The complexity of NP and NPK fertilizers in stationary and long-term system yields is proven to determine substantial changes in soil fertility, revealing interaction possibilities related to the chemistry and requirements of other nutrients (S, Ca, Mg, and microelements), while sustainable fertilization can exert control over them through rational fertilization practices and complex nutritive management. Revealing the extent of the modifying effects in the application of S, Ca, and Mg correlated to the soil-plant system conditions relates to the hypothesis of the present research in the context of long-term experiments in Romania at the Office of Pedological and Agrochemical Studies Alba (OSPA Alba) and the Turda Agricultural Research and Development Station (SCDA Turda) with 55 years of a fertilizing effect over the 1967–2022 period, but not solely. This study shows that for sustainable fertilization systems, the complementary application of S, Ca, and Mg with NP and NPK has proven to be effective and unitarily constitutes a measure for the sustainable protection and enhancement of soil fertility.

The paper hereby focuses on the essential field of soil evolution in relation to the effect of long-term fertilization on plant yields and the essential, evolutionary, and impactful changes in their fertility. NP fertilization (by application of ammonium nitrate and concentrated superphosphate) causes a change in soil reaction over time through acidification with increasing N doses, while phosphorus is able to partially mitigate this process. Acidic soil—the typical preluvosol—as a result of adsorbed acidity activation, solubilizes Al ions and may become interested in calcic amendment. Acidification tendencies are also maintained in the amended variants, but to a lesser extent due to the neutralizing capacity of the amendment and the presence of calcium ions. Complex NP fertilization in acid soil (preluvosol) cultivated with wheat can maintain a relatively constant humus content (%), but calcium amendment can cause a reduction of this indicator. In contrast, under maize, due to the effects of conventional tillage and intensive tillage, fertilization contributes to a reduction in this indicator, which is more active against the background of limestone amendment. The phenomenon can be explained by potentiation through the mineralization of the organic component of the soil. This effect is diminished in alluvial mollisol with a higher humus content, saturated in bases, and a buffering capacity. Organic and organo-mineral fertilization can control the modeling of soil humus content and its agrochemical status. The regime of essential elements (N, P, and K) becomes active in mobile forms, and the precarious supply conditions of the initial stage tend to improve. Applied superphosphate evolves, differentiated from the applied dose and pH, into non-occluded mineral forms (P-Al, P-Fe, and P-Ca), which supply the mobile forms in the soil solution for the plants, with the importance of maintaining, more of these forms at the level required by plants. In the case of potassium, the dynamics of its forms in the soil (unchangeable and exchangeable) control the soil supply state of this element and the effect of its application to plants. The data presented show that long-term experiments can effectively support the study of soil fertility through the soil-plant relationship.

The paper presents soil analytical data and their interpretation from samples in long-term experiments as a result of NP, NPK and organo-mineral fertiliser applications in a stationary system for 20 years and wheat-maize-soybean rotation system. The results show that NP fertilisation (from ammonium nitrate and superphosphate) leads to acidification of soils (regardless of soil type) depending on increasing N a.i./ha. The acidification phenomenon is higher in typical preluvosol (taxonomically acidic soil) due to activation of adsorbed (potential) acidity and solubilisation of Al ions, with devolatilisation of adsorbent complex, which updates for this soil the need for correction of the reaction (pH) by means of calcium amendments. In contrast, for the alluvial mollisol, with neutral - weakly alkaline pH and higher humus % and high buffering capacity, the multiannual acidification due to N is reduced. NP, NPK mineral fertilisation, in balanced doses, can maintain the organic-C and humus content constant, balanced, within the specific limits of the soil type over a period of 20 years, with wheat-maize-soybean crop rotation. In contrast, amendment and processing conventional intensive tillage (on maize) decrease the content of these indicators, a phenomenon that can be attributed to the enhanced mineralisation of the soil organic component. Organic and organo-mineral fertilisation can lead to a favourable modelling of humus content. The mobile forms of the essential elements of nutrition and fertilisation (N, P, K) are improved in terms of their bioavailability with the objective that these forms will maintain their quantity and quality at the level required by crops and their supply by fertilisation technologies is ensured rationally and preventively for the soil-plant system. In summary, the analytical results from long-term experiments with fertilisers are of direct benefit to soils, their productivity and fertility, with the implementation of sustainability principles.