The required dose of fertiliser is determined by the expected yield level of the plant species and the nutrient content of the soil. When determining the dose of mineral fertiliser, it is important to follow the fertiliser application instructions and to take into account soil richness and the nutritional needs of the plants.
Soil affluence depends, among other things, on the agronomic category of the soil (light, medium, heavy), and nutrient availability is derived from the proportion of humus, the abundance of available nutrients in the soil and the soil reaction (pH). The nutrient requirement of a plant depends, among other things, on the species, variety of crop grown, its stage of development and the expected yield, planned or estimated e.g. based on historical data. The nutrient requirement is the product of the expected yield and the unit uptake. The unit uptake is the amount of a nutrient that the plant needs to produce a unit of yield (tonne, deciton).
Among mineral fertilisers, so far only nitrogen fertilisation is quantitatively limited by law, to a level of 170 kg of nitrogen in pure component per 1 ha of agricultural land (Act on Fertilisers and Fertilisation – Journal of Laws 2007, No. 147 item 1033, Art. 17.3). In the near future we can expect the introduction of further phosphorus regulations.
The amount of mineral and organic fertilisation is determined by an assessment of the nutrient content of the soil resulting from the analysis. Very low and low nutrient content requires intensive fertilisation, medium requires ensuring the supply of nutrients taken up from the soil by the plants, and high and very high allows reduced fertilisation.
Recommended doses of lime fertiliser are given according to the acidity of the soil and its agronomic category (heavy, medium, light).
For more on liming and yield-dependent fertilisation rates, see this page. And the Code of Good Agricultural Practice provides, among other things, unit intake rates to help calculate nutrient requirements.
The division of fertiliser doses must be rationally justified and adapted to the nutritional requirements of the crop and take into account the risks of leaching or blocking effects of the applied fertiliser on the uptake of other nutrients. Fertiliser rates in field crops are set to make up the difference between the identified element content in the soil and the optimum content for the species concerned. However, the ability of the soil to retain and store nutrients has to be taken into account, so when high doses of fertiliser are applied that are poorly retained by the soil, this means that they have to be split into smaller doses to avoid losses.
Particular attention should be paid to nitrogen application rates. The timing of nitrogen application should be adapted to the individual needs of the plants, the growth stage and the rate of uptake from the soil. Nitrogen fertilisers should be divided into 2-3 doses, usually not exceeding 60 kg N/ha at a time (maximum 100 kg N/ha). Under plants with a very short growing season, nitrogen can only be applied pre-sowing, while the longer the growing season, the greater the proportion of nitrogen should be applied during the growing season. In meadows, the dosage of nitrogen fertilisers should be adapted to the frequency of mowing or grazing. If phosphorus and potassium doses are split, part of the calculated fertiliser dose should be applied in autumn and part the following year.
Spare fertilisation involves applying a single dose that exceeds the crop’s ability to take it up in one growing season. The main limitations of such a method are the sorption capacity of the soil, the risk of leaching, the uptake potential of the crop and the limitation of uptake of other nutrients by the component applied in excess. Phosphorus is most often applied in stock, almost always before pre-winter ploughing or sowing. This is mainly due to the very low mobility of phosphorus in the soil and, in the case of many crops, the difficulty of applying it directly near the root trichome zone, thus forcing the application rate to be higher than the nutritional needs of the plants. It is assumed that within a few years, plant utilisation of applied phosphorus fertilisers reaches only 40-60%.
Back-up fertilisation with other macronutrients is not recommended, especially with easily leached nitrogen, the upper dose of which is limited by law. In special situations, supplementary fertilisation with potassium is possible, but this is only permissible on soils with a high sorption capacity, e.g. on clay soils.
Stock fertilisation with micronutrients is rarely used. One exception is cobalt applied to pasture – ruminants require this nutrient, applied most often as cobalt sulphate. Some other micronutrients can be applied as stock, but are more often applied foliarly.
It is important to distinguish between stockpiling fertilisers and the use of long-acting fertilisers, which release the nutrients taken up by the plants throughout the growing season. The slower action of such fertilisers is achieved by: condensation of nutrients, formulation protection or granule encapsulation (with sulphur, mineral oils, plastics and other substances that are difficult or conditionally soluble), or by, for example, sintering in a glaze.
In fields that are homogeneous in terms of soil richness and characteristics, uniformity of fertiliser spreading is important and required by law, with the dose of fertiliser being appropriate to the richness of the soil and the fertiliser needs of the plants. Uneven application of fertilisers – when the spreading unevenness factor is greater than 15-20% – results in a decrease in yield, caused on the one hand by local nutrient deficiencies and on the other by local overdosing. In practice, fields are never completely uniform in terms of nutrient abundance and availability. Optimising fertilisation means adjusting fertiliser rates to actual needs, and these in turn depend on soil richness, soil moisture and the nutritional needs of the crop. Soil richness can vary due to variable granulometric composition, organic matter content, water relations, bedrock characteristics or terrain. Due to the high spatial variability of soil conditions at the field scale, the variation in terms of fertiliser needs can be considerable. With optimised fertilisation, these disadvantages can be eliminated and nutrient rates will be adapted to the actual needs of the plants. In conventional agriculture, the amount of fertiliser applied is determined for an entire plot of up to 4 ha. Fertilisation optimisation requires the identification of homogeneous soil zones, which allows the number of chemical analyses to be optimised, and various soil survey techniques (electromagnetic, spectrophotometric and others) can be used in conjunction with GPS navigation, referred to as “scanning”, which are instruments of precision agriculture.