The Effect of Inoculation Phosphate Solubilizing Bacteria and Rhizobium sp. on Plant Growth and Production of Cowpea (Vigna unguiculata)

In Malang Regency, Mediterranean soil develops from limestone, calcareous sandstone, marl and basalt andesite. The Mediterranean land is found in Karst plains, tectonic plains, tectonic hills, lower volcanic slopes, old volcanic plains and old volcanic hills. Soil cross section is medium to very deep, good drainage, slightly fine to fine texture, slightly acidic to alkaline pH, medium to very high CEC and high to very high base saturation (BS), low to medium in C-organic and total N content., P O (medium to high) and total K O low to moderate (Yatno et al., 2017). Mediterranean soil has problems with being too 2 5 2 alkaline and low inorganic matter, macro and micronutrient levels as well as low water retention capacity and poor drainage (Tri Pamungkas, 2021). Red Mediterranean soil derived from limestone has a higher soil pH value than soil derived from sandstone. The main problem with Mediterranean soil is the high soil pH which will give rise to direct problems, namely suitability for plants and availability of nutrients, namely low availability of microelements and phosphate because they are bound by Ca2+ (Supriyadi, 2007). Soil quality and fertility in Mediterranean calcareous agricultural soils are very important to be managed so that soil productivity can be sustainable. The development of Mediterranean land requires pioneer plants that can increase soil fertility, one of which is legumes. Legume crops can be a practical solution for improving calcareous soils because they contribute to nitrogen fixation, organic matter accumulation, and improved soil health. The results of Tri Pamungkas' research showed that the addition of organic fertilizer significantly increased the growth of plant height, number of leaves, and stem diameter of sugarcane seedlings planted in Mediterranean soil. Legume crops are useful as human and animal feed, wood energy, and as soil-improving components of agricultural and agroforestry systems through its association with bio-fertilizers (Havugimana et al., 2016).

Cowpea is included in the minor legume group which has prospects for future development as a source of food, animal feed, green manure, industry, and health. It belongs to the Leguminosae family and as important legume crop growing across the world mainly intropical and subtropical regions (Papa et al., 2020; Kebede and Bekeko, 2020). It is classified as food crops, feed and industrial raw materials. According to Safitri et al. (2016) stated that cowpea seeds contain a lot of nutrition, every 100 gcontains 24.4 gprotein, 56.6 gcarbohydrates, 1.9 gfat, 481 mg calcium, 399 mg phosphorus, and 2.68 gphytic acid. Cowpea also has a higher vitamin B1 content than green beans. The potential for cowpea seeds is quite high, which can reach 1.5-2 tons/ha depending on the variety, location, growing season and cultivation applied. Cowpea has the advantage of having alow fat content so it can minimize the negative effects of fatty food products (Saputro et al., 2015). Cowpea is a leguminous plant that has also the ability to bind N from the atmosphere. The ability of cowpea to fix nitrogen 2 through biological nitrogen fixation is a cheap and sustainable alternative to inorganic fertilizers. This is because the performance of cowpea depends on the rhizospheric characteristics; hence, it is able to form a beneficial association with microorganisms present in the rhizosphere (Abdel-Fattah et al., 2016). The potency of N fixation by cowpea can be optimized 2 by inoculation of Rhizobium sp. Morphologically, cowpea roots spread to a depth of soil between 30-60 cm and can be in symbiosis with Rhizobium sp. to bind free Nitrogen (N ) from the air which then forms root nodules. Inoculation of Rhizobia 2 isolates significantly increased the number of root nodules, compared to plants that were not inoculated (Kebede et al., 2020). Symbiosis between cowpea and Rhizobium sp. can increase the availability of N to support plant growth and production. Rhizobium sp is one of the bacteria that is very useful for agriculture which acts as a nitrogen fixer from the atmosphere. Nitrogen is one of the main elements in the production of food crops, especially legumes. By fixing nitrogen symbiotically, a cheap source is obtained and can help reduce production costs, especially on infertile soils (Suryantini, 2015). The development of cowpea plants in calcareous areas has several obstacles related to the level of soil fertility. In the calcareous soil, high levels of calcium can also cause problems with the presence of soil potassium. Clay colloids can adsorb and exchange base cations depending on their valence. Cations with three and two valences are more strongly adsorbed by colloids compared to cations with one valence (Putra and Hanum, 2018). As a result, potassium which has a valence of one, will be suppressed by high Ca in calcareous soils which has a valence of two. In calcareous or normal soils, P gets immobilized by cations such as Ca2+ to form a complex calcium phosphate (Ca (PO ) ). Nearly, 70 to 90% of phosphorus fertilizers applied to soils is fixed by cations 3 4 2 and converted inorganic P (Walpola and Yoon, 2012).

Optimizing the role of cowpeas as a pioneer crop planted in Mediterranean soil requires agronomic practise, one of which is the application of biological fertilizer to help increase the availability of nutrients in the soil, especially elements of N and P to support growth and production. However, the availability of Pin the soil is lacking, even though the total amount of Pin the soil is high through repeated fertilization applications. This is due to the nature of P which is easily fixed by metals, colloids and calcium which makes it unavailable for plants. Agricultural soils in general also face constraints on the availability of P elements for plants. Even though fertilization has been done many times for each planting, because Pis tightly bound by soil colloids, the availability of P element in the soil is low. Likewise with soils that have high Al-ddcontent, generally also have constraints on the availability of P elements. Low availability of phosphorus (P) in calcareous soils can effect sustainability of improvement in cereals crops yield. A higher amount of calcium in calcareous soils precipitates the P, thus making it immobile in soil. Inoculation of phosphate solubilizing bacteria (PSB) could be helpful in the sustainable management of immobile Pin soil (Wahid et al., 2020).

An alternative to increase the efficiency of fertilization in overcoming P fixation by Al, Fe and Ca is by utilizing phosphate solubilizing microorganisms. Phosphate solubilizing bacterial inoculants are microbes given to plants for the process of dissolving P which is bound to become a form available to plants. It play an important role in the biogeochemical cycle of soil P, including dissolution, mineralization, and immobilization of P, thereby increasing P bioavailability and plant nutrition. Root-associated PSB, can modulate plant root systems and stimulate root absorptive capacity and nutrient uptake, including P acquisition (Bargaz et al., 2021). PSB facilitates the conversion of stable phosphorus to active phosphorus and increases the residual phosphorus fraction in compost (Sun et al., 2023). Bacillus subtilis is one of PSB which is able to decompose P bonds in the soil and can enhance plant P uptake through remobilization of recalcitrant forms of Pin soil. The results of research conducted by Batool and Iqbal (2019) on Triticum aestivum L., recommend the use of PSB as biofertilizer, as an alternative to chemical fertilizer.

Providing N and P nutrients in calcareous soil requires technology in the form of the use of biofertilizers which can support the availability of nutrients needed by Cowpea plants to support their growth and production. This study aims to determine the optimal concentration of application of Phosphate solubilizing bacteria and Rhizobium sp. to increase biology soil fertility and the growth of cowpea cultivated in Alfisol or Mediterranean soils.