Straight fertilisers: what to apply, when, and what not to mix

Straight fertilisers are a bag with one or two nutrients, not an “all-in-one complex”. They are cheaper, easier to dose, and each one solves its own task: nitrogen drives leaf growth, phosphorus feeds roots and flowering, potassium gives fruit flavour and winter hardiness, and calcium protects against physiological disorders. A beginner’s mistake is to apply everything at once and at any time. Below is a rundown of what is in the nursery’s dry fertilisers, when to apply each and what it must not be mixed with.

What each element does

• Nitrogen (N) – growth of shoots and leaves. Needed in spring and early summer. From mid-July nitrogen is not applied: shoots will not ripen before the frosts
• Phosphorus (P) – roots, flower formation, ripening. It moves poorly in soil, so it is applied to the root zone: at planting or in autumn when digging over
• Potassium (K) – flavour and sugars in fruit, drought and frost tolerance. The need grows in the second half of summer and in autumn
• Calcium (Ca) – strength of cell walls. A shortage shows up as blossom-end rot in tomatoes and bitter pit in apples (most often the cause is uneven watering, not poor soil)
• Sulphur (S) – needed by all plants, especially brassicas; sulphate forms of fertiliser supply it as a “bonus”

The fertilisers in our catalogue, one by one

Below is the approximate composition of typical fertilisers of this kind. Exact percentages differ between manufacturers, so always check the figures on the packaging.

• Superphosphate (Baltic Agro, 1 and 4 kg) – a phosphorus fertiliser. Apply after harvest or at planting, into the root zone. On strongly acidic and on limed soils phosphorus availability drops
• Potassium sulphate (Baltic Agro, HI) – a chlorine-free potassium fertiliser, suitable for strawberries, currants, potatoes and fruit trees. In autumn it is the main helper before winter; permitted in organic growing (according to the manufacturer Baltic Agro’s description)
• Potassium nitrate (AGRONOM) – nitrogen and potassium. Suits midsummer, when fruit and berry crops need potassium but not much nitrogen any more
• Monopotassium phosphate (AGRONOM) – only phosphorus and potassium, no nitrogen. A soluble fertiliser for the second half of summer: helps fruit set and ripening without pushing leafy growth
• Calcium nitrate (Baltic Agro) – nitrogen and calcium. In spring and early summer, when there is a risk of blossom-end rot and bitter pit
• Nitrofoska (Baltic Agro) – a complex NPK fertiliser, “everything in one granule”. Handy as a spring base feed, but not suitable in autumn because of the nitrogen
• Chicken manure Organic Way (pellets, 4 kg) – organic nitrogen. In spring when digging over; fresh manure must not be applied to roots
• Sulphur and iron sulphate – not for feeding, but for acidifying soil (for example, for hydrangeas and blueberries). Use according to soil test results and the packaging

Calendar: what and when

• Spring (April–May): nitrogen – Nitrofoska, calcium nitrate, organic matter
• Early summer (June–mid-July): nitrogen feeding ends and the phosphorus-potassium regime begins
• Second half of summer: monopotassium phosphate, potassium nitrate for fruiting crops
• Autumn (September–October): superphosphate and potassium sulphate. No nitrogen

What not to mix with what

• Calcium nitrate must not be mixed in one solution with sulphates and phosphates: a precipitate forms (gypsum, calcium phosphate), the solution turns cloudy and the fertiliser loses part of its effect
• Superphosphate and lime (or ash) – do not apply at the same time: the phosphorus gets bound and becomes unavailable
• Nitrogen fertilisers and ash – do not mix: part of the nitrogen is lost as ammonia
• Do not mix different fertilisers “by eye”: read the packaging first, then decide

How to avoid doing harm

• Apply fertilisers to moist soil, and do not leave dry granules on leaves and stems – they cause burns
• Do not exceed the dose on the packaging: more is not “better”, it causes root burn and a build-up of salts
• Nitrogen is not needed in autumn: the plant will keep growing instead of preparing for winter
• If you are not sure what exactly the plant lacks, have the soil tested in a laboratory – it costs less than two bags of unneeded fertiliser

What not to do

• Do not apply nitrogen in August–September
• Do not use fresh chicken manure or raw manure under the roots
• Do not fertilise dried-out soil or plants in drought without watering
• Do not fertilise a sick or frost-damaged plant “to help it recover”: find the cause first
• Do not try to cover every need with one bag – complex and straight fertilisers solve different tasks

Frequently asked questions

What should I apply in autumn?

Phosphorus and potassium: superphosphate and potassium sulphate. Nitrogen is not applied in autumn: it stimulates growth rather than ripening.

How does potassium nitrate differ from potassium sulphate?

Nitrate contains nitrogen and potassium; sulphate contains potassium and sulphur, with no nitrogen and no chlorine. Sulphate suits autumn, nitrate suits midsummer.

Can I water plants with a solution of straight fertilisers?

Soluble forms (monopotassium phosphate, nitrates) can be used that way, but strictly at the dose on the packaging and onto moist soil. Superphosphate is better applied dry into the root zone.

Do I need to fertilise every year?

It depends on the soil and the crop. On poor sandy soils – every year, on fertile ones – less often. The best way to find out is a soil test.

Can chicken manure be applied in autumn?

The pelleted kind can be dug in, but spring suits it better: its nitrogen becomes available quickly and in autumn it would be washed out.

What to pick from the catalogue

The other fertilisers are in the “Dry fertilisers” and “Autumn fertilisers” sections.