Farmer’s Weekly – 23 August 2019

(Kiana) #1

vegetable production


Thelowering of pH is greater
whenthenitrate is leached
fromthesoil than when taken
upbythecrop. This is why
it’simportant to monitor the
depthofirrigation; apart from
thepHproblem, losing nitrate
is costlyandwasteful.

aninstant solution
If youneedto apply nitrogen
inanemergency, you can use
a ureafoliarspray. Apply it
ata rateofabout 5kg/ha.
LAN,however, is favoured
forvegetable production as
thesecropsgrow fairly quickly,
and providing nitrogen in two
forms is more likely to ensure a
good crop. Urea may be cheaper
to buy, but due to the greater
risk of loss from leaching,
it is not usually worth it.


  • Bill Kerr is a vegetable specialist
    and a breeder of a range of vegetables.
    Email him at farmersweekly@
    caxton.co.za. Subject line:
    Vegetable production. ▪FW


T


henitrateformofnitrogen
is highlymobileinthe
soil.Thewayinwhichyou
applyit willdeterminehowlong
theplantswillhaveaccesstoit.
Beinga negativelycharged
particle(ananion),nitrateis
repelledbythesoilcolloids,which
arealsonegative.Thisensures
thatit remainsinthesoilmoisture,
whereit is readilyavailableto
betakenupbytheplants.
Unfortunately,thisalsomeans
it is easilyleachedbyheavy
rainorover-irrigation,andis
transportedbeyondtherootzone.
It alsoendsupcontaminating
undergroundwaterandstreams.
Whentransplantingseedlings,
whetherbarerootsorinplugs,
frequentlightirrigationsare
appliedtoestablishtheplants.
Seedlingplugshavea different
texturetothatofsoil,somoisture
doesnotmovereadilyfromthe
soilintotheplugbycapillary
action,andtheseedlingsare
onlysafeoncetherootspenetrate
thesoilitself.Frequentlight
irrigationcarriesthenitrate
deeperthantheshallowroot
systemofsuchseedlings.

shortageofnitrogen
If nitrogenis lackinginthesoil
orunevenlydistributed,the
seedlingswill‘take’moreslowly
andgrowatdifferentrates.
A lackofnitrogenslowsdown
bothleafandrootgrowth.Many
farmersassumethattheplants,
needingwaterandnutrients,
prioritiserootdevelopment,
butinrealitytheycannotdoso
withoutnitrogen.Forthisreason,
boththenitrateandammonia
formsofnitrogenshouldbe
appliedjustbeforeplanting.
A handfulofLAN(containing
both forms) in 8ℓ to10ℓ of water
will suffice for about 2 000
seedlings. The nitrate will be
of immediate benefit, but is
vulnerabletoleaching;the
ammoniawillnotleachand
becomeavailablea littlelater.

nitr atecombinations
Nitrateis availableasammonium
nitrate,calciumnitrateand
potassiumnitrate.Although
allthreeformsarereadily
available,potassiumnitrate
andcalciumnitratearefar
moreexpensive,andbest
usedonlyinanemergency.

Potassium nitrate is two- thirds
potassium and often used
in the late developmental
stages of tomatoes and
peppers. Calcium nitrate is
useful for rapidly correcting
blossom-end rot on crops.
All forms of nitrogenous
fertiliserslowerthesoil’spH,
withammoniumsulphatebeing
themostpotentinthisregard.

ABOVE:
Nitrogenwas
notappliedto
theseedlings
ontheleft.
billkerr

lan is best for
vegetables as it
has both forms
of nitrogen

The nitrate form of nitrogen

BillkErr

42 farmer’sweekly 23 august 2019
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