Field capacity

Field capacity is defined as the amount of water held in soil after excess gravitational water has drained away and after the rate of downward movement of water has materially decreased (Veihmeyer and Hendrickson, 1931). Such a situation is normally reached 48 to 72 hours after saturation. Sandy soils reach field capacity earlier than clayey soil. The field capacity is the upper limit of available soil moisture range in soil-water-plant relationship. The force with which moisture is held in the soil ranges between 0.1-0.33 bars, (10 kpa - 33 kpa). 

Note The laboratory method is rapid and is commonly used when the water table is shallow and field method cannot be used. Value of field capacity by laboratory method usually does not coincide with that of field method since in the laboratory the natural conditions of the soil are disturbed. Normally field method is recommended for determining field capacity of the soil. 

Weight of moisture box Weight of moisture box + wet soil Weight of moisture box + over dry soil 

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Because many pesticides are appHed to the soil surface, the transport of pesticide during water infiltration is important. Water infiltration is characterized by high initial infiltration rates which decrease rapidly to a nearly constant rate. Dry soils have greater rates of infiltration than wet soils during the initial appHcation of water. Thus, perfluridone movement after appHcation of 3.8 cm of water was considerably greater in soil at a water content of <1% of field capacity than at 50% of field capacity (62). Fluometuron moved deeper into the soil in response to greater rainfall intensity or after rainfall onto a dry rather than a moist soil (63). 

Location Soil series pHb Organic matter % Field capacity % water... 

For all the essential nutrient ions, the diffusion coefficient, Du is essentially the same with a value of around 10 cm s whereas the water flux at the root surface is typically of the order 10 cm s for soils at around field capacity. The tortuosity factor typically scales with the volumetric moisture content over quite a wide range of moisture content, i.e., / 0. As the soil becomes drier, the water flux will decline much faster than the tortuosity factor due to the typi-... 

For these reasons, it is desirable to perform a series of simple calculations to determine if the field capacity for a given depth of soil is ever exceeded, rather than simply overlaying water inputs over plots of residue data. The following series of calculations addresses the primary issue of whether sufficient water was applied to the test system at appropriate intervals to create leaching opportunities ... 

More sophisticated methods that actually measure volumetric water content can also be used, such as time domain reflectometry (TDR). In Figure 14, an example of TDR results is presented. Both the calculated and measured (i.e., TDR) volumetric water contents provide a similar picture of the profile water status by depth with time. Proper soil characterization data, such as those shown in Table 6, are necessary for these calculations and improve understanding of the test system. The determination of water-holding capacity (WHC) at 0.03 MPa field capacity (FC) and 1.5 MPa... 

Depth (cm) Soil matrix potential, 0.03 MPa (field capacity) Soil matrix potential, 1.5 MPa (permanent wilting point) ... 

Other models successfully employ a simple water routing system. Each layer of soil is assumed to hold all water entering the layer up to the field capacity. When the water content of a soil layer exceeds the field capacity, water drains downward to the next layer at the rate specified by the hydraulic conductivity of the saturated soil in the layer. 

Soil strength Water-holding capacity Field capacity/wilting point Hydraulic conductivity Fertility... 

Table III illustrates the impact of adsorption on the leaching of organic chemicals in the soil. A water input of 305 cm was used, which is equivalent to a full year of precipitation in the eastern United States. In a soil with a field capacity of 30%, the water would penetrate 1017 cm. Mirex with a very large Kqc is practically immobile after a full year of precipitation, it is still on the surface. It is likely that any compound adsorbed this strongly would be carried off the land surface by soil erosion instead of being leached into the soil. In contrast, DBCP, which is very weakly adsorbed, penetrates the soil profile almost as far as the water does.

Saturation percentage-amount of water to saturate 100 g of soil (approximately twice field capacity). 

Soil solution to soil ratios also strongly affect distribution of some trace elements such as Zn speciation in arid and semi-arid soils. Fotovat et al. (1997) reported that the proportion of free hydrated Zn2+ to total Zn ranged from 20-65% at field capacity soil water content and decreased with increases in solution to soil ratios, while the proportion of Zn complexed with organic ligands increased dramatically in soils. However, solution to soil ratios do not strongly affect the distribution of Cu speciation in soil solution since Cu primarily occurs as organic complexes in these soil solutions. 

Table 6.3. Treatment levels of trace metals in the two Israeli arid soils incubated under the field capacity and wetting-drying cycle moisture regimes...

Figure 6.1. The fractional loading isotherms of Cu in a contaminated Israeli loessial soil at an initial (one hour) period and after 48 weeks. The soil was treated with increasing levels of metal nitrates and was incubated under the field capacity regime. Horizonal solid line represents the native content of Cu in the nonamended soil (Figure 6.1 - Figure 6.4, after Han and Banin, 2001. Reprinted from Commun Soil Sci Plant Anal, 32, Han F.X and Banin A.,The fractional loading isotherm of heavy metals in an arid-zone soil, pp 2700-2703, Copyright (2001), with permission from Taylor Francis)...

Following the initial fast retention, the slow redistribution of the added metals occurred over time. During one year of incubation under the field capacity regime, heavy metals were slowly transferred among solid-phase components as shown in Figs. 6.1-6.4. Added Cu and Ni were transferred from the EXC and CARB fractions into the ERO and OM fractions and Zn mainly into the ERO fraction. Chromium and Pb moved from the CARB fraction into the OM and ERO fractions, respectively. Cadmium redistributed from the EXC fraction into the CARB fraction. After one year of incubation under the field capacity regime, 65-100% of the added Cd was transferred to the CARB fraction. About 50% and 20% of the added Pb was redistributed to the CARB and ERO fractions, respectively. 

Figure 6.5. The initial reduced partition index, Ir, of six metals in two Israeli arid soils. Two soils were treated with metal nitrates at various loading levels. Soils were incubated under the field capacity moisture regime (modified after Han and Banin, 1999, with permission from Springer Science and Business Media)...

Table 6.6. Comparisons of the redistribution of metals in two Israeli soils at 3T treatment. Soils were incubated for one year under the saturated paste (SP), field capacity (FC) and wetting-drying cycle (Cycle) moisture regimes (% of the sum of fractions, as means of two replicates) (from Han et al., 2001a, with permission from Lippincott Williams Wilkins)...

Native Pb in Israeli arid soils mainly resided in the CARB and the RES fractions in the loessial soil, and it resided in the RES and the RO fractions in the sandy soil. During incubation under the field capacity regime, added Pb was slowly transferred from the CARB fraction into the ERO fraction (Fig. 6.13). However, the majority of Pb still remained in the CARB fraction. 

Figure 6.15. The average transfer fluxes of Cr and Cu among solid-phase components in Israeli soils. The soils received metal nitrates at the 3T level under the field capacity moisture regime (T total metal content in non-amended soils)...

Figure 6.17. Relationships between the initial (the first day) fluxes of metals in the soluble plus exchangeable fraction and metal loading levels in Israeli soils. Soils received metal nitrates and were incubated in the saturated-paste (SP) and field capacity (FC) moisture regimes...

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