The optimal nitrogen requirement of the plant is considered to be up to 200 kg N/ha (Bullard, M., Murphy – Bohern, D., 2000) or 13 to 15 kg N/tonnes of dry matter (Barron et al. 2003). Nitrogen deficiency may stunt growth because certain stages of plant growth are skipped. If nitrogen supply is more or less adequate, it is recommended not to add nitrogen to the soil because it could lead to soil leaching. If there is too much nitrogen, the leaves stay on the plant and the stems remain green for longer and this can affect fibre quality. Artificial nitrogen fertilisers are not recommended. However, nitrogen deficiency may result in shorter plants, smaller surface area of the leaves and lower efficiency in using sunlight, which may decrease the yield (Bullard, M., Murphy – Bohern, D., 2000).
The plants' potassium and phosphorus requirement is largest during flowering and seed formation stages. Hemp is able to absorb potassium from the deeper layers of soil but the potassium requirement is considerable (Girouard et al., 1998 cit. Barron et al. 2003), being on average between 75 and 150 kg/ha. In extreme cases, a plant may use up to 300 kg/ha. Phosporus requirement is smaller, around 50 to 70 kg/ha, but fibre elasticity and strength depend on it and thus, the availability of phosporus for producing high quality fibre is extremely important. The table below shows the main average nutritional needs of hemp:
Optimal Requirement of Nitrogen, Phosphorus and Potassium
Hemp plants need calcium in the same amount as potassium and the soil usually contains adequate amounts. If the soil is acidic, it is recommended to add lime. Hemp's magnesium requirement is approximately the same as its phosphorus requirement. Although hemp needs large amounts of nutrients to ensure good growth, it gives a considerable amount of nutrients back to the soil. Harvesting takes place after the leaves have fallen and the leaves contain plenty of minerals. In case of dew retting, a part of the minerals stays on the field. If dew retting is not done, the amount of minerals given back to the field is 5% smaller.
The table below shows the ratios of how much of the absorbed minerals goes back to the soil through different processes.
Amounts of minerals absorbed from and put back into the soil
(Baxter & Scheifele, 2000 cit. Barron, A., 2003)
||Absorbed by the plant (%)
||Share of the absorbed amount
that goes back into the soil (%)