Small amounts of micronutrients are removed in harvested parts of crop and pasture species. Relative to the rates of addition of fertilisers, removal in a single crop is usually low and ensures extended residual value of micronutrient fertilisers in many cases. Brennan (2005) calculated that only 7 % of the Zn applied at the rate of 3 kg Zn/ha had been removed in harvested grain over the subsequent 14 years. Relative annual uptake of Cu by wheat crops from an initial application of 1.38 kg/ha was estimated to be 2-3 % per annum (Brennan, 2006). However, in intensive high yield cropping systems such as rice-wheat rotations or horticultural crop production, the removal of micronutrients
in harvested crop products in a single year may account for 0.50 kg Zn/ha (Table 2.3), which necessitates either higher rates of micronutrients added in fertiliser or more frequent applications to maintain adequate supply for crop production. Similarly, in plantation forestry, high uptake rates of micronutrients and sequestration of them in bark and wood may necessitate higher rates of application than was required in previous land use systems on the same soil (Dell et al., 2003). Th erefore, it is important to account for crop removal and to determine, for a particular cropping system, the frequency with which repeat applications are needed.
Tables 2.3 and 2.4 present representative values for crop removal of micronutrients drawn from a number of sources and species. Clearly the amounts removed are dependent on yield, and values should be adjusted when expected yields diff er from those reported in Tables 2.3 and 2.4. However, a doubling of yield will not necessarily double the removal of micronutrients. Many cropping systems involve sequences of crops with two or more crops per year and, under these circumstances, the annual removal is likely to be greater than that for a single crop annually (Table 2.4).
Table 2.3. Removal of micronutrients in harvested plant parts for a range of crop species (g/ha) (Price, 2006, unless otherwise mentioned). Ni uptake is not well enough studied to assign values for removal in harvested crops but levels are likely to be similar to Mo.
Crop type Yield (t/ha) B Cu Fe Mn Mo Zn
Legume (peanut)a 2 (as pods) 154 16 1500 128 4 48
Cotton 2 - 20 158 50 - 98 Leafy vegetable (spinach) 50 (fresh leaves) 195 24 1200 175 25 100 Sugarcane 92 (stalks) - 58 3404 1472 - 258
Fruit tree (orange) Fresh fruit 157 34 168 45 - 78
Nut tree (pecan) 1.2 12 10 34 95 2 35
a From Singh et al. (2004), based on a crop yielding 2 t pods/ha and 3.6 t shoot biomass/ha.
Table 2.4. Micronutrient uptake (g/ha) by rice and wheat in a rice-wheat rotation averaged over three years in India under nil, low and high fertiliser rates (Gupta and Mehla, 1993). Fertiliser rate Biomass (t/ha/crop) Rice Wheat Rice Wheat Zn Cu Mn Fe Zn Cu Mn Fe Nil 6.5 4.1 120 90 350 1680 70 30 110 700 Low 8.8 7.5 170 130 570 2510 120 50 210 1300 High 14.5 12.4 300 220 970 4060 200 90 330 2230
Micronutrients for sustainable food, fi bre and bioenergy production
16
Erosion
Specifi c studies on erosion losses of micronutrients have not been sighted, hence most of our understanding is based on the application of principles and anecdotal information. When micronutient metals are concentrated close to the soil surface, erosion losses of soil could have a disproportionate eff ect on losses of micronutrients. For example, if 1 cm of soil was eroded and it contained 1 mg Zn/kg, this equates to a loss of 0.13 kg Zn/ ha. Minimum tillage systems and broadcasting miconutrients will tend to concentrate micronutrients close to the soil’s surface, and hence increase the risk of loss in the case of erosion events. Sub-soil Zn is commonly lower than in topsoils (Brennan et al., 1993). Th erefore, loss of topsoil commonly results in Zn defi ciency on the exposed sub-soils (Fageria et al., 2002). Copper which is strongly associated with organic matter would, like N, be lost disproportionately when surface erosion removes the humus-rich layers. According to McBride (1981), the plant available forms of Cu tend to be concentrated towards the soil surface. Th e micronutrients that are more strongly associated with the mineral soil components would tend to be depleted when erosion selectively removes mineral sediments from the soil.
Leaching
At recommended rates of application, B is the only micronutrient for which leaching is likely to be a signifi cant fl ux in the biogeochemical cycle in aerobic soils. For the micronutrient metals, the low rates applied and the rapid soil reactions in aerated soils mean that very low levels of ions exist in soil solution and the risk of leaching is low.
Boron appears to leach readily from surface soils especially in sandy textured soils with neutral to acidic pH, but much less so in heavy clay soils (Saarela, 1985). Th ree years aft er it was applied to soils in Finland, less than 25-40 % of the B was recovered in the hot water soluble B fraction from the surface 25 cm layer of sandy and loamy soils, whereas all of the added B was recovered in an heavy clay soil. In the sandy and loamy soils, signifi cant B accumulated in the 25-50 cm layer. However, even in soils with 200- 400 g clay/kg, B leaching was reported by Parker and Gardner (1982) and Wild and Mazaheri (1979). Pinyerd et al. (1984) found a linear relationship between cumulative rainfall and B leaching from the ploughed layer (0-25 cm) of a loamy sand with low organic matter levels. However, whilst leaching resulted in soil B levels in the 0-25 cm layer declining aft er 1 year to the same level as in the unfertilised soil, all the fertiliser B added (up to 10 kg B/ha) was recovered in the B horizon suggesting that it had not been lost from the rooting zone. Similarly, in the studies of Baker and Mortenson (1966), extractable B levels in soils treated with B fertiliser remained higher than untreated soils, 5 years aft er the application.
In three contrasting soils of south-east China, leaching of B below 40 cm depth generally was not evident despite the fact that sites experienced 1500-1700 mm annual rainfall, most of it concentrated in 8 months, and despite the fact that the soils contained 200-260 g clay/kg in the surface layers (Wang et al., 1997). Wang et al. (1997) showed there was more evidence of downwards movement of B when 3.3 kg B/ha was applied than with 1.65 kg B/ha, but there was no measurable increase in extractable soil B below 40 cm depth in either case. Repeat application of 3.3 kg B/ha for two successive years
increased extractable B by 0.1 mg B/kg below 60 cm depth in the sandy loam alluvial soil, but even the application of 3.3 kg B/ha for the third successive year did not increase extractable B below 60 cm in the red soil. Boron leaching below 20 cm from borax applications accounted for 0-37 % of 9.9 kg B/ha applied. However, Wang et al. (1997) presented evidence that most of the B leached below the 0-20 cm layer accumulated in the 20-40 and 40-60 cm layers where it probably remained accessible to the roots of oilseed rape. Th us, whilst fertiliser B was probably not lost from the root zone by leaching, redistribution of B in the 0-40 or 0-60 cm layers by leaching dilutes the added B in a larger volume of soil. Th e accumulation of amorphous Fe oxyhydroxides in soils, which are alternately fl ooded and drained, may decrease B leaching (Jin et al., 1987; Tsadilas et al., 1994) and may account for the limited evidence of B leaching from the B fertiliser additions reported by Wang et al. (1997; 1999).
A number of studies have examined Zn leaching and concluded that little Zn leaching occurs under most conditions at recommended rates of Zn fertiliser application. Brennan and McGrath (1988) found that most of the applied Zn was recovered within 3-5 cm of its placement on a very sandy soil (4 % clay), aft er 1438 mm of cumulative rainfall that fell mostly over a 5-month period. Th ere was no evidence of Zn movement more than 6 cm depth from an initial application of 0.75 kg Zn/ha as sulphate salt. When the Zn rate was increased tenfold to 22.4 kg/ha or greater, 12 % of the added Zn was recovered in the 5-15 cm soil layer. Th erefore, on a permeable sandy soil, only at rates of application more than 10 times higher than recommended was there clear evidence of Zn leaching but, even so, the depth of penetration of Zn in the leaching front was < 15 cm. Hence Zn leaching is unlikely except under circumstances such as described by He et al. (2006) where high Zn loadings have occurred on acid sandy soils from past use of fungicides.
For Mo, the extent of leaching depends on soil Mo sorption. On alkaline sands, Jones and Belling (1967) reported 60-95 % of added Mo was leached below 16 cm depth with only 444 mm of rainfall equivalent. In acid soils where Mo availability is lower due to Mo sorption, the extent of Mo leaching is variable. On an acid sand (pH 5.2-5.7), Jones and Belling (1967) found 50 % of the Mo added was leached from 16 cm columns by 450 mm of water. By contrast, Riley et al. (1987), found that only 10 % of Mo leached from two grey sands (< 1 % clay) and negligible Mo leached from three acid sands (pH 5-5.4; 5-14 % clay) when applied at the rate of 40 g/ha and 500 mm of water was applied. Hence, the cases where Mo leaching was reported involve higher rates of application than normally applied to correct defi ciency. Since Mo defi ciency is not encountered on alkaline soils, fertiliser application on them is unlikely, and Mo leaching would only be from native soil Mo or Mo supplied in other soil additives.
Copper is remarkably immobile in soil and hence unlikely to leach. Indeed, the immobility of Cu is such that fertiliser Cu usually has to be well mixed in the rooting zone to achieve most effi cient uptake by crops (Gartrell, 1981). However, in soils that have accumulated high levels of micronutrients like Cu and Zn from agricultural chemical additions, leaching of these micronutrients can be signifi cant and have impacts on downstream water quality (He et al., 2006).
Micronutrients for sustainable food, fi bre and bioenergy production
18
Iron and Mn also are unlikely to leach in aerobic soils. However, in anaerobic soils, such as those used for paddy rice, Fe and Mn are present at high concentrations in the soil solution as soluble Fe(II) and Mn(II) (Kirk, 2004) and hence susceptible to leaching.