Areas of Agrochemical Deepening Resulting from Long-Term Experiments with Fertilizers—Synthesis Following 20 Years of Annual and Stationary Fertilization

Abstract

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.

1. Introduction

Long-term experiments represent a superior approach to issues dependent on and controlled by soil fertility. The original long-term model founded by John Bennet Lawes at Rothamsted (1843) and subsequently undertaken by his disciple Gilbert has effectively developed a worldwide network of experiments, highly diverse in terms of location and objectives, especially since the early 20th century. The inception occurred in the early nineteenth century, following Mitscherlich’s contributions to outlining the law of action and interaction of vegetation factors (1912, 1913, and 1919). The development and diversification of long-term experimental objectives have been effectively and permanently influenced by the multiple causalities of anthropogenic and natural constraints on the soil-plant system, whose effects are difficult but highly useful to control over time.

The archive of long-term experiments worldwide [1,2,3], as well as bibliographic incursions due to the recent agreement between Bonares (www.bonares.de) and Ejpsoil (www.ejpsoil.eu) under the aegis of the European Community, provides information from 616 long-term experiments older than 20 years from 30 European countries. The geographical distribution of these experiments reveals their predominance in Europe, followed by the USA, African countries, etc. Most of the long-term experiments were conducted between 1950 and 2000, mainly on arable land, where the objectives were accompanied by tillage, fertilization, and crop rotation. In recent years, based on the reassessment of these experiments and their importance for sustainable technologies, the objectives have also converged towards issues aimed at soil and environmental protection (heavy metals, CO₂ emissions, the impact of global warming, etc.). The archiving of these experiments, carried out at the beginning of this millennium (2003 and 2022), also included an emphasis on long-term experiments in Romania [4,5,6].

The results obtained and promoted by LTEs, by the accumulation of experimental years and increased duration for experiments conducted at the end of the 19th century (Broadbalk Wheat Experiment—1843, Rothamsted; Dehrain Plots—1875, France; Eternal Rye Experiment—1878, Germany; Askov—1894, Denmark; Palace Leas Meadow—1896, UK; Static Fertilizer Experiment—1902, Germany; etc. and other like in the USA: Morrow Plots—1876; Sanborn Field—1888, Magruder Plots—1892, and others) alongside results in the soil-plant system placed or continued in the 1950–2000 period worldwide, have provided pertinent responses as regards the complex effects of nutrient accumulation in the soil and other factors, for a long-term, comprising technologies and models for their valorization in the context of sustainable agricultural systems, consumer, fertility and environmental protection [7,8,9,10,11,12,13,14,15,16,17]. As such, the long-term experimental method is fully and scientifically acknowledged as applicable to sustainable agriculture and achieving consumer and environmental protection.

Long-term experiments with fertilizers in Romania (14 archived in 2003) have been placed in adherence to a unitary concept, in ecologically differentiated but representative conditions for the agricultural area where they were located (established by Hera et al. during 1966–1967) [5]. As regards their content and experimental factors, the respective objectives were carried out in three directions: the study of the effects and interactions determined by NP, NPK, and organo-mineral (manure + mineral NP) fertilizations, by investigating plant yields and their quality, soil fertility, and environmental quality in dependence on and dynamics of pedological-agrochemical and climatic factors and applied technologies in representative 3–5 year field rotation experiments.

A comprehensive literature review of the results and recommendations promoted over 55–65 years of “long-term experiments” in sustainable fertilization systems and relevant approaches to soil fertility development and protection summarizes:

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There is a generalized effect and interaction of NP and primary nitrogen efficiency with the optimization of interactions in the alternative of a favorable modification of soil P content.

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Potassium balances and optimizes NP fertilization, making it effective on lighter soils, in high NP-consuming genotypes, in interaction with other cations, and leading to qualitative effects.

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Organo-mineral fertilization achieves maximum yields with ameliorating effects in the soil-plant system.

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Long-term fertilization brings some answers of the utmost usefulness as a consequence of its thorough approach: organic-C and humus reduction, soil acidification, optimization of the SOM regime, modeling of major and trace element supply, soil protection, and environmental protection.

Result availability and promotion to beneficiaries has been ensured through scientific studies, methods and models of promotion and application such as: production functions in crop fertilization [4]; agrochemical tables and nomograms [18]; Sistfert—Fundulea fertilization system (1978); agrochemical compendia [19,20]; methods for optimizing the soil-plant system [21]; guide for establishing up fertilization plans [22]; instructions for carrying out agrochemical studies, vol. I, II, III, 1981 [19]; monitoring soil quality status in 1998 and 1999 (ICPA); and Agrochemical mapping (agrochemical study of soils) [23], all of which have ensured the promotion and use of data from long-term experiments towards agricultural beneficiaries and research.

Moreover, it is notable that the long-term experiments in Romania, the production results, and the soil-plant system’s fertility and environmental protection have been acknowledged and appropriated at the international level, not only upon the anniversary of the Rothamsted experiments in 1993 [24], but also during worldwide events (through SISS, CIEC, IAEA, etc.). [25,26,27].

The objectives are set within the research topic of the long-term determinants of fertilization on essential soil quality properties. In this context, differentiated mineral and organo-mineral fertilization, amendment, and soil tillage can contribute to the implementation of sustainability in farming systems with positive results in soil productivity and environmental quality. Therefore, control and monitoring of reaction and acidity indices, as objectives are set, support improvement measures, including amendment of acidic soils, monitoring of acidification at high N doses, and control of organic-C content. Differentiated effects show changes in the organic component and support soil carbon modeling. Monitoring and research on the content of essential elements (N, P, and K) show an optimization of their regime in the soil-plant system over time. Long-term agrochemical control objectives of fertilized soils can support technical and economic measures with an effect on soil quality sustainability.

2. Materials and Methods

This study addresses issues related to soil agrochemical control and monitoring in NP, NPK, and organo-mineral experiments within long-term experiments at Băcăinți (45.931733, 23.278443) (of OSPA Alba) and with some partial data from the experiments at SCDA Turda (46.590030, 23.803103) and Livada (47.86048, 23.12621) (through some analytical collaborations).

The experimental designs at the three locations were unitary (Hera model, Borlan, 1966) [18] on typical preluvosol and alluvial mollisol:

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Wheat = N—0-40-80-120-160 kg a.i./ha

P₂O₅—0-40-80-120-160 kg a.i./ha

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For maize = N-0-50-100-150-200 kg a.i./ha

P₂O₅—0-40-80-120-160 kg a.i./ha

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Soya = N—0-30-60-90-120 kg a.i./ha

P₂O₅—0-40-80-120-160 kg a.i./ha

K₂O rates in NPK experiments: 0-40-80-120-160 kg a.i./ha, background 0–80/60; 160/120 NP.

The research carried out through its methodology aimed to control and monitor the agrochemical evolution of “long-term” fertilized soils in order to evaluate the conditions and possibilities to maintain their sustainability and the soil quality of the experimental plots.

Analyses performed:

• Soil: pH(H₂O); organic-C/Schollenberger; IN; P, K in AL (ammonium lactate acetate) solution, spectrophotometric (P) and photometric (K) determination; mobile-Al (Sokolov); and SB, Ah-Kappen.
• pH—in aqueous suspension, through the potentiometric method [28];
• Organic-C/Schollenberger—through the wet oxidation method and dosage titration, according to Walkley-Black [29,30];
• Mobile phosphorus and potentially accessible according to Egner-Riehm-Domingo spectro-photometrics in ammonium acetate-lactate extract (P-AL) [31];
• Mobile potassium is potentially accessible, according to Egner-Riehm-Domingo photometrics in ammonium acetate-lactate extract (K-AL) [31];
• Ah, SB—Determination of cation exchange capacity and base saturation level using barium chloride solution [32].
• Mobile Al in KCl extract, Sokolov method [33].

The paper deals exclusively with agrochemical deepening determined in the soil-plant system by long-term complex stationary fertilization, synthesized as averages of periods within 1998–2018.

3. Results and Discussion

The data presented address the broad issues of agrochemical control for soil fertility protection (a), macro-element management (b), relevant soil solution interactions, and nutrition with the engagement of macro- and trace elements (c). All these approaches aim to diagnose the determining agrochemical states as a result of long-term fertilization, with the aim of opting for the positively favorable ones in the soil and for nutrition purposes, to prevent impact states, and to support the determination of soil-plant agrochemical optimization towards the sustainability of the recommended measures.

Agrochemical control and monitoring for soil protection purposes:

Mainly focused in two directions, the study of pH and organic-C (humus) changes as an effect of long-term fertilization:

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pH changes in long-term NP experiments:

The control and monitoring of soil reactions are indispensably and indefinitely connected to the soil fertility state. Ever since the Lawes and Henry Gilbert, in the years 1881–1896, applied amendments (CaCO₃ 4 t/ha) and N fertilizers (sodium nitrate and ammonium sulfate) have been used in order to study the effect of the two nitrogen ions (ammonium and nitrate) on the soil reaction. In the 1903–1964 period, 4t CaCO₃/ha were applied periodically to maintain the soil reaction at a pH of 7.0 (Park Grass Experiment) Rothamsted [15].

The application of mineral NP fertilizers with increasing nitrogen doses and favorable nitrogen ratios causes changes in soil reaction (pH) mainly due to the availability of NH₄ ions for ion exchange (equivalent to H⁺) and nitrification (above the fixed amount without exchange), the immediate and permanent effects being protonation of the soil solution and proportional activation of adsorbed acidity (

Table 1. Change in acidity indicators (pH; Alsch, Al/S_B) following multi-year fertilization with increasing NP doses (Typical Preluvosol—NP fertilized—20 years) (synthesis—years 1998–2018).

Wheat		Un-Liming				Amended
Doze NP		pHH2O	Al—sch. m.e./100 g sol	Al/SB·100	V%	pHH2O	Al—sch. m.e./100 g sol	Al/SB·100	V%
P0	N0	5.7	0.13	1.0	76	6.0	0.12	0.8	84
	N40	5.6	0.18	1.3	78	6.0	0.13	0.9	84
	N80	5.5	0.17	1.2	77	5.9	0.11	0.7	83
	N120	5.4	0.32	2.2	75	5.8	0.15	1.0	83
	N160	5.3	0.41	3.1	78	5.7	0.17	1.1	82
P80	N0	5.5	0.10	0.7	78	5.9	0.15	0.6	85
	N40	5.6	0.13	0.9	78	5.8	0.15	0.6	83
	N80	5.6	0.15	1.0	77	5.9	0.18	0.7	83
	N120	5.5	0.27	1.7	78	5.9	0.18	0.8	80
	N160	5.5	0.29	2.0	76	5.9	0.16	0.9	78
Maize		Un-amended				Amended
P0	N0	5.7	0.17	1.6	80	6.1	0.15	0.8	85
	N50	5.6	0.30	2.3	77	6.0	0.12	0.6	86
	N100	5.4	0.26	2.5	77	5.9	0.14	0.8	85
	N150	5.3	0.42	2.8	77	5.9	017	0.7	80
	N200	5.3	0.54	3.3	75	5.8	0.13	0.8	81
P80	N0	5.5	0.28	1.6	80	6.0	0.15	0.8	82
	N50	5.5	0.31	2.1	76	6.0	0.15	0.9	82
	N100	5.4	0.37	2.5	75	6.0	0.15	0.9	80
	N150	5.4	0.38	2.6	75	5.9	0.14	0.7	80
	N200	5.4	0.49	3.1	74	5.8	0.11	0.8	81

). This phenomenon occurs due to the multiannual application of ammonium nitrate, especially unilaterally or in imbalance with phosphorus. This has also been observed in previous studies on the agrochemistry of acidic soils in our country in order to restore their fertility [34,35].

The data presented show that upon the lowering of pH values and the increase of acidity with increasing doses of N applied, molecular Al ions are mobilized. Molecular Al representation is thus enhanced in relation to the sum of cations (SB), with acidification at the expense of basification and base saturation. These are such levels and processes that deem lime amendments appropriate for the soil in question (typical preluvosol) under “long term” NP fertilization.

The data presented estimate the opportunity to apply effective amendments in the initial soil with a pH of 5.7–5.8 and a base saturation degree of V% at the limit of 75% amendable. However, through long-term fertilization (20 years), the soil acidifies below its initial values, with exceeding concentrations of mobile Al (over 0.3 m.e./100g soil) and a ratio of Alsch./S_B·100 > 1.0, conditions that can generate the phenomenon of phytotoxicity. Almost in accordance with N a.i. doses and upon their increase above 80–100 kg/ha, Al mobility becomes active and potentially phytotoxic. In the same sense, with the application of amendments and P₂O₅ doses in balance, the effects of Al mobilization are diminished. In this case, it was found that the application of amendments, at the level of 75–100% neutralization of hydrolytic acidity (Ah), cyclically, with 5t CaCO₃/ha every 5 years, proves effective and shows significant results depending on the newly created acidity and with (NP) complex fertilization (

Table 2. Effect of CaCO₃ amendment on NP background (3 cycles × 5 t/ha CaCO₃)-typical preluvosol.

Wheat
N kg/ha	0	40	80	120	160	Field diff. (+) compared to un-amended
kg/ha prod.	103	555	475	310	391	-
P2O5 kg/ha	0	40	80	120	160	-
kg/ha prod.	143	332	305	417	307	-
Total	346	887	762	727	698	Yield amended+ 3420/3 cycles/10,260
Maize
N kg/ha	0	50	100	150	200	Field diff. (+) compared to un-amended
kg/ha prod.	479	451	510	514	733	-
P2O5 kg/ha	0	40	80	120	160	-
kg/ha prod.	708	409	414	757	614	-
Total	1187	860	924	1271	1347	Yield amended + 5589/3 cycles/16,767

).

In this long-term experiment, it can be concluded that acidification rates (−ΔpH/N kg a.i./ha) in the unamended area increase with increasing N dose and duration of fertilization and maintain their acidification tendencies under amended conditions, but such parameters decrease and are controlled by re-amendments at 5 years. In this field, recent research over a long period (1968–2022) in the area of acid soils (luvisols) of north-western Transylvania (at Livada) analyzes pH changes as a function of dose (CaCO₃) and duration of amendment, differentiated according to fertilization systems, concomitant with calcium losses. In the context of non-amendment, the most significant acidifications were obtained in unilateral fertilizations (only N), in non-fertilized fertilizations, then in NP and NPK mineral fertilizations, and lower in organic and organo-mineral fertilizations. Under the conditions of calcium amendment, acidification tendencies are maintained at low values, in the order mentioned. However, it was concluded that it is possible to sustainably maintain pH indicators at 5.8 and base saturation (V%) at 75% (the limits of amendment) if the amount of calcium is balanced at 296 kg Ca/ha/year in the case of albic luvisol, annually or cumulatively every 4–6 years, respectively, on luvic brown soil, equivalent to 173 kg Ca/ha/year. The relationship between Ca content (me/100 g soil) and pH value shows that the pH value of 5.8 (which differentiates amendable acidic soils from the rest) can be maintained sustainably by soil application of 4 m.e.Ca/100 g soil [36,37].

It is shown that the quantitative effect on yield and soil chemistry is higher in maize (with higher N doses), where fertilization creates higher acidifying potential.

It must also be taken into account that there is a beneficial application effect and balance with increasing doses of P from concentrated super-phosphate with CaO and 42% P₂O₅, both ions with ameliorating effects (Ca²⁺ in neutralizing mobile Al and in base saturation, and the superphosphate anion, H₂PO₄⁻, with an inactivating effect—by retro-gradation of these phytotoxic ions).

In reconstructing these situations created by multi-year NP fertilization, it is equally effective, along with amendment and fertilization, to use alternative, annual, or periodic N products that can protect the soil reaction (pH) (nitro lime, urea, UAN, or N:P balanced complexes).

In the field of soil fertility, at least two directions with relevant results can be deduced—the first is the control of soil response in relation to fertilization systems. For each year and experimental cycle, the acidification effects are confirmed due to the permanent and annual presence of ammonium nitrate in the NP fertilization system. The ammonium nitrate first causes a significant acidification of the reaction in the typical prelluvosol (with high current and adsorbed acidity). The acidification rate (−ΔpH) is proportionally dependent on the dose of N kg a.i./ha.

Superphosphate as a source of P₂O₅, while applying monocalcium phosphate—Ca(H₂PO₄)₂ as an active substance, inactivates the actual acidity and partially immobilizes the adsorbed (potential) acidity due to Al ions³⁺. Thus, if applied to the NP formula, it partially balances the acidifying effect of ammonium nitrate.

Prevention of acidification or correction of current and adsorbed acidification is achieved in the experiment by CaCO₃ (cyclic) amendment, but in the future, protective N sources (CAN, UAN, and even urea) can be promoted as N sources.

Changes in organic C and humus content in long-term experiments:

Soil organic matter (SOM), represented primarily by humus content (% C-organic-1724), plays an essential physical, chemical, and biological role in soil fertility, and modeling this component is a requirement for the sustainability of farming systems. It is a sufficiently stable component, especially in the form of finite and specific soil compositions. However, in recent decades, due to anthropogenic and natural technological pressures, the reduction of humus and organic-C content has been discussed as a priority in terms of management practices and multifunctional models capable of determining and organizing soil organic matter components and humic balance.

In long-term experiments (for more than 20 years and in our country for 55–56 years), the control and monitoring of humus content is an analytical activity aimed at elucidating the effects of long-term fertilization (NP and NPK) but also organo-mineral fertilization on the evolution of this essential soil component, a determinant of soil quality and fertility. More recent determinations in our experiments at SCDA Turda (1998–2015) have continued and concluded that the humus in the experimental chernozem (with 3.78% initial content) (1968) shows significant reductions and represents 84.9% of the initial humus content in 1985 and 84.1% in 1999, while in recent determinations it was shown to be 82.5% (2015) of the initial humus content (since 1967). Therefore, the humus content in the Turda chernozem has been reduced from 3.78% in 1968 to 3.21% in 1995, 3.18% in 1999, and 3.12% in 2015. In the same context, it is estimated that the supplementation of N doses, predominant over phosphorus, leads to an improvement of the humus content to 3.47% due to the accumulation over time of raw organic matter in the soil plant root system.

Recently, studies on the humus content for the 1961–2002 period following experiments at Livada on a luvisol with an initial humus content of 1.88% concluded that amendment can cause a decrease in the organic-C content, on average by 0.17%, regardless of the fertilization system. Only potassium (K) had a favorable influence on the carbon content. The depressive effect of amendment on carbon and humus content was explained by its stimulation of microbiological activity, which contributed to higher mineralization [37].

The monitoring of humus content in the two soils—preluvosol (with 2.20% humus) and alluvial mollisol (with 2.60% humus)—from these experiments revealed the determining character of long-term technologies on soil maintenance but also a dependent variability of humus content in soils (

Table 3. Effect of limestone amendment of typical preluvosol on humus content in soils (long-term experiments—20 years) (synthesis—years 1998–2018).

Typical Preluvosol (Initial Humus = 2.20–2.25%)
Wheat
Un-Amended				Amended
N kg a.i./ha	Humus %	Doses P2O5 kg a.i./ha	Humus %	N kg a.i./ha	Humus %	Doze P2O5 kg a.i./ha	Humus %
0	2.28	0	2.29	0	2.18	0	2.27
40	2.22	40	2.29	40	2.30	40	2.24
80	2.38	80	2.34	80	2.44	80	2.31
120	2.35	120	2.32	120	2.38	120	2.16
160	2.32	160	2.31	160	2.42	160	2.29
Average	2.32	-	2.32	-	2.33	-	2.25
Maize
0	2.16	0	2.10	0	2.17	0	2.02
50	2.11	40	2.08	50	2.11	40	1.95
100	2.21	80	2.18	100	2.14	80	1.85
150	2.12	120	2.19	150	2.00	120	2.06
200	2.11	160	2.17	200	2.10	160	2.04
Average	2.14	-	2.14		2.07	-	1.98
Alluvial mollisol (initial humus = 2.60–2.65%)
Wheat				Maize
N kg a.i./ha	Humus %	Doses P2O5 kg a.i./ha	Humus %	N kg a.i./ha	Humus %	Doze P2O5 kg a.i./ha	Humus %
0	2.72	0	2.62	0	2.69	0	2.68
40	2.52	40	2.62	50	2.64	40	2.37
80	2.67	80	2.65	100	2.71	80	2.74
120	2.69	120	2.66	150	2.68	120	2.71
160	2.68	160	2.66	200	2.76	160	2.70
Average	2.64	-	2.64	-	2.70	-	2.64

).

The data presented confirm the effect of limestone amendment (in preluvosol) in reducing carbon and humus content in all fertilization alternatives, concurrent or periodic. This effect is also exerted by conventional tillage in terms of soil tillage (exclusively for 20 years) in the case of maize, where the soil generally shows a reduction in humus content, which is greater in the case of amended soil.

As this pressure is exerted, there is a systematic reduction of the organic-C and humus content. Thus, actions and measures to shape and achieve humic balance must be primarily based on tackling this process. This can only be achieved within rational, diversified farming systems up to conservative ones, where the contribution of organic resources (from fertilizers of this kind) is a priority alongside complex technologies to control the carbon cycle with efficient sequestration stages on a long-term or even periodic basis. A simple model, included in the experimental polygon in question, shows that organic input can function as a “humiferous amendment”, decisive in ensuring the buffering capacity of soils, while organo-mineral input, with a balanced background, has a fertilizing, nutritive role.

Data analysis supports the promotion of organo-mineral and organic fertilization for efficiency in protecting soil reactions, visibly better nutrient conditions, maintenance of initial organic-C and humus content, and support of soil buffering capacity against technological (anthropogenic) and natural pressures. In general, an optimally humified soil organic matter modeling regime supports the effects of fertilizer application on productive and quality plant yields.

In the field of soil protection, the phenomenon of reduction of organic-C content and implicitly of humus has proved to be active and has negative implications. In our experiments, the effect was mainly due to limestone amendment alongside the significant contribution of conventional tillage systems, primarily in the case of wide-row plants. NP or NPK fertilization, especially in moderate doses, had no significant effect on reducing the organic-C content in 20 years, possibly also through the supply of organic matter to the root system.

The remedy considered is organic and organo-mineral fertilization, with organic-C input and complex effects—of physical, chemical, and biological nature—that can interact and shape humus evolution.

It is unreservedly concluded that the rates of change in organic-C content reduction as well as those following amelioration measures have a particular character due to the initial soil characteristics, the agricultural practices involved in the management, and the natural and modified environmental factors.

It is frequently concluded that tillage and various fertilizer and amendment treatments increase organic-C losses and increase CO₂ emissions to the atmosphere, while rotation with organic inputs of plant residues as well as organo-mineral fertilizer combinations positively modify, maintain, and increase organic-C content [27]. As a result, the authors mentioned here recommend management decisions, including conservative organic-C sequestration works, which can result in optimal values of this indicator, even achieving a “good status” of organic-C.

On the Rothamsted and Askov experimental platforms, slow organic-C losses, regardless of fertilization treatments, are quantified at an annual average of more than 100 kg C/ha. Over the last 70 years, in rotations with 5-3-2 crops and predominantly organic treatments (FYM), the organic-C content (in t ha⁻¹) has evolved through annual applications from 30 t to 85 t/ha, compared to the unfertilized control, which has regressed to approx. 20 t C/ha⁻¹ (Johnston A.E., Poulton, P.R. 2018) [15]. In an experiment initiated in 1894 at Askov, treatments with organic N, P, and K (FYM) doses and combinations in rotations (wheat, silage maize, barley, and clover) confirmed that positive fertilization and fertilizer management enhance SOC and soil microbial activity [17].

Research at Sanborn Field Exp. contributes to the effect of fertilizer treatments on C-organic dynamics, including judgments on distribution across soil profiles benefiting from FYM application [11].

In the Morrow-Illinois experiments, over a 30-year period, SOC decreased by 15–19%, and the trend for 100 years is a reduction of this indicator not only in unfertilized but also in lower limits for the variants with ameliorating fertilization-amendment measures (FYM + CaCO₃ + P).

Long-term experiments in India support a positive balance of nutrition and fertilization/fertility management that improves SOC, organic biomass, and agricultural efficiency [38].

These experiments include integrated nutrient management (complex, including Zn and S from superphosphate), amendments, and FYM. The results show increased efficiency in sequestering organic C from FYM, plant residues, and green manures with mitigation of climate change impacts.

Compared to these findings, the results obtained in our experiments maintain a minimum balance of organic C due to NP balance, with negative effects from active mineralization of humus and plant residues, but in the typical preluvosol, attention should be paid to the rotation—wheat/maize/soybean, unchanged for 20 years. In this context, organic and organo-mineral fertilization support organic-C accumulation and balance the buffering capacity (expressed as T/Ah) (

Table 4. Agrochemical changes in typical preluvosol, organic, and organo-mineral fertilization (8 years).

Fertilisation	pH	Humus %	SB m.e./100 g Soil	Ah m.e./100 g Soil	T e.m./100 g Soil	V%	T/Ah	lg T/Ah	P-AL ppm	K-AL ppm
Unfertilised	5.90	1.92	10.2	4.2	14.4	70.8	2.42	0.3824	12	120
Manure 20 t/ha	6.11	2.32	12.2	3.8	16.0	76.2	4.21	0.6765	26	131
Manure 20 t/ha+ N50P50	6.01	2.28	13.4	3.2	16.6	80.7	5.18	0.8274	38	144
N50P50	5.80	2.10	11.0	4.4	15.4	71.4	3.50	0.4248	28	132

).

Agrochemical control in support of essential element management:

Yield results obtained in long-term experiments generally confirmed the usefulness of exploiting NP interaction, with yield dependence resting primarily on N doses and the specificity of P significance in soils poorly supplied with this element. The effect of potassium application was mainly linked to the conditioning of NP fertilization to multiannual application, even to the size of these doses, according to the needs of the genotypes cultivated in this soil. Therefore, the approaches to the introduction of potassium in fertilization have constantly taken into account the “versatile” character of the roles (according to Kraus, 1997) [39] of this element and its involvement in the N:P:K balance and in the overall quality of agricultural production.

-

Evolution of phosphorus application in the NP regime:

In the presented research, the mobile phosphorus regime in soils under multiannual fertilization with NP shows significant improvement from 4–6 ppm to 36–60 ppm. Mobile-P shows a transformation of the supply status of this element from a very poor phosphate supply to a good phosphorus supply (

Table 5. Content of non-occluded forms of phosphates (P-Al, P-Fe, and P-Ca) by long-term applications of concentrated superphosphate (0-40-80-120-160 kg P₂O₅/ha) after 20 years—1998–2018.

Preluvosol	pH	P-AL ppm	Non-Occluded Mineral Phosphates (ppm)/% of Σ
			P-Al	P-Fe	P-Ca	Σ
Wheat-un-amended
P0N0-160	5.5	5.1	15/4	47/14	273/82	335
P80N0-160	5.5	32.8	60/13	94/20	306/67	460
Wheat-amended
P0N0-160	5.9	6.4	20/7	58/20	212/73	290
P80N0-160	5.9	31.3	61/14	111/25	260/61	432
Maize-un-amended
P0N0-200	5.5	4.0	14/5	17/6	239/89	270
P80N0-200	5.4	21.6	36/9	41/10	322/81	399
Maize-amended
P0N0-200	6.0	4.2	20/6	29/9	278/85	327
P80N0-200	6.0	18.2	50/13	57/15	288/72	383
Soya-amended
P0N0-120	6.1	5.2	16/6	32/11	232/83	280
P80N0-120	6.1	25.1	48/2	63/16	290/72	401

; Figure 1).

These positive changes in the mobile-P (in AL) supply status of the soils under the three crops in the crop rotation system (after 20 years) are due to the incorporation and maintenance of increasing rates of superphosphate at an annual application rate. Obviously, the solubilization of superphosphate with 42% Ca (H₂PO₄)₂ influences the change in the evolution of the monophosphate anion into non-occluded mineral phosphates (P-Al, P-Fe, and P-Ca) through the retrogradation process over time. They are maintained annually, feeding the mobile and bio-available forms of phosphates for the soil solution and plant nutrition.

The evolutionary specificity of the P applied is dependent on its application and dose, which is distinctly positive for soil enrichment in this poorly represented element. All the non-occluded forms increase in relation to the three representative elements (Al, Fe, and Ca) in the acidic soil. On the amended soil (cyclically with CaCO₃), compared to the P-Al and P-Fe forms with representative increases, P-Ca reduces its percentage representation in the sum (Σ) of the non-occluded forms, which proves beyond any doubt that this form increases the share of P supply to the soil solution and implicitly to the crops. The physico-chemical logic of the process is at work here—the application of phosphorus on the amended soil has a priority effect on crop supply, nutrition and, secondarily, on the retrogradation and retention of phosphate desorption useful in the multi-annual reserve.

The modifications that provide a positively dependent support for the mobile P forms in the soil solution when in contact with plant roots also make the mobile forms of phosphates available upon crop harvest in a differentiated manner, not only in terms of retrogradation but also in terms of the species grown in previous and even current years. That is, the significant improvement in mobile P content is different under the wheat crop, which can be explained by the “long-lasting” contribution of the previous crop (soybean), with an ameliorating role in phosphorus solubilization for the following crop. In addition, the annual P consumption (“export”) (through the crops grown) is achieved (annually) in the following order: maize (140–160 kg P₂O₅/ha/year) > soya (80–115 kg P₂O₅/ha/year) > wheat (80–105 kg P₂O₅/ha/year). This effect of the differentiation of the mobile-P indicator (soil) over time and after crop rotation recommends differentiated fertilization systems for crops in the crop rotation system rather than unitary fertilization throughout the entire crop rotation system.

Phosphorus, with a specific chemistry, improves its performance and functions through the application of concentrated superphosphate and is efficiently productive together with nitrogen. Changes aimed at transforming a lower P supply category to a higher one are directly related to ‘phosphate retrogression’, controlled by soil pH. The P-Al, P-Fe, and P-Ca forms feed and control soil and crop P supply according to the level of solubility of these compounds, dependent on pH and the dose of P₂O₅ applied.

Through these processes, phosphorus applied on a long-term basis maintains the multi-year reserve and soluble mineral forms for soil solution and plant nutrition.

In established platforms with long-term experiments (Askov, Rothamsted, Morrow, Sanborn), with different NPK and FYM treatments, the different forms and fractions of phosphorus in the soil are maintained permanently and are growing. In these ameliorating alternatives, a special place is occupied by molecular-P reserves from organic fertilizer resources, from the composition of these fertilizers, or from the activity of microorganisms involved in phosphorus solubilization [17]. In the Rothamsted approaches, the level of availability of retrograded, conventionally physico-chemically fixed forms of phosphate in soil components has been investigated on a multi-year basis, with an emphasis in these assessments on P-residue forms and their significance for subsequent phosphate applications or assessment of the possibility of their reduction [15]. In all alternatives investigated, the optimal 1 NPK and 1½ AM rates support the regime of good mobility and availability of multiannually applied phosphorus (from superphosphate or phosphate rock). For the practical usefulness of these results, it is necessary to correctly assess, according to the soil characteristics and the varieties to be applied, the internal and external factors that can degrade the phosphorus regime and availability for plants/pH, % CaCO₃, excessive or too low N doses, and the representation of other elements—macroelements but also Zn, Cu, etc. At similar NPK mineral fertilizer input assessments and in equivalence to P from manure, even at similar phosphorus availability to plants, the higher quantitative and productivity effects are due to organic fertilization (assessed trace elements and residual N input) [40].

Evolution of long-term potassium application in an NPK system:

The multi-annual changes in mobile-K lead to good supply conditions for this element, with higher levels in the alluvial mollisol and relevant cation exchange capacity, including the non-exchangeable forms of this cation. In the two soils, preluvosol and alluvial mollisol, the mobile forms of K increase when K₂O doses are applied. Small differences are observed between the levels of concomitant NP supply, particularly in the category of newly formed reserves of non-exchangeable K, which increase concomitantly with K doses applied and with the mobile K “supply” through these reserves.

From this non-exchangeable K (higher in the alluvial mollisol), the supply of mobile forms is higher in the preluvosol (Figure 2 and Figure 3).

This dynamic model (i.e., non-exchangeable K/mobile K) differentiates soils in terms of potassium regime and may even justify the different response of crops to potassium supply when potassium is applied on the NP background (

Table 6. Influence of NPK fertilization on maize crop (average 1972–1990) (table with partial data).

Typical Preluvosol					Alluvial Mollisol
H5 308	Doses K2O kg/ha	Production kg/ha	Diff.	Diff. Signif.	H5 308	Doses K2O kg/ha	Production kg/ha	Diff.	Diff. Signif.
N80P60	0	8035	-	Mt	N80P60	0	8333	-	Mt
	40	8388	353	-		40	8622	289	-
	80	9062	1027	***		80	8997	664	*
	120	9284	1213	***		120	9140	807	*
	160	8779	744	**		160	8821	488	-
N160P120	0	7648	-	Mt	N160P120	0	8325	-	Mt
	40	8404	756	-		40	8686	361	-
	80	8610	962	**		80	9073	748	*
	120	9102	1454	***		120	9075	750	*
	160	8914	1266	***		160	8430	104	-

- not significant; * significant; ** distinctly significant; *** very significant.

).

Comparing the K effect on the two soils (preluvosol and alluvial mollisol), it can be noted that the efficiency of K application is increased on the amended acidic soil, where the nutrient regime of this element is much more dependent on the dose of K applied compared to the alluvial mollisol, which has an improved potassium regime (more exchangeable K but also non-exchangeable K). This substantially improves and feeds the regime of this element in the soil-plant system and partially diminishes the effect (dose) of the application. In much the same way, on alluvial soils this time, the soybean as a legume has a specific K consumption (kg K₂O/t production) of high K (following N in size) and higher than that of P, Ca, S, and Mg. This explains the effect of K application on the soybean crop, which is ensured in the NPK formula (that is why it only relies nutritively on its non-exchangeable form).

Potassium has a specific chemistry as well, linked to the adsorbent complex (humus and clay minerals) and the level of supply to soil and plants, and determined by the dynamic functionality of Knesch.↔Ksch.↔K soluble and the required plant consumption.

The dynamics of soil K forms, with an appreciation of the dependence of plant response to its application, are related to the clay content (%), especially the type of clay minerals that differentially maintain the representation and availability of potassium in or applied to the soil [37,41]. Fertilizer management in long-term experiments is based on NPK, or organic and organo-mineral differentiated application measures, with evaluation of the complex effects of active elements in their composition. The current K-linked application relates to K-poor soils, long-term applications of NP without K (with reduced yield, productivity, and product quality), high K-consuming genotypes, and the sustaining effects of applied or residual nitrogen [15,16,38].

Appraisal of the secondary macroelement (S, Ca, and Mg) and trace element regimes in long-term fertilization:

The diversity of soils in our country, expressed by specific indicators (pH-reaction, humus and clay content and type, CaCO₃ content, etc., at varying agrochemical states, determined by “long-term” interventions), causes profound changes in nutrition and fertility, starting with the soil solution and plant rhizosphere, throughout the soil-plant system, and ultimately in its yield and quality. Interaction and complementarity states have been demonstrated in various situations, such as N/P, N/K, N/Mg, N/S, Ca/Mg, Mg/K, etc. in macroelements and N/Mo, P/Zn, K/B, Cu/Zn, Zn/Mo, P/Mo, etc. in trace elements. Hence the need and usefulness for practical, complex, and integrated fertilization measures based on soil-plant agrochemical monitoring.

In the experiments presented, in the initial experimental framework, the application of mineral resources such as S + Mg was proposed and achieved (based on NPK modifications in the 1975–1985 period). In summary, the results were presented as follows (

Table 7. The influence of mineral resources such as S + Mg was proposed and achieved (based on NPK modifications in the 1975–1985 period).

Wheat	Prod. Kg/ha	%	Diff.	Signif. Diff.	Maize	Prod. Kg/ha	%	Diff.	Diff. Signif.	Soya	Prod. Kg/ha	%	Diff.	Signif. Diff.
Unfertilised	3056	100	-	Mt.		6591	100	-	Mt.		2253	100	-	Mt.
NPK—DOE	4699	153	1643	***		8172	123	1581	***		2734	121	481	***
NPK—DOE+S+Mg	4877	159	1821	***		8833	134	2242	***		2807	124	554	***

*** very significant.

).

The effect of S + Mg application on optimal NPK background can be assessed in relation to the reduction of humus content (concomitant with organic-S) and the interaction determined in cation chemistry (to the disadvantage of Mg). Currently, the fertilizer trade promotes NPK + S + Mg (NPK + Kiserit) assortments, which have proven effective in basic applications (on different crops).

In agrochemical approaches to long-term experiments, the analysis of trace elements (Fe, Mn, Cu, Zn, B, and Mo) in relation to agrochemical changes determined in the soil-plant system is important and topical (

Table 8. Average contents of mobile forms of trace elements (Fe, Mn, Cu, Zn, B, and Mo) (Preluvosol) (synthesis—years 1998–2018).

Culture	Trace Element Contents (ppm)
	Fertilisation	pHH2O	Humus %	Fe	Mn	Cu	Zn	B	Mo
Wheat
Un-amended	P0N0-160	5.52	2.32	4.0	73	4.1	1.7	0.20	0.30
Amended	P0N0-160	5.92	2.34	4.7	49	4.0	1.5	0.29	024
Un-amended	P80N0-160	5.50	2.32	3.2	70	3.6	1.6	0.14	0.36
Amended	P80N0-160	5.88	2.25	2.2	44	4.0	1.6	0.24	0.28
Maize
Un-amended	P0N0-200	5.52	2.03	6.1	83	5.6	1.5	0.71	0.34
Amended	P0N0-200	5.92	2.14	4.7	78	5.5	1.7	0.69	0.33
Un-amended	P80N0-200	5.48	2.05	5.6	95	5.3	1.6	0.62	0.47
Amended	P80N0-200	5.90	2.13	4.1	68	5.3	1.6	0.56	0.31

and

Table 9. Average contents of mobile forms of trace elements (Fe, Mn, Cu, Zn, B, and Mo) (Alluvial mollisol) (synthesis—years 1998–2018).

Culture	Trace Element Contents (ppm)
	Fertilisation	pHH2O	Humus %	Fe	Mn	Cu	Zn	B	Mo
Maize	P0N0-200	7.66	2.70	2.1	21	5.1	1.9	0.65	0.38
	P80N0-200	7.68	2.66	2.2	21	4.9	2.2	0.73	0.32
Wheat	P0N0-160	7.46	2.64	1.5	17	6.4	3.4	0.35	0.25
	P80N0-160	7.48	2.67	1.7	30	4.8	3.3	0.28	0.33

).

The analysis and interpretation of the content of the determined trace elements show:

• In the preluvosol:

-

Ammoniation usually reduces the mobility of trace elements (i.e., cations: Fe, Mn, Cu, and Zn).

-

The soil contains insufficient reserves of B and Mo under wheat, higher amounts of Fe, Mn, and Cu in relation to higher N doses, and higher acidification potential in maize.

• In alluvial mollisols:

-

Fe, Mn, and Zn contents are critical and pose a risk to nutrition, especially in cereals but also in sugar beet and rape.

-

Mo contents are consistently critical with the endangerment of the expected effect of nitrogen, partly of B.

-

In this instance, under conditions of soil pH > 7.5, most crops are expected to be deficient in key trace elements. Indeed, the excessive accumulation of macroelements is also controlled in terms of trace elements.

Undertaking a project for the continuation of long-term experiments with a specific research program—a “post-initial experiment project”—increases the opportunity for rigorous agrochemical control based on research directions arising from up-to-date interpretations of agrochemical changes in terms of NP, NPK, and organo-mineral supply. It is of great value to reposition laboratory methods and update current interpretations and limits for efficient use of the analytical tools involved.

Long-term experimental approaches lead to the implementation of real fertility and fertilization management, which is effective in supporting sustainable and productive farming systems. In this framework, the mentioned negative effects (acidification with degasification and reduction of organics) are controlled by effective measures and models. As such, the benefits of nutrient accumulation are evaluated and used to achieve a balanced and productive yield in the soil-plant system with nutrient monitoring measures, fertilization balance, and soil protection. It is in this context that measures and methods of nutrient, macro-, and micro-nutrient integration based on specificity, complementarity of roles, and effects can be effective [9,11,16,27,42,43,44].

Long-term experiments with fertilizers in Romania have been approached with a unified concept in order to serve the objectives of sustainability in farming systems and to aim for results in line with the principles of sustainable agriculture, where degradation processes must be known and especially balanced by soil regeneration processes.

The principles of sustainability concepts are frequently discussed and promoted theoretically, which is why the issues of sustainable agriculture have been a priority in a high-quality and unpolluted environment while preserving and revaluing soil resources efficiently. In order to take partial account of the problems of sustainable agriculture, long-term experiments with fertilizers must address the main problems of fertilization and plant nutrition in order to achieve quantitatively and qualitatively higher yields in a healthy, sustainable agricultural environment and with consumer protection.

The realization of these experiments in a stationary regime allowed for the probity of the results, their real applicability, and the assessment that the long-lasting results in Romania can constitute an accredited research method that studies the realization of plant production in close dependence on soil fertility. In this context, as the method is in a single and identical principal framework for multiple locations, it gives back to practice the productive value of soils, the methods of its realization, and the framework of measures that maintain or develop soil fertility.

In a unified concept, long-term experiments with fertilizers increase their value through the long duration of the experiments, which is why they can also benefit from a “long-term post-experiment program” that is equally effective in their implementation.

4. Conclusions

The results obtained bring forth multiple and complex measures to implement and support fertilization and fertility sustainability. The following conclusions can be drawn:

• NP fertilization, on account of the exaggerated increase in nitrogen doses, mobilizes the potential, adsorbed acidity, and increases the solubility of Al³⁺ ions, which bring into topicality the measure of calcic amendment of acid soils and moderate fertilization.
• Reduction of organic-C content is more active and significant with conventional soil tillage and CaCO₃ amendment.
• The ameliorating effects in the case of organic-C accumulation are due to organo-mineral fertilization, alternatives to more active carbon sequestration, and soils including ameliorating plants (in this case soybean).
• The effects of NP and NPK fertilization are related to the long-lasting improvement of their regimes, with a significant dependence on the duration of fertilization and the dynamics of the active forms in the soil (soluble and/or exchangeable).