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Improving Soil Fertility

Get the most from your soil.


Soil analysis is the first step towards maximising production.

Soil fertility refers to the ability of a soil to supply nutrients and sustain plant growth. It is the combined effect of three major interacting components: the chemical, physical and biological characteristics of the soil.

A fertile soil will contain all the major nutrients for basic plant nutrition;

  • Nitrogen (N)
  • Phosphorus (P)
  • Potassium (K)

as well as other secondary nutrients needed in smaller quantities:

  • Calcium (Ca)
  • Magnesium (Mg)
  • Sulphur (S)

and trace elements

  • Iron (Fe)
  • Manganese (Mn)
  • Zinc (Zn)
  • Copper (Cu)
  • Boron (B) and
  • Molybdenum (Mo).

It has been shown that in some areas about 80% of grassland soils are suboptimal in either pH, P or K. This is severely limiting potential grass production from these soils.

Nitrogen exists in many forms and is primarily introduced to the soil either through applied commercial fertiliser, fixation by legumes, animal manure or the breaking down of crop residue and soil organic matter.

Remember both N and P are restricted within the Nitrates Directive – ensure you adhere to the allowances and parameters for your farm.


1. Plan your fertiliser needs

2. Know what soil type you have

Peaty soils

On peaty soils it is best to apply P* and K during the growing season because peat has a poor capacity to retain fertiliser.

Sandy soils

Potassium shouldn’t be applied to sandy soils during late autumn/winter as it is readily leached from sandy soils. Risk of losses should be less with smaller more frequent applications of P and K during the growing season compared to a large single application out of season.

High molybdenum soils

Molybdenum (Mo) is an essential trace element but when levels in the soil are high this gives rise to copper deficiency in grazing animals. On high Mo soils, avoid increasing pH above 6.

Speak to a vet about copper supplementation to animals and also beware of copper toxicity as too little and too much have damaging effects on ruminants.

Sulphur deficient soils

Sulphur is a fundamental nutrient needed for plant establishment and there is a close relationship between nitrogen and sulphur in plant nutrition. Up to 30% of soils require sulphur for optimum yield, but there is no soil test for it.

On soils that require Sulphur –

Grazed ground requires about 20 kg/ha per year.

Silage ground requires approx 20 kg/ha per cut.

3. Test your soil

You need to know the chemical make–up of your soil and understand the nutrients required for the crops you plan to grow. Measure and monitor for best results.


Optimum soil pH:

  • Will encourage micro–organisms in the soil
  • Help in the release of N from organic matter
  • Increase earthworm activity which will in turn improve soil structure
  • Improve availability of N, P, K, S, Ca and Mg

4. Apply lime if required

Adjust soil pH based on what you plan to grow e.g. for grassland the optimum is 6.3 – 6.5. (For more information see our G–Source guide to Soil pH and liming).

5. Maintain P and K levels using soil index

This will depend on your stocking rate and production system.

Supplement low levels with chemical fertiliser and aim to meet your P and K requirements – do not exceed them.

The aim of P and K nutrient advice is to maintain all fields at the optimum soil fertility level.

A soil index 3 is considered optimum for production. To maintain the soil at this range, the P and K application should replace the P and K removed, i.e. replace offtakes in crops. The rate applied will depend on the Stocking rate and production system (grassland, arable etc.).

If your soil P or K index is 1 or 2 you will require additional nutrients to build up reserves.

your soil P or K index is above 3 you can draw on soil reserves and do not need to apply chemical P or K fertiliser.

6. Test slurry to estimate any extra nutrients required

Having slurry analysed is the best method to ensure you are not over or under fertilising a crop.



  • reduce your supplementary chemical fertiliser use
  • improve your soils
  • save money
  • protect the environment.

If silage fields are low in P or K, the silage will be low in P or K and hence slurry will also be lower in P or K nutrients. Variability in slurry dry matter will affect N, P and K content of the slurry.

Test your slurry with a slurry hydrometer as it can estimate slurry DM content on farm and be a useful tool to estimate nutrient content of slurry. Use data to determine supplementary chemical fertiliser needs. By managing the nutrient content of the manures and slurry on your farm, you can benefit from cost savings due to reduced inputs of chemical fertilisers.

As a general guide:

3000 gallons/acre at 7% DM slurry, will supply 20 kg of P and 142 kg of K

7. Always comply with any legal requirements to protect your land and watercourses.

Phosphorous is restricted within the Nitrates Directive – ensure you adhere to the allowances and parameters. The Nitrates Directive aims to improve water quality by protecting water against pollution caused by nitrates from agricultural sources.

In particular, it is about promoting better management of animal manures, chemical nitrogen fertilisers and other nitrogen–containing materials spread onto land.

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