Yes, you can.
Your IRA contributions may or may not be deductible if you're in an employer-sponsored plan (the 401k). The deductibility will depend on your filing status, modified AGI, and (if married) whether your spouse is covered under an employer plan.
Here's a link to IRS Publication 590-A, which discusses these limitations (see the tables on p.13): https://www.irs.gov/pub/irs-pdf/p590a.pdf.
You'll enter any IRA contributions in TurboTax by selecting Federal Taxes > Deductions & Credits > Retirement and Investments > Traditional & Roth IRA Contributions and following the prompts.
I am a business owner with no employees. I decided to open a solo or individual 401K plan for myself. Can I still contribute to traditional IRA in the same year?
If you are self-employed and contribute to a solo 401(k), you can also contribute to a Traditional IRA.
can you direct me to a specific page in IRS Publication 590-A that talks about this?
Here's a link to IRS retirement plan FAQ (note #3) stating that you can contribute to both an employer-sponsored plan and a Traditional or Roth IRA: <a rel="nofollow" target="_blank" href="https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-iras-contributions">https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-iras-contributions</a>. Here's a link to the IRS definition of an employer plan, which includes 401k plans: <a rel="nofollow" target="_blank" href="https://www.irs.gov/retirement-plans/are-you-covered-by-an-employers-retirement-plan">https://www.irs.gov/retirement-plans/are-you-covered-by-an-employers-retirement-plan</a>. Here's a link to the IRS page on one-participant 401k plans, which states that these plans have the same rules & requirements as any other 401k plan: <a rel="nofollow" target="_blank" href="https://www.irs.gov/retirement-plans/one-participant-401k-plans">https://www.irs.gov/retirement-plans/one-participant-401k-plans</a>. See the above-cited Pub 590-A, p.13, for Traditional IRA deductibility limitations.
Thanks. My accountant says it's one or the other. I can prove him wrong
It's likely that your accountant is wrong, but there's a small chance that your accountant is correct. The amount of your net earnings available to contribute to a traditional IRA is reduced by the amount of your deductible contributions made to your solo 401(k). If your net earnings are low enough, it's possible to contribute all of your net earnings to a solo 401(k), leaving none available to contribute to a traditional IRA.
However, with net earnings in excess of $18,000 ($24,000 if age 50 or older in 2016), there will be *some* net earnings left to contribute to a traditional IRA, perhaps enough to make the maximum traditional IRA contribution. Since your solo 401(k) contributions make you covered by a workplace retirement plan, whether or not that traditional IRA contribution will be deductible will be determined by your filing status and modified AGI.
for a single, at what AGI do I start lose the deductibility of traditional IRA? How to compute AGI?
$61,000
<a rel="nofollow" target="_blank" href="https://www.irs.gov/retirement-plans/plan-participant-employee/2016-ira-contribution-and-deduction-limits-effect-of-modified-agi-on-deductible-contributions-if-you-are-covered-by-a-retirement-plan-at-work">https://www.irs.gov/retirement-plans/plan-participant-employee/2016-ira-contribution-and-deduction-limits-effect-of-modified-agi-on-deductible-contributions-if-you-are-covered-by-a-retirement-plan-at-work</a>