Different organic manure sources and NPK fertilizer on soil chemical properties, growth, yield and quality of okra

20 Feb.,2023


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The different organic manures increased the soil organic matter (SOM), N, P, K, Ca and Mg contents of the soil compared with control. This result is consistent with the analysis recorded for the amendments in the present study that they contain these nutrients and also attested to the fact that the soil was deficient in these nutrients (Table 1). These nutrients were released into the soil as the organic manures were decomposed. Studies by13,14 have shown that animal manures and green manure increased soil OM, N, P and CEC and this was attributed to the availability and adequate supply of organic matter. The slightly lower pH of organic amended soil compared with the control could be due to the fact that during microbial decomposition of the incorporated manures, organic acids may have been released, which neutralized the alkalinity of the manures, thereby lowering the pH of the soil below their initial values. Adekiya et al.4 observed a similar trend in their work on organic amendments of soils. NPK fertilizer has the lowest pH as a result of leaching of bases from soil surface. Poultry manure significantly produced the highest soil chemical properties, this could be related to its lowest C:N ratio, lignin and lignin: N ratio (Table 2) which favours quick mineralization and release of nutrients to the soil compared with other soil amendments. Consequently, the quality of an organic amendment is defined in term of the relative content of nutrients (especially nitrogen), lignin, lignin: N and the C:N ratio14. Rabbit manure increased SOM compared with other organic manures. The increase in SOM of rabbit manure could be related to its high C:N and lignin content. Plant constituents, such as lignin retard decomposition. Organic materials with high C:N and lignin generally would favor nutrient immobilization, organic matter accumulation and humus formation14.

In both years, poultry manure increased N, P, K, Ca and Mg compared with other manures. This was due to the fact that poultry manure was low in C:N ratio, lignin and lignin/N and high in N, P, K, Ca and Mg compared with other manures. Due to high quality of this amendment, it decomposes quickly and release nutrients to the soil. The significant increase in soil nutrients especially OM, N, P and K in organic amendments compared with NPK fertilizer was due to leaching in NPK fertilizer treated plots. The reduction in Ca and Mg in NPK fertilizer plots compared with organically amended plot was due to the fact that NPK fertilizer did not contain Ca and Mg. The reduction of soil bulk density observed in both years with organic manures compared with control and NPK fertilizer could be attributed to increase in soil organic matter resulted from the degraded organic residues by soil microorganisms. Organic matter is known to improve soil structure, aeration and reduce soil bulk density15.

The decreasing order of okra yield were poultry manure > green manure > pig manure > cow dung > NPK fertilizer = rabbit manure > control. The positive effect of organic manure on growth and okra yield could be due to the contribution made by amendments to fertility status of the soils as the soils were low in organic carbon content. Manure when decomposed increases both macro and micro nutrients as well as enhances the physio-chemical properties of the soil for the betterment of okra growth.

Okra grown on poultry manure performed better in terms of growth and yield compared with other sources of organic soil amendment and NPK fertilizer. This could also be related to low C:N ratio, lignin and lignin/N values These attributes of poultry manure will lead to fast mineralization and early release of nutrients to a short gestation crop like okra, hence there was a boost in the morphological growth of the plant which translate to greater yield compared with other amendments. Wolf and Snyder16 reported that C:N ratio of organic materials markedly influences the decomposition rate and the mineralization of N because N determines the growth and turnover of the microorganisms that mineralize organic carbon.

The reduced growth and yield of okra in plots treated with other manures in comparison with poultry manure could be as a result of immobilization of soil nutrients which occurs when soils are treated with animal manure of high lignin contents which results from the feed eaten by the animal17.

Higher yields were observed in poultry manure plots compared with inorganic fertilizer because, the nitrogen content of poultry manure is released to the soil gradually and steadily over longer time for the growth of the plant compared with nitrogen from NPK fertilizer which is prone to losses by run-off, volatilization, leaching and/or denitrification. Poultry manure has been said to be a better soil amendment compared with chemical fertilizers because of the greater capability of poultry manure to preserve its N18. The superior N supply by poultry manure during okra cropping in this experiment may be the reason for better growth and yield of okra in plots with poultry manure18. The obtained results corroborated the finding of19 that poultry manure increased the height of okra relative to other amendments.

The better performance of okra under NPK fertilizer plots compared with the control was due to release of nutrients (N, P and K) from the fertilizer which are absorbed by the okra plants. Okra growth and yield in second year (2018) was better than that of first year (2017). This was due to the differences in the amount of rainfall between the two years (Table 6). Year 2017 had 1238 mm of rainfall while it was 1428 mm in 2018. There was higher moisture in the first few weeks (month of May) of incorporation of the manures in 2018 compared with 2017 which may have led to better and quicker decomposition of the organic materials in 2018. Soil biological activities which causes degradation of organic materials is severely limited during limited moisture, but with the onset of the rains (2018), there is a flush in microbial activity20.

Table 6 Meteorological data of the study area.

Full size table

The fact that organic manures and NPK fertilizer increased okra mineral contents compared with the control was attributed to increased availability of the nutrients in soil as a result of the mineralization of the manures leading to increased uptake by okra plants. In both years, the correlation coefficient between soil and okra fruit N, soil and okra fruit P, soil and okra fruit K, and okra fruit Ca, soil and okra fruit Mg were all significant with R values of 0.83, 0.71, 0.66 0.81 and 0.88, respectively at P < 0.05. Poultry manure had the highest values of N, P, K, Ca and Mg in the okra fruit compared with other amendment and NPK fertilizer. This result is also consistent with the soil chemical properties and growth and yield for this treatment. The poultry manure may have improved the availability of nutrients to the crop by enhancing the mineralization and supply of readily available nutrients to the soil21.

The increased nitrogen to the soil by the incorporation of organic manures increased the nitrogen uptake by the okra fruit thereby increasing protein. This explains the reasons for increase in crude protein values between organic amendment and control and NPK fertilizer treatment. Poultry manure has the highest value of crude protein, this is also consistent with its high soil N level (Table 3). Nitrogen is a major constituent of chlorophyll, protein, amino acids, various enzymes, nucleic acids and many other compounds in the cell of plants22. The crude protein contents in all the organically amended soils were above the critical 13–17%23,24. Therefore the organic materials sustained good nutritive quality of okra as opposed to NPK and control. The ash content in poultry manure treatment was significantly higher possibly because of the balanced nutrient in the manure, unlike NPK with inferior contents of N, P, and K and the control with lower concentration of nutrients. In this study, poultry manure gave the highest N, P and K concentrations of 3.04%, 11.9% and 1.16% in 2017 and 4.41% 13.4% and 1.87% in 2018 (Table 6) respectively. The fat content of poultry manure was lowered compared with others, this was due to its high protein content of poultry manure. It has also been reported25 that there is a negative correlation between fat and protein content. High nitrogen application reduced fat and increased protein content. Organic amendments and NPK fertilizer increased the mucilage contents of okra compared with the control. This could be as a result of increase in NPK from these amendments. This could be attributed to the increase in d-galactose, l-rhamnose and d-galacturonic acid contents in okra fruits by the application of nutrients through organic and inorganic sources which might have resulted in increase of mucilage content26. The mucilaginous polysaccharide in the okra is rich in uronic acid (65%) and consists of rhamnose, galactose, glucose, galacturonic acid and glucuronic acid in addition to 3.7% acetyl groups27. Ahmad et al.27 also reported that compost and NPK fertilizer application increased the mucilage of borage plant. The highest value of mucilage in poultry manure could be related to increased soil nutrients compared with other treatments. Okra produced under organic amendments has better qualities (N, P, K, Ca, Mg, protein, ash and mucilage) compared with NPK fertilizer and control. This is because organic manures not only increase soil nutrients but also improves the physical by prevention of erosion and leaching of nutrients (in this experiment reduce bulk density) and biological properties28. Organic manures also contains both micro and macro nutrients unlike NPK fertilizer that contains only N, P and K. Lumpkin29 also, was of the opinion that organically produced vegetables were of higher qualities than those produced using conventional methods. Cropping of okra in 2018 reduced protein, ash, fat, carbohydrate, N, P, K, Ca and Mg contents of okra fruit compared with 2017 cropping. This could be adduced to high rainfall in 2018 compared with 2017. High rainfall has been reported to reduce the nutrient and proximate content of vegetables30. Climatic conditions have a great effect on the concentration of mineral in plants. Variation in temperature and rainfall have been reported to influence the chemical composition in plants30,31.

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