The natural forest and cashew plantation are located in Agu Eke (Eke bush) in Etti village, Nanka, while the oil palm plantation is located at Umunnebo village, Ufuma, both in Orumba North Local Government Area, Anambra State.

The natural forest and cashew plantation are located in Agu Eke (Eke bush) in Etti village, Nanka ( Figure 1 ), Orumba North Local Government Area, Anambra State. The study area is situated in Orumba North LGA ( Figure 2 ), in Anambra State, Nigeria. Figure 3 shows the location of Anambra State in Nigeria. The geographic coordinates of Nanka are 6˚3'26.64" North, 7˚2'53.40" East and 6˚2'5.35" N 7˚5'1.84" E ( Ukatu, 2019 ). Its neighbours are Oko, Agulụ, Ekwulọbia, Aguluzọigbo, Isuọfia, Umuọna, and Awgbụ. Nanka comprises of seven villages including Agbiligba, Enugwu, Ifite, Amakor, Umudala, Ubahu, and Etti, in that order ( Ukatu, 2019 ).

The geological setting in the study area is that of layered sequences in which a predominantly sandstone formation is underlain by a predominantly shale formation ( Obiadi et al., 2011 ). The area lies between the tropical rain forests which dominate nearly half of southern Nigeria and is characterized by luxuriant vegetation and abundant plant species but many parts have been subjected to severe deforestation due to anthropogenic activities thereby reducing the area to savannah vegetation in many parts ( Obi & Okekeogbu, 2017 ). It is bounded by fresh water swamp forest in the south and Guinea Savanna in the North, although the area is marked by continuous growth of trees, shrubs and climbing plants, the land is almost bare during the dry season exposing critical areas of the slopes to the impacts of rains at the commencement of wet season ( Igwe, 2018 ). The vegetation in the study area is influenced by hydrogeological factors including relief and lithology as well as other anthropogenic factors ( Obiadi et al., 2011 ). Although Anambra State has a high yearly rainfall, ranging from 1400 mm in the north to 2700 mm in the south, it is concentrated in one season, November to February, with nearly four months of dryness.

This objective was met by comparing soil physical parameter of natural forest with oil palm and cashew plantations, using one-way analysis of variance (ANOVA). The aim was to find out if there is significant difference in mean physical effects of natural forest, oil palm and cashew plantations respectively. The physical effects measured here are moisture content (in percentage), bulk density (g/ml), and particle density (g/ml). Each test is significant when the p-value is less than .05, otherwise the test is not significant. Significance implies that there is significant variation in soil physical parameters of the natural forest and plantations. Post hoc tests were used to determine which of the land use, natural forest, oil palm, and cashew plantations were significant.

From the ANOVA table, the p-value for the comparison of moisture contents of the natural forest and plantations is .211, which is greater than .05. This implied that there is no significant difference (variation) between the moisture contents of natural forest and the plantations; that is, the moisture contents were the same.

The table also showed that the p-value of the comparison of bulk densities of natural forest and the plantations is .000. It implied that there were significant difference (variation) in the bulk densities of the natural forest and plantations. The p-value for the test of variation between particle densities of natural forest and plantations is .548, which is greater than .05. It implied that there were no significant variation between the bulk densities of the natural forest, oil plantation and cashew plantation, meaning that the bulk densities were significantly the same.

The post hoc table showed that the bulk density of the natural forest is higher than that of the oil palm and cashew plantations.

To meet this objective, one way ANOVA was used to assess variations in loss of organic carbon, nitrogen, sodium, calcium, phosphate, pH and calcium, which all constituted the soil chemical parameters. The variations were between natural forest, oil palm and cashew plantations. Like stated earlier, each test is significant if the p-value is less than .05, otherwise, the test is not significant. Significance implies that there is variation, while non-significant means that there is no variation.

Post hoc tests were also done for significant tests, in order to ascertain which of natural forest, oil palm and cashew plantations is significant.

Result of the ANOVA showed that the p-values of variations in loss of organic carbon (sig = .000), nitrogen (sig = .000), phosphate (sig = .000), pH (sig = .022) and potassium (sig = .010) are all less than .05. This implied that there were significant variations in these parameters in natural forest, oil palm and cashew plantations.

The p-values of sodium and calcium are .470 and .249 respectively. The values are greater than .05, implying that there were no significant variations in sodium and calcium in the natural forest and plantations.

For the post hoc results, Table 4 showed that the loss of organic carbon of the cashew plantation varied significantly more than the others with p-value = .000, whereas for the variation in nitrogen of the natural forest varied significantly more than the others with p-value = .000.

Further comparison of the phosphate, pH and potassium, revealed that phosphate and pH of the natural forest varied more significantly than those of oil palm and cashew plantations, while the potassium of the cashew plantation varied most significantly than others.

The result of this research on moisture revealed that the p-value for the comparison of moisture contents of the natural forest and plantations is .211, which is greater than .05 ( Table 1 ). This implies that there were no significant difference (variation) between the moisture contents of natural forest and the plantations; that is, the moisture contents were the same. This is in contrast with reviews from Gorems and Goshal (2020) , Jackson et al. (2005) and van Dijk and Keenan (2007) , where moisture content had significant relationship between natural forest and plantations. The reason for this variation in soil moisture might be due to the net deficiency between evapotranspiration and precipitation which are two major processes in ecosystem water cycles. The amount of water lost through evapotranspiration is greater than the amount of water gained through precipitation in plantations ( Benyon et al., 2006 ; Stape et al., 2008 ). According to Jackson et al. (2005) , plantations afforested in croplands, grasslands and shrub lands decreased stream flow by 180 mm year-1 and 38% on average globally, and the climate feedbacks were unlikely to offset the soil water loss. It is important to note that a decrease in soil moisture content may limit root growth and the increment of stand biomass in plantations.

Bulk density tends to be lower in organic soil (loamy soil) and higher in sandy soil ( Cresswell & Hamilton, 2002 ). In contrast, the result of this research showed that natural forest has higher bulk density than plantations because the p-value of the comparison of bulk densities of natural forests and plantations were significantly different (p-value .000 < .05) ( Table 1 ). This was so because from the Post hoc (multiple comparisons) test, oil palm plantation has lower bulk density, which resulted to the decrease in cashew plantation when computed together ( Table 2 ). The reason for the high bulk density in natural forest may be due to; vegetations, economic activities, geology and soil settings of the research area. The main geologic units (soils) of the Etti village Nnaka are derived from the Nanka Sand (Eocene), overlain by Ogwashi-Asaba, formation (Oligocene) and underlain by Imo Shale and as such comprised mainly of porous, red and brown sandy soils, and brown and pale clay soils ( Obiadi et al., 2011 ). Also the economic activities in the Etti village Nnaka emanating from population pressure and need to provide energy and sustenance for a vast majority, the ecosystems are being altered through: industrialization, built up areas, grazing, slash-burn, firewood gathering etc. This may expose the soil to solar radiation and soil compaction thus leading to higher bulk density in natural forest. Increase in soil bulk density, i.e. soil compaction, is a global concern due to adverse effects on soil environments and productivity ( FAO, 2005 ). Soil compaction may limit the access of roots to water and nutrients, destroy soil structural units, slow gaseous diffusion and reduce rates of root respiration and litter decomposition ( Tokunaga, 2006 ), thereby reducing soil quality.

*. The mean difference is significant at the .05 level.

For particle density, the result showed that there were no significant relationship between the natural forest and plantations because the p-value for the test of variation between particle densities of natural forest and plantations was .548, which is greater than .05 ( Table 1 ). This is in contrast with Selassie and Ayanna (2013) where there was a significant relationship between particle density of natural forest and plantation, the reason for this contradiction maybe due to the geographical area of study which is Ethiopia. Particle density shows the chemical composition and structure of minerals in the soil. Therefore any increase in particle density will lead to increase in soil quality vice versa.

There were significant relationship between loss of organic carbon (sig = .000), nitrogen (sig = .000), phosphate (sig = .000), pH (sig = .022) and potassium (sig = .010) on natural forest and plantations ( Table 3 ). However, the post hoc result showed that the loss of organic carbon of the cashew plantation varied significantly more than the others ( Table 4 ). This means that introducing cashew plantation to natural forest may reduce the soil organic carbon. This affirms with the studies of Igu et al. (2023) ; Hartemink (2005) and Kopittke et al. (2019) . Hartemink (2005) posits that the main impacts of plantation agriculture using perennials crops like palm oil, cashew, rubber and cocoa include: soil erosion, soil fertility decline and pollution. In contradiction, natural forest landscapes are expected to sequester more carbon since they have sufficient amounts of litter falls and biogeochemical processes from trees, herbs and grasses, as well as a relatively undisturbed carbon cycle and processes ( Igu et al., 2023 ). Equally, the soil in forest ecosystems is relatively less disturbed and exposed than would be the case in the other land use category and so, has a higher tendency to retain and conserve carbon in the soil ( Igu et al., 2023 ). Anthropogenic activities such as agriculture generally degrade the soil and affect its ability to provide ecosystem services such as carbon storage ( Kopittke et al., 2019 ). Though plantation agriculture of tree crops (such as palm or cashew) could protect the soil against direct impacts of sunshine and carbon loss, natural forests have higher capacities for such and have more under-canopy covers ( Igu et al., 2023 ).

However, it is important to note that certain factors may also contribute to lower soil organic carbon content in plantations such as plantation site preparation, climate, soil texture, land use, drainage, vegetation ( European Community, 2009 ). The reason for the loss of organic carbon in cashew plantation might be due to high bulk density of cashew plantation in the research area. This is consistent with many previous field studies, in which soil bulk density increase leads to soil Carbon and Nitrogen concentrations decrease in plantations relative to natural forests ( Aborisade & Aweto, 1990 ; Solomon et al., 2002 ; Nsabimana et al., 2008 ).

*. The mean difference is significant at the .05 level.

Another reason for loss of organic carbon in cashew plantation might be due to the soil formation in Etti village Nnaka which is predominantly sandy with thin clay stone and silt stone bands/laminations. The sand is poorly sorted. These units, separated by shale-siltstone-fine sand layers, may be as thick as 30 m in some places. The deposits also exhibit well-developed patterns of alternating cross-bedded sands and layers of dark-grey shales. The shale units generally occur in beds 40 - 50 cm thick alternating with fine sand and siltstone. The units generally have a low dip ranging between 70 and 90 wests ( Egboka & Okpoko, 1984 ). Therefore, the soil types are recognized as hydromorphic ferrallictic soils. This shows why Nanka soil formation appears the most prone to severe erosion gulling and land sliding because it consists of a thick succession of very loose, friable and barely cemented sands ( Ofomata, 1975 ; Egboka & Nwankwo, 1983 ; Obiadi et al., 2011 ; Igwe, 2015 ; Obi & Okekeogbu, 2017 ). According to European Community (2009) , loss of organic carbon also occurs because erosion washes away top soil and humus. Overall, plantation returns less organic matter to the soil than natural forest ( European Community, 2009 ). Fine texture soils tends to have more organic matter than coarse soils because they hold nutrients and water better thus providing good conditions for plant growth ( European Community, 2009 ). Loss of soil organic carbon content can limit the soil’s ability to provide nutrient for sustainable plant production. This may lead to lower yield and affect food security. Loss of organic carbon also means less food for the living organisms present in the soil, thus reducing soil biodiversity ( European Communities, 2009 ).

Looking further at the comparisons of the nitrogen and phosphate, the result showed that the nitrogen and phosphate of the natural forest varied more significantly than those of oil palm and cashew plantations. The decrease in plantation soil Nitrogen, Carbon and Phosphate concentrations could be attributable to the differences in ecosystem properties between plantations and natural forests. Plantations had significantly lower net primary production (percentage change ±95% CI = –10.6 ± 3.1, n = 9), aboveground biomass (–21.5 ± 4.1, n = 16), litterfall (–36.3 ± 1.8, n = 27), aboveground litter mass (–29.1 ± 3.5, n = 29) and fine root biomass (–75.8 ± 10.5, n = 19) than natural forests ( Liao et al., 2010 ). These differences may result in lower Carbon input into soils under plantations than under natural forests. The decrease in soil Carbon can lead to a reduction of soil Nitrogen due to the tight coupling between ecosystem Carbon and Nitrogen cycles ( Luo et al., 2006 ). Additionally, site preparations involving burning for establishing plantations might also lead to Phosphate and Nitrogen losses in plantation. This shows why nitrogen and phosphate of natural forest is more significant than plantations.

Our results showed that soil pH differ significantly between plantations and natural forests, with natural forest having the highest value ( Table 4 ). It was also shown that the oil palm plantations reduces the _pH deposit of natural forest while there is no significant variation between the _pH value of natural forest and cashew plantation. This implies that oil palm plantation when introduced to natural forest has the tendency of reducing the soil pH of natural forest. The high increase in soil pH of natural forest is in line with results from previous reviews by Jackson et al. (2005) and Berthrong et al. (2009) . They showed that mean soil pH significantly decreased by .3 units below plantations compared with non-forested lands. They also found that soil sodium (Na) concentration was 71% higher below plantations than below non-forested lands. The increase in soil Na concentration might be responsible for the reduction in soil pH. However, the soils from regions with the same climate tend to be more acidic on natural forests than non-forested lands such as grasslands ( Chapin et al., 2002 ; Berthrong et al., 2009 ). As the strongest predictor of pH soil, soil Na concentration did not differ between plantations and natural forests in this research ( Table 3 ). Therefore, we conclude that other factors might have contributed to the acidity of natural forests. Soil acidity can occur naturally in higher rainfall areas and can vary according to the landscape geology, clay mineralogy, soil texture, buffering capacity ( Plant and Soil Sciences eLibrary, 2024 ). The study area of the natural forest is characterized by heavy rainfall, which usually start by April and ends around October ( Igwe, 2018 ). Soil acidity can also occur by the removal of plant and animal products, leaching of excess nitrate, addition of some nitrogen based fertilizers, build-up in mostly plant-based organic matter ( Plant and Soil Sciences eLibrary, 2024 ). Animal grazing in the research area could be associated with the loss of vegetation cover, which causes soil degradation and leaching. Soil acidity is a potentially serious land degradation issue. When soil becomes too acidic it can: decrease the availability of essential nutrients, increase the impact of toxic elements, decrease plant production and water use, affect essential soil biological functions like nitrogen fixation and make soil more vulnerable to soil structure decline and erosion ( Plant and Soil Sciences eLibrary, 2024 ). Without treatment, soil acidification can impact agricultural productivity and sustainable farming systems. Acidification can also extend into subsoil layers, posing serious problems for plant root development ( Plant and Soil Sciences eLibrary, 2024 ).

Potassium is essential for plant growth because it plays a crucial role in absorption, stomatal movement, enzyme activation and protein synthesis ( Li et al., 2023 ). Plant growth is inseparable from nutrient elements in nature because plants commonly experience stress from the lack of many of these nutrient elements especially potassium ( Li et al., 2023 ). The potassium concentration of the cashew plantation varied more significantly among others in this research ( Table 3 and Table 4 ). This is in contrast with review from Tellen and Yerima (2018) , where potassium concentration did not significantly vary among the land use types (natural forest, farmland and plantation) although soils from natural forest had the highest potassium concentrations (4.00 cmol (+)/kg soil) at (p < .01) followed by farmland (2.94 cmol (+)/kg soil) while those under plantation had the lowest concentrations (1.05 cmol (+)/kg soil). This variation from the two studies showed that land use change alone cannot determine the potassium concentration present in the soil. Other factors such as ecosystem differences will also have to be put into consideration. However, an increase in the potassium concentration of cashew plantation in this research may be due to the observed frequent application of household wastes, particularly wood ash. This is consistent with the findings of Bohn et al. (2001) .

Furthermore, the result showed that there were no significant variations in sodium (.470), calcium (.249) and magnesium (.713) ( Table 3 and Table 4 ) of the natural forest and plantations. This affirms the findings of Tellen and Yerima (2018) . Magnesium and calcium are among the secondary nutrients required by plants for normal and healthy growth, that is, all plants need calcium and magnesium rich soil to grow. Calcium is a non-leaching mineral that helps improve water penetrability and reduce soil salinity. It is used by the plant to develop cell walls and membranes ( Tellen & Yerima, 2018 ). This insignificant variation in the calcium of soil of natural forest and plantations indicates the reason for the insignificant sodium concentration in natural forest and plantations because calcium reduces soil salinity. That is, the higher the calcium the lower the sodium and vice versa.

From the above discussions this research was able to establish the variations in the chemical parameters of natural forest and plantations and the possible reasons for such variations.