Author: Oliver Knight

With the high fertiliser prices this year it will be prudent to soil test, in particular to measure soil pH, and correct deficiencies by liming as lime prices have stayed relatively similar to last year.

A lot of farms in New Zealand’s Hill Country have low soil pH levels. Dairy pastures that do not perform are usually associated with low soil pH.  As it stands, there has been more emphasis on fertiliser at the expense of liming. However, both can be achieved within the farmer’s budget.

Pasture species do not like low pH soils as there is increased Aluminium in the soil solution which retards root growth, which in turn leads to less growth of herbage on top. Soil pH levels below 5.5 tend to have soil solution Aluminium levels >3.0 mg/kg, ideally you want the aluminium levels < 0.1 mg/kg. One of the cheapest ways to alleviate this issue is liming.  

There is scientific evidence to show that as soil pH increases up to ~soil pH 6.5 there is an increase in microbial activity.  Microbial activity plays a major role in nutrient cycling in the soil.  You may have 3 (500 kg) cattle running on top of your dirt, but you have about the equivalent (in weight) of 25 cattle or 250 sheep, in underground fauna, running below ground.

MAF’s recommendation is for soil pH to be between 5.8 and 6.0 based on the dry matter production.  The ideal pH is around 6.2 – 6.4 if you consider quality of feed, more clover growth, greater earth worms (which aerate the soil), more calcium in the diet, and phosphate less tightly held by the soil.

Because of the cost of flying lime on Hill Country, a lot of farmers are put off from trying it. Lime is normally $30 -$35 per tonne, cartage is typically ~$20 – 40 per tonne (depending on how far the farm is from the lime pit), but flying is ~$120/tonne.  There is not enough in the farmers’ budget when a normal Hill Country farm fertiliser and lime budget is ~$120 -$150 /hectare/per year. However, there are solutions to address this price issue.

Normally it takes 1 tonne of lime to lift soil pH by 0.1 unit, but much depends on the Cation exchange capacity (CEC) of the soil. A low CEC soil, such as a sandy soil, requires much less lime to lift pH by 0.1 unit.

With the greater rainfall this spring there will be greater leaching occurring and soil nitrates that leach below the root zone will take a positive ion such as calcium with it.  Generally, ~150 kg/ha/yr of Calcium is leached if no nitrogen is applied as fertiliser but increases as more nitrogen fertiliser is applied.

The problem with New Zealand farms is that farmers are not applying lime because of the above, so many soils are getting very acidic (< 5.5 pH), with soils getting down to as low as 5.0 pH.  This has a significant detrimental impact on pasture production and clover growth, which ultimately leads to substandard animal performance.

The solution lies in having proper soil and pasture testing as without testing you cannot know what minerals need to be added to your soil and in what amounts. Otherwise, it becomes a guessing game.  Expert advice is needed at the right time, with a plan to address the issue within the budget allocated.

If these steps are taken, there is enough in the farmers’ budget to both lime and fertilise in order to reduce soil acidification. 

Written by Gordon Rajendram – The Soil Scientist https://gordonrajendramsoilscientist.co.nz/