The chemical form of fertilisers determines their availability to plants. The nutrients contained in mineral fertilisers can be supplied in a form that is directly available to plants or available only after transformation in the soil. They may be available in a single season or over several seasons.
Most often they contain simple chemical compounds with a high nutrient content. In some situations, as a result of soil transformations, nutrients change into a form that is more difficult to assimilate or completely unavailable to plants. The availability of nutrients to plants is always determined by soil characteristics such as pH, organic matter content or granulometric composition – the content of clay minerals (so-called floatable parts).
A good example of the variation in chemical forms may be found in nitrogenous fertilisers, which contain nitrogen as: ammonium ions, nitrate ions, nitrite ions or amide groups (urea). The effect of fertilisers that contain nitrogen in the nitrate form (NO3-) is described as fast, while the others are slower. The ammonium form (NH4+) is well retained in the soil and taken up more slowly by the plants. It is advisable to mix such fertilisers into the soil, so it is the typical pre-sowing form of nitrogen fertiliser. The nitrate form NO3- – is not retained in the soil and is leached out very easily. It is recommended for use during intensive plant growth phases. The nitrate-ammonium form combines features of the nitrate – post- and ammonium – pre-sowing forms, i.e. it is the most universal form of nitrogen fertilisers. The amide form (C-NH2), found in urea, is mainly taken up after an enzymatic decomposition process in the soil, first to the ammonium form and later also to the nitrate form. It is therefore a slow-acting form, recommended for spring fertilisation of plants and autumn fertilisation of winter crops.
Potassium fertilisers behave differently in soil than nitrogen and usually contain potassium chloride. After acidification, soil deficiencies in assimilable potassium are the second most important factor reducing the fertility of Polish soils and limiting crop yields. The total content of potassium in soils ranges from 0.8 to 2.5% and depends on the amount of clay (clay content) and the mineral composition of the soil. The majority of potassium in soils is in the form of aluminosilicate and silicate, but these forms are not available to plants. Heavy soils contain the most potassium, organic soils the least. In soil, potassium occurs in 4 forms: active, exchangeable, bound and in the crystal lattice. Plants take up potassium in the form of the K+ ion present as the active form. Increasing potassium abundance is difficult, as it is easily leached from light soils, while it becomes more volatile in heavy soils. At low soil pH, up to 14% higher leaching of potassium ions has been observed compared to soil with neutral pH. An increase in potassium content in the soil must be preceded by a pH adjustment, an increase in magnesium content and an increase in soil organic matter:
Phosphorus is a low-mobility element, available to plants only from the immediate vicinity of the roots (up to 2 mm), and its uptake is strongly dependent on soil reaction and temperature. When using conventional phosphorus fertilisers: superphosphates, ammonium phosphates and their derivatives, or Nitrophoska, a pH of 6.5 – 7.2 is required to achieve adequate fertilisation efficiency. Plants mainly take up phosphorus in the form of H2PO4- ions and also HPO42- ions. For phosphorus in the form of ions to be taken up by plants, it must be in direct contact with the root hair zone. Organic forms of phosphorus make up 15% to 80% of the total phosphorus content in the soil. The total phosphorus content of the arable layer is mostly 0.03 – 0.15%. Phosphorus losses from the soil are low – the element does not form volatile compounds and the mobility of phosphates in the soil profile is very low. Phosphorus fertilisers left on the soil surface do not penetrate the deeper soil layers, and the reduction in mineral phosphorus content in the soil occurs mainly through plant uptake and surface runoff. Its deactivation in the soil occurs mainly by precipitation as hardly soluble salts with aluminium, iron and calcium.