The fertiliser plan to set your farm up for success this growing season
The amount of phosphorous (P) and potassium (K) for required largely depends on how much grass is going to be utilised according to Stan Lalor, speaking to the audience at the Irish Grassland Association conference this week.
Uptake required to grow 1t DM/ha of healthy sward of rye grass is 4kg/ha of P and 30kg/ha K, according to Stan. Thus, if a farmer plans to grow 15t DM/ha, the total uptake will be 60kg/ha of P and 450kg/ha of K.
Allowing for 80pc utilisation of grass, these figures are then adjusted to 48kg/ha of P and 360kg/ha of K, this is before taking into consideration how much is being recycled back into the field by the cow.
Some 60pc of P and 90pc of K is recycled back into the field, meaning 40pc of P and 10pc of K needs to be replaced in order to keep the levels right for optimum grass growth, Stan said.
Based on these sums, the typical nutrient removal by grazing animals for soil fertility maintenance is 19kg/ha of P and 36kg/ha of K, at 12t DM/ha utilised, assuming grass indices are high, he said.
When to apply P and K
According to Stan, P should be spread in earlier spring to achieve the best results, with a target to get between 50-75pc of the annual P requirement applied in the first two rounds of fertiliser. The remaining P should then be spread little and often throughout the summer months.
Applying K in spring is not as critical as applying P, according to Stan. Although he admitted they usually go hand in hand when spreading, he said to avoid spreading excess K in spring as it can have a negative effect on the uptake of magnesium and resulting in the risk of grass tetany.