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The Importance of Soil Fertility


The maintenance and improvement of soil fertility is essential to maximise grass growth on your farm. The pH status of a soil plays a hugely important role as nutrients may only become available to the grass plant at certain pH levels. Ronan Mulligan, Teagasc Walsh Scholar, Claremorris tells us more

Why is Soil Fertility Important?

The maintenance and improvement of soil fertility is essential to maximise grass growth on your farm. Approximately 90% of the soils sampled throughout Ireland are sub-optimal in either Phosphorus, Potassium or soil pH. The pH status of a soil plays a hugely important role as nutrients may only become available to the grass plant at certain pH levels. The ideal pH for mineral soils ranges from 6.3- 6.5. A soil pH between 6.3 and 6.5 allows maximum nutrient uptake by the roots of the grass. Correcting the pH of your soil is the first step in optimising grass growth on your farm. Recent studies have shown that grassland soils maintained at pH 6.3-6.5 have the potential to release approximately 60kg to 80kg/ha more nitrogen (N) than soils with pH 5.0, thus, representing a significant cost saving on Irish farms. Having the correct soil pH means you will also make better use of the chemical fertilizer applied to your farm. Today, approximately 60% of mineral soils are below pH 6.0 and 25% of soils have a pH below 5.5 so this is an area that needs attention out on farms.

When to apply Lime?

August and September is an excellent time of year to apply lime when grazing rotations start to lengthen, when ground conditions are still good and all silage is cut so the risk of lime having any negative impact on silage fermentation is reduced. Getting the lime spread in autumn allows the lime time to get to work raising the soil pH over the winter months. Spreading lime in autumn also removes the complication of worrying about avoiding spreading slurry or urea after spring applied lime. Ideally apply lime to bare ground. Lime is available as ground limestone or granulated limestone. Limestone is the number one method to correct the pH within a soil. Ground limestone usually costs €25-30 per tonne delivered and spread with soils usually requiring between 2.5 - 7.5 tonnes of ground lime per hectare depending on the soil pH.

Phosphorous and Potassium

Phosphorous and Potassium are two hugely important macronutrients within the soil. Phosphorus is very important for crop establishment and root development and it also plays an important role in the nutrition of livestock. Therefore Phosphorous is very important in reseeded fields to achieve good grass establishment.

Potassium increases stem strength, improves drought resistance, cold tolerance and most importantly increases yield. Potassium is very important in silage fields. Cattle slurry is an excellent source of potassium. There are approximately 30 units of Potassium in 1000 gallons of cattle slurry. This is why it is best to re-cycle slurry back out onto silage fields. High applications of potassium fertiliser in spring can lead to suppression of Magnesium uptake leading to grass tetany. Therefore, spreading potassium in August or September using Muriate of Potash is an excellent way to build up the level of Potassium within the soil and try to avoid issues with grass tetany.

Importance of Soil Testing

The only way to know the soil fertility status on your farm is to carry out a full farm soil analysis. The most favourable time to do soil testing is between September and March. Consider soil testing all or some of your farm this autumn. If you only sample some of the farm target the silage fields and any fields that maybe re-seeded in the near future. Farmers can get their soil tested through their local Teagasc office.