Nutritional value of soybean under phosphorus fertilization
The method of fertilizer application affected the chemical composition of soybean seeds. The phosphate fertilizer placement methods used had a significant effect only on CF content and non-protein nitrogen compounds (NPM) (Table 2). Significantly more CF was found when fertilizers were placed brodcast, while for NPM this was the case with band fertilization.
The results are consistent with the working hypothesis that fertilization with STR would significantly increase the CA content. The highest CA content was found after STR application (6.73%) regardless of the fertilizer application site (p < 0.01). Seed content of other nutrients did not varry according to the source of P fertilization. The interaction of factors caused a modification in the majority of parameters and affected the nutritional value of soybean seeds, such as CA, CP, TP, CF, and EE content, and also GE value (Table 2).
Struvite fertilization, both band and broadcast, contributed to an increase in CA content, while it did not significantly increase CP content. The situation is different for TP. Broadcast fertilization with STR caused an increase in TP content.
According to our working hypothesis, phosphorus fertilization significantly affected protein value. Indeed, the lowest CP and TP values were recorded when STR was applied band. On the other hand, when STR was applied broadcast, CP and TP values were the highest (p < 0.001). When the fertilizer was applied broadcast, the proportion of TP was 97% and N-PNF was 3% in CP; however, when the fertilizer was applied band, the proportion of TP was 96% and N-PNF was 4% in CP.
In addition to CP values, fat content (EE) is an also an important characteristic. Soybean seeds are characterized by high CP (over 30%) and EE (over 20%). Analyzing the fat content (EE) of the seeds studied, a different correlation was found than that for protein (CP). Thus, EE was higher where STR was placed band, and lower where it was applied broadcast. Fat content affects gross energy in the seeds (the greater the gross energy, the higher the fat content). These results were reflected in the yield. Protein and fat yield were not dependent on the placement of phosphorus fertilizers or its type. Interaction between factors affected the protein yield with a positive tendency, while struvite application regardless of the its placement also affected this characteristic.. Analyzing the results, it can be assumed that broadcast application of P fertilization was insignificantly more advantageous compared to band application (Fig. 1).
The effect of phosphorus fertilization on the yield of protein and fat in the seeds of soybean. Control—without phosphorus fertilization; SUP—triple superphosphate as Super FOS DAR 40; STR—struvite as Crystal Green; band—P fertilizer placed 5 cm below seeds; broadcast—P fertilizer placed on surface of the soil.
The level of NFE compounds in the tested samples ranged from 23 to 28%. The content of this fraction was calculated on the basis of the determined basic chemical composition. This includes easily hydrolyzable sugars, such as simple sugars, disaccharides, polysaccharides (starch), and volatile fatty acids.
The biological value depends on the amount and type of fiber, among other factors. The crude fiber content ranged from 5.1% to 6.6%. Phosphorus fertilizer placement had a significant effect on this parameter. The highest content was found under broadcast fertilization.
There was a positive correlation between CF and EE (the higher the fat (EE) content, the higher the fiber (CF) content increases), and also between TC and CP (Table 3). A negative correlation was found for CP content and EE, as well as EE and TP.
To better explain the obtained chemical composition data for soybean seeds and similarities and differences observed between the samples, PCA was used, and the extracted results are presented in Fig. 2. The first PC explained 42.56%, and the second explained 23.30% of the total variance within the observed data.
The PCA biplot diagram, showing the relationships among chemical composition of soybean seeds. Method of placing the fertilizer: band—P fertilizer placed 5 cm below seeds; broadcast—P fertilizer placed on surface of the soil. Type of fertilizers: SUP—triple superphosphate as Super FOS DAR 40; STR—struvite as Crystal Green. DM dry matter, CP crude protein, CA crude ash, EE ether extract, CF crude fibre.
Amino acid composition of soybean under phosphorus fertilization
Soybeans have a high protein content (ca 33% CP) and also a favorable amino acid profile (Tables 3 and 4). Taking into account the biological value of feed material, the content of amino acids is pivotal. This is especially the case for the content of EAA that are important in monogastric nutrition. This is why expression of the content of an amino acid relative to lysine is a conventional means of expressing dietary protein quality for monogastric animals (Figs. 3 and 4). Soybean seeds are a good source of LYS, although it is relatively poor in MET. The first amino acid limiting the biological value in our study was VAL. In order, the next limiting amino acid was ILE and the sulfur amino acids MET with CYS.
The essential amino acid profile relative to lysine: (a) band; (b) broadcast. Type of fertilizers: SUP—triple superphosphate as Super FOS DAR 40, STR—struvite as Crystal Green.
Non-essential amino acid profile relative to lysine: (a) band; (b) broadcast. Control—without phosphorus fertilization; SUP—triple superphosphate as Super FOS DAR 40; STR—struvite as Crystal Green.
Fertilizer placement significantly affected LYS, LEU, VAL, PHE, and TYR. Significantly higher contents of these amino acids were found under band fertilization. Struvite significantly increased the content of PHE (Table 4). Band fertilization with STR caused a significant increase in ILE, LEU, TYR, HIS, LYS, CYS and PHY, MET.
The method of P fertilizer led to changes in GLU, GLY, ALA, and ARG content in the seeds (Fig. 4). A significantly higher content of GLY and ALA was found under broadcast fertilization, while GLU and ARG content increased with band fertilization. Phosphorus fertilizer had a significant impact only on ASP, PRO, and ARG content. Struvite caused an increase in ASP and ARG content, while it had no impact on PRO content. Interaction between the factors affected all the examined amino acids (Table 5). In the majority of cases, striuvite caused an increase in the amino acid content (ASP, SER, GLU, ARG) irrespective of fertilization method (Fig. 4).
The proportion of GLU increased by 2.5% in the seeds where STR fertilization was applied band. However, when broadcast fertilizers were applied, there was a decrease in this amino acid: by about 5% (with SUP application) and by about 4% (with STR application), respectively. In line with our working hypothesis, fertilization also contributed to an increase in the proportion of PRO: by about 11% (with SUP application) and by about 8.5% (with STR application). The increase in less valuable protein fractions may have influenced the lower values of indices determining the biological value of soybean seed protein (Table 5).
Amino acids (mainly EAA) determine the biological value of a protein. The sum of all amino acids averaged 89.56 g per 16 g N. Phosphorus fertilizer application had a significant impact only on the sum of essential amino acids (Table 6), with a higher content under band placement. Phosphorus fertilization had no significant effect on any sum of amino acids. Interaction between factors significantly affected all determined parameters. Struvite application using the band method significantly increased the content of the sum of all amino acids, sum of essential amino acids and non-essential amino acids. On the other hand, phosphorus fertilizer application caused a decrease in the above paramaters.
The CS ratio averaged 46% according to WE. The essential amino acids (EAA) averaged 43% of the total of all amino acids, as reflected in the high essential amino acid index (EAAI). This index, determined according to the accepted standard, was the highest for the band application and amounted to 71.8%. Based on the calculated indices (expressing the biological value of the protein), it can be concluded that the application of band fertilization caused an increase in EAA compared to their content in the standard protein (WE).
The method of placing the fertilizer significantly affected the following parameters: EAAI, PER 1, PER 2, PER 3, and BV. Phosphorus fertilization had no significant effect on the determined parameters. Taking into account the interaction between factors, STR applied band caused an increase in all the evaluated parameters. The calculated predicted PERs (PER 1—2.27%; PER 2—2.37%; PER 3—2.21%) confirm the highest nutritional value for soy protein when STR was applied band. A positive correlation was found between the selected amino acids as shown in Table 7.
For the graphical representation of the amino acid profile data for soybean seeds across the samples, PCA was used, and the extracted results are presented in Fig. 5. The first PC explained 32.54%, and the second explained 23.98% of the total variance within the observed data.
The PCA biplot diagram, showing the relationships among amino acid profiles of soybean seeds. Method of placing the fertilizer: band—P fertilizer placed 5 cm below seeds, broadcast—P fertilizer placed on surface of the soil. Type of fertilizers: SUP—triple superphosphate as Super FOS DAR 40, STR—struvite as Crystal Green. ASP aspartic acid, THR threonine, SER serine, GLU glutamic acid, PRO proline, GLY glycine, ALA alanine, CYS cystine, VAL valine, MET methionine, ILE isoleucine, LEU leucine, TYR tyrosine, PHE phenyloalanine, HIS histidine, LYS lysine, ARG arginine.
Macro- and microelements in soybean seeds under phosphorus fertilization
Placement of P fertilization had a significant effect on Ca, Na, and K content, with a higher content being noted with band fertilization. Phosphorus fertilization had a significant impact on Ca, Na, and P content in soybean seeds. In line with our working hypothesis, struvite caused an increase in P and Ca content. Interaction between factors had a significant impact on all examined elements. Broadcast fertilization with STR led to an increase in the N, Ca, and P content in soybean seeds (Table 8). Na content was found to be the highest in seeds when using in fertilization both while SUP and STR band fertilization.
Placement of P fertilizer had a significant impact on Fe and Zn content. In line with our working hypothesis, phosphorus fertilization in the form of STR caused an increase in Fe and Mn content in soybean seeds. Interactions between factors, in the case of Cu, Fe, and Mn with strivite applied broadcast, increased the content of these elements. However, the highest content of Zn was found in those pots where SUP was applied band (Table 9).
