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‘2,000-2,500 gallons of cattle slurry will replace most of the P and K coming out in first-cut silage’

Mark Plunkett, a Teagasc soil and plant nutritionist specialist, discussed the impact of lime on maintaining soil pH, alongside the benefits of sulphur, protected urea, and LESS, in a recent podcast Teagasc produced.

Soil fertility

When it comes to growing grass, Mark recommends “getting lime, P, and K right in terms of producing tonnes of grass at key times during the growing season”.

To maintain soil pH, the first thing to consider is lime.

“It is very important that we get our pH in that sweet spot that is 6.3 to  6.5 in terms of nutrient deficiency, especially in terms of nitrogen”, Mark told Ciaran Lynch on Teagasc’s Ovicast podcast.

“The one thing about drystock farms, especially on the grazing side, there is a lower requirement for both P and K.”

“On the silage side, we must be aiming for optimum indexes number one in terms of index 3 for P and K. Then there is a greater requirement for P and K anywhere you either cut hay or grass silage.”

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Surplus silage removing P and K

Ciaran points out that there has been a lot of grass growth in recent weeks.

“Maybe the option of taking out that surplus grass is compensating for a low silage yield earlier in the year. The need to replenish the P and K in those offtakes is probably underestimated how much P and K will remove with surplus silage”, Ciaran suggested.

“If you take your first cut of grass silage, you are taking out somewhere in the region of eight tonnes of fresh-cut grass per acre which is the equivalent of 16 units/acre of phosphorus and approximately 100 units/acre of potassium”, Mark stated.

In Mark’s opinion, “anywhere you take out strong paddocks, you are taking off significant amounts of P and K in those cuts”.

No slurry, no problem

According to Mark, “2,000 to 2,500 gallons of good quality cattle slurry will replace most of the P and K coming out in that first-cut of grass silage”.

Where there is no cattle slurry available, farmers should use a fertiliser compound instead.

“Something like a 13:6:20 which is quite available out there or 15:3:20, depending on your P limits or your P allowances on the farm,” Mark advises.

Fertilisers, according to Ciaran, are more readily available than in previous years.

NPK requirements 

For those considering a second cut to bulk up supply due to a lower yield in the first cut, Mark advises the following target recommendations:

“If we are growing three tonnes of dry matter grass or six tonnes of fresh grass, the NPK requirement is 60 units of nitrogen, 10 units of phosphorus and 60 units of potassium.”

“If cattle slurry was available on the farm, the programme would be two thousand gallons per acre of cattle slurry plus a bag and a quarter of protected urea plus sulphur, 40% nitrogen plus 60% sulphur.”

“Where cattle slurry is not available on the farm, a good fertiliser compound is 15:3:20 plus sulphur, topping it up with 0.4 to half a bag of protected urea plus sulphur.”

Sulphur – an overlooked compound

Sulphur is a nutrient that is less abundant in the atmosphere or in the rain we receive throughout the year; thus, it is important to apply the recommended sulphur rate.

Mark believes sulphur is “very important” in terms of nitrogen efficiency.

“It makes the nitrogen work for us more efficiently, and it tends to be the lighter soils that are most responsive.”

In Johnstown Castle, they are seeing a yield response of a tonne per hectare on heavy soils.

“A 10% yield response to applying sulphur during the growing season, on loam soils a 20% to 25% yield response to sulphur.”

“If you are growing ten tonnes of dry matter, you will grow an extra two and a half tonnes of dry matter by putting in sulphur.”

On light soils and anywhere grass was intensively cut, they are seeing up to a 35% yield response.

“That is a big yield response on those light soils. They are very free-draining, low in soil organic matter, and have low nutrient retention. That is where we are seeing big responses.”

Selecting a fertiliser for second-cut silage 

“When picking a fertiliser compound for second cut silage, it is important to get one that contains sulphur in the blend or just straight nitrogen, like the protected urea plus sulphur is a good option.”

“The 40% plus 60% sulphur is a great way of getting sulphur into the mix.”

In Mark’s opinion, you do not need a lot of sulphur as “on grazing ground, ten to fifteen units maximum per year”.

The best response to sulphur is seen “once you hit peak grass growth in April, May, June and July”.

“Our advice for second-cut silage; six tonnes of fresh grass per acre would have a requirement of about 10 units per acre.”

“This is why I recommend something like 15:3:20 plus sulphur, it would have 3% sulphur, and you can add some protected urea with sulphur as well to get those ten units in.”

LESS – 50% improvement in nitrogen efficiency and recovery 

Ciaran believes at this time of year, there are many benefits such as “slurry not crusting on top of the ground”.

“A splash plate recovers three units per thousand gallons, whereas a low emissions slurry spreader with a band spreader or a trailing shoe can double that by getting six units of nitrogen per thousand gallons.”

“You are placing the slurry in a band; you are placing it under the grass; it is very close to the soil surface. That is how we are reducing the ammonia or increasing the recovery of nitrogen from that slurry.”

“Another benefit is the precise application of NPK across the spread width of the spreader.”

LESS allows more nitrogen to be utilised from slurry while cutting down on both the farm costs and the use of chemical nitrogen.

In Mark’s eyes, this is a “double win in terms of it is a lower expense on-farm and also environmentally it is a big win”.

Lime – 20% increase nationally

“We have seen an increase in the usage of lime over the last five years, using approximately 20% more lime nationally.”

“Even though we are hitting close to a million tonnes, this increase is not enough,” Mark said.

“We were around 700,000 to 800,000 tonnes per year over the last five years, but we need to spread another fifty per cent. We need to hit the 1.5 million tonnes or above to get the pH to where it should be.”

“There are tonnes of nitrogen in soil organic matter. By getting the pH right, we release and provide the conditions for the fungi, bacteria and earthworms to mineralise that nitrogen.”

“We can mineralise somewhere in the region of 70kg of nitrogen per hectare per year.”

“By spreading lime and getting the pH right, we unlock that nitrogen making it more available.”


“Phosphorus unlocks and makes phosphorus and potassium more available in the soil.”

“We are now at a key time in the year; an opportune time to spread lime after first-cut silage applying what is recommended based on the soil test report and the same on grazing ground.”

“We are getting more out of nitrogen on all fronts whether it is the soil, the bag, farmyard manure or cattle slurry; it is more available for the grass to utilise.”

Ciaran believes particularly for dry stock farms out there with a lower overall fertiliser, input “it is the obvious one to go with; it is probably the cheapest source”.

According to Mark, the right pH causes grass growth to increase by an extra 10% to 15%.

“We can grow an extra tonne, tonne and a half of dry matter annually for a very low input.”

Protected urea 

Protected urea is an “efficient form of nitrogen”.

“It is urea plus a urease inhibitor giving the same results as CAN, but is slightly cheaper.”

“From an environmental point of view, it is very beneficial because it is an ammonia-based fertiliser compared to CAN, reducing nitrous oxide emissions.”

“Potash and sulphur would be very suitable for silage ground as it is a direct replacement for CAN.”

“Under the conditions that we have at present, this is when it works best in the drier, more challenging conditions you get the same results as you would with a CAN-based fertiliser.”

“Maintaining soil fertility is one of the main key tasks on farms each year. It is a key driver behind grass production and the development of most systems,” Ciaran concludes.

Further information

Listen to this episode of the OviCast podcast.

For more farming tips and advice

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