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PK additives primarily contain phosphate (“P”) and potassium (“K”). These elements are essential during the flowering and fruiting phase. Although bloom nutrients contain phosphorus and potassium, heavy fruiting plants such as tomatoes and peppers have an especially high demand for these elements. Hence, PK additives are used in conjunction with bloom nutrients to help ensure no deficiencies occur. They are also a more economical alternative compared to using extra bloom nutrient.
PK additives should generally be used from the onset of budding until flowers or fruit are fully formed.
When choosing a PK additive choose a brand that is relatively pH 'neutral'. Many brands are quite ‘alkaline’ which causes a significant increase in nutrient solution pH. These can quickly destabilize the nutrient if pH is not adjusted immediately after dosing.