Mark said that a major contributor to lower grassland productivity in recent years has been a switch away from the traditional practice of regular lime application and applying straight nitrogen over compound fertilizers.

The trend has seen soil fertility levels gradually decline resulting in lower grassland productivity, poorer persistency of perennial rye grass and a lower response to nutrients applied.

Soil pH

While numerous factors can influence the pH of a soil, Mark says that the greatest contributor leading to the loss of lime and deterioration in pH is due to high rainfall.

“A small volume of lime goes away in the trailer when you are selling livestock. It gets taken up when harvesting crops of hay and silage but, most of all, it is removed from our soils through drainage. The rate of loss depends on the volume of rainfall but is generally between 0.5t and 1t each year in grassland swards.’’

Mark (pictured) says in high rainfall areas, soils will become acidic over time. Failing to correct soil pH reduces grass growth which, in turn, gives an advantage to wild grasses and weed species to compete with ryegrasses and establish in the sward.

Soils with below optimum pH will also lock up more nutrients with crops achieving a poorer response to nutrients applied in chemical fertilizer and organic manures. “We are standing on tonnes of organic nitrogen. Putting out lime and correcting your pH increases biological activity and can release up to 60kg to 80kg nitrogen per hectare per year. This is the equivalent of a bag of urea (per acre) that is not costing you anything and will more than cover the cost of liming over two years.”

Increasing soil pH

The target pH for optimum grassland production on mineral soils is pH 6.3 to 6.5 and 5.5 to 5.8 on peat soils. Applying ground limestone is a longer-term approach to increasing soil pH to the target pH 6.3 and is a good value for money solution.

Granulated lime, on the other hand, will give an immediate response but is shorter acting and has to be applied each year.

Soil test results will show the recommended rate of lime required. Liming grassland should be done at least once every five years. Mark advises not to exceed 3t/acre in any one application. He also highlighted that it is important not to raise the pH above 6.2 on soils high in molybdenum (Mo), as increasing the pH above this level increases the availability of Mo. This, in turn, may induce copper deficiency in ruminant animals.

Reseeding and lime

Mark says that, at reseeding time, lime should be applied based on a recent soil test result and worked well into the seedbed to aid the establishment of new grass seeds. Where direct reseeding takes place, in the absence of the plough, lime will reduce acidity created by decaying organic matter on the surface.

Lime should preferably be applied after burning off vegetation if minimum cultivation is being practised and after ploughing where traditional cultivation is being practised and worked into the top 2-3in of the soil.

Mark adds that, ideally, cultivation techniques that avoid ploughing should be considered if possible as ploughing will bury the most fertile layer of soil. The exception to this is where a break crop is sown and the most fertile layer of soil will be ploughed back up before sowing grass seed.

P and K index

The aim is to have soil P and K levels at the optimum soil index 3 to maximise grass production. Fertilizer advice at index 3 replaces nutrients removed (meat and wool). Lower index soils (1 and 2) will require additional nutrients annually to build soils to index 3 over time (Table 1). Rectifying soil pH will help to increase the availability of soil nutrients (especially P) but targeted use of compound fertilizers and organic manures will be required to bring about any real change in soil fertility over a number of years. Mark says, in general, increasing the P levels in soils tends to be much slower than K levels, but is dependent on soil type.

The recommended P and K application rates for soils with varying indices and stocked at 130kg to 170kg organic nitrogen per hectare, or 10 to 13 ewes per hectare, is detailed in Table 1.

The table also includes an example fertilizer programme.

The best time to target compound P and K fertilizers to lift indices is early in the year prior to rapid spring growth. Doing so will also help boost grass growth throughout the season. For soils low in K, now is a good time to target soils with a compound fertilizer or muriate of potash (50% K). Access to cattle slurry will aid in increasing K levels faster.

Once fertility levels have been brought to Index 3, Mark says that required P and K rates for maintenance are low.

He says that for him the most suitable fertilizer for sheep farms that are not cutting excessive silage/hay crops is something like two bags of 18-6-12 or a bag and a half of 10-10-20, followed by 24s, 27s or straight nitrogen fertilizers. These rates can be reduced if soils are also receiving organic manures (values shown in Table 2).

Making it pay

Mark concluded by saying that correcting soil fertility will reap rewards in terms of increased grass growth at the shoulders of the year and throughout the main growing season. “If you can take one message from today, test your soil and get lime right.”

He advises farmers that correcting a significant soil fertility deficit is not cheap and to get the full value from your investment, management practices need to be put in place, as is the case on Anthony McShane’s farm, to utilise the extra grass grown (for example; rotational or paddock grazing, increasing stocking rate, reduced concentrate feeding).

The five steps to improving soil fertility:

  • Soil test.
  • Rectify pH and lime.
  • Address P and K index.
  • Targeted use of organic manure.
  • N, P and K balance to maintain fertility.
  • Read more

    Sheep management: fertiliser application