The soil is able to supply crops with phosphate and potassium, provided the soil index has been maintained and is close to the target index. However, there is often a gap or lag between crop demand and soil availability.
When is crop demand for P and K greatest?
This graph 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, rather than the more conventional method of applying them on their own in the autumn makes sense.
So, when is soil nutrient supply at its lowest?
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:
- The more clay and humus in the soil, the higher the nutrient holding capacity.
- Increased organic matter levels, improve the nutrient availability
- The higher the levels of calcium, iron, aluminium, the poorer the availability
- The optimum pH is around 6.4.
- Cold, waterlogged soils have the lowest nutrient availability
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.