Arable News

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Soil management myth-busting August 2018

Liming is soil husbandry specialist, Steve Townsend’s focus this month as he attempts to sort out the soil management wheat from the chaff.

‘You only need lime to correct your soil pH’

If we know only one thing about soils it’s that keeping them from becoming too acidic is fundamental to keeping them in good heart. And liming is the best way of doing this.

We’ve learnt, almost from the cradle, that soils just on the acid side of neutral (pH 6–6.5) mean the greatest plant nutrient availability and the least susceptibility to soil-borne diseases like clubroot. A roughly neutral pH is also recognised as important for maintaining the healthiest population of earthworms and soil micro-organisms.

So we test our soils regularly and add a good dose of calcium in the most handy and least costly limestone form we can get wherever they fall too low. Job done.

Well, it would be if (a) it’s only calcium that affects soil pH and (b) the only thing that calcium affects is soil pH. Unfortunately, both these general assumptions are false.

We need to appreciate that calcium is very far from the only positively-charged element that can balance the hydrogen ions responsible for soil acidity. Other cations like magnesium, potassium and, to a lesser extent, sodium also play their part. As they do in the whole soil cation-exchange mechanism that underpins our soil fertility.

At the same time, we have to understand the number of other vital roles calcium plays in crop production. First and foremost, it’s an essential plant nutrient; and interestingly, given the sort of climatic variability we are seeing these days, one that’s more likely to cause deficiencies during periods of drought and when soil moisture is inconsistent due to increased crop susceptibility.

Calcium is also important in assisting the movement of other nutrients from the soil into the plant, as well as in maintaining the healthiest, aerobic soil biology.

Soil structure

Perhaps the most valuable – as well as least appreciated – contribution calcium makes, though, is to soil structure; specifically to the clumping of clay particles so essential to both drainage and tilth production. Which, in turn, increases the soil’s resilience in challenging seasons and creates a far less favourable environment for black-grass.

Because of this contribution, calcium is more important overall than either P or K in my book. I’ve seen too many instances where soil conditions have improved markedly and black-grass levels have declined massively following liming to be in any doubt here.

As a soil improver, calcium is completely different to magnesium which encourages clay particles to separate so they pack together tighter. This interferes with drainage, encourages black-grass, restricts the most favourable soil biology and makes effective cultivation difficult – if not impossible – in both very wet and very dry conditions.

So what? Well, on the one hand, it means our soils can have perfectly acceptable pHs but be critically short of calcium. And on the other, that high magnesium limes can be very effective in combatting soil acidity but about the worst thing to use on difficult clay soils.

I come across both these issues almost daily in my soil improvement work; in one case, indeed, I’ve found a calcium-deficient soil with a pH of 7.8 (due to excessive levels of magnesium and sodium). I’ve also seen a clear tendency for calcium to sink over time on heavy soils with little or no tillage, meaning there’s often nowhere near enough in the rooting zone.

Where liming is concerned, therefore, we should always think calcium rather than just pH. We also need to be aware that, as with everything else in life, we can have too much of a good thing. As Mulder’s classic chart underlines, too much calcium interferes with the availability of just about every other important plant nutrient. So it’s the balance that really matters.

The best soil balance between calcium and magnesium, for instance, is generally agreed to be between 5:1 and 7:1, with the higher calcium ratios required on heavier soils.

Because so many clay soils don’t seem to have enough calcium, those of us with heavy land really shouldn’t rely on conventional pH testing alone. Our routine soil tests should also assess the contents of all three principal cations – calcium, magnesium and potassium; especially so where soil magnesium indices are 3.0 or more.

That way, even though our soil pHs may be 6.5 or more, we won’t unknowingly allow them to fall short of calcium. Either that or do our low pH clay soils more harm than good by correcting their acidity in the wrong way. 

Between exploring more of what he sees as today’s most damaging soil management myths with us, Steve Townsend is more than happy to discuss any tillage or soil management issues, interests or concerns with Farmers Guide readers by email on [email protected] or by phone on 01452 862696.

  • Written by: Farmers Guide
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