After the oil has been pressed from the seed kernels, the remaining material is called "neem cake". This material has been used for many centuries throughout India as a soil amendment. Experience has taught farmers there that working the leftover neem cake into the soil of a garden produces larger, healthier plants that have few problems with insect pests.
Several studies were done to find out why plants grew better in soils mixed with neem cake. The studies discovered that neem cake was richer in plants nutrients than manure killed damaging nematodes, promoted larger populations of earthworms, helped keep nitrogen in the soil available for the plants, and provided significant protection from insects. This combination of effects provides an almost ideal growing condition for the plants ( Khan, 1974); ( Vizayalakshmi,et ai, 1985).
By killing nematodes in the soil, a major plants pests is eliminated. Nematodes suck the juices from the roots of plants to the point where they are unable to supply sufficient nutrients to the plant. The plants look sickly, fail to grow and may eventually die despite sufficient food, water and care. One the other hand, by promoting larger populations of earthworms, neem cake helps keep soil loose so that water and nutrients can more easily be absorbed by the roots. Earthworms also enrich the soil by creating readily absorbable nutrients as it feeds on decaying plant material. Neem cake also reduces the nitrification rate of the soil by suppressing nitrifying bacteria such as nitrosomonas and nitrobacter bacteria. This reduces the need for applications of external nutrients. Some studies have shown that mixing neem cake with regularly scheduled applications of manure can almost double crop yield over manure alone. Neem compounds are also slowly absorbed into the plant to augment plant natural defenses with the proven nutritive, antifungal and insect repellent properties of neem.