Magnesium locking up potash?

Jgs

Member
Location
SW Scotland
I have a field with a high mg index (5.3) and moderate k (1.7) ph 6.1, it's fairly heavy land but last year it just didn't perform the grass was kinda yellowy and next to no yield compared to other fields. It was reseeded in 2011. now the yara rep tells me to put potash on ( he would) but I have for the last two years and it's done no use as far as I can see and was wondering if the mg would lock up the availability of k? Would gypsum bring down the mg?
Cheers
 

DrDunc

Member
Livestock Farmer
Location
Dunsyre
You shouldn't apply potash to grassland in the spring as it risks locking up Mg and causing grass staggers. High potash will lock up Mg.

The converse is also true, high magnesium levels will lock up K, as your commission earning fert pusher has said.

However the real problem with high Mg is its balance with Calcium. When the Ca to Mg ratio reaches 1:1 the Mg is too high. You are correct that applying gypsum is the right thing to do. This will increase the soil Ca, reduce the ratio of magnesium, and release more potash to the plant.

Don't use calcium lime as it's not soluble enough to alter the Ca:Mg ratio.

I don't know what amount of gypsum to apply to the acre without knowing your soil test results, but I wouldn't go asking the yara salesman;)
 

Steevo

Member
Location
Gloucestershire
Don't use calcium lime as it's not soluble enough to alter the Ca:Mg ratio.


Not soluble enough? Could you expand on this please?

If soil tests show that your calcium levels are too low, surely the most logical thing to do it add calcium limestone, as the Ca will be held most tightly by the soil colloid and will displace some of the Mg ions.
 

Jgs

Member
Location
SW Scotland
Thanks for that, I'm maybe getting more cynical but I just wasn't buying the same old "u need to spread more potash" at £xxx!il try get a hold of some gypsum and see how it goes.
Cheers
 

DrDunc

Member
Livestock Farmer
Location
Dunsyre
Not soluble enough? Could you expand on this please?

If soil tests show that your calcium levels are too low, surely the most logical thing to do it add calcium limestone, as the Ca will be held most tightly by the soil colloid and will displace some of the Mg ions.
The OP soil is pH 6.1 with a very high Mg level indicative of low calcium, though a further soil test is needed.

To significantly alter the Ca:Mg ratio, the quantity of Ca lime required would make the soil excessively alkaline, preventing the release of Ca, hence why you use gypsum.
 

Sir loin

Member
Location
North Yorkshire
You shouldn't apply potash to grassland in the spring as it risks locking up Mg and causing grass staggers. High potash will lock up Mg.

The converse is also true, high magnesium levels will lock up K, as your commission earning fert pusher has said.

However the real problem with high Mg is its balance with Calcium. When the Ca to Mg ratio reaches 1:1 the Mg is too high. You are correct that applying gypsum is the right thing to do. This will increase the soil Ca, reduce the ratio of magnesium, and release more potash to the plant.

Don't use calcium lime as it's not soluble enough to alter the Ca:Mg ratio.

I don't know what amount of gypsum to apply to the acre without knowing your soil test results, but I wouldn't go asking the yara salesman;)

My soil tests have shown very high mag and low potash this is for haylage grass land, (PH fine phosphate good) if the potash has been locked up by the magnesium then am I right in assuming that it would show up in the soil sample that potash levels were ok and that the grass would exhibit potash deficiency symptoms, or if locked up it wouldn't show up in the actual sample.
 

Warnesworth

Member
BASE UK Member
Location
Chipping Norton
Not soluble enough? Could you expand on this please?

If soil tests show that your calcium levels are too low, surely the most logical thing to do it add calcium limestone, as the Ca will be held most tightly by the soil colloid and will displace some of the Mg ions.
Excess calcium is more likely chemically to displace the potash, sodium and other cations before mg. You need sulphur to ideally which will work on the mg but will also work on the dominant cation, most probably calcium. Gypsum may be useful unless the calcium saturation is already in excess in which case you need straight sulphur. All of this is academic without a decent soil test.
 

Warnesworth

Member
BASE UK Member
Location
Chipping Norton
My soil tests have shown very high mag and low potash this is for haylage grass land, (PH fine phosphate good) if the potash has been locked up by the magnesium then am I right in assuming that it would show up in the soil sample that potash levels were ok and that the grass would exhibit potash deficiency symptoms, or if locked up it wouldn't show up in the actual sample.
The high mg will also be locking itself up to an extent. You may find that you are also low in mg especially if you have been applying any quantity of N.
 

Renaultman

Member
Arable Farmer
Location
Darlington
As a stock farmer i do not want to risk my cows going down with staggers. if I bring my Mg levels down then surely that may be a risk especially grazing .
I understand, as has been stated above, that high soil mg levels doesn't necessarily means it is getting in to the plant, must confess, I am looking at it with an arable head on so apologies if wrong
 

milkloss

Member
Livestock Farmer
Location
East Sussex
Excess calcium is more likely chemically to displace the potash, sodium and other cations before mg. You need sulphur to ideally which will work on the mg but will also work on the dominant cation, most probably calcium. Gypsum may be useful unless the calcium saturation is already in excess in which case you need straight sulphur. All of this is academic without a decent soil test.

Raising the issue again...... what sort of soil test would be required to establish a bad calcium:magnesium ratio? Livestock farmer here so be gentle! What other tests might be useful for a livestock man for soil health?
 

marco

Member
Raising the issue again...... what sort of soil test would be required to establish a bad calcium:magnesium ratio? Livestock farmer here so be gentle! What other tests might be useful for a livestock man for soil health?
Someone with an understanding of animals and Albrecht type base saturation tests. Is @York still on here. @Warnesworth seems to have all the info, is he a farmer or consultant?
 
Raising the issue again...... what sort of soil test would be required to establish a bad calcium:magnesium ratio? Livestock farmer here so be gentle! What other tests might be useful for a livestock man for soil health?

as said above a Albrecht soil test,

Ssm Should be able to sort them for you. Straight forward enough to read also. Get a good field tested and a bad grass field tested and post the results. Will be able to break it down then.
 

Warnesworth

Member
BASE UK Member
Location
Chipping Norton

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