Average interest earning assets produced a tax-equivalent yield of 4.20% for the nine months ended September 30, 2021, compared to 4.53% for the nine months ended September 30, 2020. The average rate paid on interest bearing liabilities was 0.95% for the nine months ended September 30, 2021, compared to 1.63% for the nine months ended September 30, 2020.
Interest Income. Total interest income on a tax-equivalent basis was $95.7 million for the nine months ended September 30, 2021, compared to $84.8 million for the nine months ended September 30, 2020. The $10.9 million, or 12.8%, increase in total interest income on a tax-equivalent basis was primarily due to continued organic growth in the loan portfolio and PPP loan income.
Interest income on the investment securities portfolio (on a fully-tax equivalent basis) increased $582,000, or 8.3%, during the nine months ended September 30, 2021, compared to the nine months ended September 30, 2020, due to a $80.6 million, or 26.4%, increase in average balances between the periods, which was offset partially by a 44 basis point decrease in the aggregate portfolio yield.
Interest income on loans (on a fully-tax equivalent basis) for the nine months ended September 30, 2021 was $87.8 million, compared to $77.3 million for the nine months ended September 30, 2020. The $10.5 million, or 13.5%, increase was primarily due to a 20.1% increase in the average balance of loans outstanding, which was offset partially by a 26 basis point decrease in the average yield on loans. The increase in the average balance of loans outstanding was due to strong organic loan growth. The decrease in yield on the loan portfolio was primarily driven by lower market interest rates and lower loan fee recognition, offset partially by increased recognition of PPP loan fees.
Interest Expense. Interest expense on interest bearing liabilities decreased $6.3 million, or 29.8%, to $14.7 million for the nine months ended September 30, 2021, compared to $21.0 million for the nine months ended September 30, 2020, due to lower interest rates paid on both deposits and borrowings.
Interest expense on deposits decreased to $10.6 million for the nine months ended September 30, 2021, compared to $15.7 million for the nine months ended September 30, 2020. The $5.1 million, or 32.6%, decrease in interest expense on deposits was primarily due to a 67 basis point decrease in the average rate paid, even as the average balance of deposits increased 29.0%. The decrease in the average rate paid was primarily due to the impact of lower market interest rates. The increase in the average balance of interest bearing deposits resulted primarily from increases in interest bearing transaction deposits, savings and money market deposits and brokered deposits, offset partially by a decline in time deposits.
Interest expense on borrowings decreased $1.1 million to $4.1 million for the nine months ended September 30, 2021, compared to $5.3 million for the nine months ended September 30, 2020. This decrease was primarily due to lower average balance of FHLB advances and federal funds purchased, as well as the payoff of the Company's notes payable, offset partially by the issuance of additional subordinated debentures in July 2021.
Provision for Loan Losses
The provision for loan losses was $1.3 million for the third quarter of 2021, a decrease of $2.5 million, compared to the provision for loan losses of $3.8 million for the third quarter of 2020. The provision for loan losses was $4.0 million for the nine months ended September 30, 2021, a decrease of $4.9 million compared to the provision for loan losses of $8.9 million for the nine months ended September 30, 2020. The decrease in the provision for loan losses compared to both prior periods related to improving economic conditions.
The allowance for loan losses to total loans was 1.43% at September 30, 2021, compared to 1.39% at September 30, 2020. The allowance for loan losses to total loans, excluding PPP loans, was 1.46% at September 30, 2021, compared to 1.51% at September 30, 2020.
As an emerging growth company, the Company is not subject to Accounting Standards Update No. 2016-13 “Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses of Financial Instruments,” or CECL, until January 1, 2023.