The principles of organic farming dictate that nutrient cycling in the agricultural system should be closed and therefore not reliant on additional inputs. However, due to the removal of both vegetable and animal matter from the system, a proportion of the soil reserves will be depleted with every harvest resulting in an export of nutrient. Furthermore it is not always possible or practical to return these nutrients by way of the addition of organic manures.
In order to ensure an adequate supply of nutrients to crops, and in order to safeguard long-term fertility of agricultural soils, the EU-Eco-Regulation No 834/2007 and No 889/2008 allows mineral fertilisation, if needed (liable to registration). The demand for such treatment is assessed by the competent regulatory authority.
The requirement for mineral supplementation depends on the farm system as well as on the crops harvested and their nutrient requirement. Land used for field crops and vegetables usually requires more fertilisation than does pastureland. Based on soil analyses and the nutrient balance (assessed by comparing and monitoring soil nutrient levels over time) the need for supplementary fertilisation can be determined. Even if the annual deficit seems negligible, the effect is cumulative and over time, a significant loss of soil fertility can occur by depleting nutrient reserves.
Crop rotation: Silage maize – winter rye +US – clover – potato – winter rye – blue lupin – winter triticale
| Balance parameters | Manure application3), kg ha-1+year | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| incl. | excl. | ||||||||
| N | P2O5 | K2O | MgO | | | N | P2O5 | K2O | MgO | |
| Nutrient supply total | 90 | 30 | 45 | 17 | | | 31 | |||
| of this, manure | 57 | 30 | 45 | 17 | | | ||||
| N from N-fixation |
33 | | | 31 | ||||||
| of this, mineral fertiliser4) | 80 | 27 | |||||||
| Nutrient export, total | 87 | 37 | 101 | 15 | | | 40 | 21 | 40 | 7 |
| of this, primary products | 821) | 35 | 87 | 13 | | | 402) | 21 | 40 | 7 |
| of this, co-products | 5 | 2 | 14 | 2 | | | ||||
| Nutrient balance |
3 | -7 | -56 | 2 | | | -9 | -21 | 40 | 20 |
| Nutrient balance without mineral fertiliser | 3 | -7 | -56 | 2 | | | -9 | -21 | -40 | -7 |
| 1) | Clover removed |
| 2) | Clover remains on field |
| 3) | 20 t ha-1 manure on potatoes and silage maize, 10 t ha-1 as top-dressing on winter rye and triticale |
| 4) | 240 kg ha-1 Patentkali as an annual rotation average |
Source: Dittmann B., Zimmer J.: LVFL Brandenburg, Güterfelde, 2008
As is apparent from the nutrient balance, if there is no supplementation from mineral fertiliser, the application of only manure results in phosphate and potassium deficiencies, while operations without any livestock suffer deficiencies of all nutrients. Even though it is possible to positively influence these balances by crop selection, such as by raising the proportion of N-fixating legumes, in order to increase the N-balance. However, this is not sufficient for balancing the needs within the crop rotation. It also needs to be taken into consideration that the legume-proportion within the crop rotation may not be increased indefinitely. Next to raised costs for seed, there are also crop protection and agricultural aspects to take into account when growing more legumes
The soil levels of potassium, magnesium and phosphorus are continuously reduced by a non-livestock based system. Soil analysis before and after one full crop rotation has shown that the soil’s potassium content is seriously depleted as a result of several years of K-removal. Not only does this jeopardise the potential crop yield but results in degredation of the soil fertility in the longer term and therefore reduces the efficiency of the entire farm system.
In order to avoid this, soil analyses need to be conducted periodically. This helps to detect nutrient deficiencies early on, so that mineral fertiliser may be applied in compliance with the EU-Eco Regulation and the local associations governing organic agriculture. Another important topic is the supply of sulphur. Due to efforts made to purify the air, S supply from the air has been reduced considerably. Sulphur return from liquid manure only applies to livestock-operations; while only 5-10% is actually available during the given vegetation period. For this reason, sulphur deficiency has become a major yield-limiting factor in many areas. Sulphur is essential for the yield and the quality of oilseeds, legumes, onions, leeks and other crops.
The approval of fertilisers in organic agriculture is governed by European Council regulation No 834/2007 and No 889/2008, which among others lists the following fertilisers as admissible:
