Soil pH

Read on for the results of research supported by NZSTX into the effect of lime on soil pH and aluminium, as well the effect of lime on the establishment of legumes.

Effect of lime on soil pH and aluminium

South Island high country soils have high levels of aluminium. Where high levels of aluminium are present in the soil, soil pH is strongly related to levels of plant-available aluminium. As soil pH decreases, the amount of plant-available aluminium increases to a point where it limits legume root growth (see the graph below). The pH of these soils needs to be managed to minimise any potential toxicity to legumes caused by the aluminium. 

In acidic soils with high aluminium content, a toxic level of plant-available aluminium in the soil will limit the establishment and persistence of legumes. Restricted legume growth will limit the production of nitrogen. Reduced nitrogen levels will cause the pasture to become dominated by species that are tolerant of low nitrogen (such as browntop), which reduces overall production and digestibility of the pasture. 

Trial results

Trials undertaken by Lincoln University showed that applying high levels of lime to the soil surface resulted in an increase in soil pH and a reduction in plant-available aluminium. Liming was most effective at the soil surface and less effective below a depth of 15 centimetres.

The results of the investigation indicated that the effect of lime on soil pH and plant-available aluminium varied between sites, highlighting the lack of understanding around the mechanisms driving soil aluminium levels. Therefore any on-farm decisions on liming rates and legume suitability need to be based on individual blocks.

Further information about the effect of lime on soil pH and aluminium

Click on the following link for more information about the effect of lime on soil pH and aluminium:

Establishment and persistence of perennial legumes in response to lime

Applying lime at a rate of five tonnes per hectare (the highest rate used in this trial) delivered the highest yield from each of the legumes tested (lucerne, Caucasian clover and Russell lupin). This highlights the critical role of pH (and the subsequent aluminium concentration) in the establishment of legumes where aluminium is present in the soil.

Specific conclusions from the trial:

  • Due to its slow establishing nature, Caucasian clover was least competitive at establishment but it had greater root growth into the low pH soil than the other legume species.
  • Lucerne yield continued to decrease over time due to a deformed root system and lack of nodulation.
  • The highest yields were produced by perennial lupin, regardless of lime rate. Perennial lupin also had the highest percentage of nodulated plants at low lime rates. While the trial results suggested that perennial lupin may have agronomic potential in low pH, high aluminium soils, particularly where applying lime is not a viable option, the risk of seed spread into environmentally sensitive areas must be adequately addressed. 
  • Successful establishment of all of the legumes relied on removal of existing vegetation, reducing competition from resident species for light, nutrients and water. Each of the legumes (lucerne, Caucasian clover and perennial lupin) had significantly higher establishment and persistence when herbicide was used to kill resident species prior to drilling.
  • Even after four years, application of up to four tonnes per hectare of surface-applied lime only reduced aluminium to potentially safe levels (<3 mg/kg) in the top 7.5 centimetres. There was no evidence that soil pH or soil aluminium levels had altered below this depth. 

Further information about decreased soil pH and aluminium toxicity in high country environments  

Click here for the report from Lincoln University, Soil pH and Aluminium Toxicity, which gives more detail on the challenges of decreased soil pH and aluminium toxicity in high country environments, and provides recommendations for effectively sampling soil for aluminium levels. 

This research, including the trials undertaken in collaboration with Lincoln University, is part of the New Zealand Sheep Industry Transformation Project (NZSTX), a Primary Growth Partnership with the Ministry for Primary Industries (MPI). Click here for more information about NZSTX.