Lime takes on a new importance as fertiliser prices soar

Thursday, 7 October 2021

As fertiliser prices rise to new highs, the case for liming grassland has never been greater. Grassland consultant, George Fisher, explains how liming can increase grass yields by up to 40%. 

The economic case for liming grassland has never been greater and the time to do so is now, according to independent grassland consultant, George Fisher. 

As fertiliser prices rise to new highs, he says the importance of creating optimum soil conditions for the uptake of nutrients grows in importance, and liming can be essential to setting this in train. 

Raising the soil’s pH to its optimum by liming will not only increase its ability to release nutrients to the growing crop, but it will also optimise conditions for the soil’s biology and health,” he says. This results in less wastage of expensive nutrients, less pollution into water courses and an improved soil structure, with all its associated benefits. 

Further gains from optimising pH include the more efficient capture of nitrogen by legumes while a growing body of evidence is beginning to suggest soils with a low or high pH can lose more carbon. 

All of this points towards the importance of measuring the pH of all grassland soils, and optimising pH at close to 6.5. 

“This ensures that all nutrients, whether from the soil or a bag or from livestock, are at the optimum availability for the plant to take up, provided everything else is in place, he says. 

Quite apart from the environmental impact of nutrient loss, the financial cost of the waste has escalated this year, as fertiliser prices continue to soar. 

In contrast, the price of liming materials remains relatively stable, providing an even better return on investment than in previous years. 

Dr Fisher therefore urges grassland growers to test their soils at the earliest opportunity to ensure lime is spread before Christmas if pH needs to be raised. 

“It doesn’t matter when you lime if ground conditions are suitable, but it’s not advisable when cutting or grazing as you will be covering the sward in lime,” he says. “However, if you do it now, at the end of the grazing season, it has the winter to work its way into the soil and positively impact the pH next season.” 

How much lime to apply 

Once the pH is known, the amount of lime required can be determined from the Agricultural Lime Association’s website where a ‘lime calculator’ will specify the tonnage to be applied to different soil types and through a choice of products. 

Again, he says, it’s not so much an issue of which product is used – with choices ranging from the basic crushed lime direct from a quarry to enhanced, granulated products – but of opting for calcium-based products, rather than those based on magnesium. 

“Calcium adds structure to the soil whereas magnesium makes it sticky and easier to compact, he explains. “I find that soils in a structurally bad state are often high in magnesium.” 

However, farmers may have historically used lime products high in magnesium if this is the natural product from their local quarry. 

“A lot of quarries have high levels of magnesium in their lime,” he warns. “This may have raised a farm’s magnesium index to 4 or 5, which can take decades to get back to the more desirable index of 2 to 3.”  

Other considerations to be borne in mind include the farming system in operation, with high input dairy farming in particular tending to produce slurry which is high in magnesium, as well as other minerals. This means a close watch should be kept on the mineral status of these soils. 

A further consideration is the amount of nitrogen fertiliser that’s been applied to the sward as this in itself makes soils more acidic. 

“For very kilogram of nitrogen applied you need to put on 2-3kg of lime as calcium carbonate, he says.   

However, this fact in itself makes a powerful case for the use of legumes which require no artificial nitrogen, so don’t pull pH down. 

“A beef and sheep farmer using low applications of fertiliser would fix far more nitrogen by using a clover sward than supplied through bagged nitrogen,” he says. 

However, this too could depend on liming as the root nodules of legumes, where the nitrogen is fixed, also require a pH of 6.5 to take up nitrogen from the atmosphere and return it to the soil. 

“The bacteria in the root nodules have evolved over millennia to take up nutrients at the pH that’s optimal for the soil,” he says. “In other words, the bugs are most efficient when the soil is most efficient, and that includes pH. 

Getting the pH right is not a silver bullet but it’s an essential link in the process of soil management on which all growth depends. 

He says soil health is about chemistry (its nutrients), physics (its structure) and biology (its microbial life). 

“You can work away on the physics and biology, but if you don’t have the pH right, you will still lose nutrients,” he says. “But get them all right and you reduce your chances of nutrient loss, leaching and pollution and increase your chances of a healthy and high-yielding crop.” 

This is backed by long-standing research which shows that grass yield can be 30-40% lower as pH drops from 6.5 to 5.5. 

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