Effect of Fertilisation Regime on Maise Yields

Abstract

Using natural fertilisers in agriculture improves quantity and quality yields. They introduce macronutrients (nitrogen, phosphorus, and potassium) and micronutrients into the soil. Enriching the soil with organic substances through fertilisation with digestates requires the farmer to have considerable knowledge and accuracy in dosing due to the need to comply with permissible concentrations of macronutrients. The availability of nutrients in a digestate is closely dependent on the substrates used in the biogas plant, and it cannot be stated unequivocally that better yields of field crops will be achieved by using it as manure. Therefore, the authors conducted a two-year study of the effect of the fertilisation method on maise yields. Based on the research carried out, the fertiliser suitability of the digest was confirmed. Plants fertilised with it were characterised by the highest (compared to other fertilisation methods) grain yield (of 12.07 Mg per hectare on average). In addition, they were characterised by adequate plant height (3.15 m on average). The observations also indicate good emergence and satisfactory early vigour.

1. Introduction

Fertilisation involves the introduction of substances into the soil to increase the nutrient content necessary for plant growth and improve the soil’s chemical and physical properties. In doing so, four types of fertilisers are distinguished [1,2]:

• Natural—manure from livestock, in the form of manure, slurry, intended for agricultural use;
• Mineral (inorganic)—produced by physicochemical transformations or the processing of mineral raw materials;
• Organic—made from organic substances and mixtures of organic substances (e.g., composts);
• Organic-mineral—mixtures of mineral and organic fertilisers.

Using natural fertilisers in agriculture improves the yield obtained in quantity and quality [3,4]. They introduce macronutrients (nitrogen, phosphorus, and potassium) and micronutrients into the soil [5]. The use of organic fertilisers also reduces the occurrence of nitrates and nitrites in plants. Nitrates are compounds harmless to human health but are quickly reduced to the more toxic nitrites. At elevated temperatures, nitrites react with secondary and tertiary amines to form nitrosamines, which have mutagenic and carcinogenic effects. Organic fertilisation increases plants’ carbohydrates, easily digestible proteins, and vitamin B content. An increased iron, magnesium, phosphorus, and potassium content was also observed in carrots, potatoes, savoy cabbage, spinach, leeks, and lettuce fertilised with organic fertilisers [6].

Organic fertilisers are usually characterised by low contents of arsenic, mercury, lead, and other heavy metals, unlike some mineral fertilisers [7,8].

Table 1. Maximum concentrations of heavy metals allowed in manures [9].

Metal	Maximum Content
	[mg·kg−1]
Cadmium	5
Chrome	100
Nickel	60
Lead	140
Mercury	2

shows the maximum levels of heavy metals allowed in Poland for organic fertilisers. If exceeded, such fertilisers are disposed of, as they can endanger crops and the environment.

The chemical properties of manure vary and depend mainly on the litter used and the type of animal—

Table 2. Macro- and micronutrient content of livestock manure [10].

Manure Origin	Macronutrients [%]					Micronutrients [mg·kg−1]
	N	P2O5	K2O	CaO	MgO	B	Cu	Mn	Mo	Zn
Cattle	0.47	0.29	0.67	0.45	0.16	4.46	4.46	64.65	0.29	34.60
Pigs	0.49	0.71	0.68	0.44	0.16	3.60	5.38	63.22	0.33	48.51
Horses	0.54	0.29	0.95	0.45	0.16	3.51	3.36	70.41	0.25	25.91
Sheep	0.76	0.40	1.25	0.61	0.21	5.81	5.14	84.22	0.34	32.37
Poultry	1.20	0.79	0.80	0.73	0.21	9.59	8.73	76.63	0.52	66.60
Mixed manure	0.49	0.31	0.68	0.44	0.16	4.65	5.05	73.31	0.34	40.50

.

In recent years, organic agricultural products have become increasingly popular. Such products are provided by organic farms, where the use of chemical plant protection products and mineral fertilisers has been reduced as much as possible. One way to reduce the use of mineral fertilisers is to replace them with a digestate from biogas plants [11]. A digestate, which contains organic matter and essential mineral compounds, is an alternative to mineral fertilisers and can compete with natural fertilisers. In biogas production, electricity and heat are also obtained, which can cover the farm’s energy needs at which the biogas plant is built [12,13]. A positive aspect of anaerobic digestion in creating the digestate is that it reduces pathogens, kills viruses, fungi, Listeria, Salmonella, and Escherichia coli bacteria, and inactivates plant seeds [14,15,16,17]. Digested slurry has a less unpleasant odour and a more favourable consistency for further processing than slurry [18].

The digestate residues from agricultural biogas plants can be treated as waste, effluent, or organic fertiliser. If the digestate is treated as waste, it can be spread on the land surface for fertilisation or soil improvement [19]. If the digestate is treated as wastewater, it should be managed in accordance with the water permit obtained in this respect, meeting sanitary standards and pollution limits [20]. If the digestate is treated as a fertiliser, permits must be obtained to market it by meeting the requirements of the Act on Fertilisers and Fertilisation [1].

The fertilising qualities of a digestate include the following [18]:

• High biological activity of beneficial microflora;
• Neutralised effect of pathogenic micro-organisms (e.g., Salmonella);
• The high content of readily available ammoniacal nitrogen;
• Reducing the germination capacity of weed seeds contained in the digestate;
• No risk to groundwater.

The application of a digestate on agricultural land as an organic fertiliser is already considered a standard way of using it [21,22]. Enriching the soil with organic matter through digestate fertilisation requires the farmer to have considerable knowledge and accuracy in dosing due to the need to comply with acceptable concentrations of macronutrients [23].

Table 3. Basic fertiliser properties of the digestates obtained from selected substrate mixtures [24].

Digestate	Dry Matter	Total Nitrogen	Nitrogen NH4	P	K
	[%]	[kg·m−3]	[kg·m−3]	[kg·m−3]	[kg·m−3]
bovine slurry—100%	5.1	5	3.3	1.8	6.5
maise silage—70% + cattle slurry—30%	9	5.8	3.8	2.3	9.1
maise silage—40% + pig slurry—60%	6.3	5.5	3.6	2.6	5.2
maise silage—85% + pig slurry—10% + wheat grain—5%	10.5	7.5	4.9	3.6	10.1
maise silage—80% + rye silage—20%	10.9	7	4.6	2.8	11.1
cattle slurry—84.4% + oat and maise silage—11.6%	3.1	4	2.4	0.8	3.1
pig slurry—94.6% + maise residues—5.4%	2.8	3.4	2.7	1.2	2.7
pig slurry—91.4% + rapeseed residues—9.6%	4.4	3.6	2.9	1.1	3.1
pig slurry—95.5% + sunflower residue—4.5%	3.8	3.5	2.6	1.1	3.1

summarises the properties available in the literature for digestates obtained from different substrate mixtures.

The digest obtained from animal slurry has an alkaline reaction with a pH of about 7–8 [24,25]. The digest with the highest content of the inorganic form of nitrogen (NH₄) has the highest fertilising value. Unfortunately, this form is perishable and can volatilise rapidly in an alkaline environment. Nitrogen in NH₄ undergoes rapid nitrification (under favourable conditions), facilitating its availability to plants. Unfortunately, it is also possible for it to leach deep into the soil profile into groundwater, posing a risk of contamination.

In order to demonstrate the changes in the composition of the digestate, a summary of the chemical compositions of the various substances used as fertilisers in agriculture is presented in

Table 4. Summary of basic chemical components of natural fertilisers [26].

Fertiliser	Dry Matter	N	P2O2	K2O
	[%]	[g·kg−1 Fresh Weight]
Manure	21–24	4.6–5.4	2.7–4.4	6.5–6.7
Slurry	5–9.5	0.6–8.2	0.2–9.6	0.1–5.1
Digestate	4–7	3.0–5.0	1.0–1.5	3.5–5.5

, compared to conventional organic fertilisers.

Comparing the composition of organic fertilisers with digestates, it can be seen that a digestate contains more nitrogen than slurry but less potassium and phosphorus than untreated manure. Plants can better utilise the nutrients in slurry once it has been digested.

A digestate can be used as a fertiliser in both liquid and solid form. Mechanical or thermal water separation from the digestate usually forms the solid phase. The macronutrient and micronutrient content of a solid digestate depends on the composition of the input raw materials to the fermentation process and the retention time of the raw materials in the fermenter [27]. The dry matter content of a solid digestate is in the range of 21–30% [28], with the nitrogen content in the range of 2.2–3%, phosphorus 1.9%, and potassium 3.6% of the dry weight of the solid part of the digestate [29,30]. Due to its chemical composition and physical properties, applied solid digest can positively influence the biomass yield and soil structure [31].

Liquid digest can be considered as a diluted substrate solution containing a wide range of nutrients in a form acceptable to plants [32]. Micronutrient contents for the liquid part of a digestate are 7.7–9.2% for nitrogen, 0.4–0.7% for phosphorus, and 3.9% for potassium [29]. The digest in liquid form appears to be a suitable feedstock for use on arable land during the growing season, both in terms of fertilisation and irrigation [33,34]. The dry matter in the liquid digest is in the range of 0.8–4%. Nitrogen is mainly present in mineral form, with a concentration of 0.15–0.30%, comparable to the potassium content. As the N–P–K ratios are variable in each digestate, it is necessary to analyse the individual components before applying such a fertiliser in the field [35].

The best yield-forming effect will be obtained if the digestate is applied in doses calculated according to the rules for drawing up a fertiliser plan. It allows for a precise calculation of the nutrients necessary to be introduced in the digestate into the individual soils. To do this, a chemical analysis of the soil must first be carried out to determine the reaction and the content of bioavailable forms of phosphorus (P), potassium (K), magnesium (Mg), and mineral nitrogen (N). Then, knowing the chemical composition of the digestate, the possibility of covering the nutritional requirements of individual plant species using the digestate and possibly other organic and mineral fertilisers is determined.

Studies show that using a digestate from agricultural biogas plants reduces the environmental risks [36] that are generally associated with using mineral fertilisers [37] while achieving comparable crop yield parameters. At the same time, it should be emphasised that the availability of nutrients very much depends on the substrates used in the biogas plant, and it cannot be stated unequivocally that the use of anaerobic digestion by-products will achieve better yields of field crops [38,39,40,41]. In order to fill this research gap, the authors conducted a two-year study of the effect of the fertilisation method on maise yields.

The research presented in this article aimed to determine the influence of the type of fertiliser applied to maise fields on selected plant parameters. Particular attention was paid to the suitability of the digestate as a natural fertiliser used in agriculture.

2. Materials and Methods

Each year, a 2700 m² field divided into 15 test plots of 180 m² each (6 m × 30 m in size) was used for this study. The research was conducted in Poland, in the Podlaskie Province. The field experiment was conducted using the independent randomised block method, where the location of test plots with a dedicated fertilisation method was randomly generated. In the first year of this study, research plots of 2700 m² were used, while in the second year, plots of the same total area were used but located next to the area used for this study in the first year. This change was dictated by the need to obtain similar starting conditions in each year of this study by reducing the influence of the fertilisation method in the previous year. Each year, the forecrop was winter triticale. The maise variety Vistula (FAO 210-220) was selected as the test crop. Lumax 537.5 SE (at 3.5 L/ha) and Elumis 105 OD (at 1.5 L/ha) (produced by Syngenta, Warsaw, Poland) were used for herbicide protection. Five different ways of fertilising the plots were adopted for this study:

• The plot was fertilised with a digestate (at a rate of 40 t per hectare).
• The plot was fertilised with a digestate (at a rate of 40 t per hectare) and an NPK fertiliser (N—87 kg per hectare, P—60 kg per hectare, and K—90 kg per hectare).
• The plot was fertilised with a digestate (at a rate of 40 t per hectare), an NPK fertiliser (at a rate of N—87 kg per hectare, P—60 kg per hectare, and K—90 kg per hectare), and micronutrients (in the form of Plonvit NPK at a rate of 3.0 l per hectare).
• The plot was fertilised with an NPK fertiliser (at a rate of N—140 kg per hectare, P—42 kg per hectare, and K—119 kg per hectare).
• The plot was fertilised with an NPK fertiliser (at a dose of N—140 kg per hectare, P—42 kg per hectare, and K—119 kg per hectare) and micronutrients (in the form of Plonvit NPK at a dose of 3.0 l per hectare).

The amount of fertiliser was determined based on the nutritional requirements of corn, in relation to individual nutrients, for the assumed yield. Each plot occurred in triplicate. The layout of the plots is shown in

Table 5. Layout of experimental plots in successive years (own study).

Block No.	No. of Application of Fertiliser to the Plot
First year of study
1	4	1	3	2	5
2	2	5	4	3	1
3	1	3	5	4	2
Second year of study
1	4	3	2	1	5
2	1	2	5	4	3
3	3	4	1	5	2

.

The sowing of maise in the test plots was carried out on 18 May 2021 and 16 May 2022. Fertilisers and the digestate were applied on the day of sowing each time. Harvesting took place, respectively, on 23 September 2021 and 26 September 2022. The results of the pre-sowing and post-harvest soil analyses carried out in each research year are shown in

Table 6. Soil parameters in the test plots by year of study.

Parameter	Unit	First Year of Study		Second Year of Study
		Before Sowing	After Harvesting	Before Sowing	After Harvesting
pH	-	6.7	6.7	6.7	6.7
P2O5	mg per 100 g of soil	13.2	14.4	13.8	14.0
K2O	mg per 100 g of soil	22.0	16.0	20.8	14.9
Mg	mg per 100 g of soil	5.7	4.9	5.4	4.9

.

The growth and development of crop plants and their yield depend to a large extent on environmental conditions. Any disturbance causes an imbalance in the vital functions of plants and becomes a cause of disease, which affects the condition and growth of plants—the research period varied in terms of the pattern of weather conditions, as shown in

Table 7. Weather conditions’ characteristics during the Podlaskie Voivodeship study period (own elaboration).

Specification	Year of Study	Months
		V	VI	VII	VIII	IX	V–IX
Average daily temperature [°C]	1	12.2	19.4	21.8	17.0	12.4	16.6
	2	12.2	18.3	18.4	21.0	10.9	16.2
Total precipitation [mm]	1	77.9	61.2	122.8	114.2	54.8	430.9
	2	64.9	46.4	115.3	31.4	93.8	333.8

.

In order to determine the amount of the elements supplied to the plants during their fertilisation with the digest, tests were carried out on the chemical composition of the digest, which was later fed to the plants in the research fields. The results obtained are shown in

Table 8. Results of tests on the chemical composition of the digest used to fertilise the plants studied (own study).

Year of Study	-	1	2	Average	Deflection
Dry matter	%	4.71	4.86	4.785	0.075
Organic dry matter	%	73.58	76.4	74.99	1.41
Calcium (Ca)	g·kg−1 f.w.	34.1	24.7	29.4	4.7
Magnesium (Mg)	g·kg−1 f.w.	10.4	11.17	10.785	0.385
Sodium (Na)	g·kg−1 f.w.	10.3	8.52	9.41	0.89
Phosphorus (P)	g·kg−1 f.w.	11.2	9.24	10.22	0.98
Potassium (K)	g·kg−1 f.w.	51.5	49.9	50.7	0.8
Molybdenum (Mo)	mg·kg−1 f.w.	2.24	2.91	2.575	0.335
Cobalt (Co)	mg·kg−1 f.w.	1.94	1.83	1.885	0.055
Manganese (Mn)	g·kg−1 f.w.	0.333	0.362	0.3475	0.0145
Zinc (Zn)	mg·kg−1 f.w.	401	398	399.5	1.5
Lead (Pb)	mg·kg−1 f.w.	3.65	3.44	3.545	0.105
Cadmium (Cd)	mg·kg−1 f.w.	0.325	0.328	0.3265	0.0015
Chromium (Cr)	mg·kg−1 f.w.	2.78	2.98	2.88	0.1
Copper (Cu)	mg·kg−1 f.w.	167	167.7	167.35	0.35
Nickel (Ni)	mg·kg−1 f.w.	8.08	7.98	8.03	0.05
Nitrogen (N)	g·kg−1 f.w.	3.1	2.8	2.95	0.15
Carbon (C)	g·kg−1 f.w.	29.4	14.56	21.98	7.42
Mercury (Hg)	mg·kg−1 f.w.	0.019	0.016	0.0175	0.0015
Ammonium nitrogen (NNH4)	mg·kg−1 f.w.	931	834	882.5	48.5

f.w.—fresh weight.

. Based on the information received about the chemical composition of the digestate and the soil, as well as the nutritional needs of the maise, the amount of digestate to be used in the test plots was selected.

3. Results and Discussion

On the basis of the field tests carried out over a period of two years, information was obtained on the amount of maise grain harvested, plant height, and dry matter content depending on the fertiliser applied. The amount of maise grain harvested from the experimental fields in the first research year ranged from 261.29 to 498.5 kg, while in the second year, it ranged from 287.61 to 456.14 kg. Based on the averages determined, it was observed that the values obtained were similar in the two research years for each fertiliser. However, in each case, they were lower in the second research year compared to the first. The highest yields were obtained from the field fertilised with the digestate, while the lowest yields were obtained from the field fertilised with digestate + NPK + micronutrients (Figure 1 and

Table 9. Analysis of the amount of grain harvested in each research plot in each year (own study).

Fertiliser	Plot No.	First Year of Study			Second Year of Study			Total of Two Years
		Grain Weight	Average	Deviation	Grain Weight	Average	Deviation	Deviation	Deviation
		[Mg/ha]	[Mg/ha]	[Mg/ha]	[Mg/ha]	[Mg/ha]	[Mg/ha]	[Mg/ha]	[Mg/ha]
Digestate	1	11.82	12.19	1.24	11.73	11.95	0.52	12.07	0.12
	2	13.85			12.67
	3	10.89			11.45
Digestate + NPK	1	10.15	10.50	1.63	11.42	9.93	1.15	10.21	0.28
	2	12.64			9.75
	3	8.69			8.62
Digestate + NPK + micronutrients	1	10.36	9.15	1.36	9.76	8.64	0.79	8.90	0.26
	2	9.85			8.19
	3	7.26			7.99
NPK	1	9.42	9.56	0.13	8.94	9.29	0.29	9.42	0.13
	2	9.52			9.26
	3	9.73			9.66
Control NPK + microelements	1	9.60	10.10	0.36	9.20	9.58	0.28	9.84	0.26
	2	10.25			9.66
	3	10.44			9.87

).

The average values for the amount of maise grain harvested from plots fertilised with the various types of fertilisers are very similar in both years of this study—the deviation in no case exceeds 280 kg per hectare. The slightest deviation (120 kg per hectare) was observed with the application of the digestate. Considering the weight of grain harvested over two years in the plots fertilised with a particular type of fertiliser, the best yields were achieved with the application of the digestate (average 12.07 Mg per hectare), while the digestate + NPK + micronutrient fertilisation performed the worst (average 8.9 Mg per hectare). Perhaps in this case, the plant received too much fertiliser (over-fertilised), reducing its production capacity. The differences in the amount of maise grain harvested when fertilised with the other fertilisers were no longer significant and did not exceed 800 kg per hectare. Fertilisation is one of the most critical yield-forming factors. Appropriate doses, as well as the balance of nutrients, determine the yield obtained [42]. According to Gołębiewska and Wróbel [43], correct nitrogen fertilisation significantly influences maise yields, as it has high nutrient requirements. However, fertilisation should be optimised because of the possibility of yield reduction when certain levels of the component dosage are exceeded. Therefore, it was found that the content of nitrogen and other nutrients in the digest were appropriate, resulting in the best yields compared to the amount of yield after fertilisation with other fertilisers. The significant reduction in yields collected from the field after applying digestate + NPK + micronutrients may have been mainly due to the exceeded nutrient requirements of maise. Also, according to Beresia et al. [44], too high of a dose of nitrogen results in an increased vegetative weight with an excessive leaf mass and weak stalks, which also translates into an increased susceptibility to lodging. Maise over-fertilised with the mentioned component enters the flowering stage later and, at the same time, matures more slowly, which is reflected in a decrease in the grain yield.

When analysing plant height, it was noted that NPK fertilisation contributed to the lowest results over the two research years (Figure 2). The dose of digestate + NPK and digestate + NPK + micronutrients contributed to higher plants than the others. Maise is a problematic crop to over-fertilise with nitrogen; however, too high doses of this nutrient can contribute to lower yields. In such a case, the plant forms too much green mass and forms too many cobs, which it cannot feed later. Furthermore, over-fertilisation with nitrogen prolongs the vegetation period, makes the plants more sensitive to fungal diseases, and gives the grain a higher moisture content at harvest. This phenomenon was observed in the research plots, as the dry matter content of the plants decreased with an increasing plant height, followed by an increased moisture content. This also translates into yields with a lower content of bioactive components.

The dry matter content of the harvested maise in the first research year ranged from 26.12 to 30.05%, while in the second year it ranged from 26.09 to 29.75%. The highest results were obtained after harvesting maise fertilised with NPK, while the lowest results were obtained for maise fertilised with digest + NPK + micronutrients (Figure 3). It is worth mentioning that plants fertilised only with the digestate ranked second in this classification. From the results of the experiment, it can be seen that the significant intensification of fertilisation has contributed to a reduction in dry matter accumulation in maise. The use of increasing mineral fertilisers translates into a greater susceptibility to water accumulation in plant tissues from these crops, as well as an increase in yields, which ultimately has a negative effect on the dry matter content [45,46]. As reported by Księżak et al. [47], increasing the level of the nitrogen fertilisation of maise in studies conducted in 2008 and 2009 reduced the dry matter content. A trend of decreasing dry matter content under the influence of increasing nitrogen fertilisation was also noted in their study by Zieleniewicz [48].

An ANOVA analysis of variance was used to determine whether the fertiliser affected the amount of maise grain harvested and plant height. In the first year of this study, it was noted that the significance level reached p ≤ 0.05 when the relationship between plant height was tested (

Table 10. Results of ANOVA test for number of grains harvested and plant height (for a significance level of p ≤ 0.05, the result is shown in red) (own study).

Variable under Study	Significance Level p in First Year of Study	Significance Level p in Second Year of Study
Quantity of grain harvested	0.139153	0.007452
Plant height	0.000003	0.000000
Dry matter content	0.000000	0.000000

). Thus, on this basis, it was concluded that the fertiliser significantly affected the results obtained. On the other hand, in the second year of this study, the results were statistically significant for all the traits tested.

As a result of the tests, it became apparent that not all the averages associated with the fertilisers used were equal. Therefore, a post-hoc test, the Tukey Test, was applied to isolate statistically different averages (

Table 11. Results of the Tukey Test (results highlighted in red indicate means that are statistically different at p ≤ 0.05) (own study).

Quantity of Grain Harvested
First year of study
	Fertiliser	1	2	3	4	5
1	Digestate		0.574081	0.119222	0.202293	0.386015
2	Digestate + NPK	0.574081		0.748282	0.909924	0.995848
3	Digestate + NPK + micronutrients	0.119222	0.748282		0.995811	0.909640
4	NPK	0.202293	0.909924	0.995811		0.986975
5	NPK + micronutrient control	0.386015	0.995848	0.909640	0.986975
Second year of study
	Fertiliser	1	2	3	4	5
1	Digestate		0.088697	0.005314	0.021103	0.040396
2	Digestate + NPK	0.088697		0.394353	0.878812	0.984503
3	Digestate + NPK + micronutrients	0.005314	0.394353		0.878812	0.668698
4	NPK	0.021103	0.878812	0.878812		0.992448
5	NPK + micronutrient control	0.040396	0.984503	0.668698	0.992448
Plant height
First year of study
	Fertiliser	1	2	3	4	5
1	Digestate		0.148652	0.955973	0.000202	0.001920
2	Digestate + NPK	0.148652		0.378238	0.000177	0.000226
3	Digestate + NPK + micronutrients	0.955973	0.378238		0.000186	0.000848
4	NPK	0.000202	0.000177	0.000186		0.074699
5	NPK + micronutrient control	0.001920	0.000226	0.000848	0.074699
Second year of study
	Fertiliser	1	2	3	4	5
1	Digestate		0.044766	0.395070	0.000178	0.000886
2	Digestate + NPK	0.044766		0.582223	0.000176	0.000182
3	Digestate + NPK + micronutrients	0.395070	0.582223		0.000176	0.000231
4	NPK	0.000178	0.000176	0.000176		0.014398
5	NPK + micronutrient control	0.000886	0.000182	0.000231	0.014398
Dry matter content
First year of study
	Fertiliser	1	2	3	4	5
1	Digestate		0.000177	0.000176	0.000177	0.000176
2	Digestate + NPK	0.000177		0.000176	0.000176	0.082564
3	Digestate + NPK + micronutrients	0.000176	0.000176		0.000176	0.000186
4	NPK	0.000177	0.000176	0.000176		0.000176
5	NPK + micronutrient control	0.000176	0.082564	0.000186	0.000176
Second year of study
	Fertiliser	1	2	3	4	5
1	Digestate		0.000413	0.000176	0.000276	0.000276
2	Digestate + NPK	0.000413		0.001204	0.000176	0.965942
3	Digestate + NPK + micronutrients	0.000176	0.001204		0.000176	0.002657
4	NPK	0.000276	0.000176	0.000176		0.000176
5	NPK + micronutrient control	0.000276	0.965942	0.002657	0.000176

). Considering the amount of maise grain harvested in the second year of this study, it was noted that the mean after the application of the digestate was significantly different from the mean after the application of the digestate + NPK, NPK, and NPK + micronutrient control. A significant variation in the statistically significant results was noted for plant height.

A correlation analysis was used to determine the interrelationships between the variables depending on the fertiliser used. A correlation coefficient scale, according to Stanisz [49], was adopted:

• rxy = 0—variables are not correlated;
• 0 < rxy < 0.1—weak correlation;
• 0.1 ≤ rxy < 0.3—weak correlation;
• 0.3 ≤ rxy < 0.5—average correlation;
• 0.5 ≤ rxy < 0.7—high correlation;
• 0.7 ≤ rxy < 0.9—very high correlation;
• 0.9 ≤ rxy < 1—correlation almost certain;
• rxy = 1—correlation certain.

Table 12. Correlation analysis of variables after the application of a type of fertiliser (own elaboration).

Fertiliser	Quantity of Maise Grain Harvested—Dry Matter Content	Quantity of Maise Grain Harvested—Plant Height	Quantity of Maise Grain Harvested—Flask Seating Height	Dry Matter Content—Plant Height	Dry Matter Content—Flask Seating Height	Plant Height—Flask Seating Height
First year of study
Digestate	0.951199	−0.123258	0.197431	0.188982	−0.114708	−0.997176
Digestate + NPK	0.623842	0.623842	−0.469310	1.000000	0.397360	0.397360
Digestate + NPK + mikroelementy	0.988519	0.846167	0.036710	0.755929	−0.114708	0.563621
NPK	0.509771	0.997621	−0.979778	0.449252	−0.327327	−0.991241
NPK + micronutrient control	0.952158	0.986050	0.919830	0.989743	0.755929	0.841698
Second year of study
Digestate	−0.67843	−0.980059	−0.297024	0.810885	−0.5	0.101361
Digestate + NPK	0.973936	−0.956864	−0.999968	−0.866025	−0.972088	0.959161
Digestate + NPK + mikroelementy	−0.408848	0.971793	0.800001	−0.182092	−0.874639	0.635934
NPK	0.895558	0.984045	0.895508	0.960434	0.603957	0.802036
NPK + micronutrient control	0.895558	0.984045	0.895508	0.960434	0.603957	0.802036

shows the results of the correlation analysis of the variables after the application of a particular type of fertiliser. Analysing the amount of maise grain harvested and the height of the plant after the digestate’s application, it was noted that a significant negative correlation exists between the amount of maise grain harvested and the height of the plant in the second year of this study, and this result shows an almost certain correlation. Thus, on this basis, it can be concluded that the higher the plant height, the lower the grain harvested. There is a significant negative correlation between the height of the cob establishment and the height of the plant in year 1 of this study, and this obtained result shows an almost certain correlation. Therefore, on this basis, it can be concluded that the higher the plant height, the lower the cob seating height.

When the plants were fertilised with digest + NPK in the second year of this study, it was also found that there is a statistically significant negative correlation that is almost certain between plant height and the amount of grain harvested. When plants were fertilised with digestate + NPK in the first year of this study, it was found that there is a statistically significant positive and definite correlation between plant height and the dry matter content. In contrast, in the second year of this study, it was noted that as the amount of the maise grain harvested increased, the Flask seating height decreased.

The extracted variables characterising maise after the application of digestate + NPK + micronutrients show statistically significant correlations, as in previous studies, only in the second year of this study.

In the case of the NPK fertilisation of maise, there is a positive correlation that is almost certain between the amount of grain harvested and plant height.

The variables studied after the fertilisation of maise with NPK + micronutrients are characterised by positive correlations between plant height and the amount of grain harvested.

Among the factors influencing maximum maise yield are variety selection (30%), cultivation treatments (40%), and climatic conditions (30%). The grower has total influence on the first of these two factors. In order to take full advantage of the possibilities offered by plant fertilisation, it is first necessary to know the chemical parameters of the soil in question and the energy needs of the crop to be grown. With this information, the right amount of fertiliser needs to be applied. In the case of natural fertilisers such as digestates, it is also necessary to know their chemical composition. This makes it possible to correctly select the amount of digestate necessary for a given area and to determine the need to supplement the digestate with another type of fertiliser.

4. Conclusions

Thanks to skilful management and the application of various agro-technical measures, including, above all, fertilisation, it is also possible to some extent to regulate weather conditions favourably and to contribute to increasing the efficiency of agricultural activity, as well as to improve the surrounding environment, given that the use of the digestate for fertiliser purposes makes it possible to close the nutrient cycle in the agricultural production space, which is a part of the principle of sustainable development. The use of a beneficial digestate reduces the consumption of mineral fertilisers and limits the extraction of the fossil raw materials (phosphate and natural gas) used in fertiliser production [50]. It is also seen as a method of reducing greenhouse gas emissions [51].

The fertiliser usefulness of the digest was confirmed based on the tests carried out. The plants fertilised with it were characterised by the highest grain yield (434 kg on average). In addition, they were characterised by an adequate plant height (3.15 m on average). The observations also indicate good emergence and satisfactory early vigour. Studies have not shown the superiority of artificial fertilisers over natural fertilisers, and by fertilising maise with digestates, yields are no worse than with other types of fertilisers. An additional attribute of using digestates in liquid form is the speed of delivery of nutrients to the plant.