Nitrogen Form – Impacts on Crop Physiology
Crops in the most part have come through winter in better shape than 2020 and it is again time to consider how to manage crops to achieve optimum yields. In many cases nitrogen applications will have already been applied and/or a ring roll employed in an attempt to promote tiller proliferation and [also] in the case of the roll, consolidation of puffy seed beds to ensure close contact in the rhizosphere to promote efficient nutrient scavenging from the soil by roots, essential to meet crop requirements and maintain healthy crop growth.
It is also the second year in a row where excess winter rainfall (EWR) has been higher than the long-term average, indicating that more nitrogen (N) as nitrate (NO3) may have been leached down through the soil profile. This will undoubtedly lead to the temptation to apply more nitrogen in an attempt to ensure crops are not left hungry. However, whilst N is the element required in the largest quantity by crops (excluding carbon) we should be paying careful attention to how we administer it, the form in which it is taken up can have implications for energy expenditure, plant growth physiology and ultimately yield.
The Energy Cost.
Plants can metabolise N in the following forms; nitrate (NO3), ammonium (NH4) and amine (NH2), whilst all three forms will promote growth they will do so at differing carbon (energy) cost. NO3 which is generally the most readily available form (due to the nitrogen cycle and bacteria that convert ureic forms of N to nitrate in the soil) has a higher energy cost to your crop due to the requirement for NO3 to be first converted to NH4 before being metabolised into amino acids and proteins. So it would seem perverse that we should be applying large quantities of nitrate fertilisers in only a few doses within the season, especially if a crop is recovering from stress, i.e. water logging, putting crops under more stress due to energy requirements as opposed to drip feeding smaller quantities. So whilst crops may look better than last year, following the higher than average EWR they may still be recovering from water stress and careful consideration should be given to N rates early on. You may also want to consider the use of a stabilised amine form of N as a foliar feed delivered alongside essential micronutrients as found in Elona to help crops through periods of stress and aid recovery, feeding crops on a form of N which has a lower energy requirement whilst also addressing micronutrient shortfalls, such as manganese, copper and zinc.
The form of N a plant metabolises can have an impact on where the plant places its resources, known as ‘growth partitioning’, when crops take up large quantities of nitrogen in the NO3 form this is transported to and metabolised in the leaf (as opposed to the root), this creates apically dominant top growth due to the production of the growth hormone ‘auxin’. This has the effect of creating very top heavy plants with lower root to shoot ratio’s leading to root systems less able to supply the demand of crop canopies, especially during warm, dry conditions [later in the season] ultimately leading to sub optimal yields and stress leading to higher susceptibility to disease, whilst crops are also more susceptible to lodging.
Crops that are exposed to and metabolise N in the amine (NH2) form, where metabolism takes place in the root, will build a larger root system and be more influenced by the hormone ‘cytokinin’ [produced in the roots], conveying more lateral shoot (tiller) growth reducing the dominance of the apical shoot. This gives the effect of a more bushy top growth improving the root to shoot ratio (see figure 1) which is less wasteful, whilst the roots are more capable of scavenging for soil nutrients and water, being less susceptible to stress during drier periods, especially later in the season.
So whilst considering your spring fertiliser regime, consider the use of ‘LimiN Technology’ within the Elona crop range from Levity Crop Science which stabilises nitrogen in the amine form (SAN) supporting robust canopy growth by promoting strong lateral rooting. Research carried out by Levity Crop Science (2020) by exposing winter wheat plants to bursts (foliar applications) of Elona containing ‘LimiN Technology’ demonstrates how it is possible to improve the root to shoot ratio of a modern winter wheat cultivar (figure 1).
Figure 1. Comparison between the effects of SAN (Elona), control and industry standard (IS) foliar N fertilisation treatments on root length (A) and root length – canopy height ration (B).
(121g/l N, 22g/l MgO, 80g/l Mn, 5g/l Zn, 1.3g/l Cu) – May be applied at 1.5-3l/ha with repeat applications as required.
(180g/l N, 84g/l K2O) – May be applied at 2-4l/ha with repeat applications as required.
(135g/l N, 150g/l MgO, 15g/l Zn) – May be applied at 2-4l/ha with repeat applications as required.