STEP 5 Determining Acceptable Application Rates
Safe rates of manure application depend on the nutrient value of the manure, nutrients available in the soil, and the expected nutrient uptake of the crop to be grown (Saskatchewan Agriculture and Food 1997, Johnson and Eckert 1995). These values are determined by testing of manure and soil as described in Steps 1 and 2 above.
Crops requiring nitrogen, phosphorus and potassium will benefit most from manure application. Nutrient levels that exceed critical levels (Table 4) will lower crop production or threaten the environment. The health of crops planted in soils with nutrient levels below the critical levels will be agronomically compromised. Soils with nutrient levels above the critical levels represent potential contaminants for surface water and groundwater, and may cause some micronutrients to be less available to crops or be toxic to growing crops. These critical levels are still being researched, and they are provided only as guidelines, and must be adjusted based on specific field, soil and landscape situations.
Table 4. Critical Nutrient Levels (Based on Ross McKenzie, pers. comm. 1999).
|Nutrient||Critical Nutrient Levels|
|NO3-N (0-60cm) (0-24")||<56||< 50||>280-390||> 250 300|
|P (0-15cm) (0-6")||<22.5||< 20||>225-450||> 200 400|
|K (0-15cm) (0-6")||<280-390||< 250 350||>3350||> 3000|
|SO4- (0-15cm) (0-6")||<11.2||< 10||>112||> 100|
Application Rates Based on Nitrogen
Animal manure has traditionally been applied at rates designed to meet the nitrogen requirements of a crop. Previous nitrogen application rates recommended for Alberta were 30 and 60 Mg/ha (13 and 26 tons/ac) on dryland and irrigated fields, respectively. However, studies by Chang et al (1991) and Chang and Entz (1996) at the Agriculture and Agri-Food Canada Research Centre in Lethbridge found that long-term application at these rates caused significant loss of nitrate by leaching to shallow groundwater under irrigation. Chang and Travis (1995) concluded that heavy rates of cattle manure have minimal agronomic benefits for barley production, but significant negative impacts on soil, water and atmospheric quality.
In a study conducted by Alberta Agriculture in southern Alberta, the recommended annual application rate of manure with a 60% moisture content was less than 60 Mg/ha (26 tons/ac) in the short term (3-5 years), especially on coarse-textured, irrigated soils over shallow groundwater. Lower annual rates of 20 or less Mg/ha (9 tons/ac) may be required if applications are continued over a longer period (5 -10 years) (Olson et.al.,1999).
The County of Lethbridge and Alberta Agriculture, Food and Rural Development are conducting research to determine what manure rates can be safely applied to an established alfalfa crop. Allowing manure application to alfalfa would allow an increase in the land base on which manure could be applied (T. Ormann pers. comm. 1999).
When manure is applied at rates designed to meet crop requirements, the operator must estimate the expected amount of N to be mineralized from the manure each year. Research conducted in Alberta indicates that about 35% of the N in beef manure is available for plant growth in the first year (McGill 1996). Research from Texas indicated that about 40% of the total nitrogen applied from manure is available the first year, 20% the second year, and 10% the third year (Glover et. al. 1994). It is generally thought that after 4-5 years very little nitrogen is released from the remaining manure. Data from Ohio (Table 5) extends the benefits of residual organic nitrogen from manure over ten years (Johnson and Eckert 1995).
Table 5. Percent of residual organic nitrogen made available from manure applied in previous years (Johnson and Eckert 1995).
|Years After Application||Percent of Residual N Available|
application equipment evaluation
The application equipment must be able to apply the manure:
A manure application rate of 16.4 tonnes/ha (7.3 tons/ac) dry weight would be needed to supply the 100 kg/ha of N needed to meet the crop requirements for a silage barley yield of 3200 kg/ha. Because N continues to be mineralized from manure for about five years, the annual application rate would be progressively decreased until a sustainable manure application rate of 6.0-6.5 tonnes/ha (2.7-2.9 tons/ac) is reached after 11 to 14 years of manure application. The amount of N supplied by manure in the first year of application, for a given N%, is summarized in Table 6.
Table 6. Application rates for solid beef manure to satisfy varying crop requirements. Manure application rates are expressed in tons/acre, and assume the manure has a moisture content of 50% (West 1996b).
|Total N||N Supplied by Manure for Crop Production (lbs/acre)*|
*lbs/acre x 1.1206 = kg/ha
Saskatchewan Agriculture and Food (1997) summarize three approaches to manure application based on nitrogen. Based on work in Alberta, there is still a need to monitor phosphorus build up.
1. Apply the manure annually to the same field at a rate that supplies all the nitrogen requirements of the crop in the first year of application. Decrease the annual rate of manure application each year until the combination of annual manure addition and mineralization from the previous application of manure meets the nitrogen requirements of the crop. Once this balance is reached, the application rate of manure remains virtually constant for all successive years under an established crop rotation plan.
2. Apply the manure annually to the same field at a constant rate that will not meet all the nitrogen requirements of the crop in the year of growth. Commercial nitrogen fertilizer must be added to meet the deficiency in the crops nitrogen requirements. This results in a decreasing annual addition of commercial nitrogen until the manure supplies all the nitrogen requirements of the crop. Once this balance is reached, the application rate of manure remains virtually constant for all successive years under an established crop rotation plan.
3. Apply the manure to a field every two to four years. There may be excess available nitrogen in the initial years, and a need will develop for commercial nitrogen in the latter years before the next manure application. This management system is commonly used to reduce the application costs of the manure. When fields are manured only periodically under this management approach, the application rate should not exceed 200% of the initial crop nutrient requirement. This practice is not recommended, since it does not use manure nitrogen efficiently, and could promote nitrate leaching to groundwater, and can promote other nutrient contamination such as phosphorus movement via runoff.
Application Rates Based on Phosphorus
Manure application rates have traditionally been based on crop nitrogen requirements. Although average manure contains about three times more nitrogen than phosphorus, major hay and grain crops use eight times more nitrogen than phosphorus, so excess phosphorus is supplied when manure is used to meet all the nitrogen needs of the crop (Daniel et. al. 1994, Simard 1998). About 40% to 60% of total P in manure becomes plant-available in the first year after application and nearly all the phosphorus becomes available within three years after application (Ewanek 1996). Manure application rates based on meeting nitrogen requirements have resulted in high phosphorus levels in soil, and the potential for contamination of surface water and groundwater. Excess levels of potassium, copper and zinc can also result when manure is used to meet crop nitrogen requirements (Hao et. al. 2000).
|Phosphorus application should take into account:
1. slopes and slope length
In view of the concerns over phosphorus and salts, Gillund et. al. (1996) recommend that manure application rates should be based on the constituent in manure that requires the greatest land area for crop uptake. They recommend that this constituent should be phosphorus, potassium or sodium (Gillund et. al. 1996). Gillund et. al. (1996) documented the nutrient uptake needs of a typical barley crop (Table 7).
Table 7. Nutrient uptake by a 3.2 tonnes/ha (60 bu/acre) crop of barley (Gillund et al. 1996).
Twenty years of manure application based on rates necessary to achieve nitrogen requirements could potentially result in an accumulation of 553 kg/ha (493 lbs/ac) of phosphorus (Gillund et. al. 1996). Any manure application rate over 2.4 tonnes/ ha (1 ton/ac) for barley silage production in the thin black soil zone of Alberta supplies more P than is needed for plant growth (Gillund et. al. 1996). Soil phosphorus levels on Alberta cropped or pasture fields should not exceed 225 kg/ha (200 lbs/ac) on inclined landscapes, and 450 kg/ha (400 lbs/ac) on normal soils and landscapes. If manure must be applied to fields at levels greater than 30 ppm [(67 kg/ha)(60 lbs/ac) ] of phosphorus, Johnson and Eckert (1995) recommend the following.
For cropland, manure must not be applied where the Bray P1 level in the top 20 cm (8") of soil exceeds 150 ppm 335 kg/ha (300 lb/ac) of phosphorus.
Manure application rates should not cause long-term phosphorus levels to increase appreciably.
Incorporate manure below the depth of tillage, generally deeper than 20 cm (8"), using rates great enough to satisfy nitrogen requirements for a succeeding grass crop.
Crops should be monitored for nutrient deficiencies using plant-tissue analysis. Increasing soil test phosphorus and potassium levels above recommended levels increases the probability of yield-reducing nutrient imbalances.
Relationships between soil test P levels and the total quantity of P lost in runoff are complex and not fully understood at this time (Snyder et. al. 1998). Although dissolved P in surface runoff tends to increase with increasing levels of soil test P, the relationship varies with many factors, including soil type, depth of soil sampled, soil test extraction method, crop grown, soil management, rainfall amount, intensity and duration, soil slope, water infiltration rate, and ground cover.
Fields most at risk for high losses of P are those where high soil P levels coincide with zones of active surface runoff or erosion. The P Index (Table 8) is a crude estimator to rank sites on the relative risk of loss of P to surface water. The application of the P Index is region specific, and varies with factors such as water use.
Research being conducted by the County of Lethbridge, is studying the potential of alfalfa to utilize P that has accumulated in soils due to heavy applications of beef manure. Soil P removal by tap-rooted alfalfa, fibrous alfalfa and silage is being compared (T. Ormann pers. comm., 1999). Amendments to manure can increase the N: P ratio of manure, which brings it closer to the N: P ratio of crop uptake. In this way, less N is lost by ammonia volatilization and manure application can be based on N requirements without adding more P than is necessary.
Table 8. Phosphorus Index Interpretations (Beegle 1999).
|Low||N based nutrient
Low potential for P movement from the site.
Current farming practices should protect water quality.
|Medium||N based nutrient
management planning with P considerations.
Medium potential for P movement from the site and environmental degradation.
Some remedial action may be required to protect water quality.
|High||P based nutrient management planning.
High potential for P movement from the site and adverse impact on the environment.
Changes in P management will be required to protect water quality.
Researchers in Alberta are currently working on a project titled "Implementing Phosphorus Standards for Agriculture Land in Alberta: Implications, Evaluation and Production Management Options". The expert Technical Committee, commissioned by the Livestock Regulatory Stakeholder Advisory Group, has requested a phosphorus management strategy be developed by 2001 (Howard et. al. 1999).
Application Rates Based on Salts
The application of excess manure can cause salt levels to accumulate and cause potential problems for soil and water. Sodium, potassium, chloride, and magnesium all tend to increase with increasing rates of manure application (Olson et. al.1999). As noted previously, soil productivity is reduced when the SAR (sodium) is greater than 7 and the EC (total salt content) is greater than 4 dS/m. One suggestion for manure application rates is that soil receiving manure should not have an SAR greater than 4, and added manure should not increase the SAR of the system by more than one unit (Gillund et. al. 1996).
In a survey of beef manure from six feedlots in southern Alberta, the salt and sodium contents were high enough to be potentially problematic for crop nutrition (Chang 1997). Twenty years of manure application based on rates necessary to achieve nitrogen requirements for silage barley would cause a net accumulation of a large amount of sodium and chloride (Gillund et. al. 1996). Based on oilfield guidelines of a lifetime Cl loading of 1600 kg/ha (1430 lbs/ac), the manure spreading area has a life of 11.4 years. Higher manure spreading rates reduce the environmentally sustainable life span of the receiving soil (Gillund et. al. 1996).
On grain land, the application of more than 1.3 tonnes/ha of manure (1160 lbs/ac) will result in the application of more potassium than the crop will use (Gillund et. al 1996).