Teagasc soil fertility researcher Mark Plunkett presented a paper on the importance of potash for growing crops of silage at the Teagasc soil fertility conference in Kilkenny on Wednesday. The messages were particularly relevant given the shortage of silage on many farms after the tough spring and a summer drought. While the weather was undoubtedly to blame for the fodder shortage, Mark presented some startling figures on the use of potash (K) fertiliser and soil fertility trends in Ireland.

  • Imports of potash fertiliser declined by 300% in the 20 years between 1989 and 2009.
  • The amount of potash fertiliser being spread on Irish farms has increased by 50% since 2009.
  • Despite this, the number of soil samples showing potassium at index 1 and 2 has increased by 50% over the last decade.
  • Over the last 10 years, the average application rate for potash on dairy farms was just 23kg/ha/year – well below the recommended rates for maintenance, not to mention soil build-up.
  • Why K

    Mark said that unlike in a grazing situation where K is recycled from dung and urine, silage swards don’t get any additional K other than what is applied by the farmer, so K deficiencies are more pronounced in silage fields. He says that K is necessary to drive nitrogen efficiency and where K is limiting there will be a rapid drop in silage yields.

    On top of this, silage is a heavy user of K, with every tonne of silage dry matter using up 25kg of K. So a typical crop of first-cut silage yielding 10t/acre removes 125kg of K per hectare (100 units/acre). Mark said that a typical two- or three-cut silage system yielding 10t DM/ha will remove 250kg of K per hectare (200 units/acre).

    Mark presented figures showing the response to potash in silage crops. There was work done in Johnstown Castle between 2005 and 2009 on low soil index swards with between one- and two-thirds perennial ryegrass. These swards were cut four times per year and had an average response of 9kg of grass dry matter for every 1kg of potash fertiliser applied. This response was measured when up to 180kg K/ha is applied. The response dropped when 240kg K/ha was applied. Interestingly, the response to potash was lower on 100% perennial ryegrass swards at 6kg of grass dry matter for every 1kg of potash fertiliser applied.

    See Table 1 for the potash requirements for high-yielding silage crops. Fields at index 3, cut three times per year need 250kg/ha of K (200 units/acre). Fields at index 1 will require 310kg/ha of K (248 units/acre). Mark said no potash should be spread on index 4 fields for one year and then revert back to index 3 recommendations until the next soil test. He said that no more than 90kg/ha of K (72 units/acre) should be applied in one application in spring to prevent the risk of grass tetany. Where more than this is required, split the application to the aftergrass or to the autumn.

    Slurry

    On the use of slurry, Mark said high dry matter (7%) cattle slurry will provide almost all the potash requirements for silage. But low dry matter (4%) will only provide between 50% and 60% of the potash requirements even when spread at 3,000 gallons/acre.

    Patrick Forrestal was also speaking at the conference and he said there is a huge variation in the nutrient content of slurry. He said in samples analysed at Johnstown Castle, there was a 17-fold difference in available nitrogen, an 11-fold difference in phosphorus and a 15-fold difference in K between slurry samples. He said measuring the dry matter is a good indicator of nutrient value in slurry. This can be done using a slurry hydrometer.

    Message

    Considering the offtake in silage and the amount being applied in chemical fertiliser and slurry, it is not hard to see why potash levels in Irish soils are depleting so much. It’s not just silage ground that is getting depleted, more and more milking platforms are running low in K. With no limitations on spreading rates or dates and no environmental risk from K, farmers are shooting themselves in the foot by letting K levels slide. Those hoping for bumper crops of silage next year will be severely disappointed unless they correct potash levels first.

    The good news is that potash and lime can still be spread this year. Despite heavy rain last weekend, ground conditions are holding up very well after a dry week. The message I got from this session was to correct the pH and potash levels on your farm as soon as possible.

    Conference in brief

    Phosphorus levels decline in Agricultural Catchments Programme

    Noeleen McDonald from the Department of Agriculture presented results on soil samples from the three areas in the Agricultural Catchments Programme (ACP). The programme was established in 2008 to look at the effect of the nitrates directive on water quality and soil fertility in three catchment areas in Ireland. The catchments were chosen based on soil type, with Timoleague in Cork being a well-drained catchment, Dunleer in Louth a moderately drained catchment and Ballycanew in Wexford a poorly drained catchement.

    Each field in the catchment is soil sampled every three to four years and all the farmers’ inputs and outputs are recorded so accurate analysis of inputs and offtakes are recorded. Noeleen said the percentage of fields at index 1 and 2 for phosphorus (P) was highest in the poorly drained catchment and it increased from 67% in 2009 to 85% in 2017. The percentage of fields at index 3 declined in all three catchments over time. The percentage of fields at index 4 for phosphorus declined in the well drained and poorly drained catchment but increased slightly in the moderately drained catchment.

    On average, between slurry and chemical phosphorus, farmers in the poorly drained catchment applied 24kg/ha of P per year. Farmers in the moderately drained catchment applied 30kg/ha of P per year while those in the well-drained catchment applied 37kg/ha of P per year. She said the P balance (P input minus P offtake) varied between fields and soil properties and management. Fields cut for silage usually had a P balance of -14kg/ha per year on average while grazed fields had a P surplus of 10kg/ha per year on average.