Freshly ripened fruits were collected from beneath a tree domesticated in a farmer’s agricultural field in the Chivi district of Zimbabwe at 20° 24′S, and 30° 13′E in May 2020. The tree had been retained on the farm for its big fruits with high juice content. The fruits were brought to the laboratory at the Midlands State University where they were peeled and pulp was removed from the seeds. The seeds were washed, dried, and then stored at room temperature until setting up the trials. Before germination, the viability of the seeds was determined using the dens metric test with tap water to remove the nonviable seed.

A greenhouse experiment was conducted at Midlands State University Kwekwe farm. The area falls under Natural Region III of Zimbabwe’s Agro-ecological zones (Manatsa et al., 2020) and is located 18 ̊ 55̍ 04.5 ̊ S, 29 ̊ 45̍ 03.2 ̊ E with an altitude of 1355 m above sea level. The average annual rainfall is 883mm, and the mean temperature is around 23 °C–30 °C.

To perform initial soil characterization, twenty soil samples were collected from randomly selected points of the greenhouse using a soil auger at a depth of 20 cm. The twenty soil samples were then mixed thoroughly in a clean plastic bucket to obtain a composite sample. The composite sample was air-dried, sieved (<2 mm) and characterized (Table 1). The content of soil organic carbon (Table 2) in the soil sample was measured using the Walkely and Black method (Nelson and Sommers, 1996). The texture class of the soil sample was measured using the Bouyocous hydrometer method (Bouzoukis, 1962). The pH of the composite soil sample was measured by weighing a 15 g soil sample in a 200 mL honey jar and adding 75 mL 0.1M CaCl₂ to the sample. The mixture was shaken for 30 min and pH was determined using a digital pH meter (Model: Orion 701, Orion Manufacturing, MI, USA). The total nitrogen content was calculated through the Kjeldahl method using concentrated H₂SO₄, K₂SO₄ and HgO to digest the sample (Bremmer, 1996).

The bulk density of the experimental soil sample was measured using the core method (Blake and Hartge, 1986). Twenty soil cores were randomly collected from the experimental greenhouse for bulk density determination. Bulk density (Db) was calculated using the following formula: Db = MsVt

Where Ms is the mass of oven-dry solids and Vt is total soil volume. The soil cores were oven-dried at 105 °C (to constant weight) to ascertain the mean gravimetric water content.

Taking particle density (Pd) of soil to be 2.65 g cm^-3 total porosity was calculated as: 1 –  DbPd

The different combinations of topsoil, sand, and cattle manure were the treatments. Treatment 1 comprised 5 parts topsoil, 1 part sand and 4 parts cattle manure (5 TS:1 S:4 CM), T2- 6 parts topsoil, 1 part sand and 3 parts cattle manure (6 TS:1 S:3 CM), T3- 4 parts topsoil, 2 parts sand and 4 parts cattle manure (4 TS:2 S:3CM), T4- 5 parts topsoil, 2 parts sand and 3 parts cattle manure (5 TS:2 S:3 CM), T5- 4 parts topsoil, 4 parts sand and 2 parts cattle manure (4 TS:4 S:2 CM). T6 was topsoil only (TS), T7-cattle manure only (CM), and T8 – only sand (S). Trenches with dimensions 30 cm length x 10 cm width x 10 cm depth were dug and filled with the different cattle manure amended soil. The trenches were dug along drip lines to ensure that each plant got uniform and adequate moisture. Irrigation was done for 1 hour twice everyday. Each trench represented an experimental plot. The treatments were arranged in an RCBD and replicated four times. Twelve marula seeds were sowed into each experimental plot 5 cm apart, at a depth of 5cm, and then covered with the respective media.

Germination parameters were observed at weekly intervals until no further germination was observed. A seed was considered to have germinated where there was a visible protrusion of split seed coat with the cotyledons, hypocotyls, and epicotyl above the soil’s surface. Weekly germination percentages were calculated and summed up to get cumulative germination for each treatment using the formula: Germination Percentage = Number   of   seeds   germinated Total   number   of   seeds   sown × 100

At the end of the germination experiment, we calculated the germination index (GI) as described by the Association of Official Seed Analysis (1983): Germination Index = Number   of   germinated   seeds Days   to   the   first   count + Number   of   germinated   seeds Days   to   the   last   count

The rate of germination was calculated using a modified formula from Islam et al. (2009): Rate of germination = Number   of   seeds   emerged   14   days   after   planting Number   of   seeds   emerged   21   days   after   planting × 100

The collection of germination data was stopped when no further germination was observed for a sustained period of 21 days. Seedling growth performance was monitored from 4 weeks after emergence up to 12 weeks after emergence to assess the effect of soil media composition on plant growth. Five seedlings were randomly selected in each plot and seedling height and the number of leaves was measured and counted after every 4 weeks. Seedling height was measured using a string and ruler to the nearest cm from below the cotyledons. At the end of the experiment (12 weeks after planting), three plants were uprooted in each experimental unit and the length of the roots was measured. The vigor index was calculated as final germination × total plant length.

Analysis of Variance (ANOVA) was done on data of all germination and growth parameters using GenStat 18th Edition, and differences between treatments means were separated using the Least Significant Difference (LSD) test at the 5% significance level.

There was a significant difference among the different soil media compositions in germination percentage (p < 0.001) 2 weeks after planting. The highest germination percentage was observed in T7 (cattle manure only) (45.75%), while the lowest (%) was in T1 (5 parts topsoil: 1 part sand: 4 parts manure) (Figure 1). Again, there was a significant difference in germination percentage (p < 0.001) 3 weeks after planting. The highest germination percentage at 3 weeks was T3 (4 parts topsoil: 2 parts sand: 4 parts cattle manure) with 58%, and the lowest was T1 (5 parts topsoil: 1 part sand: 4 parts manure) with 12.66% (Figure 1). Finally, there were significant differences (p < 0.001) in germination percentages 4 weeks after planting among different soil media compositions. The highest germination percentage was 92.25%, recorded in T7 (cattle manure only). The lowest germination percentage was 29% recorded in T1 (5 parts topsoil: 1 part sand: 4 parts manure) (Figure 1). Thereafter, there was no further germination for the next 3 weeks.

The effects of media composition on the germination index were significant (p < 0.001). The highest germination index (0.825) was recorded in seeds sown in T7 (cattle manure only) and the lowest (0.1875) in T1 (5 parts topsoil: 1 part sand: 4 parts manure) (Table 3). In addition, there was a significant difference (p < 0.001) in seedling germination rate on different media compositions. Seeds that were sown in T7 (cattle manure only) emerged fastest (50%), while those sown in T2 (6 parts topsoil: 1 part sand: 3 parts cattle manure) had the lowest (17.67%) rate of emergence (Table 3). The effects of media composition on the vigor index were significant (p < 0.001). The highest vigor index (59.38) was recorded in seeds sown in T7 (cattle manure only) and the lowest (16.2) in seeds sown in T1 (5 parts topsoil: 1 part sand: 4 parts manure) (Table 3).

At 4 weeks after planting, the effects of growth media composition on plant height were not significant (p = 0.062). However, at 8 weeks after planting, the effects of soil media compositions on plant height were significant (p < 0.001). Seedlings growing in T4 (5 parts topsoil: 2 parts sand: 3 parts cattle manure) recorded the highest plant height (46.25 cm), and T1 (5 parts topsoil: 1 part sand: 4 parts manure) recorded the lowest plant height (31.75 cm) (Figure 2). At 12 weeks after planting, the effect of soil media composition on plant height (final height) was significant (p < 0.05). Seedlings growing in T4 (5 parts topsoil: 2 parts sand: 3 parts cattle manure) had the highest final plant height (57.5 cm), while the lowest plant height (46.75 cm) was recorded in T1 (5 parts topsoil: 1 part sand: 4 parts manure) (Figure 2).

There were no significant differences in the number of leaves from different growth media compositions at four, eight (p = 0.117), and twelve (p = 0.473) weeks after emergence.

The effect of growth media composition on root length was significant (p < 0.001). The longest root length (11.05 cm) was of seedlings grown in T4 (5 parts topsoil: 2 parts sand: 3 parts cattle manure), and the shortest (8.60 cm) was recorded in T2 (6 parts topsoil: 1 part sand: 3 parts cattle manure) (Figure 3).

This study shows that growth media compositions influence seed germination percentage and the rate of germination. The early and better germination percentage, index, and rate of germination in the cow dung slurry treatment might be because of growth promoters present in cattle manure such as bacteria, yeast, actinomycetes, and certain fungi have been reported in cow dung which help in softening the seed coat hence increasing the permeability of the seed to water (Basavaraj et al., 2002; Lokesh, 2007; Swain and Ray, 2009). The cattle manure also improves physical conditions such as porosity and water holding capacity and helps maintain soil temperatures, enhancing higher germination percentages and emergence rates (Srivastava, 1998). This also explains the high germination percentages and rate of emergence found in seeds grown in the medium composed of 4 parts topsoil: 2 parts sand: 4 parts cattle manure. The presence of growth-promoting substances (auxins) and nutrients in the cattle manure gives seedlings emerging in this treatment a competitive advantage in early growth, survival, and development (Sankaranarayanan et al., 1994; Swamy et al., 1999). Seeds that were grown in a medium of sand only had a relatively low final germination percentage (42%). This might be due to the poor water holding capacity of sand. These results do not agree with the findings of Dolor, (2011), who observed high germination percentages in Irvingia wombolu seeds grown in river sand. Anber, (2010) also reiterated the high germination percentage observed in sand media in a study on Bauhinia variegate and Delonix regia. In a growth medium composed of sand and cattle manure, the properties of the soil’s biology, physics and chemistry are improved, leading to increased germination and growth, as the organic matter acts as a glue leading to soil aggregation and also increases the fertility of the soil (Das et al., 2017). This can help explain the high final germination in the medium with 4 parts topsoil, 2 parts sand and 4 parts cattle manure where the sand component helps make the medium more penetrable by the radicle and roots of the plant. In contrast, the cattle manure component provides the needed nutrients to support plant growth.

Seedling height in Sclerocarya birrea seedlings is influenced by the volume of soil organic matter, as Okunomo et al., 2000 stated, who recorded higher plant heights in plants grown in a medium of poultry droppings than in topsoil, sawdust, and clay soil with Pittosporum bicolor. Yadav and Sidhu, 2016 also supported this when they reported that the addition of organic matter to the soil medium increased the availability of nutrients that facilitate plant growth. This results from the increased photosynthetic activity, partitioning of photosynthates, and improved permeability of the cell membrane in the plants (Shukla et al., 1997). The high plant heights observed in sand media confirm the findings of Anber (2010), who also got the highest seedling heights of Bauhinia variegata and Drosera regia in the sand. Leaf production and growth are important indicators of the plant’s efficiency in trapping sunlight for photosynthesis (Detpiratmongkol et al., 2014). The cattle manure component of the soil media increases the carbon, nitrogen, pH, Cation exchange capacity, and exchangeable Ca, Mg and K in the soil, consequently enhancing vegetative growth (Ayoola and Makinde, 2008).

The longer root lengths of seedlings grown in the media with 4 parts topsoil, 2 parts sand and 4 parts cattle manure and 5 parts topsoil, 2 parts sand and 3 parts cattle manure are because of the efficient assimilation and partitioning of assimilates within the marula seedlings, which promotes luxurious growth of above and below-ground parts of the tree (Pandiyan et al., 2011). Water retention capacity and nutrition retention in the topsoil and cattle manure components in 4 parts topsoil, 2 parts sand and 4 parts cattle manure and 5 parts topsoil, 2 parts sand and 3 parts cattle manure could be the reason for the higher root lengths of these treatments (Malamy and Ryan, 2001). A closer observation of the results shows that plants that are grown in the sand-only medium (T8) and media with a higher proportion of sand 4 parts topsoil, 4 parts sand and 2 parts cattle manure also recorded longer root length, and this can be because of the porosity of the sand and the media with a high percentage of sand. The higher percentage of topsoil in the medium with 6 parts topsoil, 1 part sand and 3 parts cattle manure and topsoil only medium give the media higher water holding capacities. This unfavorable characteristic leads to reduced oxygen diffusion to the root and consequently reduced water availability if the matric potential is too low and mechanical impedance if the soil is too compacted or dry. Hence, the slower root growth compared with other treatments (Chen et al., 2010).