| | Manure as a crop nutrient source | Impact of manure on forms and fractions of P | Other organic P sources
Manure As a Crop Nutrient Source
A major difference between animal manure and commercial fertilizer is that some of the nutrients in manure are in organic form and must be mineralized before they are available to the plant. Therefore manure acts as a slowly available nutrient source. While manure is an inexpensive source of nutrients, its nutrient content is variable, and nutrient availability depends on the manure characteristics and their relationship with the soil. Manure application guidelines strongly recommend regular manure sampling and analysis for nutrient content.
Commercial fertilizer is designed in physical form and nutrient content for the best placement and utilization by crops, such as banding or seed placement. Use of manure reduces placement options, since banding and placement with the seed are impractical for raw manure. The nutrient balance of manure is also not matched to crop requirements. Low N:P ratios can result in a need for fertilizer N when manure is applied at rates to match crop P requirements, or in soil P loading with time when manure is applied at rates to meet crop N requirements. Manure has relatively low concentrations of nutrients compared to commercial fertilizers, resulting in the need to handle large volumes of water and organic carbon material with little or no nutrient value. Thus, producers using manure instead of commercial fertilizer need different equipment, must move higher volumes per unit weight of nutrient, and must consider issues such as odour. Good crop management requires additional sampling and analysis of manure, usually at an extra cost.
Manure, however, has benefits beyond being a nutrient source. The addition of organic carbon to soil can improve physical and chemical characteristics. Robertson and McGill (1989) reported that long-term annual application of 9 tonnes ha-1 to a Luvisolic soil at the University of Alberta’s long-term research plots at Breton, Alberta, resulted in one of the highest yielding treatments and did not change the pH. Izaurralde et al. (2001) determined that for the 51-year treatment history at the Breton plots, manure additions to a wheat-fallow system resulted in a net increase in soil organic carbon, whereas the control and fertilizer treatments resulted in a net loss. Manure treatments also resulted in the highest above-ground carbon productivity.
Impact of Manure on Forms and Fractions of P
For nutrient management, it is important to understand not only the amount of P in manure but also the forms of P in the manure and how they influence the P forms in the soil once manure has been added. Phosphorus content of manure is variable between the species of animal, within a given species, and within a management practice for a given species. Similarly, P forms vary from one species of manure to another, as illustrated in Table 4.1.
DeLuca and DeLuca (1997), citing values from the literature, reported that the total P content of beef feedlot manure ranged from 0.1 to 0.8% by weight, averaging 0.4%. Sharpley and Moyer (2000) stated that P concentrations varied as a result of diet, manure collection, storage and treatment. In citing literature, they reported coefficients of variation ranging from 30 to 100%.
Table 1: Forms of P, inorganic plus organic, as a percentage of total P in various manure and compost samples (Sharpley and Moyer 2000).
P Form | Percentage of total P |
Dairy | Poultry | Swine
Slurry |
Manure | Compost | Manure | Compost |
| Water soluble | 63 | 16 | 34 | 22 | 23 |
| NaHCO3 (labile) | 11 | 37 | 29 | 39 | 15 |
| NaOH (moderately labile) | 13 | 11 | 3 | 11 | 51 |
| Acid (slowly available) | 1 | 33 | 32 | 27 | 10 |
Manure application has resulted in increased levels of moderately labile P and total P in the soil after a single application (Qian and Schoenau 2000a) and after long-term applications (Campbell et al. 1986; Dormaar and Chang 1995; Tran and N’dayegamiye 1995). Campbell et al. (1986) and Qian and Schoenau (2000a) reported that long-term manure application gradually improves the P-supplying capacity of soils. Dormaar and Chang (1995) reported that relatively high levels of P, 15 to 46%, were in soluble or labile forms following 20 years of manure application.
On previously non-manured soils, however, Qian and Schoenau (2000a) found that a single hog manure addition had little effect on P availability in the soil. Hog manure is high in labile forms of P (Sharpley and Moyer 2000; Qian and Schoenau 2000a); however, despite the high proportion of labile P forms, application of the liquid hog manure did not appreciably increase the labile P forms in the soil (Table 4.2). This suggests that the most soluble forms of P in the soil are not affected by manure during the short term, but long-term repeated applications result in increased levels of these forms. MnKeni and MacKenzie (1985) explained the increases in more available forms of P in long-term manure studies as the result of the cumulative effect of repeated manure additions saturating P fixation sites in the soil, therefore reducing P adsorption.
Qian and Schoenau (2000a) reported that P from manure had changed rapidly into more stable forms within two weeks. They suggested that environmental risk of P from manured lands degrading water quality is probably lower in the early years of manure application and increased with repeated applications, where manure was applied in excess of crop removal rates for several years.
When compared to fertilizer applications, Qian and Schoenau (2000a) reported that even high additions of urea had little influence on the P forms in the soil, suggesting the addition of N alone was not impacting P transformations. Campbell et al. (1986) reported variable effects of P in commercial fertilizer and manure on different forms of soil P, depending on rates of application and the initial level and distribution of P in the soil profile. McKenzie et al. (1992 a, b) reported that for both a Luvisolic and a Chernozemic soil, addition of P fertilizer increased Pi forms, whereas addition of N fertilizer indirectly led to an increase of Po forms. Cropping with no fertilizer or manure inputs resulted in conversion of more stable P forms to more labile forms and a continuous drain on all P forms (McKenzie et al. 1992a, b).
Table 2: Forms of P two weeks and 16 weeks following a single application of (a) manure and (b) urea, in a Black Chernozem (Qian and Schoenau 2000a).
a. Manure
P form
following manure application | Soil P content (mg kg-1) |
2 weeks | 16 weeks |
No
manure | 100 ppm N | 400 ppm N | No
manure | 100 ppm N | 400 ppm N |
| Resin (labile Pi) | 37 | 36 | 36 | 36 | 35 | 36 |
| NaHCO3 (labile) | Pi | 20 | 25 | 24 | 21 | 25 | 25 |
| Po | 6.0 | 1.7 | 2.4 | 7.6 | 5.1 | 6.2 |
| NaOH (mod. labile) | Pi | 39 | 39 | 36 | 32 | 32 | 33 |
| Po | 120 | 139 | 133 | 103 | 116 | 120 |
| Dil. HCl (slowly available) | 217 | 226 | 229 | 196 | 190 | 193 |
| Conc. HCl (least avail.) | Pi | 50 | 46 | 44 | 42 | 46 | 46 |
| Po | 132 | 135 | 141 | 165 | 171 | 173 |
| Modified Kelowna | | | | 16.7 | 17.1 | 16.9 |
| Total P | 710 | 723 | 728 | 708 | 723 | 728 |
b. Urea
P form
following urea application | Soil P content (mg kg-1) |
2 weeks | 16 weeks |
No
Urea | 100 ppm N | 400 ppm N | No
Urea | 100 ppm N | 400 ppm N |
| Resin | 37 | 36 | 36 | 36 | 35 | 34 |
| NaHCO3 (labile) | Pi | 20 | 21 | 22 | 21 | 24 | 23 |
| Po | 6.0 | 4.1 | 4.3 | 7.6 | 6.1 | 7.0 |
| NaOH (mod. labile) | Pi | 39 | 39 | 42 | 32 | 33 | 32 |
| Po | 120 | 123 | 122 | 103 | 103 | 113 |
| Dil. HCl (slowly available) | 217 | 210 | 200 | 196 | 192 | 185 |
| Conc. HCl (least avail.) | Pi | 50 | 45 | 47 | 42 | 46 | 44 |
| Po | 132 | 131 | 127 | 165 | 167 | 167 |
| Modified Kelowna | | | | 16.7 | 16.6 | 15.9 |
| Total P | 710 | 719 | 709 | 708 | 717 | 716 |
Qian and Schoenau (2000b) reported that when swine manure had been applied to two Saskatchewan soils, the P supply to a canola crop was constant or increased through the growing season. Sharpley (1996) found that release of P from manured soils initially dropped rapidly, followed by a more gradual release of P with time. The rate of P release depended upon soil type, and more specifically on the P sorption saturation of the soil. Their results suggest that under similar STP levels and cropping, soil type would control the rate of P availability to crop from manure P sources.
Beauchemin and Simard (2000) compared an area where P additions were primarily from long-term fertilizer sources to an area where P sources were primarily from long-term manure application. They reported higher concentrations of Pt, Po, water-soluble P, and STP in the B and C horizons in the area dominated by manure application. In the A horizon, STP levels were essentially the same, but Pt and Po levels were higher in the manured areas. Their study indicated that in medium and coarse textured soils of eastern Canada, long-term manure applications have contributed to downward movement of a wide variety of P forms.
Application of composted beef manure has resulted in higher STP levels and increased the long-term levels of bioavailable P compared to that of manure (Eghball and Power 1999; Schwartz and Dao 2000). Schwartz and Dao (2000) calculated that 47% and 71% of manure and compost P, respectively, become extractable and/or available during a 25-month cropping period.
Although regular manure application results in a different proportion of the inorganic and organic soil P forms, for soil testing and crop fertility recommendations, STP levels are interpreted the same way regardless of whether they are from fertilized or manured soils (J. Ashworth, personal communication).
Other Organic P Sources
Municipal biosolids, in the absence of heavy metal or other contaminant concerns, are generally applied to land at rates in balance with the N requirements of the agricultural crop (Zebarth et al. 2000). Cogger et al. (1999) reported higher N recovery in the second year of application than in the first, when biosolids were applied to forage crops. The fertilizer equivalency (i.e. the ratio of the apparent recovery of biosolid N in comparison to the apparent recovery of fertilizer N) averaged 32% and 50% for the first and second years of application, respectively. Higher N recovery in the second year was also observed in dryland wheat (Cogger et al.1998). In both cases, delayed mineralization of N was a factor.
Zebarth et al. (2000) found application of municipal biosolids to forage grass resulted in an increased cumulative uptake of micro-nutrients, especially copper (Cu). Forage Cu values in the first year reached levels of possible concern for some animal species.
Food processing effluent nutrient distribution must be known before it is used in long-term application to land. Barl and McKenzie (1995) reported that processing effluent from a potato processing plant had potassium levels that ranged from 466 to 707 mg kg-1, values that are about 10 times the level of P and six times the level of N in the effluent. If this effluent was applied at the conventional N or P rates, it would quickly accumulate potassium levels in soils. Excessive potassium levels are capable of having a negative impact on crops (R.H. McKenzie, personal communication) and are capable of leaching to groundwater in sandy soil (Havlin et al.1999).
Because nutrient levels in organic P sources tend not to be matched to crop uptake, and nutrient movement into the water system can degrade water quality, care must be taken to understand the complete nutrient status of both the soil and the organic P source. Regular testing and nutrient planning should be part of any long-term management of organic P sources. |
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