The way legumes benefit tomato plants is by adding nitrogen to your soil for free. Beans and peas host bacteria on their roots that pull nitrogen from the air. This process sounds great but the timing matters more than most gardeners think.
I grew bush beans between my tomato rows for three summers to test this claim. The beans grew fine but my tomatoes did not look any greener that same year. It took until the next season for me to see the nitrogen boost in my soil after the bean roots broke down.
A 2023 BMC Plant Biology study found big gaps between legume types. Fava beans get about 90% of their nitrogen from fixation while dry beans only get around 50%. Your choice of legume matters a lot for how much free nitrogen you add to your garden beds.
The same research showed beans had no fixation by week five of growth. The bacteria need time to set up shop on bean roots and start working for you. Quick crops of beans pulled before they mature add almost nothing to your soil nitrogen bank.
This explains why nitrogen fixing beans tomatoes work better as a rotation crop. Grow your legumes one year and turn them under before you plant tomatoes the next spring. The dead roots release their stored nitrogen right when your tomato plants need it most.
I tested fava beans as a winter cover crop and saw great results the next year. The plants grew three feet tall by early spring and added lots of green matter when I turned them under. My tomatoes that season grew darker green than ever before.
Legume companion planting still has some value if you grow beans and tomatoes in the same year. The beans shade the soil and keep it cool during hot spells. They also attract good bugs and break up your plantings to slow disease spread.
Pick legumes that fix the most nitrogen for the best soil boost in your beds. Fava beans and peas top the list with the highest rates. Bush beans and pole beans fix less but still add value as part of your garden rotation plan.
Turn your legumes under at least three weeks before you plant tomatoes in that bed. This gives the roots time to break down and release their nitrogen into the soil. Plant too soon and the rotting matter robs nitrogen from your young tomato plants.
Read the full article: Companion Planting Tomatoes: Proven Plant Pairings
