Spring applications of NPKS fertilisers have consistently outperformed traditional autumn applications of P&K fertilisers in independent trials. So why do so many growers still apply phosphate and potash in the autumn rather than in the spring.
The traditional approach to phosphate and potash management has been through the autumn application of products such as a 0-24-24, 0-20-30 or straights (eg TSP and MOP). This strategy is a consequence of the soil being the focus as a level of fertility is looked to be maintained or in some cases built up. This is still appropriate on those low Index (0) fields, or areas of fields, that require larger application rates and crop establishment is at risk due to poor nutrient supply. However, the majority of fields now have adequate soil phosphate and potassium reserves (Index 1 and above) to satisfy the early establishment crop nutrient demand, enabling a change of focus towards the crop and when it requires the bulk of its nutrient for optimal growth and development. Such a change of focus soon highlights the need for a change of strategy as the traditional approach does not meet the crop requirements.
Spring applied NPKS has been shown to give a yield response in wheat and barley when compared to autumn applied phosphate and potash.
Trials by NIAB/TAG have shown that spring applications of NPKS compound fertilisers can increase yields by, on average, 0.3t/ha (0.12t/acre) in winter wheat and winter barley. More recent trials by the Royal Agricultural University (RAU, Cirencester) in 2016 have shown responses of more than double this – see here - with an increase in wheat yields of one tonne/hectare
The graph below indicates the nutrient demand of a winter wheat crop and clearly shows that the all important demand happens in the spring for all the macronutrients. The months of March, April and May are so critical as the crop goes through its ‘grand growth phase’, putting on large amounts of biomass that ultimately becomes yield.
In spring, crops such as oilseed rape go through a period of rapid growth during which their uptake requirement of several nutrients increases, especially potassium (K). It is a vital building block for yield development and also helps protect the crop against disease. The demand for K in OSR may be in excess of 12kg/ha/day with a total requirement of up to 300kg K20 by the end of flowering, for a 3t/ha crop. In wheat the demand for K may exceed 10kg/ha/day with a total requirement of up to 250kg K20 by the end of flowering. Therefore, applying P and K in the spring, as part of a ‘spring starter’ compound granular fertiliser – rather than the more conventional method of applying them on their own in the autumn – certainly makes sense.
Having established that nutrient demand is greatest during the spring, it is then important to consider soil nutrient supply as this needs to be synchronised, meaning that demand and supply are not compromised. So, what factors impact on the soil nutrient supply? There are clearly a number of factors here including a soils physical state and its properties (texture, organic matter levels etc), its pH, chemical composition, moisture and temperature. These can be summarised by the following:
Whilst some of the factors above are site specific, the final point (cold and wet soils) are more generic, meaning that for most soils, nutrient supply will be at its lowest as it comes out of winter and into the spring months. It follows therefore that crop nutrient demand and soil supply are most likely to be out of sync on most farms.
Having established the facts then the most important next step with your strategy is to address the issue with an appropriate solution. This solution is very simple and can actually bring other advantages often not considered. All that is required is a change of nutrient application timing, and to select a product that has some key features:
Such a solution will enable you to supply all your key macronutrients (NPKS) in two or three applications (dependent on your total nitrogen application rate). This compares to the more traditional approach where very often the number of fertiliser applications will be four to six dependent on product choice, adding up to £45/ha in application costs (ref. ABC May 2020 contractor charges).
There are a number of considerations that will help product selection including:
These compound fertilisers are recommended for all crops and supply a combination of nitrogen and sulphur (NPK and S) in appropriate ratios for a spring application of all nutrients.