The last day farmers can spread chemical fertiliser is coming up on Monday, September 14, so what should farmers do between now and then?

Now that grass has finally turned green following a long, harsh drought, and growth rates are above 35kg of dry matter (DM)/ha, many farmers will be eager to get fertiliser out.

Getting an application of 25kg of nitrogen (N)/ha out on grass this weekend should keep grass growing right up until mid to late October.

However, farms are being reminded not to put surplus out, but rather just enough for what you need.

Fertiliser advice

Farmers will be half sick of hearing about mineralisation at this stage, but Dr. David Wall, head of enterprise at Teagasc, Johnstown Castle, said that it is a "very real" concern.

Dr. Wall noted that any organic material in the soil is broken down by microorganisms.

For every 2,000gal of slurry spread, only 1,000gal is available for nutrient uptake.

The other 1,000gal is used to feed the microorganisms.

When these microorganisms eat, they are only interested in carbon molecules, but each carbon has has two to three nitrogen (N) molecules attached, which the microorganisms discard.

Once moisture is available in warm soils, these broken-down nitrogen molecules cause mineralisation worth roughly 1kg of N per day, enough to maintain a growth rate of 35kg DM/ha.

Farms must also be aware that grass growth will not realistically surpass the 55kg of DM/ha mark again.

Therefore the plant only needs enough nitrogen to grow 20kg of DM/ha, as the mineralisation is suppling enough nitrogen for the other 35kg DM/ha.

Farms looking to apply 40 units of nitrogen now will only be a wasting money, as the nitrogen will go below the root zone and get flushed away.

Dr. Wall noted the work of mineralisation by highlighting that the average farm spreads 150kg of N.

As this is only enough to grow approximately 6t of grass, the rest of the growth comes from mineralisation.

He also noted that having slurry tanks on the floor by October 1 is more impoartant than getting fertiliser out.

Therefore farms should be spreading 2,000gal of slurry diluted with washings if possible instead of fertiliser.

As well as this, farms should be trying to spread lime over the autumn months to improve their soil fertility.

Dr. Wall noted that lifting soil pH from an acidic 5.8 to the target of 6.3 will provide farms with a free 80kg of N/ha in the back end of the year.

Calendar farming

Dr. Wall recently acknowledged the topic of 'calendar farming' and how it can be a thorn in the side in years where there is a good back end.

"That's not what it's about, it's about the time taken to take up the nutrients before they are lost from the root zone," Dr. Wall said.

Dr. Wall noted how the likes of ryegrass plants have deep root to connect to water.

However, the majority of roots are within 30-60cm of the plant, meaning any nutrients below these will not get utilised in the back end of the year, and ultimately end up being washed away.

He highlighted how there will be nutrient uptake right through the winter, but in miniscule quantities from mid-October.

The September 15 deadline is in place because of these uptake rates, as the plant will not physically be able to use any more nutrients than what is spread currently, Dr. Wall said.

He went on to say that deadlines are not dictated by rainfall, but by light.

As daylight falls back in the mornings and evening, there is less sun to drive photosynthesis, which means grass growth falls and with it, so does nutrient uptake.