How Deep Does You Garden Soil Need To Be Before Hardpan

Edaphic Factor

N. Rajakaruna , R.S. Boyd , in Encyclopedia of Ecology, 2008

Depth

Soil depth can greatly influence the types of plants that can grow in them. Deeper soils generally can provide more water and nutrients to plants than more shallow soils. Furthermore, most plants rely on soil for mechanical support and this is especially true for tall woody plants (e.g., shrubs, trees). A classic example of the influence of soil depth on plant communities is seen on granite rock outcrops in the southeastern US. As the granite weathers, it can form pools of soil that vary in depth from a few millimeters at the margin to tens of centimeters in the middle. The shallow marginal soils support certain annual plants, whereas deeper soils support herbaceous perennials and still deeper soils are colonized by woody plants. Plant zonation in these soil pools can be striking ( Figure 1 ). Some soils can develop special soil horizons (horizontal soil layers characterized by distinct chemical and physical features) that limit the soil depth available to support plants. These special soil horizons include claypans, zones of soil which contain large amounts of clay, and hardpans, layers of soil particles that have been cemented together by the deposition of mineral materials. Hardpans include calcic horizons (commonly called caliche), in which calcium carbonate cements the soil particles. The net effect of these dense horizons is to impede or prevent root growth and thus limit the effective depth of the soil. They also may affect soil oxygenation by restricting drainage at times in which large amounts of water are present.

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Site Selection and Climate

Ronald S. Jackson PhD , in Wine Science (Third Edition), 2008

Soil Depth

Soil depth, in addition to soil texture and structure, can influence water availability. Shallow hardpans reduce the usable soil depth, and enhance the tendency of soil to waterlog in heavy rains, and fall below the permanent wilting percentage under drought conditions. Limiting root growth to surface layers also can influence nutrient access. For example, potassium and available phosphorus tend to predominate near the surface, especially in clay soils, whereas magnesium and calcium more commonly characterize the lower horizons. Soils vary in the accumulation of nutrients through their soil horizons.

Effective soil depth is best achieved before planting. Breaking up hardpans by soil ripping is a standard technique in several countries. An alternate procedure is mounding topsoil in regions where vines are planted. It is particularly useful in situations where high water tables are unavoidable and under saline conditions. Planting a permanent groundcover or mulching helps to minimize erosion from the mounds. Where root penetration is limited by high acidity in one or more soil horizons, soil slotting can significantly increase root soil-exploration.

Effective soil depth may decrease as a consequence of various viticultural techniques. For example, cultivation promotes microbial metabolism and degradation of organic material. This weakens the crumb structure of the soil, leading to the release and downward movement of clay particles. In addition, salinization as a result of improper irrigation can disrupt aggregate structure, releasing clay particles. If clay particles flow downward, they tend to plug soil capillaries. Over time, this can result in the formation of a claypan.

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Nanoparticle Ecotoxicology

Ashok K. Singh PhD , in Engineered Nanoparticles, 2016

7.1.4.2.2 Distribution of Bacteria in Soil

Effects of soil depth on aerobic and anaerobic bacterial populations have been shown by Xu et al. (2014), who measured vertical distribution of methane-oxidizing bacteria (MOBs) expressing methane monooxygenase and sulfate-reducing bacteria (SRBs) expressing α-subunit of adenosine-5′-phosphosulfate (APS) reductase in soil from surface to 738 mm depth. As shown in Figure 76, from surface to 160 cm depth, MOB's population was several thousand-fold higher than APS's population, indicating mostly aerobic oxidation of methane to methanol.

Then, the MOB's population decreased, while the APS's population increased, reaching comparable populations from 230 to 370 cm depths, indicating a balance between methane oxidation and reduction of sulfate. As the depth increased, a proportional increase in MOB and a decrease in APS populations occurred. This may reflect anaerobic oxidation of methane using another acceptor. Then, a proportional increase in APS and a decrease in MOB occurred and the APS population surpassed the MOB population, indication of sulfate reduction. This suggests that depth plays a critical role in the distribution of aerobic and anaerobic bacteria in soil.

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ENZYMES IN SOILS

R.P. Dick , E. Kandeler , in Encyclopedia of Soils in the Environment, 2005

Macroscale Distribution

Enzyme activity declines with soil depth and generally is correlated with organic C and microbial biomass distribution in the soil profile. This has been shown in many studies and is to be expected, as most of the biological activity and organic matter is in the surface soil horizons.

Plot-scale investigations have determined that organic matter turnover rates, microbial biomass, and enzyme activities of soil samples vary with vegetation and soil type. Relatively little is known about the topographic, pedogenic, soil mineralogy, and other properties that control microbial and enzyme distribution at landscape levels. The characterization of these interactions is essential to achieve a better understanding of complex ecosystem processes. This latter scale is of particular importance for developing a soil-quality indicator, because enzyme activities can vary more as a function of soil type than the differences caused by soil management.

Fuzzy operations (which use logic inference procedures by allowing individuals, or soil properties in this case, to be assigned into continuous class membership values instead of exact hard classes in order to make interpretations based on several or many soil properties) and multivariate analysis (factor analysis of many soil properties simultaneously; in this case to see which ones are most important in explaining soil variation in space or between management systems) have shown that land use is the strongest factor and soil contamination the weakest factor governing the level of soil enzyme activities at the ecosystem level in Central Europe. Soil type is an important site factor, as it summarizes climatic, topographic, and geologic conditions, acidification, and vegetation influence on soil biology and enzyme activity.

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Quantifying and Managing Soil Functions in Earth's Critical Zone

Y.-L. Liu , ... S.A. Banwart , in Advances in Agronomy, 2017

4.4 The Relationship Between Soil Depth and Clay Mineralogy

The principal component analysis demonstrated that soil depth had a stronger effect in relation to soil mineral composition than the field treatments and that the effects varied with particle size class and H ₂O₂ treatment (Fig. 5). The stratifications by soil depth were largely attributed to the variation in the intensities of the soils processes modifying soil minerals and their complex interactions. These processes have been discussed in the previous sections and are summarized up in Fig. 8. The stratification by soil depth was mainly driven by contrasting changes in the (1) well-crystallized illite for both the 2- to 5- and < 2-μm particle size class at the 0–0.2 m depth, (2) the vermiculite from the 2- to 5-μm particle size class, and (3) the smectite for the < 2-μm particle size class at the 0.3–0.4 m depth.

This pattern with depth corresponded to the transformation of these minerals through K⁺ transformations that were strongly differentiated with plant types. This differentiation with depth results from plant having different root depths, K fertilization, and ion transport as well as vertical transport of soil silt and clay particles. After removal of soil organic matter, the stratification by soil depth became much weaker for the 2- to 5-μm particle size class and no clear pattern among the soils for the < 2-μm particle size class. The stratification was significant only for the soils such as Alfa, F1C1, and F1C2 that exhibited the strongest variations in SOC, EK, and pH with soil depth. The absence of a clear pattern among the soils suggested complex interactions between soil processes and the effects of the field treatments in the soil profile.

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HYDROLOGY | Ground and Surface Water

S. Ge , in Encyclopedia of Atmospheric Sciences, 2003

Water in Soils

Near the land surface at shallow depths, soils are often partially saturated; such a region is known as the unsaturated zone. The degree of saturation is defined as the fraction of pores that contain water, and varies from 0, representing a dry condition, to 1, representing a fully saturated condition. The water in the partially saturated soils clings to soil particle surfaces and is sustained by suction or tension. Pore pressure in the unsaturated zone is conventionally expressed in negative values, reflecting the use of atmospheric pressure as the zero reference pressure. The pore pressure distribution and the rate of moisture movement vary spatially depending on soil types and weather conditions and temporally in response to rainstorms, seasonal changes, and long-term climate change.

Infiltration is an important process in the unsaturated soil zones and involves downward movement of moisture under wet climatic conditions. The infiltration rate over a small area can be measured using a ring infiltrometer. This is a portable cylindrical ring, with a diameter from a few centimeters to 20 cm, extending several centimeters above and below the surface of the soil. The rate of water dissipating from the ring infiltrometer into the soil can easily be converted to an infiltration rate.

In contrast to infiltration, evaporation and transpiration draw moisture upward under dry climatic conditions. Evaporation causes water loss from surface waters, such as lakes and rivers, and from shallow-depth soils. Water evaporates as a vapor diffusion process that is largely controlled by the energy exchange between radiation or sensible heat from the atmosphere or ground, and the heat energy change in the evaporating body. A direct method for determining the evaporation rate has been developed and is known as the pan-evaporation approach. It involves exposing a cylindrical pan of water to the atmosphere in clearings where precipitation can be monitored. The standard US National Weather Service Class A pan is 1.22 m in diameter and 25.4 cm deep. Transpiration is a process whereby water is lost to the atmosphere through the vascular systems of plants. The transpiration process works by absorption of water by plant roots, translocation of liquid through the plant vascular system, and transpiration into the atmosphere through openings in the leaf surface. Although transpiration is also considered a diffusion process, water is first pulled through the plant by a potential energy gradient before diffusing into the air in response to a vapor pressure difference.

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The Source of Water Transpired by Eucalyptus camaldulensis: Soil, Groundwater, or Streams?

Peter J. Thorburn , Glen R. Walker , in Stable Isotopes and Plant Carbon-water Relations, 1993

1 Activity of Surface Roots of the Inland Trees

Uptake of water from near-surface soil depths by the inland trees was expected in the early part of the study period, because matric (and total) potentials were high in this region ( Fig. 6a). However, at the end of the study period surface soil matric potentials were very low (–2.5 MPa, Fig. 6b), yet around half the water transpired by the trees was derived from these soil depths (Table III). There may be some error in matric potentials and mixing model results due to factors such as spatial variability in soil water matric potentials or δ values, or the travel time of sap from roots to twigs. However, it is unlikely that these errors would have been large enough to have caused substantial changes in the range of surface soil matric potentials or the mixing model results.

There is other evidence that the trees were extracting water from shallow soil layers to low potentials at this site. Thorburn et al. (1993) sampled water from roots of these trees (and nearby E. largiflorens) in January, 1991, and compared its isotopic composition with that of soil around the roots. The roots were 10–20 mm in diameter and were taken from the top 0.2 m of soil which had very low matric potentials (–2.5 to –5.0 MPa). In 10 of the 14 roots sampled, δD values of the root waters were similar to those of the soil around the roots, but different from twig sap δD values (twig sap δD values were the same as groundwater δD values). A further two roots had δD values intermediate between those of the groundwater and surrounding soil. The similarity between the soil and root δD values showed that these large shallow roots were generally not participating in groundwater uptake, but were in communication with water in the surrounding dry soil.

Shallow roots of these trees may be active in dry soils to facilitate uptake of nutrients. Additionally, high salinity levels in the lower profile may reduce root activity, therefore making the surface roots relatively more important in water uptake.

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Terroir: the effect of the physical environment on vine growth, grape ripening and wine sensory attributes

C. van Leeuwen , in Managing Wine Quality: Viticulture and Wine Quality, 2010

9.6.1 Soil depth

Much confusion exists about terroir expression in relation to soil depth. To soil scientists, soil depth is the depth to which the parent material is altered by pedological processes. Viticulturists consider rooting depth as soil depth. In general, shallow soils allow only shallow rooting, while deep soils allow deep rooting. However, if the parent material is not hard rock, roots may extend beyond the layers altered by pedological processes. This might increase the amount of water available to the vines. The first comprehensive terroir studies were carried out by Seguin (1969, 1975, 1983, 1986) in gravelly soils in the Haut-Médoc (Bordeaux, France). This author showed that in these poor, free draining soils, rooting depth is a quality factor in red wine production, because it regulates vine water uptake conditions. These observations have been incorrectly extended to other soil types, and in many popular wine books, the terroir effect is presented to be mediated trough vine roots exploring deep soil layers in search for minerals. In reality, on most substrates shallow soils enhance terroir expression.

This aspect was extensively studied by Bodin and Morlat (2003, 2006) in the Loire Valley (France), who developed a field model based on soil depth and average clay content, in relation to the level of weathering of the parent rock. Sites were characterized as weakly weathered rock (WWR), moderately weathered rock (MWR) and strongly weathered rock (SWR). Vine vigour was high and phenology was delayed on SWR (deep soils); vigour was low and phenological stages were reached earlier on WWR (shallow soils). This effect was largely mediated through vine water status, as is shown by the authors through pre-dawn leaf water potential measurements and carbon isotope discrimination (δ¹³C) measurements on grape sugar at ripeness (Fig. 9.11). On WWR, grape berries were smaller, richer in sugars and anthocyanin and had a higher total phenolics than those of the vines cultivated in SWR (Morlat and Bodin, 2006). On WWR, grapes contained less malic acid, resulting in a lower titratable acidity. Thus, shallow soils have higher grape quality potential for red wine production because their low water holding capacity is more likely to induce water deficit stress compared to deep soils. Vine nitrogen status was not controlled in this study. However, it is likely that soil nitrogen offer is also lower on shallow, weakly weathered soils and that this reinforces the devigorating effect of low water supply on these soils. Similar results were published by Coipel et al. (2006) in the very different environment of the Rhône Valley (France). Even in a very dry vintage, vine vigour was lower and grape quality potential higher on shallow soils compared to deep soils, because water and nitrogen supply were restricted on shallow soils. Soil depth can thus be an integrative variable in terroir studies. This approach might suffer some exceptions in alluvial soils.

Several soil variables influence vine rooting. The quantity of roots and their vertical distribution is positively correlated to water supply, and negatively to penetrometer soil strength, bulk density and water logging (Morlat and Jacquet, 1993).

Limitation in soil depth reduces the soil water holding capacity, which can be a quality enhancing factor. However, the first 20 cm of the soil are relatively rich, because they contain most of the organic matter and it is not desirable to have most of the roots in this part of the soil. Weed destruction by ploughing, or the use of cover crop, prevents roots from colonizing the surface layer of the soil, while the use of herbicides promotes them to colonize this layer (Soyer et al., 1984).

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Agroforestry: Fertilizer Trees

G.W. Sileshi , ... O. Jiri , in Encyclopedia of Agriculture and Food Systems, 2014

Improvement in soil physical properties

Among the commonly used indicators of soil physical properties are soil depth, bulk density, aggregate stability, infiltration rates, water-holding capacity, and penetration resistance. Soil bulk density is a direct measure of soil compaction. Soils with low bulk density, although open-textured and porous, are susceptible to erosion, poor water retention, and oxidation of SOM and loss of SOC. In contrast, soils with high bulk density have lower porosity. Various studies indicate improvement in bulk density, aggregate stability, and porosity due to fertilizer trees. In sandy loam soil in the pre-Amazon region of Brazil, bulk density, total porosity, and soil aeration were substantially improved in alley cropping with leucaena , pigeon pea, acacia, and their mixtures over a period of three years (Aguiar et al., 2010). In gliricidia, leucaena, Vachellia, and sesbania rotational fallows in Zimbabwe and Zambia soil bulk density was up to 12% lower and aggregate stability was higher by 18–36% compared to sole maize crops (Table 4). Pore density was also significantly higher in vachellia and sesbania fallows (285–443 m^–2) compared to continuous maize (256 m^–2). The pore density was significantly higher in Vachellia and sesbania fallows (4521–8911 m^–2) compared to continuous maize (2689–3938 m^–2). The mean pore sizes were lower in continuous maize and higher in the fertilizer tree fallows (Nyamadzawo et al., 2008a). The mean pore sizes at 5 cm tension were 0.07–0.12 mm in fallows relative to continuous maize, which were 0.03 mm.

Table 4. Changes in soil physical properties (0–20 cm) due to fertilizer trees (FT) in improved fallow and the control (sole maize) and the % change (%Δ) at Msekera, Kagoro, and Kalunga sites in Zambia and Domboshawa in Zimbabwe

Variable	Tree species	Site	FT	Control	(%Δ)	Reference
Bulk density	Gliricidia	Msekera	1.39	1.53	−9.2	Sileshi and Mafongoya (2006)
(Mg m⁻³)			1.40	1.42	−1.4	Mafongoya et al. (2006)
	Leucaena		1.35	1.53	−11.8	Sileshi and Mafongoya (2006)
	Vachelia	Domboshawa	1.33	1.41	−5.7	Nyamdzawo et al. (2008)
	Sesbania	Msekera	1.35	1.42	−4.9	Mafongoya et al. (2006)
			1.59	1.66	−4.2	Phiri (2002)
		Domboshawa	1.36	1.41	−3.5	Nyamdzawo et al. (2008)
Aggregate stability	Sesbania	Msekera	83.3	61.2	36.1	Chirwa et al. (2004)
(mm)			65.0	55.0	18.2	Phiri (2002)
			38.0	32.0	18.8	Phiri (2002)
	Pigeon pea		80.0	61.2	30.7	Chirwa et al. (2004)
Infiltration rate	Gliricidia	Kagoro	4.4	2.9	51.7	Chirwa et al. (2003)
(mm hr⁻¹)		Msekera	16	4.0	300.0	Mafongoya et al. (2006)
	Leucaena	Kagoro	3.7	2.9	27.6	Chirwa et al. (2003)
	Vachelia	Kagoro	5.5	2.9	89.7	Chirwa et al. (2003)
		Domboshawa	>35	5.0	600.0	Nyamdzawo et al. (2007)
	Sesbania	Msekera	20.0	4.0	400.0	Mafongoya et al. (2006)
			0.13	0.08	62.5	Phiri (2002)
			4.4	2.1	109.5	Chirwa et al. (2004)
		Kagoro	9.5	2.9	227.6	Chirwa et al. (2003)
	Sesbania	Kalunga	21.0	7.0	200.0	Nyamadzawo et al. (2006)
		Msekera	8.0	5.0	60.0	Nyamadzawo et al. (2006)
		Domboshawa	12	5.0	140.0	Nyamadzawo et al. (2007)
	Pigeon pea	Msekera	5.2	2.1	147.6	Chirwa et al. (2004)
	Tephrosia	Kalunga	16.0	7.0	128.6	Nyamadzawo et al. (2006)
		Msekera	7.1	5.0	42.0	Nyamadzawo et al. (2006)
Time to runoff	Vachelia	Domboshawa	30.0	15.0	76.5	Nyamadzawo et al. (2006)
(min)	Sesbania	Kalunga	21.0	9.0	133.3	Nyamadzawo et al. (2006)
		Msekera	7.0	3.0	133.3	Nyamadzawo et al. (2006)
		Domboshawa	21.0	15.0	40.0	Nyamadzawo et al. (2006)
	Tephrosia	Kalunga	14.0	9.0	55.6	Nyamadzawo et al. (2006)
	Tephrosia	Msekera	7.0	3.0	133.3	Nyamadzawo et al. (2006)
Drainage	Sesbania	Msekera-1a	56.4	15.8	257.0	Phiri (2002)
(mm)		Msekera-1b	10.9	1.0	990.0	Phiri (2002)
		Msekera-2a	61.1	7.6	703.9	Phiri (2002)
		Msekera-2b	10.7	5.7	87.7	Phiri (2002)
Penetrometer resist	Gliricidia	Kagoro	0.6	1.2	−50.0	Chirwa et al. (2003)
(Mpa)	Leucaena	Kagoro	0.8	1.2	−33.3	Chirwa et al. (2003)
	Vachelia	Kagoro	1.0	1.2	−16.7	Chirwa et al. (2003)
	Sesbania	Kagoro	0.9	1.2	−25.0	Chirwa et al. (2003)
		Msekera	2.2	3.2	−31.3	Chirwa et al. (2004)
	Pigeon pea	Msekera	2.9	3.2	−9.4	Chirwa et al. (2004)
Runoff loss (%)	Vachelia	Domboshawa	0	57.0	−100.0	Nyamadzawo et al. (2006)
	Sesbania	Domboshawa	21.0	57.0	−63.2	Nyamadzawo et al. (2006)

The improvement in soil structure was also associated with increased drainage, especially during wet periods. In eastern Zambia and Zimbabwe, steady-state infiltration rates were 42–600% higher when maize was rotated with gliricidia, leucaena, vachellia, sesbania, and tephrosia compared to continuously grown sole maize (Table 4). Time to water runoff was also longer by 40–133% and drainage was improved by 88–900% compared to continuous sole maize. The soil in maize planted following improved fallows had lower penetration resistance compared with monoculture maize at various sites in eastern Zambia (Table 4).

Reduced penetrometer resistance and increased water infiltration imply reduced water runoff and soil erosion. The improvement under fertilizer trees was evident from the longer time to runoff measured in maize fertilizer tree rotations compared to sole maize in Zambia and Zimbabwe (Table 4). Land under fertilizer trees has been shown to be less susceptible to runoff and erosion than continuous maize. According to Fagerström et al. (2002), in an upland rice cropping system in northern Vietnam, tephrosia fallows and hedgerows effectively prevented nutrient losses by erosion. Runoff and soil losses were also lower in maize grown with fertilizer trees compared to continuous maize in Zimbabwe (Table 4). Soil loss was 30–100% higher under continuous maize than under fertilizer tree fallows (Nyamadzawo et al., 2006; Nyamadzawo et al., 2012).

As they increase hydraulic conductivity and reduce runoff losses, fertilizer trees improve water retention, storage, and availability to associated crops. At Domboshawa in Zimbabwe 75–80% of the total available water was retained at suction <33 kPa in the top 0–15 cm depth under vachellia fallows (Nyamadzawo et al., 2012). Soil water stored in 2-year sesbania-improved fallows was greater than in continuously cropped fertilized or unfertilized maize in eastern Zambia (Phiri et al., 2003). In parklands in Ethiopia, the amount of available water under faidherbia was twice that outside the tree canopy (Kamara and Haque, 1992). Similarly, in Malawi, soil moisture in the 0–15 cm soil was 4–53% higher under faidherbia than outside the tree canopy (Rhoades, 1995). The trees canopy also intercepts water and channels it down to the soil, thus contributing soil water recharge through macropores created by roots and increased microbial activities. Phiri (2002) recorded greater rainfall interception by sesbania tree canopies indicated by increased moisture storage and sub-soil moisture recharge.

The role of fertilizer trees in improving water use efficiency (WUE) has recently been demonstrated with long-term field studies in Africa (Sileshi et al., 2011). In rain-fed agriculture, rain use efficiency (RUE) defined as the ratio of aboveground net primary production to annual rainfall provides information similar to WUE. Sileshi et al. (2011) analyzed variations in RUE with leucaena in three long-term experiments conducted in Zambia and Nigeria. At the two sites in Zambia, maize intercropped with leucaena achieved 191–197% higher RUE compared to sole maize continuously cropped without nutrient inputs. At the Nigerian site, RUE was 139–202% higher in maize planted between leucaena hedgerows compared to the control (Sileshi et al., 2011). According to another study at Makoka (Chirwa et al., 2007), WUE was higher in maize intercropped with gliricidia than in the sole maize and maize+pigeon pea intercropping. At another site in eastern Zambia, WUE was 202% higher in sesbania fallows compared to continuous sole maize (Phiri, 2002).

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Global Change and Forest Soils

Chuck Bulmer , ... Grant M. Domke , in Developments in Soil Science, 2019

Defining the population of interest: mapping objectives

Defining the population of interest (e.g., geographic extent, soil depth, soil properties) helps to determine whether there are sufficient data for classification and/or prediction and ultimately to achieve the mapping objectives. A sample is a set of population units or individuals (e.g., soil pits, soil cores), with the whole set of individuals considered as a population. Understanding the nature of data required to classify and/or predict soil properties is an essential part of developing a map or estimating a population parameter. Typically these data come from surveys which may be extensive (e.g., national forest inventories [NFIs]) or project-specific (e.g., stands or landscapes).

For each population unit, a set of variables are measured or qualitative attributes assigned which can take on several different forms. In some cases, a variable describing a soil characteristic can take on one of two states (binary variables). In other cases, a soil characteristic may take on more than two states (multi-state variables). Finally, there are soil properties with quantitative values that fall along a continuous scale (continuous variables). Collectively, these binary, multi-state, and continuous variables are used along with auxiliary information (e.g., climate data, remotely sensed data) to classify and/or predict soil properties for a population.

In many cases, data that were collected in the field, measured in the laboratory, or observed from remote instruments were not intended to be used together. Fortunately, there are many data fusion techniques available to resample remotely sensed data, transform variables collected in the field or laboratory, and standardize units of area, volume, and/or mass so that multiple data sources can be harmonized for use in classification and prediction (Castanedo, 2013).

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