplied(kg ha"*)
applied(kgha-1)
P-0
P-37.5
A .Cropyear1952-1953
N-0
N-37.5
N-75.0
N-150.0
B. Cropyear1953-1954
N-0
N-37.5
N-75.0
N-150.0
C. Cropyear1954-1955
N-0
N-37.5
N-75.0
N-150.0
P-75.0
P-150.0
2107
2211
2389
2232
2394
2484
2493
2551
2727
2706
3074
3159
3048
3220
3665
3164
3303
3649
3547
4188
4682
3686
4166
4647
3649
4291
4691
1214.4
1529.4
1755.6
threeconsecutiveseasonsatBangkhenExperimentStation.
- Identify the control plot yields obtained in the three consecutive crop
years.
- Can you explain why the control yields obtained in the three consecutive
yearsaredifferent?
- Experience with traditional Thai rice varieties has shown that their rough
grain*-straw ratio is normally close to 0.5 ('long straw varieties'). The
minimumnitrogenconcentrations of roughgrainandstraware0.0078 and
0.0036, respectively.
Calculate with these numbers the slope of the rough grain yield N uptake
curveandusethisslopetocalculatethenitrogenuptakefromthecontrol plots
inthethreeconsecutivecropyears.
Rough grain pertains to unhusked rice with 12percent water; one kilogram of
roughriceisequivalentto0.8kghuskedrice(Section5.4).
186
4.2.3
The recovery of nutrients applied in fertilizers
In Section 4.1 it was shown that the increase in nutrient uptake following
fertilizer application is generally proportional to the quantity of that nutrient
added, at least in the case of nitrogen and potassium. It was also shown that
there are exceptions to this rule, e.g. in the case of immobilization or fixation
of the added element by the soil. In this section not all possible exceptions will
be treated, but element recovery in a normal situation where application- uptake relations are linear in the relevant range will be discussed. It was also
explained in Section 4.1 that the slope of the application - uptake relation, i.e.
the quantity of a certain element taken up from a fertilized soil minus the
quantity taken up from the same but unfertilized soil divided by the quantity
of that element contained in the applied fertilizer, is called the recovery fraction, or in amathematical notation:
Rx = (u f , x -u 0 , x )/A x
(79)
where
Rx
uf x
u 0x
Ax
isthe recovery fraction of element x (kg kg"')
isthe uptake of nutrient x from fertilized field (kg ha"x)
isthe uptake of nutrient x from control field (kg ha" ! )
isthe application of nutrient x to fertilized field (kg ha"*)
Theoretically, Rxranges in value from close to 0to close to 1.0;it expresses the
efficiency with which acertain fertilizer is used.
The actual recovery of an element depends on the competitive position of
the plant relative to processes in the plant- soil- atmosphere system that
render the element unavailable to the plant. Taking nitrogen as an example,
volatilization of ammoniacal nitrogen, leaching of nitrogen in nitrate form,
denitrification to gaseous N forms and immobilization of nitrogen by soil
microbes are processes lowering the recovery of applied fertilizer N. It follows, therefore, that a low recovery can be improved by either increasing the
uptake activity of the plant roots or by decreasing the impact of the competing
processes, or both. In the case of nitrogen, about o n e - t e n t h of the nitrogen
taken up is needed in the root system, so that the maximum N recovery in the
above-ground parts of a crop would be of the order of 0.9 kg kg" 1 . In
practice, it is often difficult to reach a higher recovery than 0.5 kg kg" 1 . If the
actual N recovery is well below this value it is worthwhile to try to improve the
situation. It is this aspect, viz. indication of the need for adaptation of variety
and/or cultural practice, that makes it so important to quantify the recovery
of applied fertilizer nutrients in the analysis of acropping situation.
The recovery of a certain element can be calculated from fertilizer experiments, if the biomass components are analysed for that element. It was
187
explainedwhysuchcompleteexperimentsareratherscarce.Theirnumberand
regionaldistributionmaybesufficient toindicatewhetherinacertainregiona
situation exists or has been created where shortage of a certain element is
prominent, butcompleteexperiments arenearlyalwaystoo few innumberto
allow sufficiently reliable estimation of the recovery of that element from
appliedfertilizers. Inpractical farming, nutrientrecoverydependsnotonlyon
plant properties and environmental conditions but also on management factors, such as the type of fertilizer used andthe timing andmode of fertilizer
application. If, forinstance,ahighdoseofanitrogenfertilizer isbroadcastat
thebeginningofthecroppingseason,whenthenitrogendemandofthecropis
still low, high losses of nitrogen from the root zone can be expected and,
consequently, lowNrecoveryvalues.Adaptedapplicationmethodsandgood
timingof fertilizerapplication mayleadtoincreasedNrecoveryinsuchsituations,aswillbeexplainedlater.
Where complete experimental information is scarce, the limited amount
available must beusedasefficiently aspossible. Ifitisknownwhichelement
ismost likely limiting thegrowth of acertaincropinaparticular situation,a
first estimateof nutrient recoverycanbemadewithout further chemicalanalysis of plant components. This is possible because it may be assumed then
thatthelimitingelementinthecropisdilutedtoitsminimumconcentrationin
thevarious plant parts. The simplest experiment that still yields therequired
information, consists of acontrol plotandafertilized plot whose production
components, viz.economicyieldandcropresidues,areseparatelyweighed.It
must be realized that the reliability of only one recovery value, calculated in
thisway, islow andthatsuch resultsareonly indicative. Theresultsofproperly conducted experimental series, normally published as average yieldsper
treatment with specified standard deviations, are a sound basis for recovery
calculations.
Exercise59
InTable44average yields of rough ricearegiven for threeconsecutiveyears
ofN- PfertilizerexperimentsatBangkhenExperimentStation,Thailand.
- Fromtheaverageyieldvaluesfor 1954 - 1955,itcanbeobservedthatata
givenlevelof nitrogenapplicationdifferent Papplicationsdonotresultin
significant yield differences. Which conclusion can bedrawn withregard
tothePsupplytothecropinthissituation?
- What hascaused thissituation? (Note:look alsoattheeffects of Papplicationonyieldsin 1952- 1953and 1953- 1954).Isitlogical thatthesame
situationhasapparentlynotbeenreachedforthesoil'snitrogenstatus?
- Calculate the nitrogen recovery fractions realized in 1954-1955 for the
fertilizer combinations N 37.5-P 37.5, N 75.0-P 37.5 and N 150.0-P
188
37.5. Assumethesamegrain-strawratioandminimumNconcentrations
asinExercise58.
Earlierinthissectionitwasarguedthatunsatisfactory recoveryof fertilizer
nutrientscanberemediedbyadaptation of cropcharacteristics,environmentalconditionsormanagement, aloneorincombination.Agriculturalresearch
hasdeveloped cropvarietiesthatcanrecovernutrientsandrealizeanacceptable yield level under conditions that would be prohibitive for unimproved
varieties, for example acidity-tolerant varieties andvarieties whichcansuccessfully begrowninabrackishenvironment. Betternutrient recoveryisnot,
or only marginally, involved in the success of the modern high yielding rice
varieties(HYV's)developedattheInternational RiceResearchInstituteinthe
Philippines.Thekeytothesuccessofthesevarietiesistheirhighgrain- straw
ratio, which is of the order of 1.0; HYV's areso-called 'short strawvarieties'.Thehighgrain-straw ratioisassociated withahighinitialefficiency of
theyield- uptakerelation, i.e. ahighincrement ingrainyieldperkgnitrogen
taken up. Their short posture allows uptake of considerable quantities of
nitrogenwithlessriskof lodging, whichquickly follows tooluxuriantvegetativegrowthoftraditional long- strawrices.Incombination, thesetwo effects
result in very high grain yields per hectare, provided that the crop is grown
under favourable environmental and management conditions that allow a
highnutrientrecovery.Underunfavourableconditionssuchasnutrientshortageorheavyweedinfestation, traditional ricevarieties,whichhaveevolvedin
thecourseof centuriesof naturalselection byricefarmingcommunities,may
wellperformbetter.
Increasing nutrient recovery by adaptation of the environment has been
practiced as long as agriculture exists. Bunded rice fields are an example:
nutrient recovery is promoted by the artificially created conditions in the
puddledtoplayerof awetricefield. Often, manipulation of theenvironment
involves management measures. Inthemanagement sphere, too, wetlandrice
cultivation offers many practical examples of how nutrient recovery can be
manipulated and improved. Illustrative of the great practical significance of
good management is the mode of nitrogen fertilizer application to bunded
rice.Inmanyriceareas,ureaisusedasanitrogensourceandbroadcastonthe
flooded fieldatthetimeoftransplanting.Theaveragerecoveryfractionofthe
fertilizer N is then often only 0.2, or even lower. This low recovery may be
understood as follows: the favourable temperature conditions and oxygenrich environment of the shallow water layer on top of the rice field promote
rapidtransformation of ureaNto ammonium ions(NH4+) and subsequently
to nitrate ions (N03~) . The nitrate ions are highly mobile and move downward into the soil with percolating water or by diffusion. The oxygen inthe
waterlogged soil is rapidly depleted by microbes decomposing soil organic
matter, and subsequently some bacterial species use the nitrate ions as an
189
oxygensource. NitrateNisthenconvertedto gaseous forms (N2orN20)and
escapes to the atmosphere. A good way to combat this nitrogen loss and
improve thenitrogen recovery isplacement of theureadirectly into thelowoxygen layer. There urea N is only converted to NH4+ ions that are, incontrastwithanionslikeN03~ tosomeextent protectedagainst leachingbecause
they areretained bythesoil through adsorption atthesurfaces of negatively
charged clay and organic matter particles (the process of ion adsorption will
bediscussed insomedetail inSection 5.3). Placement of ureainthepuddled
surface layer may well bring the recovery of nitrogen to a value of 0.5 or
higher. Ingeneral, losses of fertilizer Ncanbereduced andrecoveryimproved, if only small doses of fertilizer are given at a time, so that most of the
nutrientappliedcanbeabsorbedbytherootsinarelativelyshorttime.Therefore, repeated application of smaller doses spread over the growing season
('split application') is often superior to application of all fertilizer in one
dressing.
Exercise60
TheTechnical DivisionoftheThaiRiceDepartmentinBangkok hasconductedfertilizer experiments withbunded ricetoinvestigatetherecoveryofnitrogen from different types of fertilizer, applied at different moments and in
different ways (Lusanandana et al., 1966). Nitrogen application was 60 kg
ha on all fields. Inaddition, each field received phosphorus atarateof 26
kgha"1andpotassium atarateof 50kgha"1atthebeginningof thegrowing
seasontoeliminatethepossibilityofPorKshortages.
Table45presentsessential information onthedifferent treatments, aswellas
grainandstrawyieldsandtheresultsofchemicalplantanalyses.
- Examine carefully thedifferent treatments tested inthis experiment. Note
that four different nitrogen fertilizers were used, applied either at transplantingoratthetimeof earprimordiuminitiation, andatthreemodesof
application.
- Calculate for each treatment the P/N ratio at the time of primordium
initiation.Answerthefollowingquestions:
Do the P/N ratios indicate that nitrogen availability isthegrowth- limiting factor in this experiment? Are there indications that N fertilization at
transplanting (16July)hasameasurablebeneficial effect onproductionat
thetimeofprimordiuminitiation(21September)?
- Calculatethegrain- strawratiosatharvesttime.Arethericevarietiesused
intheexperimentmodernHYV'sortraditionallong- strawvarieties?
Whatdoesthismeanfortheslopeoftheyield-Nuptakecurve?Checkyour
answer bycalculating this slope with theresults obtained for one ormore
ofthetreatments.
190
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- Calculate thetotal nitrogen uptake for each of thetreatments and forthe
control(onthebasisofabove-ground parts).Then,calculatethenitrogen
recovery realized ineachof theeight treatments andanswerthe following
questions:
a. Arethe recoveries realized underTreatments A and Bsatisfactory? If so,
canyouexplainwhy(forbothtreatments)?
b. ComparetherecoveriesrealizedinTreatmentsCandD.
Is there reason to expect high losses of fertilizer nitrogen in these treatments?Canyouexplainthedifference inNrecoverybetweenTreatmentC
andTreatmentD?
c. Compare the N recoveries realized inTreatments Dand H, wheresodium
nitratewasappliedatdifferent stagesof thegrowthcycle.Canyouexplain
the difference in nitrogen recovery if you consider for each treatment the
competitivepositionoftheplantrelativetootherprocessesremovingnitrogenfromtherootzone?
d. It has been stated in this section that N recovery from urea, broadcast at
thetimeof transplanting isoften lowerthan0.2. Consider theNrecovery
realized under Treatment F and explain why you have found a different
recoveryvalue.
4.2.4 Thenutrientrequirementfromfertilizer
Thequantity of fertilizer that must beapplied to realize Production Situation2,cannowbeestablished. Ithasbeendiscussedinwhichwaythelimiting
element canbe identified. InSection 4.1 the minimum concentrations of the
three elements in the relevant plant parts are given. The total uptake of a
certainelement required formaximumproductioncantherefore becalculated
bymultiplyingthedryweightsofeconomicproductandcropresiduecalculated for Production Situation 2, withtherespective minimum nutrientconcentrations. Part of thetotal required uptakeiscovered bythenatural soil fertility or base uptake. This base uptake is established through analysis of the
production obtained on an unfertilized field. The difference between therequireduptakeat Production Situation 2andbaseuptakemust besuppliedas
fertilizer nutrient, therecoveryof whichisestablished foragivensituationby
makinguseofpublishedfertilizerexperiments.
This recapitulation demonstrates that the quantity of fertilizer nutrient
needed to create Production Situation 2, i.e. the 'fertilizer nutrient requirement', Dx, can bequantified bysubtracting from thecalculated total uptake
under Production Situation 2, umx, the uptake under unfertilized (control)
conditions, uox, and subsequent division by the recovery fraction, Rx. Ina
mathematicalnotation:
193
Dx = (um,x-u0,x)/Rx
(80)
Earlierinthissection itwasshownthatthesamenutrient canbesuppliedby
different fertilizer materials. Inthecaseof nitrogen, urea,ammonium fertilizersandnitratefertilizershavebeenmentioned,butothers,bothchemicaland
'natural' (e.g. farmyard manure) areavailable. Table 46 lists some common
commercial fertilizers and their concentration of pure nutrient. From this
tableitmaybededucedthatacalculatednitrogenfertilizerrequirementof 100
kgha"1canbemet withanapplication of 100/0.21 kgammonium sulphate,
but also with 100/0.45 kg urea, provided that nitrogen recovery is the same
from bothsources.Thisdifference partlyexplainsthepopularityof ureaasa
nitrogensource,especiallywhentransportcostsarehigh.
Exercise61
In Exercise 57 an experiment with a HYV near Paramaribo, Suriname was
treated. Assume that in that experiment nitrogen availability is the growth
determining factor atanylevel of production. Assume furthermore thaturea
isusedasafertilizer andthattherecoveryfraction fromthisNsourceis0.55.
Dry-grainyieldofanunfertilized fieldwas1000kgha"1.
- Calculate the total N uptake required to realize the production calculated
for Production Situation 2. Grain weight and total above-ground dry
weightatmaturityaregiveninSubsection2.3.2.
- Calculatetheslopeoftheyield- Nuptakerelationandthenitrogenuptake
iffertilizersarenotused.
- Calculatethenitrogenrequirementof thecropanddecidehowmany25kg
bagsofureayoumustreserveforonehectare.(UseTable46).
So fartheconcept of nutrient requirement wasillustrated usingnitrogenas
the limiting element and Production Situation 2 as the pursued production
target. Itiswellpossible,however,thatafarmerisnotinterestedinproducing
themaximumyieldof ProductionSituation2,e.g. ifthereisnomarketforall
the produce. He may then aim at a lower yield and adapt his fertilizer use
accordingly.
Itisalso possible thatcropgrowthisnot limited bynitrogen supplybutby
the supply of some other nutrient. Experience has shown that in manysuch
casesphosphorussupplyisthelimiting factor. Then, thephosphorusrequirement can becalculated with Equation 80, i.e. by quantifying the total phosphorus uptake needed to realize the production target, subtracting fromthat
amount the uptake of P from an unfertilized field'and dividing by the P
recovery fraction. Thecalculation procedure is identical for anynutrientbut
194
Table46.CommoncommercialNandP-fertilizersandtheir
nutrientconcentration.
Nitrogenfertilizers
Ammoniumsulphate
Urea
Ammoniumnitrate
Sodiumnitrate
Calciumnitrate
Calciumammonium
nitrate
Phosphorusfertilizers0*
N concentration(kg kg'1)
0.21
0.45
0.33
0.16
0.155
0.205
P concentration(kgkg~v
Superphosphate
0.08
Triplesuperphosphate(T.S.P.) 0.20
Rockposphateb)
0.16
Data from DeGeus, 1973;ILRI, 1972;JacobandV.Uexkull,1958.
a) Phosphorus contents areoften expressed in P2O5;1 kg
P205correspondswith0.44kgP.
b) Phosphorus contents of phosphate rocks varygreatly;
the P-content given refers to Christmas Island Rock
Phosphate(C.I.R.P.)
thebehaviourof thedifferent nutrientsinthesoil- plant- atmospheresystem
is not. Unlike nitrogen, phosphorus is in general not lost from the system in
significant amounts. Itschemistryinthesoilis,onthewhole, morecomplicatedthanthat of nitrogen andthat isoften reflected innon-linear applicationuptakecurves. Levellingoff athigherapplication rates,assignalledinSection
4.1 andillustrated inFigure45, isnot uncommon. Itshouldberealizedthatin
situationswithnon- linearapplication- uptakerelations,nutrientrecoveryis
not independent of the fertilizer application rate anymore! Most problems
with phosphorus recovery are related to the low solubility of many P compounds. Low P solubility may be due to a secondary reaction in the soil
solution, e.g. precipitation of phosphorus fromasolublefertilizerasinsoluble
calcium, ironoraluminium phosphates, buttherearealsoPfertilizersthatare
themselves hardly soluble such as rock phosphates. P recovery from, rock
phosphate is therefore commonly less than 0.05 (Penning de Vries & van
Keulen, 1982), but it does have the advantage that the effect may persist for
quite some years even after only one application. Obviously such low recoveries are inadequate to make rock phosphate a suitable fertilizer for realizing
Production Situation 2. By treating rock phosphate with sulphuric acid, the
195
fertilizer industry produces so- called superphosphate which has a higher P
concentration than rock phosphate and ismore readilysoluble. Consequently,
P recovery from superphosphate is higher than from rock phosphate. A recovery of 0.3 may be obtained on some soils, but on soils that strongly immobilize phosphorus, recovery from superphosphate is still very low. Phosphorus
immobilization in soils is complex and difficult to predict. As a first approximation, Table47 listsanumber of common soil groups arranged according to
the phosphorus recovery that may be achieved if they are cropped with an
annual crop, optimally supplied withwaterand nitrogen.
In production situations where water may at times be in suboptimum supply,
recovery may belowerdueto thereduced activity of plants underwaterstress.
Theuncertainty with respect to phosphorus availability insoils makesitgenerally advisable to be generous in the estimation of P requirements. Especially
as too much phosphorus does not do any harm and what is not taken up on
short notice isadded to the phosphorus stock of the soil and may be used bya
subsequent crop. This does not apply to soils that strongly immobilize phosTable 47. Indicative P recovery fractions of superphosphate applied to a
graincrop.ThesoilsarearrangedaccordingtodecreasingPrecovery.
Class
Recovery
fraction
Soiltype
I
0.30-0.15
II
0.15-0.08
III
0.08-0.02
— quartziticsandysoils
— organicsoils/peats
— young,neutral,coarseand
medium-texturedalluvialsoils
— weakly acid to neutral alluvial clay soils of
intermediateage
— weaklyacidtoneutral soilswithathickblack
organicsurfacelayer
— weakly or medium acid well structured clay
soils
— churningheavyclaysoils
— neutraltoweaklyalkaline,calcareoussoils
— permanently waterlogged mucky soils, low in
bases
— old acid red and/or yellow soils, rich in iron
andaluminium
— veryacidleached'podzoPsoils,richiniron
— strongly acidified soils in pyrite-containing
marinesediments,richinaluminium
— young brown or black volcanic soils, rich in
allophanes
196
phorus. With them the availability of fertilizer- P decreases dramatically in
thecourseoftime.
Exercise62
In Exercise 61 the nitrogen requirement of a HYV rice near Paramaribo,
Suriname, wascalculated undertheassumption that nitrogen availabilitydetermined crop growth. In this exercise it is assumed that the availability of
phosphorusisthelimitingfactoratanylevelofproduction.
- Calculate the total P uptake under Production Situation 2. Grain weight
and above-ground dry weight at maturity aregiven in Subsection 2.3.2.
Minimum P concentrations in grain and straw dry matter are 0.0011 kg
kg"1and0.0005 kgkg"1,respectively.
- Calculate the slope of the yield- P uptake relation and the phosphorus
uptakeif fertilizers arenotused.Assumeadry-grain yieldonthecontrol
plotof 1000kgha"1.
- CalculatethePrequirementofthecropandthequantityofrockphosphate
thatmustbeappliedforrealizationof ProductionSituation2.Assumethat
Precoveryfromrockphosphateis0.02kgkg"1(ConsultTable46).
- Calculatehowmuchtriplesuperphosphate (TSP)wouldhavetobeapplied
to realize Production Situation 2. Assume aPrecovery from TSP of 0.15
kgkg"1(ConsultTable46).
- Assumethatthereisariceglutandthattheproductiontargetisloweredto
0.8 timestheproductioncalculated forProductionSituation2.Howmuch
TSPwould havetobeappliedthen? Explainwhythisquantityislessthan
0.8 timesthe amount needed to realizethe production calculated forProductionSituation2.
Sofaritwasassumedthatonlyonenutrientisinshortsupply:thenitrogen
requirement wasdiscussed for asituation withphosphorus sufficiency orthe
phosphorusrequirement forasituationwherenitrogensupplywasoptimal.It
is, of course,possibletocreatesuchconditions,e.g. byapplyingagenerousP
dressing at the beginning of cropping inasituation where nitrogen supplyis
knownto belimiting. Therewould belittlerisk involved inahigh Papplication, but the same cannot be said of a high blanket dressing of nitrogen if
phosphorusisknowntobeinshortsupply.Excessivenitrogenlossescouldbe
theundesirable result. Itistherefore necessarytoestablishtheneed fornitrogenfertilizers, ifthePconcentrationof theplantisknowntobelimiting,and
the P fertilizer requirement in the reverse case. It was shown in Section 4.1
that relative nutrient shortage iswitnessed bytheratio of element concentrationsintheplanttissue.Inthecaseofphosphorusandnitrogen,theP/N ratio
197
variesbetweenamaximumof about0.15 andaminimumof about0.04.This
knowledgecanbeputtouse:if, forinstance,theavailabilityof phosphorusis
limiting,theoverall Pconcentrationof anunfertilized graincropwithagrain
strawratioof 1.0hasaminimumvalueof:
IX 0.0011 - M X 0.0005 = 0 Q 0 8
2
Its overall N concentration cannot be higher than 0.0008/0.04 = 0.02 kg
kg"1. It is also unlikely that the nitrogen concentration is much lower than
0.02, becauseNuptakeisnotdictatedbynitrogenavailabilitybutisentirelya
function of P availability. Addition of P would probably result in further
nitrogenuptakewithout anyadditionof nitrogen fertilizer (seealsoFigure50
inSection4.1).
Assume that in a particular situation the dry-grain yield and dry-straw
production of anunfertilized cropare 1000kgha"1 and that for Production
Situation 2thecalculated grainyieldequals 5000kgha"1andthestrawyield
also5000kgha"1. LetTSPbeusedasaPfertilizer andassumearecoveryof
0.1 kgkg"1.Fromthisinformationthefertilizerphosphorusrequirement, DP,
canbeestablishedaccordingtoEquation80:
Dp= ((5000x0.0011 + 5000x0.0005) - (1000x0.0011 + 1000x0.0005))/0.1
Hence, DPamountsto64kgha"1,whichcanbesatisfied withTSPatarateof
320kgha"1.
To quantify an accompanying N fertilizer application to ensure optimum
nitrogen supply at the high production level of Production Situation 2, the
same reasoning is followed. However, it would be unrealistic to estimate N
uptake from anunfertilized field onthebasisof minimum Nconcentrations,
becausethereal(overall) Nconcentration of thecontrol material hasalready
beenestimated. LetammoniumsulphatebetheselectedNfertilizerwithanN
recovery (established underconditions of mineral element sufficiency) of 0.5
kgkg"1.Thenthemaximumpossible fertilizer nitrogenrequirement amounts
to:
__5000x0.01 + 5000x0.004)-(2000x0.02) _ 6 0 k g h a - i
0.5
Thisrequirement canbesatisfied withanammonium sulphateapplication of
286 kg ha"1. Compared with a calculation based on assumed minimum N
concentrations inanunfertilized situation, this represents areduction incalculatedfertilizerinputbynolessthan 114kgha"1.
It should be realized that the so calculated N requirement guarantees N
sufficiency at aproduction ascalculated for Production Situation 2, butthat
thereisstillalikelyoverestimation of theNrequirement. Thetotal Nuptake
D
198
fromanunfertilized field (nowestimatedat2000x0.02 = 40kgNha"1)was
limited byPdeficiency andcould conceivably havebeenhigherinasituation
with Psufficiency. Inotherwords:natural soil fertility maycontributemore
than40kgNha"1iftheTSPhasbeenappliedascalculated.
The practical consequences of this - mild- overestimation arenormally
not prohibitive. It appears that, in general, the stock of readily available
nitrogenisrapidlyexhaustedandthatwithholdingNfertilization foronlyone
ortwoyearscreatesalreadyasituationwherePisinsufficient supplyandNis
limiting. Innormal soilsthat donot stronglyimmobilize phosphorus, acomparatively small maintenance application of P fertilizer will suffice to maintainPsufficiency onceafavourablephosphoruslevelhasbeenestablished.To
whatextent acertainsoil immobilizes P, howhighthemaintenancedoseofa
certain P fertilizer should be, and for howlongthat maintenance dosewould
probablyensurePsufficiency, canonlybejudged(inaratherqualitativeway)
if analysesof soil chemical conditions andsoil mineralogical compositionare
available. Some attention will be paid to this in Section 5.3. In practice,
maintenance applications are largely a matter of experience, gained from
years of fertilizer experiments and transferred to the farming community by
agriculturalextensionservices.
Above, a situation was considered in which P availability limits plant
growthinanunfertilized situation(sufficient Navailabletorealizethecontrol
yield but noguaranteed Nsufficiency atProduction Situation 2).Thereisno
needtoarguethatthesamereasoningapplies - withtherespective differences
considered- to asituation in which the control yield is dictated by nitrogen
shortageandPsufficiency atProductionSituation2isuncertain.
Exercise63
InExercise60,anexperimentwithtraditional ricevarietiesatBangkhenExperiment Station, Thailand, was considered. In that experiment different N
fertilizers were applied at various moments and with different application
techniques. To make sure that the effects of nitrogen application were not
blurred by P shortage, superphosphate was given to all fields at the time of
transplanting. ThePapplication of thisblanket dressingwas26kgha"1.The
valueofthatPapplicationcannowbejudged(refertoExercise60andTables
45and47).
- Calculatethetotal Nuptake from thecontrol plot anddividethisvalueby
thetotaldryweightofgrainandstrawtoestablishtheoverallNconcentrationof thedrymatterproduced undercontrol conditions. Assumeamoisturecontentof0.12kgkg"1inthefieldproduce.
- Wasthecontrol yield limited bynitrogenshortage? Ifso, approximatethe
maximum possible P concentration of the dry control material at harvest
199
-
-
-
bymultiplyingtheoverallNconcentrationwiththeappropriateP/Nratio.
ThetotalPuptakebythecontrolplotcannowbeestimated.
The crop under treatment A (Table 45) produced the highest amount of
plant matter, viz. 3335 kggrain plus 12410 kgstraw. Assume anaverage
moisturecontent of 0.12 kgkg"1andcalculatethetotal Puptake,whichis
minimally required to realize the dry matter production in Treatment A
(minimum P concentrations of drygrainand straware0.0011 and 0.0005
kgkg"1,respectively).
Bangkhen Experiment Station issituated on soilsbelonging totheRangsit
Soil Series. This soil has, under inundated conditions, properties which
placeitintheupperhalfof Precoveryclass IIinTable47. CalculatetheP
requirement fortheproduction asobtained undenTreatmentA. Assumea
Precoveryfromsuperphosphateof0.15 kgkg"1.
Wastheblanketphosphorusdressingof26kgha"1arealisticone?
200