How Neem trees affect soil insects and seed germination

Neem seeds smell bitter, almost like medicine and if you get your fingers on them, the smell seems to stick to them. I always thought the neem tree did something special, like providing shade for smaller plants and making leaves that would help break down in the soil. In some ways, I am correct. The shade from a neem tree is beneficial to small plants during the hottest part of the afternoon. Also, over time the leaves that fall from the tree create mulch that adds to the soil.

The part about controlling pests is much more complicated. The substance called azadirachtin, which is likely responsible for controlling the different insect populations, is primarily contained in the neem seed. Therefore, a distance of 40 feet from a neem tree does not release any substance into the air to control the aphids.

As for the mulch from the neem leaves, it adds to the richness of the soil but can also inhibit germination in the seed bed when placed too thickly. Something in the chemistry of the leaf mulch will slow down the germination process if the timing and amount of leaf mulch isn't carefully watched.

This doesn't change the usefulness of the neem tree, the neem cake when worked into the soil before planting will help suppress certain soil insects. The shade will give you results if placed properly and a benefit of the neem tree lies in its specific intentional use rather than just proximity.