(Ref: Optimizing the Nitrogen Supply in Biological Farming, Louis Bolk Institute, Nutrient Management Institute, Wageningen, April 2003)
The Figure shows the results of a model study with different fertilizing scenarios for early lettuce on sandy soil, with an average gross yield of 25 ton and an N-uptake of 65 kg N/ha.
Solid manure scenario: The use of 40 tons of manure (240 kg N, 123 kg P2O5), a few weeks before transplanting, will not supply enough available N and this almost during the whole growing cycle. Also the possible second crop will have insufficient nitrogen.
Slurry scenario: With 30 tons of slurry (134 kg N, 54 kg P2O5), incorporated within 12 hours after application, the early crop can be sufficiently supplied with N. At the end of the growing cycle the mineral N drops to a low level. The second crop will probably suffer from shortage in available N.
Side dressing scenario: A supply of 15 tons of slurry in the first crop, incorporated within 12 hours after application, combined with a side dressing of 60 kg fast acting nitrogen delivers sufficient nitrogen. Repeating this for the second crop again provides enough nitrogen. Most probably the 2nd side dressing can even be omitted.
Results :
- Unsatisfactory results with 240 kg N as manure and 134 kg N as slurry both applied in one time as the buffer level of mineral N towards the end of the second crop cycle is not attained.
- Good results with 2 x 67 kg N of slurry together with 2x 60 kg of quick acting N side dressing (total 250 kg N) even though at the time of harvest a low level of mineral N is quickly reached limiting the hazard of losses.
This strategy also limits the risks of excess application rates of other main nutrients (P, K) present in manure and slurry.
Ideally the solid manure and the side dressing application should be combined to obtain the optimum result. More examples can be found in the book : Use of Natural Chilean Nitrate in Organic Farming