We estimate the weak winter weather season in both the first quarter and fourth quarter of last year negatively impacted our full-year 2020 operating income by approximately $40 million to $45 million.
In addition, our South American Plant Nutrition business experienced stronger year-over-year agriculture sales volumes and, in local currency, achieved a 16% increase in fourth quarter operating earnings versus 2019.
However, the Brazilian currency weakened by approximately 33% during the year compared to the U.S. dollar, which ultimately hurt our bottom-line in terms of U.S. results.
As we look at full year 2020 on a consolidated basis, net income for the year decreased by approximately 5% and adjusted EBITDA decreased by approximately 8% when compared to 2019 results.
On the positive side, we continue to generate strong positive cash flow from operations totaling over $175 million for the full year.
We also took an aggressive approach to managing our capital plan and I'm pleased we were able to come in 13% below the midpoint of our original guidance for a total spend of roughly $85 million for the year.
Our free cash flow for the full year was just over $90 million and we returned at $99 million to shareholders through our dividend program, which reflects our confidence in the company's ability to deliver cash flow through varying economic and weather-related cycles.
Full-year adjusted EBITDA margins increased approximately 3 percentage points to 29% despite our adjusted EBITDA being lower by 2%.
On a full year basis, production tons out of Goderich have increased 17% from 2019 results and production costs are down 16%.
These storms resulted in approximately 11 lost production days during the year.
Our team worked to ensure we were well-positioned to capture those sales volumes in the fourth quarter, ultimately, delivering strong year-over-year revenue growth of 16%.
Our South American Plant Nutrition business continued to achieve measured growth in local currency, with sales revenue up 18% for the full year 2020.
During its initial year in this new structure, the department executed over 65 initiatives, driving as much as 10% annualized savings in a number of specific procurement categories such as contractor services, packaging raw materials and equipment spare parts.
In addition to achieving a multi-year low for our 12 months rolling TCIR average in 2020, we ended the year with an average of 1.53 and I'm happy to share that our TCIR in December was among the lowest of any month in the history of the company, coming in at 1.23.
We now estimate a combined negative impact to our original operating earnings forecast of roughly $67 million due specifically to weak winter weather in both the first and fourth quarters, a Brazilian currency that progressively weakened throughout the year and COVID-19 impacts, including both cost preventative measures at our sites and reduced demand within certain higher-margin end markets.
For the full year, sales revenue and operating income were also lower as we dealt with over $100 million in sales revenue impact and more than $40 million of operating earnings and tax due to weak winter weather.
Total sales in the quarter were $228.5 million, down from $310.9 million in the fourth quarter of 2019, largely due to lower weather-driven demand for deicing products and the effects of customer carryover inventories.
Total Salt segment sales volumes dropped to 23% compared to fourth quarter of 2019.
Within our Salt segment, highway deicing experienced a 25% sales volume decline and consumer and industrial sales volumes dropped 16% year-over-year.
Highway deicing prices were down 11% versus the prior year quarter at $59.20 per ton.
However, consumer and industrial average selling prices increased 1% to $169.30 per ton as a broad-based price increases across all non-deicing product groups was mostly offset by lower sales mix of our higher priced deicing products.
Operating earnings for the Salt segment totaled $50.2 million for the fourth quarter versus $80.5 million last year, while EBITDA for the Salt segment totaled $67.6 million compared to $96.5 million in the prior year quarter.
When stepping back and looking at our fourth quarter Salt costs, we ended up at $41 per ton, which is flat with the 2019 fourth quarter.
However, on a mix-adjusted basis, our unit cost is about $1.25 per ton lower than prior year.
So we absorbed a 25% decline in year-over-year fourth quarter Salt sales volume and we were still able to decrease our mix adjusted Salt unit costs versus the prior year.
Improved production and logistics costs in our North American highway business for the full year 2020 helped to offset a 12.4% lower salt revenue and resulted in adjusted operating income declining just 6% and an adjusted EBITDA decrease of only 2% year-over-year.
These efforts contributed to the expansion of the Salt segment adjusted operating margin to nearly 21% from about 19% last year and, at the same timem driving adjusted EBITDA margin to 29.3% compared to 26.1% for the full year 2019.
Fourth quarter total sales revenue increased 15.9% from the prior year to $88.7 million.
We achieved this by delivering a 23% increase in sales volumes, partially offset by a 6% lower average selling prices.
In turn, our Plant Nutrition North America EBITDA margin compressed to about 20% in the quarter compared to nearly 34% in the prior year, with operating margins declining about 10 percentage points quarter-over-quarter.
Strong full year sales volumes, partially offset by lower sales prices, helped us deliver a 16.2% improvement in 2020 full year Plant Nutrition North America revenue versus 2019.
These revenue results, coupled with the short-term fourth quarter cost pressure and the previously disclosed $7.4 million inventory adjustment in the third quarter, resulted in a $10.4 million decline in operating income and a $14.6 million decrease in full-year EBITDA.
Excluding the inventory adjustment, full-year operating margin would have been 8.1% compared to 10.9% in 2019, while full-year EBITDA margin would have been 25% versus 32.5% last year.
Our Plant Nutrition South America segment delivered a 24% year-over-year increase in fourth quarter 2020 revenue and an 18% increase for the full year, both in local currency.
Fourth quarter agro revenue was up about 29% versus 2019 and up nearly 23% for the full year.
Even more impressive was our fast growing Ag B2C business unit where strong sales volumes and price drove a 37% increase in both our 2020 fourth quarter and full year revenue when compared to prior year, again, all in local currency.
In local currency, our Plant Nutrition South America fourth quarter and full year 2020 operating earnings increased 16% and 35%, respectively, while EBITDA increased in lockstep by 15% and 25%, as well.
While we're obviously disappointed with our fourth quarter and full year 2020 results, it's important to again point out that the combination of weak winter weather, Brazilian currency devaluation and COVID-19 impacts in both mitigation costs and end market deterioration negatively affected our 2020 full-year operating income by nearly $70 million.
Despite this impact, we were able to hold year-over-year adjusted EBITDA margins flat at 21% and generate more than $175 million cash flow from operations and $90 million of free cash flow.
For the full year 2021, we are expecting consolidated adjusted EBITDA of between $330 million and $360 million, which is a year-over-year increase of about 20%.
While the midpoint of our guidance is at the lower end of last year's full-year guidance, it's important to note that weak winter weather impacts, like those in 2020, are never immediately reset as prior bid season pricing almost always has a significant influence on the following years' average selling prices.
Our annual operating plan anticipates approximately $100 million in 2021 capital spending, as well as free cash flow at levels similar to 2020.
While our net debt to adjusted EBITDA ratio ended 2020 at about 4.3 times, it is expected to end 2021 below 4 times.

Summary:
While the midpoint of our guidance is at the lower end of last year's full-year guidance, it's important to note that weak winter weather impacts, like those in 2020, are never immediately reset as prior bid season pricing almost always has a significant influence on the following years' average selling prices.