Testing shows low pH levels

Published 11 October 13

 

Key messages

  • More than four fifth of areas may have incorrect  soil pH
  • This can reduce crop yields by 30%
  • The situation has deteriorated over the past 40 years since testing began
  • Make sure you have all the information you need about your product before you apply it.

Eighty-two percent of recent soil pH tests, using SWARM  funding, showed that results were not at target levels for grass production.  

The results, discussed at a recent BGS Nutrient Wise Liming Event at Duchy College in Cornwall, show that the UK finds itself in a worse position than 40 years ago when soil pH tests were undertaken regularly.

With incorrect pH resulting in drops of crop yield of up to 30%, the wrong pH level can have large financial ramifications for dairy businesses through reduced soil quality, lower uptake of nutrients in slurry, manures and purchased fertilisers is lower, and poorer competition of grass and clover species against weed species.

pH is a measurement of the Hydrogen (H) ions in the soil, which are essential to nutrients crossing the cell wall of the root of the plant. At a low pH, Aluminium (Al) and Manganese (M), which are toxic, are released into the soil, hindering the plants’ uptake of key nutrients. At a high pH, the uptake of some key nutrients is reduced.

Target pH 

  • Grass, permanent pasture = 6 (range of 5.6 – 6.8)
  • Grass leys = 6 (range of 5.8 – 7)
  • Forage legumes = 6.5
  • Arable crops, wheat, oats, rye = 6.5 (range of 6 – 7.5)
  • Barley = 6.8 (range of 6.5 – 7.5)

 

 

soil ph 2

pH testing

Soil sample and pH test every three to four years in a W-pattern across the field, avoiding gateways and water troughs,  in order to give representative samples. Using a soil core (available from a local lime supplier) take about 25 samples per area, mix them all up and submit a sample for analysis.

Samples need to be taken from the top 7.5cm in grassland soils. Try to avoid sampling close to any fertiliser application as this will affect the results, hence sampling during the winter period or before nutrient application in the spring is advisable.   

You don’t need to sample every paddock on the farm. Sample every three to four acres or so, as long as the area has been managed in the same way.

Correcting pH

Best results are gained from applying and incorporating lime during the working of a seed bed.  Application on grass swards are best made in autumn or early spring. Large application rates (5t/ha) should, if possible, be split between autumn and spring applications. A finer liming product tends to work more effectively and check the ‘reactivity’ of the product as this can impact on efficacy. Soil compaction reduces the effect of liming.

After surface application and no ploughing it takes nine to 12 months for lime to work into grassland soil and alter pH. It is therefore not worth testing soil for at least 12 months after liming.

Do not apply more than five tonnes per hectare in one go and, if soil pH has become very low you may need to apply lime over a number of seasons to correct it.

There are many lime products available on the market and they can vary hugely in their ability to increase pH. Check the neutralising value (NV) of a product and use standard tables or the Agricultural Lime Association online Lime calculator to calculate application rates.

Make sure your supplier provides you with all the information you require about the product;

  • The declared name of the lime
  • Its chemical composition
  • The fineness of the material
  • The NV.

The British Grassland Society has a good book called Grassland soils and fertilisers: Digging out the answers, which has lots of useful information about soil pH and liming, orderable from their website (link).

Grass+ has a fact sheet on grassland liming.