Once the optimum canopy has been constructed then the focus must be on maintaining its health, and keeping it green for as long as possible ensuring maximum seed fill which will deliver the final yield. Whilst the youngest leaves will be the most photosynthetically active, the pod canopy will also contribute towards yield so their health must also be maintained.
Nutrients that are essential for maintaining the canopy are nitrogen and magnesium.
Magnesium plays a crucial role at the heart of the chlorophyll molecule despite being considered a secondary nutrient. Any magnesium deficiency quickly entails a drop in the chlorophyll content explained by reduced protein synthesis. 20 to 30% of the plant’s magnesium is connected to the chlorophyll; the remainder is in soluble form associated with various organic or mineral ions.
High deficiencies are still fairly rare however and can result either from a magnesium depleted soil, or be induced from the compaction of the soil or through an imbalance with other cations in the soil such as potassium, calcium or ammonium.
Compared to the total uptake, oilseed rape returns 90% of the potash and calcium, 83% of the magnesium, 66% of the sulphur, 55% of the nitrogen and 46% of the phosphorus.
Manganese and iron are the two micro nutrients with the largest requirement by oilseed rape, however, boron and molybdenum deficiencies are those most frequently encountered.
A boron deficiency greatly reduces the viability of the pollen and seed formation, thereby reducing flowering and fertilization. Protein content is also affected. A slight deficiency may show no symptoms but nonetheless leads to poor seed formation.
Distinguished by a very low requirement in the order of 20g/ha with contents of 0.4 to 0.6 ppm in the plant, oilseed rape is nonetheless susceptible to molybdenum deficiencies as with all crucifers. Any molybdenum deficiency reduces protein synthesis, leads to an accumulation of nitrate and reduces the effectiveness of the nitrogen.
There is an antagonism between sulphur and molybdenum: high sulphur uptake may reduce the uptake of molybdenum. Molybdenum is the only micro nutrient whose plant availability is increased by raising the pH of the soil.
Sulphur has been the subject of much research, considering oilseed rape’s great demand for it and the risk of significant loss of yield where it is deficient; as well as its role in seed and oilcake quality.