*2.1. Study Area*

The study was conducted within the UPM, a governmen<sup>t</sup> tertiary institution in Serdang, Selangor, Peninsular Malaysia (Figure 1a), a land surface of about 1108 hectares made up of different natural and artificial facilities in buildings, agricultural lands, trees, and lakes. The entire area is within latitude: 2◦5934.19 N and longitude: 101◦4216.79 E. It is mostly warm with a sunny tropical rainforest climate and abundant rainfall, particularly in the northeast monsoon season between October and March. It has a constant annual temperature with a maximum between 31 and 33 ◦C, while minimums are usually between 22 and 23.5 ◦C. The average yearly rainfall is about 2400 mm, with relative dryness between June and July. The geology of Serdang consists of three different rock formations, which include the Kajang, Kuala Lumpur, and Kenny hills formations. Kajang formation consists of schist and some intercalations of limestone and phyllites. On the other hand, the Kuala Lumpur formation consists of limestones with intercalation of phyllite, while quartzes and phyllite make up the Kenny hills formation [79]. The experiment was carried out on the 11 August 2020 on a 65◦ slope gradient within 2◦5934.3 N 101◦4330.1 E. The area's soil is a mix of sandy, clay, and loam collectively identified under Serdang and Malacca. The surrounding vegetation in the site consisted of native and introduced lower grasslands species. The land is irregular, highly degraded with a uniform slope subject to severe erosion, and prone to landslide, with no vegetation. Before carrying out the field tests, a calibration test was conducted to identify the machine calibrations and settings. This test showed that the spray mechanism was reliable and ready to go with the tests.

The site was cleared of stumps, rock debris, stones, and unwanted materials of more than 4–5 cm (Figure 1c). A hand rake was used to rough grade the entire area to uncover debris and level the site. Initial tiling to a depth of 5 cm was done to reduce earth compaction, allow bonding of the topsoil and the subsoil, and improve vegetation root penetration and groundwater movement. We also studied the soil's physical and chemical properties to reveal the soil nutrient level, pH, acidity, and alkalinity level. The site was measured and marked and ready for hydroseeding. Table 1 presents the description of the study area.

**Figure 1.** Study area (**a**) map of Malaysia; (**b**) LiDAR image of the study area; (**c**) hydroseeded site.

**Table 1.** Site description of the study area.


Physical and Chemical Properties of Serdang and Malacca Series

The soil of the study area was predominantly sand. The soil textures were primarily sandy clay loam to sandy clay. The clayey property of the soil increased slightly with depth. This is one of the characteristics of the Serdang series, developed from sandstone parent material [80]. The Serdang and Malacca Series consisted of topsoil at 0.1 m and 1.5 m subsurface depth. At a depth of 1.5 m, the value of the soils of the Serdang series was about 57.21% sand, 10.21% silt, and 32.53% clay. For the Malacca series, at between 1.5 m, the values of the soils samples collected were about 41.58% sand, 2.94% silt, and 55.49% clay.

The physical and chemical properties of soils depend primarily on soil texture, which gives more ideas on soil classification (a nutrient required by plants), crop suitability, and soil interpretation [81]. The particle size distribution of the Malacca soil series observed in the study area reveals that the Malacca series's soil is dominated by clay. Sand content decreases with depth while clay content increases with depth. High clay content on the B horizon shows that illuviation has occurred and that the Malacca series contains more ferromagnesium minerals, which produces more clay [82]. The soil had a lower bulk density on the topsoil, which means that the soil organic matter content was lesser on topsoil with values between 1.47 and 1.42 grcm<sup>−</sup><sup>3</sup> and from 1.48 to 1.49 grcm<sup>−</sup><sup>3</sup> and 1.44 to 1.45 grcm<sup>−</sup><sup>3</sup> on the subsoil. The bulk density of the Serdang series was moderate and can improve crop production. The value of bulk density of the Malacca series was 1.36 and 1.27 grcm<sup>−</sup><sup>3</sup> at the topsoil, and between 1.37 to 1.41 grcm<sup>−</sup><sup>3</sup> and 1.30 to 1.32 grcm<sup>−</sup><sup>3</sup> on subsoil. The bulk density was higher on the subsoil. This is due to the compaction of soil at the oxic horizon as a result of the illuviation process. It could also be a result of the decrease in organic matter content with depth.

The water holding capacity of the topsoil and subsoil at 33 kPa ranged from 23.02 to 16.61 *v/v* and 21.16 to 17.01 *v/v* while the water holding capacity of the topsoil and subsoil at 1500 kPa ranged from 12.59 to 7.58 *v/v* and 11.97 to 8.36 *<sup>v</sup>*/*<sup>v</sup>*. The value of soil moisture content at the potential 33 kPa (FC) and 1500 kPa (PWP) was higher on topsoil than subsoil. More organic matter content in the topsoil increases the soil's ability to hold water. At 33 kPa, the water holding capacity of the Malacca series' was 23.83% to 21.51% *v/v* on the topsoil and 23.72% to 21.81% *v/v* on subsoil, while at 1500 kPa, water was held from 10.75% to 10.77% *v/v* on topsoil and from 10.29% to 13.23% *v/v* on the subsoil. The low available water holding capacity is due to low organic matter, which reduces the specific surface area of the soil. Both soil series show very strong to moderate pH values from 4.99 to 4.90 and 5.68 to 5.22. The pH value of the topsoil is higher than the subsoil, probably as a result of the H+ supply to the soil by organic matter. The pH values were between 4 and 5 and are considered low. This value is typical for tropical soil, where soil erosion, weathering, and leaching are considered very high. The pH of Ultisols in Malaysia is acidic below 7, which ranges from 4 to 5 in the B horizon due to climatic conditions that wash away the soil's cations such as Ca2+ Ma2+, P, K, and Na, and causes the accumulation of sesquioxide and has an impact on low productivity [83]. Table 2 summarizes the properties of the soil of the study location.

**Table 2.** The soil physical and chemical properties of the study area.

