Farmer’s Weekly – 23 August 2019

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