Sustainable management of potassium nutrition in alfalfa crop production is one of the major key factors for achieving optimum seed and biomass yields. An inappropriate supply of mineral potassium nutrition in alfalfa production could result in a decrease in biomass and grain yield production, leading to luxury consumption with cost implications. Alfalfa (Medicago sativa L.) is a perennial leguminous forage crop known for its high protein content, nutritive value, biomass yield production, soil-improving abilities, and livestock feed. Potassium nutrition plays a crucial role in alfalfa production by influencing several physiological processes essential for biomass yield, growth, development, photosynthesis, nutrient uptake, and stress tolerance. Although several studies have been conducted regarding the role of potassium nutrition in agriculture productivity, only limited research has focused on crop-specific impacts. Therefore, this paper reviews (i) the significant role potassium nutrition plays in alfalfa production along with its implications for quality, yield, growth, and resistance to abiotic stress; (ii) the factors affecting the availability, absorption, and transport of potassium; (iii) the source of potassium and the consequences of inadequate availability; and (iv) highlights some strategies for mitigating potassium nutrient deficiency to optimize alfalfa productivity and sustainability in agricultural systems.

The effects of intensive and reduced tillage, fertilization, and irrigation on soil chemical and microbiological parameters were studied in a long-term field experiment in Hungary. The treatments were plowing tillage, ripper tillage, strip tillage; control (without fertilization), NPK fertilization (N: 160 kg/ha; P: 26 kg/ha; K: 74 kg/ha); and non-irrigation and irrigation. Soil samples were collected through maize monoculture in the fall of 2021 in the 30th year of the experiment. The soil organic carbon, total nitrogen, soil microbial biomass (based on PLFA analysis), and soil enzyme activity were observed to be significantly high in the strip tillage plots, but were lower in the ripper tillage plots, and even lower in the plowing tillage plots. The fungal, arbuscular mycorrhiza fungal, and bacterial biomasses were significantly higher in the strip tillage and ripper tillage plots compared to the plowing tillage plots. The strip tillage treatment was found to be the most favorable cultivation method for improving the microbial biomass and activity of Chernozem soil, followed by the ripper tillage and plowing tillage treatments. The long-term use of chemical fertilizers greatly reduced the soil microbial biomass and negatively impacted the soil microbial community, leading to a decrease in fungi and Gram-negative bacteria. The ratio of cyclopropyl PLFA precursors to cyclopropyl PLFAs, as a “stress factor”, indicated the most stressful bacterial environment was that found in the fertilized, non-irrigated plowed soil.

A greenhouse experiment was conducted on a calcareous chernozem soil to investigate the effects of nitrogen (N) and sulfur (S) fertilization and their ratios on the yield and quality of white mustard (Sinapis alba L.). Four levels of N [0.5, 1, 1.5, and 2 g pot⁻¹ N as ammonium nitrate (NH₄NO₃) and ammonium phosphate (NH₄H₂PO₄)] in combination with three levels of applied N–S ratios [8, 4, 2; S as potassium sulfate (K₂SO₄) and sodium sulphate (Na₂SO₄)] were tested as treatments. Results indicated that a significant response to seed yield was observed for N and S application. Maximum yield, 24.8 g pot⁻¹, was found when full doses of N and S were applied. Increasing N supply from 0.5 g pot⁻¹ to 1 g pot⁻¹ had little increasing effect in the oil content of the seed. Higher application of N doses (>1 g pot⁻¹) decreased these values significantly. The maximum oil content (28%) was achieved with the 1 g N pot⁻¹ treatment; the lowest values were observed in the pots applied with the highest N doses. Changes in the N content of the seed and straw showed a statistically significant increase with increasing N and S fertilization. Highest values in the seed and straw (5.96% and 0.87%, respectively) were observed by applying highest N and S doses. Seed and straw S levels were also observed to increase with increasing N rates and decreasing N–S ratio. Nitrogen doses significantly improved the quantities of essential amino acids with the exception of threonine, tyrosine, phenylalanine, and methionine. The amounts of these amino acids decreased with increasing N supply. When the N–S ratio decreased by increasing S, the quantities of valine, isoleucine, leucine, lysine, methionine, and cysteine increased significantly and the amount of tyrosine decreased. The quantities of nonessential amino acids with the exception of proline, histidine, and glycine increased with increasing N doses. In contrast, N rates decreased the amount of glycine. Decreasing the N–S ratio lowered the proline and arginine contents. The total amounts of essential amino acids slightly increased with increasing N rates and decreasing N–S ratio.

The development of critical levels for sap nitrate and chlorophyll meter reading (SPAD test) in the case of various crops is of great importance for growers in characterizing a plant’s N status. A field experiment with spring oat (Avena sativa L.) was carried out on loamy soil in Debrecen, Hungary, using a small-plot design. Ammonium nitrate was broadcast at rates of 0, 30, 60, and 90 kg N/ha in three replicates. The total N content of the plant, sap nitrate content, and SPAD values were measured at jointing when the first node appeared above the soil surface (Feekes 6) and at boot stage (Feekes 10). Regression analysis between total N content and sap nitrate showed cubic and linear relationships with r² = 0.7982 (Feekes 6, whole plant) and 0.9625 (Feekes 10, upper developed leaves), respectively. Optimal grain yield was obtained when sap nitrate exceeded 650 mg/L and 540 mg/L at Feekes 6 and Feekes 10, respectively. There were linear and logarithmic relationships between total N content and SPAD values with r² = 0.8058 and 0.6258 at Feekes 6 and 10. Optimal grain yield occurred over SPAD values of 43 and 48 at Feekes 6 and 10, respectively. Optimal N rate was 60 kg N/ha on the experimental site.

The combined effects of increasing sulfur (S) fertilization rates and drought stress on the yield and compositional parameters of spring wheat on Chernozem soil were studied. In a greenhouse pot experiment, increasing S doses (22.4, 28, 56 kg S/ha) were used with a constant nitrogen (N) dose (112 kg N/ha), resulting in different N:S ratios (1:0.2; 1:0.25; 1:0.5). Water supply treatments included optimal irrigation, maintaining 60% of field capacity, and a water stress treatment where irrigation was withheld until wilting symptoms appeared, followed by irrigation to 40% of field capacity. By measuring the dry biomass production; plant N and S%; and inorganic sulfate-S content, the N/S ratio; harvest index (HI); and organic S, N and S uptake were determined. Our findings indicate that, under water stress, S incorporation into plants is limited, as it tends to remain in an inorganic form. Furthermore, results showed an increase in the N/S ratio under drought conditions, suggesting that drought stress impedes S uptake more significantly than N uptake. In this experiment, fertilization with 112 kg N/ha and 56 kg S/ha (N:S = 1:0.5) proved to be most effective under adequate water supply. In this treatment, grain N and S% were 1.80% and 0.18%, respectively.

Soil microbial communities are crucial for ecosystem services, soil fertility, and the resilience of agroecosystems. This study investigated how long-term (31 years) agronomic practices—tillage, NPK fertilization, and cropping system—along with measured environmental variables influence the microbial biomass and its community composition in Chernozem soil under corn cultivation. The polyfactorial field experiment included three tillage treatments ((moldboard (MT), ripped (RT), strip (ST)), two fertilization regimes (NPK (N: 160; P: 26; K: 74 kg/ha), and unfertilized control) and two cropping systems (corn monoculture and corn–wheat biculture). The soil samples (0–30 cm) were collected in June and September 2023. Microbial biomass and community structure were quantified using phospholipid fatty acid (PLFA) analysis, which allowed the estimation of total microbial biomass and community composition (arbuscular mycorrhizal (AM) fungi, fungi, Gram-negative (GN) and Gram-positive (GP) bacteria, actinomycetes). Our results showed that microbial biomass increased from June to September, rising by 270% in unfertilized plots and by 135% in NPK-fertilized plots, due to higher soil moisture. Reduced tillage, especially ST, promoted significantly higher microbial biomass, with biomass reaching 290% and 182% of that in MT plots in June and September, respectively. MT had a higher ratio of bacteria-to-fungi compared to RT and ST, indicating a greater sensitivity of fungi to disturbance. NPK fertilization lowered soil pH by about one unit (to 4.1–4.8) and reduced microbial biomass—by 2% in June and 48% in September—compared to the control, with the particular suppression of AM fungi. The cropping system had a smaller overall effect on microbial biomass.