Potassium and Sulfur Fertilizer Sources Influence Alfalfa Yield and Nutritive Value and Residual Soil Characteristics in an Arid, Moderately Low-Potassium Soil

Abstract

Alfalfa (Medicago sativa L.) requires a large amount of potassium (K) for maintaining forage yield and stand persistence. Therefore, soil inherently low in K levels must be supplemented with K fertilizers. Although several commercial K fertilizers are available on the market, choosing an appropriate K-supplementing source for a particular soil can be crucial in boosting alfalfa yield and nutritive value. A two-year study was conducted in an arid southern New Mexico location near Tularosa to evaluate the efficacy of three K commercial fertilizer sources in improving alfalfa yield and nutritive value in a moderately low potassium (84 ppm) soil. Nine K treatments or combinations and a no-K control were tested in a randomized complete-block design with four replications. Overall, supplementation of 160 kg ha⁻¹ of K₂O in all treatments resulted in a significant increase in alfalfa seasonal forage yield and nutritive value compared to the control. Sulfate of potash combined with ammonium sulfate (AS) produced greater seasonal yield compared to other fertilizers (muriate of potash and potassium nitrate) or combinations with gypsum (p < 0.05). This treatment combination also showed greater crude protein, neutral detergent fiber, tissue Fe and S, and soil residual sulfate-S and nitrate-N. A positive correlation between sulfur abundance and nitrogen availability in alfalfa production was evident in the study. Addition of AS to all three K fertilizers yielded better seasonal forage yield than K fertilizers alone or K fertilizers in combination with gypsum. Adding gypsum to the K-fertilizers appeared to have negative effect on the seasonal forage yield. Future research should evaluate various combinations of the levels of K and S to determine the most reasonable balance.

1. Introduction

Potassium is of one of the plant-essential macro-elements required for alfalfa growth and development [1]. Potassium supplementation with alfalfa promotes better stand establishment in the early stages, better stand persistence, and better stand density in the later stages [2,3,4]. Stand persistence due to K fertilization has been partly attributed to stress tolerance and carbohydrate storage [5,6,7,8]. However, the positive correlations of K fertilization with alfalfa yield and stand persistence have not always been consistent in the literature [9,10]. Potassium nutrition is also an essential component of alfalfa nutrient management for better alfalfa yield and forage nutritive value [11,12]. In a greenhouse experiment, it was found that alfalfa with adequate K had twice the number of shoots plant⁻¹ compared to K-deficient plants [13]. The positive response of alfalfa yield and shoot biomass due to K fertilization has been also reported in several investigations [4,10]. It has been reported that the minimum plant tissue concentration of K required for healthy growth of alfalfa was 2.5% (w/w) [12], which translated to a cumulative requirement of 270 kg ha⁻¹ of K to produce about 10 ton ha⁻¹ yield target in an irrigated arid-zone environment. Although, the response of alfalfa to K fertilization is generally greater in low-K soils [4,14,15,16], the response curve of alfalfa did not seem to decline, even with a 178 mg kg⁻¹ residual soil exchangeable K concentration [17]. In that study [17], application of 300 kg ha⁻¹ K₂O maximized the alfalfa yield without compromising the forage nutritive value.

It has been reported that K supplementation in sandy soils, which are prominent in arid and semiarid regions, increased the N fixation by 300% [18] by enhancing root nodulation [19], which suggests a positive correlation between K fertilization and N fixation in alfalfa. Potassium fertilization has been also found to reduce stresses due to water shortage [8,20,21] and disease [8,22] in alfalfa. Excessive tissue concentrations of K (>3% dry wt.) in alfalfa can have a negative impact on crude protein (CP) and some essential plant nutrients, especially Ca, Mg, and Na [15,23]. Utilization of Ca-deficient forage as induced by luxurious K uptake in rations may lead to malnutrition and low productivity in cattle [24]. It has been also reported that a decline in alfalfa’s nutritive value, especially CP and neutral detergent fiber (NDF) digestibility (NDFD), occurs with an increase in the applied-K rate beyond 300 kg ha⁻¹ [25].

In general, most soils in arid and semi-arid regions, such as New Mexico (NM) in the USA, have abundant soil-extractable K concentrations [16,26]. However, sporadic pockets of alfalfa-producing soils in NM and other arid and semiarid regions with high proportions of sand have shown lower exchangeable K levels (<60 ppm NH₄OAc extractable K) (unpublished data from the current study’s analyses). In addition, these sandy soils are predominantly calcareous, which contributes significantly to the fixation of K in the soils [27]. Previous research has suggested that light textured soils with high proportions of sand content and low cation exchange capacity exhibited lower extractable K levels than heavy clay soils [28]. For low-K soils, the exchangeable K saturation rate for optimum K response has been found to be 3–4% [29]. The response curve of K supplementation is generally more satisfactory in K-deficient soils than in soils rich in K [16,30]. The soils with low exchangeable K often require additional K supplementation for optimal plant growth, especially for perennials like alfalfa [10,31]. Currently, muriate of potash (MOP), sulfate of potash (SOP), and potassium nitrate (KNO₃) are the common types of K fertilizer sources used for alfalfa production. Choosing the appropriate fertilizer source, based on soil texture and chemistry, to supplement K in K-deficient soils has tremendous potential for boosting alfalfa yield. Therefore, this research evaluates the efficacy of the three prominent commercial K fertilizers in promoting alfalfa growth and nutritive value in moderately low-K soils of arid regions, such as southern New Mexico. This project also evaluates the possible effects of the use of commercial K fertilizers on the residual soil chemistry after harvesting alfalfa.

2. Materials and Methods

2.1. Site Selection and Experimental Design

A two-year study was conducted in a producer’s field near Tularosa, NM, USA (33.0624° N, 106.0327° W, Elevation 1348.5 m; Prelo fine sandy loam), during 2019 and 2020 to evaluate the effect of various K commercial fertilizers on alfalfa yield and quality. Weather data were collected from https://www.wunderground.com/ (accessed on 11 February 2023). The soil K level at the Tularosa experiment site was around 84 ppm at the beginning of the study. An experiment was designed with 10 fertilizer treatments (

Table 1. Fertilizer treatments tested on alfalfa during 2019 and 2020 in Tularosa, NM, USA.

Treatment	Abbreviation
Control	Control
Muriate of potash	MOP
Sulfate of potash	SOP
Potassium nitrate	KNO3
MOP + ammonium sulfate	MOP + AS
SOP + ammonium sulfate	SOP + AS
KNO3 + ammonium sulfate	KNO3 + AS
MOP + gypsum	MOP + gypsum
SOP + gypsum	SOP + gypsum
KNO3 + gypsum	KNO3 + gypsum

) that included a control and 9 various K sources in combination with S fertilizer sources. All treatments included urea (46-0-0) and monoammonium phosphate (11-52-0) as N and P sources in the experiment to meet the requirements for alfalfa and to equalize applied N and P levels among the various treatments. The fertilizers were applied to supplement the target dosage of 180 kg ha⁻¹ of N, 100 kg ha⁻¹ of P₂O₅, and 160 kg ha⁻¹ K₂O, except for the control, which included no added K. The SOP, ammonium sulfate (AS), and gypsum treatment combinations added 54, 80, and 80 kg ha⁻¹ of sulfur, respectively, to the soils. Gypsum added Ca, and MOP added chloride to the soils. The experimental treatments were in 4 randomized complete blocks of 3.7 m × 4.6 m experimental units.

Alfalfa variety ‘WL 440 HQ’ at the seeding rate of 45 kg ha⁻¹ was planted with a Brillion seeder (Brillion Farm Equipment, Brillion, WI, USA) on 21 October 2018, in a conventionally tilled soil. Prowl (0.76 L ha⁻¹) was sprayed to prevent weed emergence in the early spring each year. Alfalfa was uniformly irrigated with center-pivot sprinklers throughout each growing season using groundwater to prevent moisture stress.

The fertilizer treatments were imposed on 5 March 2019, and 4 March 2020, when the alfalfa was at the 3–5 leaf stage. All fertilizers were solid formulations and were applied simultaneously by hand broadcasting method to the same plots each year. Prior to imposing treatments, soil samples were collected at two incremental depths of 0–30 and 30–60 cm to determine the initial nutrient status of the soil (

Table 2. Initial nutrient content of the soil composite (60 cm depth) at the time of the trial’s establishment in Tularosa, NM, USA, in 2018.

Soil pH, 1:1	8.05
Excess Lime	HIGH
Organic Matter, %	1.97
Nitrate-N, ppm N	23.4
Olsen P, ppm P	4.3
Potassium, ppm K	84.1
Sulfate, ppm S	35.1
Zinc, ppm Zn	1.76
Iron, ppm Fe	3.2
Manganese, ppm Mn	4.7
Copper, ppm Cu	0.26
Calcium, ppm Ca	19,070
Magnesium, ppm Mg	477
Sodium, ppm Na	271
CEC/Sum of Cations me/100 g	120.8

). Another soil sampling was conducted at the end of each study year to estimate the nutrient balance and possible effects of treatments on plant nutrient uptake and residual nutrient status. The following chemical analytical methods were performed for the study: pH and Soluble Salts-1:1 method; Organic Matter-LOI (Loss of Ignition) method; Nitrate-N-KCl Extractant; P-Olsen Method; K-Ammonium Acetate and Water Extractant; Sulphate-S-Mehlich-3 Method; Mn, Zn, Cu, Fe-DTPA Method, Na, Ca, Mg-Ammonium Acetate Extractant.

Because of the late planting in 2018, the producer considered 2019 to be the establishment year and delayed the first harvest; during the 2019 season, four harvests of alfalfa were performed, on 1 July, 5 August, 16 September, and 11 November. During the 2020 season, six harvests of alfalfa were performed, on 28 April, 29 May, 30 June, 30 July, 2 September, and 12 November. At each harvest, a sample from a 1 × 1 m² area was hand-clipped, bagged, weighed, and stored individually in a container for transportation to the lab at Tucumcari, NM, USA. After transportation, the samples were dried in a forced-air oven at 60 °C for 48 h, reweighed for calculation of dry matter mass [32], and delivered to a commercial lab (Ward Laboratories, Kearney, NE, USA) for NIRS analysis of nutritive value and tissue nutrient content. Nutritive value variables of each year’s total yield were calculated as the weighted average of the individual harvest’s nutritive values based on the forage mass of the individual harvests [32].

2.2. Statistical Analysis

Alfalfa yield and nutritive value and soil data were combined across years and subjected to SAS MIXED procedures [32] for tests of significance requiring an alpha level of p < 0.05. Treatments and years were considered fixed, and replications were treated as random. If p values were less than 0.05, a mean separation test was conducted using PDMIX800 analysis in SAS [33,34]. The soil data was presented in separate years due to heterogeneity of variance (P[χ²] 0.0016, α = 0.05). Correlation analyses were conducted using SAS procedures [33].

3. Results and Discussion

3.1. Forage Yield and Nutritive Value

Weather conditions (

Table 3. Monthly weather variables during the alfalfa season in 2019 and 2020 in Tularosa, NM, USA.

	2019				2020
Month	Monthly Precip. (cm)	Monthly Max. Temp. (°C)	Monthly Min. Temp. (°C)	Monthly Avg. Temp. (°C)	Monthly Precip. (cm)	Monthly Max. Temp. (°C)	Monthly Min. Temp. (°C)	Monthly Avg. Temp. (°C)
April	0.64	26.0	7.7	17.8	0.00	26.1	7.8	17.8
May	0.15	28.3	10.6	20.0	0.03	31.7	13.9	23.9
June	4.85	33.9	17.8	26.1	0.46	35.6	18.3	27.2
July	2.77	36.1	20.6	28.3	2.74	37.2	20.0	28.9
August	3.02	35.6	20.0	27.8	2.06	36.7	20.6	28.9
September	2.92	31.1	17.2	23.9	1.85	30.6	14.4	22.2
October	0.74	23.9	8.3	16.1	0.28	27.2	8.3	17.8
November	4.17	17.8	2.8	10.0	0.00	21.1	12.2	3.9
Seasonal Total	19.25	-	-	-	7.42	-	-	-
Seasonal Avg.	-	29.1	13.1	21.3	-	30.8	14.4	21.3

) caused forage yield to be significantly affected by the year (12,310 and 22,076 kg ha⁻¹ for 2019 and 2020, respectively; p < 0.002) without any interaction between the year and the treatment (

Table 4. Weighted average of nutritive value parameters with respect to various K fertilizer applications during 2019–2020 at the Tularosa, NM, USA site. Values are the LS means of two years and four replicates. NS = non significant.

Treatment (TRT)	Total Yield	CP	ADF	NDF	RFV	TDN	NDFD	IVTDMD	RFQ	P	K	Ca	Mg	S	Zn	Fe
	kg ha−1	%				%				%					ppm
Control	19,864	28.4	22.1	26.7	267	65.8	49.5	85.6	273	0.35	2.3	1.86	0.49	0.54	33.9	311
MOP	19,771	28.9	20.3	24.7	287	67.1	49.6	86.6	293	0.35	2.2	1.88	0.46	0.60	34.3	429
SOP	20,923	28.4	21.9	26.1	270	65.9	51.6	85.9	284	0.36	2.5	1.73	0.47	0.68	33.2	214
KNO3	19,858	29.0	20.7	25.1	280	66.9	51.5	86.6	292	0.35	2.4	1.85	0.48	0.61	35.6	274
MOP + AS	20,784	28.2	21.9	26.5	263	65.9	49.9	85.7	273	0.35	2.4	1.79	0.48	0.62	35.0	248
SOP + AS	22,046	31.2	22.7	28.8	252	65.3	44.9	84.8	247	0.34	2.3	1.91	0.42	0.78	36.0	518
KNO3 + AS	20,855	29.0	20.7	24.9	292	66.8	53.8	86.9	310	0.36	2.4	1.78	0.48	0.60	34.1	203
MOP + Gypsum	18,286	29.7	20.8	25.6	280	66.7	52.4	86.7	293	0.36	2.4	2.07	0.49	0.57	37.5	249
SOP + Gypsum	19,904	27.8	22.4	27.0	254	65.5	49.3	85.3	263	0.34	2.4	1.99	0.46	0.71	34.1	205
KNO3 + Gypsum	18,732	27.8	22.5	26.9	255	65.5	49.1	85.2	264	0.34	2.4	2.04	0.47	0.58	34.2	215
LSD (0.05)	801	1.4	NS	1.8	NS	NS	5.4	NS	42	NS	NS	0.12	NS	0.07	NS	244
p-Values
TRT	<0.001	0.023	0.672	0.042	0.934	0.328	0.021	0.485	0.018	0.558	0.811	0.032	0.414	0.037	0.741	0.017
Year	0.002	0.563	0.956	0.347	0.735	0.251	0.453	0.890	0.114	0.214	0.575	0.286	0.351	0.682	0.532	0.489
TRT × Year	0.816	0.472	0.648	0.156	0.954	0.743	0.491	0.738	0.143	0.540	0.278	0.618	0.753	0.372	0.562	0.945

). The 2020 season was drier and warmer than the 2019 season (Table 3); as a result, the alfalfa crop in the 2020 season was irrigated more often than in the 2019 season. In addition, irrigation water in the later season of the crop’s growth (October and November) in 2019 was limited due to a mechanical failure of the irrigation system. The total irrigation amount supplemented in 2020 was 21.24 cm greater than in the 2019 season. All these factors contributed significantly to the greater forage yields in 2020 (Table 4). In 2020, two additional harvests were conducted during the crop season, compared to 2019 crop season. The total seasonal forage yield in 2020 was about 80% greater than the 2019 forage yield. It is also worth noting that 2019 was an establishment year, with associated delayed growth due to slower root growth and development. Statistical analysis showed non-significant ‘Treatment × Year’ interactions affecting total forage yield (Table 4).

Fertilizer treatments used in the study had a significant effect on total forage yield (Table 4). In general, SOP in combination with AS yielded consistently greater forage yield in five out of six harvests. Overall, the results also indicated that treatment combinations of SOP had a relative advantage in increasing the total forage yield (seasonal yield) compared to the corresponding treatment combinations of MOP (chlorinated K) and KNO₃. Supplementation of K with MOP and KNO₃ sources had no total seasonal yield advantage compared to the control with no applied K. However, the SOP source showed a significant increase in total yield compared to the control and the MOP and KNO₃ treatment combinations (Table 4). Similarly, the SOP treatment combination with AS or gypsum yielded significantly greater total seasonal forage yield compared to the corresponding combinations of MOP and KNO₃. Overall, SOP in combination with AS had the greatest total yield compared to all other treatment combinations.

Addition of AS to the MOP and KNO₃ yielded significantly greater forage than did MOP and KNO₃ alone or in combination with gypsum. The results suggested that supplementation of sulfur through sulfur-based fertilizer, especially AS, may have an added advantage in increasing the forage yield of alfalfa. The soils are originally moderate in alkalinity (pH = 8). Adding AS might have lowered the pH, thus resulting in favorable nutrient absorption and increased forage yield. This observation is supported by the research results of [35,36], which determined a linear response of S supplementation to the N-fixation and alfalfa forage yield. All K treatment combinations with gypsum appeared to have lower total forage yield. The soils are originally rich in Ca (Table 2). Adding more Ca (basic cation) in the form of gypsum might have resulted in the fixation of more nutrients, for example, P and K [27,37], and reduction of other acidic essential trace nutrients, such as Fe, Zn, Cu and Mn [38,39], and, hence, a decrease in the forage yield. However, the findings of this research were in contrast with previous study [40], which reported the synergetic effect of gypsum and K on increasing forage yield. This may be due to a difference in the magnitudes of original Ca content in the two soils.

Most of the forage-quality and mineral-composition parameters were unaffected by fertilizer treatments, with a few exceptions (Table 4). Crude protein and NDF were significantly greater in the SOP and AS fertilizer combination compared to other fertilizer treatments. Although NDFD and relative forage quality (RFQ) values were numerically greater in the KNO₃ and AS combination, they were not statistically different from the other treatments. Forage tissue S and Fe were significantly greater in the SOP and AS fertilizer combination, compared to other treatments. The tissue Fe, being an acidic cation, was increased with increased S concentration, which supports the idea of a positive correlation between S and Fe (r = 0.65, p = 0.04). Significantly greater tissue S concentration in the SOP + AS treatment combination contributed to increased N-assimilation, as evidenced by the greater CP and total forage yield (Table 4) in this treatment. Total forage Ca was significantly increased in all gypsum treatment combinations, compared to other treatment combinations; however, reduction in tissue P or K concentrations as result of increased Ca was not evident, contrary to the expectation. Tissue K concentrations were not affected by the source of K fertilizer in the study (Table 4). Year and ‘Year × Treatment’ interactions had no significant impact on the forage nutritive value.

3.2. Residual Soil Characteristics

Most of the post-study residual chemical characteristics at 0–30 cm depth in 2019 were not affected by source of the K fertilizer or K-S fertilizer combinations used in the study, the exceptions being nitrate-N, K, and sulfate-S (

Table 5. Chemical characteristics of soil samples collected after final alfalfa harvest (post-study) in 2019 at Tularosa, NM, USA. Values are the LS means of four replicates. NS = non significant.

TRT	pH	S.Salts (mmho/cm)	OM, %	NO3-N	P	K	SO4-S	Zn	Fe	Mn	Cu	Ca	Mg	Na	CEC	Water Sol K, ppm
0–30 cm depth
ppm
Control	7.7	2.10	1.65	1.50	7.93	67	5410	2.32	1.80	2.00	0.24	21,385	414	121	112	19
MOP	7.7	3.84	1.73	1.30	7.33	95	5531	2.20	1.50	1.98	0.26	22,165	408	124	115	34
SOP	7.8	1.56	1.88	3.25	7.70	100	5982	2.29	1.83	1.73	0.28	21,955	375	115	114	43
KNO3	7.7	1.61	1.70	1.48	6.55	80	5584	1.66	1.65	2.00	0.19	21,973	409	129	114	27
MOP + AS	7.7	1.51	1.65	2.25	8.33	92	5795	2.09	1.90	2.25	0.23	22,028	414	138	114	31
SOP + AS	7.8	3.68	1.80	3.72	6.83	93	6168	1.67	1.85	2.63	0.25	22,683	373	137	117	36
KNO3 + AS	7.7	2.79	1.80	2.10	7.93	98	5791	1.65	1.78	2.15	0.21	22,118	396	128	115	39
MOP + Gypsum	7.9	1.59	1.63	1.99	6.63	77	5711	2.00	2.05	2.20	0.24	21,810	404	131	113	37
SOP + Gypsum	7.9	1.83	1.70	3.15	7.73	94	5875	1.45	1.53	2.40	0.21	22,743	428	130	118	39
KNO3 + Gypsum	7.9	1.69	1.82	1.93	6.25	81	5769	2.04	1.83	1.98	0.23	21,858	439	146	114	29
LSD (0.05)	NS	NS	NS	1.29	NS	30	358	NS	NS	NS	NS	NS	NS	NS	NS	17
30–60 cm depth
Control	7.7	1.49	0.80	1.20	6.15	45	5087	0.52	2.20	3.68	0.36	19,815	437	177	105	26
MOP	7.7	1.16	0.80	0.70	5.28	68	5700	0.51	2.95	2.70	0.26	20,020	492	182	105	26
SOP	7.8	3.15	0.83	2.43	4.90	69	5987	0.50	2.58	2.85	0.28	21,310	424	125	111	22
KNO3	7.7	2.45	0.83	1.08	5.38	67	5494	0.61	4.80	3.93	0.37	19,050	413	126	99	25
MOP + AS	7.8	1.46	0.70	1.28	4.60	62	6127	0.58	3.43	3.08	0.22	19,870	429	162	104	25
SOP + AS	7.8	1.36	0.75	2.30	7.15	67	6437	0.48	2.70	2.53	0.30	20,358	474	166	107	23
KNO3 + AS	7.7	1.60	0.75	1.08	5.05	72	6281	0.40	2.53	2.68	0.24	20,105	402	117	105	28
MOP + Gypsum	7.9	1.62	0.73	1.03	3.83	74	5620	0.45	2.63	2.48	0.30	19,730	452	157	103	20
SOP + Gypsum	7.9	1.46	0.77	1.23	4.20	59	5798	0.32	3.00	2.43	0.22	20,183	450	156	105	23
KNO3 + Gypsum	7.8	1.22	0.80	2.23	3.65	77	5621	0.39	2.35	2.18	0.28	20,918	425	177	110	22
LSD (0.05)	NS	1.65	NS	1.04	3.10	19	431	NS	NS	1.41	NS	NS	NS	59	NS	NS

). It was also observed that supplementation of K or S fertilizer combinations at 0–30 cm depth did not result in a change in these residual chemical characteristics compared to the control. However, fertilizer treatments at 30–60 cm depth showed significant differences in several residual chemical characteristics (Table 5). Nitrate-N concentration at 0–30 cm depth was greatest when SOP and AS were applied together. It was also noted that sulfate-S at this depth was significantly greater, which demonstrates the positive correlation of S and N (r = 0.72, p = 0.004). The same trend was also observed at 30–60 cm depth. Mineralization of N and availability of more nitrate-N in the presence of sufficient amounts of sulfur has been demonstrated in the previous research [34,35]. Both total K and water-soluble K at both depths were significantly greater in all K fertilizer treatments compared to control; however, the differences in residual K concentrations were not significant among the different K-fertilizer sources tested in the study (Table 5). At the 30–60 cm depth, the residual soluble salts, Mn, and Na, were significantly greater in all K- and S-supplemented treatments compared to the control (Table 5). A significant increase in residual Ca due to gypsum application was observed at a 30–60 cm depth.

As in 2019, in 2020, most of the residual chemical characteristics were unaffected by application of K and S fertilizers but the variables affected were different (

Table 6. Chemical characteristics of soil samples collected after the final alfalfa harvest (post-study) in 2020 at Tularosa, NM, USA. Values are the LS means of four replicates. NS = non significant.

TRT	pH	S.Salts (mmho/cm)	OM, %	NO3-N	P	K	SO4-S	Zn	Fe	Mn	Cu	Ca	Mg	Na	CEC	Water Sol K, ppm
0–30 cm depth
ppm
Control	7.9	1.33	1.60	2.45	6.75	62	6540	1.68	2.75	3.83	0.25	22,205	321	63	114	11
MOP	7.9	1.45	1.63	2.83	6.43	72	6653	1.54	2.38	3.63	0.27	22,320	359	63	115	36
SOP	7.9	1.61	1.65	3.98	6.98	90	6899	1.19	2.95	4.03	0.32	22,325	395	68	116	30
KNO3	7.9	1.48	1.65	3.28	4.68	76	6691	1.07	2.60	3.43	0.24	22,415	397	74	116	29
MOP + AS	7.9	1.48	1.63	3.13	6.08	74	6798	1.34	2.78	3.83	0.25	22,025	318	81	114	22
SOP + AS	8.0	1.48	1.90	3.96	7.35	82	7059	1.73	2.68	3.90	0.29	22,255	378	84	115	27
KNO3 + AS	7.9	1.48	1.78	3.65	8.20	80	6574	1.52	2.88	4.40	0.33	22,008	474	94	115	27
MOP + Gypsum	8.0	1.48	1.25	2.53	6.20	81	6777	1.24	2.75	3.30	0.28	22,638	431	81	118	45
SOP + Gypsum	7.9	1.48	1.45	2.55	5.73	84	6835	1.20	2.95	3.58	0.31	21,855	401	69	113	46
KNO3 + Gypsum	8.0	1.48	1.28	2.45	6.70	85	6646	1.23	2.65	3.03	0.28	22,533	420	75	117	30
LSD (0.05)	NS	1.48	NS	NS	2.12	18	234	NS	NS	NS	NS	NS	NS	42	NS	14
30–60 cm depth
Control	8.0	0.78	1.37	3.13	6.60	65	6957	1.55	2.43	3.13	0.21	22,617	335	61	117	24
MOP	7.9	1.51	1.48	4.45	5.35	70	6748	1.29	2.55	3.43	0.25	22,488	419	88	117	24
SOP	7.9	1.57	1.73	3.23	7.03	84	6513	1.71	2.63	3.65	0.29	22,248	412	74	115	27
KNO3	7.9	1.54	1.58	3.78	12.85	74	6755	2.22	2.68	3.70	0.30	22,285	418	105	116	79
MOP + AS	7.9	1.47	1.53	3.75	5.28	67	6678	1.33	2.65	3.45	0.26	22,358	380	84	116	22
SOP + AS	8.0	1.62	1.30	3.95	4.70	75	6453	1.05	2.60	3.03	0.29	22,713	422	89	118	31
KNO3 + AS	7.8	1.63	2.00	4.58	9.38	91	6754	2.42	2.75	3.63	0.29	21,868	351	77	113	33
MOP + Gypsum	7.9	1.66	1.78	4.33	6.58	76	6969	1.38	2.68	3.70	0.26	22,483	386	95	117	33
SOP + Gypsum	7.9	3.37	1.73	3.38	14.85	78	6848	1.07	2.90	4.05	0.31	21,243	325	75	110	33
KNO3 + Gypsum	7.9	2.27	1.28	2.78	5.75	57	6792	1.14	2.68	3.28	0.22	22,233	298	62	114	22
LSD (0.05)	NS	0.64	NS	NS	4.60	30	NS	NS	NS	NS	NS	NS	89	NS	NS	12

). Soluble salts at both depths were significantly increased by the application of K and S fertilizers compared to the control. There was a difference in the P content at both depths due to application of different K and S fertilizers; however, it appeared to be random, and not necessarily due to one particular type of fertilizer application. Total K and water-soluble-K were significantly greater in all K-fertilizer applications, except for soluble-K at the 30–60 cm depth. Unlike the results in 2019, nitrate-N concentration was unaffected by the fertilizer treatment applications in 2020 at both depths. Residual Sulfate-S at 0–30 cm was significantly greater in the SOP and AS treatment combination, as had been observed in 2019; however, this difference was not significant among the treatments at the 30–60 cm depth.

4. Conclusions

The results of this study and the pertinent reviewed literature indicated that sulfate of potash (SOP), in combination with ammonium sulfate (AS), produced greater seasonal yield compared to other fertilizers or fertilizer combinations. This combination also showed greater crude protein, neutral detergent fiber, tissue Fe and S, and soil residual sulfate-S and nitrate-N. A positive correlation between sulfur abundance and nitrogen availability in alfalfa production was observed in the study. Addition of AS to all three K fertilizers yielded better seasonal forage yield than K fertilizers alone or K fertilizers in combination with gypsum. Adding gypsum to the K-fertilizers appeared to have a negative effect on the seasonal forage yield. Future research should evaluate various combinations of levels of K and S to determine the most reasonable balance.