The decrease in average yield on loans was due to the declining interest rate environment as well as an increase in payoffs of higher interest rate loans, as customers refinanced loans at lower interest rates.
The average balance of investment securities increased $4.6 million, or 22.2%, to $25.4 million for the three months ended March 31, 2022, from $20.7 million for the three months ended March 31, 2021, while the average yield on investment securities increased by 31 basis points to 1.37% for the three months ended March 31, 2022, from 1.06% for the three months ended March 31, 2021. This increase in yields resulted from the effects of management’s sale of lower yielding investments in December 2021 and purchases of higher yielding securities during the three months ended March 31, 2022.
The average balance of other interest-bearing deposits, comprised primarily of certificates of deposit in other financial institutions, overnight deposits and stock in the Federal Home Loan Bank, decreased $442,000, or 1.6%, for the three months ended March 31, 2022, along with a decrease in the average yield of 20 basis points, to 0.29% for the three months ended March 31, 2022, from 0.49% for the three months ended March 31, 2021 reflecting the lower interest rate environment.
Interest Expense. Total interest expense decreased $27,000, or 26.4%, to $75,000 for the three months ended March 31, 2022, from $102,000 for the three months ended March 31, 2021. The decrease was primarily due to a decrease in the average cost of deposits to 0.27% for the three months ended March 31, 2022, from 0.42% for the three months ended March 31, 2021, reflecting the declining interest rate environment, which was partially offset by an increase of $13.3 million, or 13.7%, in the average balance of deposits, to $110.0 million for the three months ended March 31, 2022, from $96.7 million for the three months ended March 31, 2021.
Net Interest Income. Net interest income increased $43,000, or 6.4%, to $710,000 for the three months ended March 31, 2022, compared to $667,000 for the three months ended March 31, 2021. The increase was due to an increase in the average net interest earning assets of $2.5 million period to period, while the interest rate spread decreased to 2.04% for the three months ended March 31, 2022, from 2.13% for the three months ended March 31, 2021. The net interest margin decreased to 2.09% for the three months ended March 31, 2022, from 2.22% for the three months ended March 31, 2021.
Provision for Loan Losses. Based on an analysis of the factors described in “Critical Accounting Policies and Use of Critical Accounting Estimates – Allowance for Loan Losses,” management concluded that a provision for loan losses was not required for each of the three months ended March 31, 2022 and 2021. The allowance for loan losses was $223,000 at both March 31, 2022 and 2021 and represented 0.27% of total loans at March 31, 2022, and 0.31% of total loans at March 31, 2021. The determination over the adequacy of the allowance for loan losses was due primarily to the low balances of nonperforming loans, delinquent loans and no net charge-offs in both periods.
Total nonperforming loans were $231,000 at March 31, 2022, compared to $7,000 at March 31, 2021. Classified loans totaled $72,000 at March 31, 2022, compared to no classified loans at March 31, 2021, and total loans past due greater than 30 days were $1.2 million and $1.4 million at those respective dates. As a percentage of nonperforming loans, the allowance for loan losses was 96.42% at March 31, 2022, compared to 3,327.62% at March 31, 2021.
The allowance for loan losses reflects the estimate management believes to be appropriate to cover incurred probable losses which were inherent in the loan portfolio at March 31, 2022 and 2021. While management believes the estimates and assumptions used in the determination of the adequacy of the allowance are reasonable, such estimates and assumptions could be proven incorrect in the future, and the actual amount of future provisions may exceed the amount of past provisions. Furthermore, as an integral part of its examination process, the OCC will periodically review our allowance for loan losses. The OCC may have judgments different than those of management, and we may determine to increase our allowance as a result of these regulatory reviews. Any material increase in the allowance for loan losses may adversely affect our financial condition and results of operations.
Non-Interest Income. Non-interest income was consistent between the three months ended March 31, 2022 and 2021, totaling $53,000 for the three months ended March 31, 2022, a decrease of $1,000, or 1.9%, from $54,000 for the three months ended March 31, 2021.
Noninterest Expense. Noninterest expense increased $55,000, or 8.6%, to $698,000 for the three months ended March 31, 2022, compared to the three months ended March 31, 2021. The increase was due primarily to an $11,000, or 22.0%, increase in data processing fees, a $24,000, or 68.7%, increase in professional services and a $12,000, or 16.2%, increase in other expenses. The increase in data processing was due to costs associated with enhancements in service capabilities for customers.