Introduction. Determination of the spatial variability of the nutrient levels in soil facilitated adaptation of the fertilizer doses to the soluble forms availability. Nowadays, an increasing use of this method of the fertilizer application is observed, with this being associated with both economical and environmental advantages, as well as, with growing assortment of the purpose-built agricultural instrumentation. An accurate determination of the spatial distribution of nutrients, on the other hand, requires collection of an adequate number of the soil samples, which in turn results in rising expenses of the method. The quantity of the samples is conditional on the numerous factors, one of which is soil granulometric composition.

 

Aim. The aim of the study was to determine the effects of the variable potassium fertilization on the K soluble forms content in the soil. We postulated that employment of the variable fertilization techniques will significantly improve distribution uniformity of this element in the soil. To test this the statistical measure of variability of the potassium levels in the soil was taken for both the classic and variable fertilization approach using Statistica software.

 

Test methodology. The field experiments were conducted within the period 2007-2013 in farms located in southwestern region of Poland (Charbielin and Przeworno). The soil samples were collected in the manner in which a single soil sample represented 0.5 ha area. Soil samples collections were performed in the period between harvest of plants in the field and the planting of the next crops, and subsequent fertilization of the plants. The soil sampling location was determined with an accuracy of 0.5 m, and an average sample test consisted of 10 individual tests. The contents of 'available' potassium forms were determined in accordance with the Egner-Riehm DL method – the one widely used in Poland. Based on the results, the soil sample intervals were set to measure the fertility of the soil. Afterward, the maps of the soil fertility levels were created by interpolation using the Kriging method and the Agro-Map program and potassium fertilization application maps. ACT control terminals (Agrocom Computer Terminals), which were connected to the fertilizer spreader and a GPS antenna, were used to apply variable fertilization doses. The doses of fertilizer were adjusted by changing the operating speed of the spreader rollers.

 

Results and conclusions. The variable potassium fertilization resulted in a significant reduction of spatial variability of K levels in both light and heavy soils tested. Importantly however, the potassium content equalization was sooner in the soils classified as ‘light’, as reflected by decrease in variance, standard deviation and range of the K contents in the soil. Furthermore, the variable fertilization techniques used for potassium decreased participation of the determined K levels in the individual fertility classes for the benefit of the improved contribution to the optimal range.