Long-term experiments are those, that are conducted on the same experimental units, over a sequence of years, with a pre-planned sequence of treatments or crops or both, mainly carried out to study the long-term effects of given treatments or crops on soil fertility and economic returns. Long term experiments include fertilizer experiments with cereal crops, irrigation experiments, tillage experiments, fertility status of soil, experiments on biomass production of cereals, on crop rotation effects of green manure etc.

It is mainly to study the effects of given treatments and crops on soil fertility and economic returns over the long term. Manures and fertilizers have a similar and very large effect on the long-term productivity of soil relative to applying no nutrients.

Thus, the addition of nutrients in either form must be regarded as essential for the maintenance of soil health. The long-term effects of organic amendments have a greater effect on increasing soil organic matter levels, soil physical properties and soil biological activity relative to fertilizers.

Long-term trials are the best indicators of both organic material utilization and sustainability. .