Yes, you can run into trouble with too much green manure, but the real fault is almost never the amount of seed you sowed. The problem shows up when you let the crop grow too big and woody before you dig it in. Thick, tough stems take a long time to break down, and while they rot they can tie up the very nitrogen your next crop needs. So the cure is rarely sowing less. It is cutting and turning in the growth at the right stage.
Here is what happens under the soil. When you bury woody, high-carbon growth, soil microbes swarm in to eat it. But to digest all that tough material they need nitrogen of their own. So they pull it straight out of the soil around them. This is nitrogen immobilization, and it starves your seedlings for a few weeks. The plants you put in next can turn pale and stall, even though you just added what looks like rich green organic matter.
Overgrown, woody green manure briefly locks up nitrogen as it rots. Kill rye at about 6 inches (15 centimeters) tall and legumes at bloom to avoid it, per University of Missouri IPM.
The deciding factor is the carbon to nitrogen ratio of the growth you turn under. Think of it as the balance between woody bulk and soft, green tissue. Young, leafy plants hold more nitrogen and less carbon. So they rot fast and feed the soil right away. Old, stemmy plants swing the other way. They carry a lot of carbon and little nitrogen. That mix is the trigger. It forces the microbes to borrow nitrogen to finish the job. The taller and tougher the crop gets, the worse that balance tips against you. A crop you let bolt waist high can sit in the soil for a month or more before it gives anything back.
University of Missouri IPM gives a clear line to follow. Kill cereal rye when it is about 6 inches (15 centimeters) tall, because rye that grows past that point can tie up nitrogen as it breaks down. For legumes like clover, vetch, or field peas, cut them at bloom, when their leafy growth still holds plenty of nitrogen. Pass those stages and the same crop that should have fed your soil starts to compete with it instead.
So work with timing, not seeding rate, to stay safe. Dig in your green manure before it flowers while the growth is still soft and full of sap. Chop the leaves and stems small with a spade or shears so the bits break down faster and more evenly. Smaller pieces give the microbes more surface to grab onto. That shortens the hungry window when nitrogen sits locked away from your crops. A clean, shallow chop also keeps the material in the top few inches of soil, where it rots best. Bury it too deep and it just sits there, cold and slow.
Then give the soil time before you plant again. Wait two to three weeks after digging in so the material can rot and hand its nitrogen back. By then the microbes have finished feeding. The nitrogen they borrowed flows back into the soil for your seeds. Skip the wait and you plant straight into that hungry gap. Say you missed the window and your crop grew tall and woody anyway. You still have options. Plant something nitrogen hungry a little later, once the rot has caught up. Or top up with a quick liquid feed to carry seedlings through the lean stretch. Get the timing right and too much green manure simply stops being a worry.
Read the full article: Green Manure: A Practical Soil Guide
