Soil sampling – how to read your soil test results

With 90% of Irish soils having sub optimal fertility (pH, Phosphorous (P) or Potassium (K)), Teagasc has recommended September as the ideal month to soil test on Irish farms.

Soil sampling has been highlighted as one of the key steps in addressing this issue, and identifying which nutrient is lacking and where they are required.

Once the test has been completed the next step is interpreting the results. According to Teagasc’s Mark Plunkett the pH should be addressed first.

Soil pH

The ideal pH for grass and cereal soils vary, the optimum grassland pH is 6.3, while cereals perform best when the pH rests at 6.5.

Soils with a pH lower than these values will require a buffer to stop further leeching of nutrients, this is generally in the form of lime.

Soils with low or high pH will reduce the release of macro-nutrients (N, P, K and S) which will have a negative impact on yield, whether this be grass or a cereal crop.

“Soil samples act as a good guide to addressing soil pH issues. Currently 60-70% of soils tested have a below adequate pH,” said Plunkett.

“Lime is a cost effective soil conditioner and is in effect a fertiliser as it can unlock major soil nutrients,” he said.

The autumn and winter are the ideal time to spread lime he added. Spreading lime during this period avoids the interaction with urea and cattle slurry therefore reduces potential issues with N loss.

Soil fertility – P and K

Once the pH issues have been addressed farmers can focus on identifying fields lacking in P and K, he said.

“This is essential as this will allow the targeted application of slurry and farm yard manure to fields lacking in P and K.

“Fields with low readings for P and K should be targeted first,” he said.

According to Plunkett, soils with Index 1 and 2 results should be targeted first and these soils will have the greatest response to fertiliser applications, which will give the farmer the most benefit.

The process of identifying fields requiring P and K from soil tests uses a scale of 1-4. Across both nutrients Index 1 and 2 soils will have the greatest response to P and K applications.

These are the areas which should be target as nutrient levels are low in these soils, which will have a negative impact on yield.

The soils classified as index 3 are target, these soils have adequate levels of these important nutrients, while index 4 soils have sufficient/high levels of both P and K.

Soil sampling table

Soil sampling

The soil fertility of the farm above has dropped in recent years, if we take a quick look at the soil test results we can easily identify that four out of the five fields are lacking in P (all bar field 2) and K (all bar field 2).

Generally, the pH levels of the five fields are good and range from 6.33-6.55. But the farmer should consider taking action on the P and K issue.

If we take the first sample (1-Peggies) this field is index 2 for both P and K. Out of the five samples taken only one field had adequate levels of P and K (field 2), which is grazing only.

All of the samples above with the exception of sample 2 had low levels of P and K and these must be addressed through targeted slurry and farm yard manure use.

If these nutrient sources are limiting the use of compounds should be considered.

When the background of each individual field is looked at it becomes clear why the four out of the five samples are lacking in P and K, three fields have been used as the farms silage source for the last decade, while sample five has recently been reclaimed.

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