New research uncovers how much damage pine forests are doing to our soil

New research is showing just how much damage is being done to our land by pine forests.

It suggests it can take 30 years for the soil to be suitable for pastoral use once again, after the trees are chopped down.

Ravensdown chief scientific officer Dr Ants Roberts - known as Dr Dirt - has dug up statistics that may make farmers think twice about converting pastoral land to forest.

"It takes time and considerable time, 20 to 30 years, to get it back to full, what we would regard as soil health for pastoral farming," he explained to Newshub.

That's because the soil in pine forests tends to be acidic, unlike the soil on a lifestyle block, and in the forest you don't get nearly as much organic matter.

"You won't find these in pine forests, these are pastoral earthworms," Dr Roberts said, holding some up in his hand.

Additionally, forestry soil is looser and may be subject to erosion.

Experts always suspected that growing pines would have a huge impact on the soil but they didn't know for how long.

Ravensdown and AgResearch scientists crunched the numbers from Ngāi Tahu land converted from forest to pastoral.

"Converting good pastoral land to pine forests - as an agricultural soil sample, er, soil scientist, I cry when I see that," Dr Roberts said.

He's upset at the loss of top-quality pastoral soil.

"That horse has bolted but yes I wished we had taken a breath and weighed up the consequences of locking up a lot of good pastoral land into forestry," he said.

We've seen plenty of pastoral land turned to forests under the Emissions Trading Scheme, about 210,000 hectares since 2017.

"Pastoral agriculture, the production of meat, wool, meat, fibre and milk is what drives the New Zealand economy. Yeah, we wouldn't have half the stuff we have in this country if it wasn't for agriculture earning overseas funds," Dr Roberts said.

Agricultural economist Phil Journeaux said "you're losing those exports and you're gaining nothing from the carbon per se".

However, Journeaux said you do get the benefits if and when you harvest but you still have to replant to retain the carbon credits.

"In the long term you will get something from that, it just depends whether they're production forests or permanent forests," he said.

Despite the degradation of the soil, watch for more forests on the slopes of farms - just not the steepest one with the biggest risk of slash, which the Government may prevent farmers from planting.

Scion principal scientist Dr Peter Clinton said soils that have naturally developed under forest, like most soils in New Zealand, are acidic in their natural state.

“When forest is cleared to make pasture, soils need to be made less acidic through application of lime to reach a pH level that is best for pasture. It’s no surprise to see those changes reversing when forest is reestablished.”

 These changes have been well documented in long-term studies, Dr Clinton said in a statement to Newshub.

 “When we have measured soil health under pasture, planted pine forest and indigenous forest, we have found that soils under pine are much more similar to those under indigenous forest than they are to soils under pasture. In fact, lime needs to be regularly added to pasture soils to maintain the pH suitable for pasture growth.

 “We see these similarities in a range of measures. Nutrient and water runoff under pine forest are much more like the nutrient and water runoff under natural forest than pasture too.”

The New Zealand Institute of Forestry (NZIF) President, James Treadwell said in a statement:

"The prevalent belief pines contribute to lower soil pH and this is damaging is misleading. Contrarily, all forest soils tend to be acidic; indigenous New Zealand forests, deciduous forests and conifer forests. Our most acidic soils occur under our mighty Kauri forests.

"The transformation of pasture into native or planted forests naturally tends towards increased acidity, especially when reverting from pasture, which is generally top-dressed with lime and fertiliser every few years in perpetuity.  

This move towards acidity is the soil reverting to its natural state.”

This article was amended on December 19 to add in comment from Scion and the New Zealand Institute of Forestry.