Cap­tur­ing soil ni­tro­gen

The Western Star - - RURALWEEKL­Y - PAUL MCIN­TOSH

I WAS con­tem­plat­ing the sum­mer sea­son many of us have had and how much sub soil mois­ture there may be in our fu­ture win­ter crop blocks.

Those who were un­der the big rain events in late Fe­bru­ary and early March would be rea­son­ably happy with their pro­files of mois­ture.

Con­grat­u­la­tions on get­ting un­der­neath these big storms, and given your soil struc­ture and stub­ble lev­els were ap­pro­pri­ate at the time, sub soil mois­ture lev­els would be very handy now.

What do any of us think about our ni­tro­gen lev­els?

It is the ma­jor nu­tri­ent which all plants re­quire to sur­vive and of course, as farm­ers and agros, we do not just need the wheat or bar­ley crops to sur­vive, we want them to yield as much grain as pos­si­ble.

It is then an­other step up to achieve a higher pro­tein level in the grain to give bet­ter re­turns to your bank ac­count.

So it is a fairly im­por­tant de­ci­sion is the ni­tro­gen one.

How­ever it is not just about rate is it?

Ap­ply 120kg of ni­tro­gen (260kg urea) per hectare with about 50kg per hectare of ni­trate ni­tro­gen al­ready in the root grow­ing zone of the soil pro­file and you could pro­duce just over four tonne per hectare of wheat at 12 per cent pro­tein.

Sounds sim­ple an equa­tion and it is nearly the end of April, so in the next cou­ple of months, surely we will get some rain to give us an op­por­tu­nity of plant­ing of a ce­real crop.

Have you ever given much thought as to how the ni­tro­gen spreads through the soil from where you place it and just as im­por­tantly, how long does it take?

The yield equa­tion in the above ni­tro­gen bud­get it­self, is what we have been as­sum­ing for many many years.

It is quite cor­rect, how­ever as­sumes your ni­tro­gen ef­fi­ciency is run­ning at the 50 per cent level and that is prob­a­bly low enough to hor­rify many of you.

So fac­ing up right now to con­sider a ni­tro­gen ap­pli­ca­tion for this win­ter crop is not so sim­ple and could be some­what au­da­cious to as­sume you will get any­where near 50 per cent ef­fi­cacy and it could be as low as 20 per cent use­ful­ness to your wheat or bar­ley crop.

First let us as­sume un­for­tu­nately due to lack of the sum­mer rain, you have a dry­ish top­soil down to 25-30cm and have re­cently ap­plied ni­tro­gen be­neath the soil sur­face, then you are go­ing to need some se­ri­ous rain events to spread and mo­bilise down­wards, the ni­tro­gen into a fu­ture de­vel­op­ing root zone.

So I be­lieve a 25mm rain­fall event is not go­ing to cut it, even in very light soil types.

You are go­ing to need mois­ture and warm soil to start and speed the con­ver­sion from urea to a plant avail­able form of ni­tro­gen.

The warm soil is not a prob­lem, how­ever the lack of rain is the is­sue.

In a typ­i­cal soil type with around 25 to 50 cation ex­change ca­pac­ity, which describes much of our soil of our win­ter crop re­gion, I would sug­gest you are go­ing to need 75mm of steady pen­e­trat­ing rain­fall to fa­cil­i­tate con­ver­sion and most im­por­tantly, for the ni­tro­gen to be moved ben­e­fi­cially deeper into the soil pro­file.

This wet­ting front can be more than half again in front of your ni­tro­gen pas­sage.

In other words the wet­ting front could be at 30cm and the ni­tro­gen may be only half way or less even.

These rough fig­ures could be very de­pen­dent on your soil type and I sus­pect that the heav­ier the soil, then slower the ni­tro­gen fronts de­scent.

Let us con­sider that you got un­der all that rain of 125-200mm in Fe­bru­ary and March this year and you did not have your ni­tro­gen ap­plied.

That would have been a fair call not to ap­ply ex­tra N be­fore then, as it was fairly dry and our sum­mer crop prof­itable eco­nom­ics was fairly sad.

So you have a good pro­file of mois­ture now and in­tend or have ap­plied post rain ni­tro­gen to these fu­ture win­ter crop blocks.

So we need an­other good rain event and none of us would knock that back.

It is a real co­nun­drum when to ap­ply ni­tro­gen to your ce­real grain pad­docks and I will not state it is an easy de­ci­sion time.

Var­i­ous parts of Queens­land re­ceived a big rain­fall in this mid-Fe­bru­ary to late April pe­riod.

We need to ap­ply ni­tro­gen for our win­ter crops be­fore these huge down­pours oc­cur, to get a re­li­able and more ef­fi­cient N re­sponse from our wheat and bar­ley crops.

PHOTO: FILE

WIN­TER CROPS: It is im­por­tant to take ad­van­tage of rain­fall events to get ni­tro­gen lev­els right for plant­ing win­ter ce­real crops.

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