For decades, farmers in Ireland have been able to have good information on their soil fertility regarding phosphorous (P), potassium (K), pH and lime status from soil test results. This estimate of nutrient levels is the foundation that is necessary for good fertiliser planning that will allow a farmer to:
Identify fields that have poor fertility and allow targeted fertiliser applications to address the issue.Identify fields that are rich in nutrients and that can be managed in a way that reduces the expenditure on fertilisers.Unfortunately, in the case of N, no soil test is available that can reliably predict the ability of the soil to supply N. Most of the N on earth is in the atmosphere as gaseous N. Gaseous N accounts for almost 80% of the air that we breathe. The manufacture of fertiliser or biological fixation by legumes involves the conversion of atmospheric N to plant available N (ammonium and nitrate). These compounds are highly volatile in the soil and are readily broken down to gaseous N. Indeed, much of the N applied in fertilisers ends up as gaseous N in the atmosphere, which completes what is known as the N cycle.
Difference between soils
Irish soils typically contain between two and 7.5t/ha of N in the top 10cm of soil. However, most of this N is in the soil organic matter, and is not available to plants for uptake until it has been broken down in the soil. This process releases the N in the soils for uptake by plant roots.
Soil organic matter is variable and consists of fractions with differeing composition and stability. Therefore, the rate at which soil organic matter is broken down to release N is also variable.
Figure 1 shows results from a study in Johnstown Castle that measured grass growth on 28 soils under optimised growth conditions in a lab. Over a five-week period, the grass growth potential of the different soils varied from less than 1t/ha to more than 3t/ha of grass DM.
The large range in DM yield highlights the differences between soils in potential soil N supply for grass growth. This equates to a difference in soil N supply of over 80kg/ha between the lowest and highest yielding soil. Previous studies conducted in Ireland show that the total annual N release to grass can range from 70kg/ha up to 270kg/ha. This difference of up to 200kg/ha could make a substantial difference to a fertiliser plan if it could be reliably predicted. It is potentially a difference of over €200/ha in the fertiliser bill on a farm.
Predicting N supply
Nitrogen is involved in a large number of interacting processes in the soil. It is constantly moving between available and unavailable forms in the soil. Available forms of N are also highly prone to being lost from the soil to the air, or in drainage water.
There are a number of tests available that can estimate the available pool of N present in the soil at the time of sampling, but because N can move so quickly in the soil and also change into different forms, and because it can be lost quickly out of the soil, these tests are of little value to predict the long term availability of soil N. This is particularly problematic in wetter climates such as Ireland. These tests are used in some countries where conditions are drier, resulting in greater stability of the forms of N in the soil.
Various soil N testing methods have been developed, all of which have had mixed levels of success in the field. There are a number of biological N techniques available, but they are slow to process and results can take too long to be returned to be useful for fertiliser planning.
More recently, the focus has been placed on identifying and quantifying the fractions of N in the soil organic matter that can act as an indicator of the N release from the soil over a longer period. One such test is the Illinois soil nitrogen test (ISNT), which has been used successfully to estimate soil N supply on arable soils in North America. Work is ongoing at Johnstown Castle to see if this test might be applicable for Irish grassland.
So far, an evaluation of a range of chemical N tests to predict soil N supply in 35 Irish grassland soils has shown that this test has a strong relationship with some of the standard biological incubation tests. In a follow-up study, the test was positively correlated to grass DM yield and N uptake on these soils over five-week growth intervals. This study indicates that the ISNT may be applicable in the future as a way of helping farmers better match fertiliser N applications to the N supply potential of their soils. This would be a major breakthrough in improving fertiliser efficiency on grassland farms in Ireland.
Timing of N applications
The timing of N application should be based on maintaining a constant supply of good quality grass for grazing livestock. It is important to plan ahead to have optimum grass covers at each stage of the grazing season.
A good start is important so plans for the first N application should be in place in advance of stock turnout. In the early spring, grass growth is dependent on soil temperature and an adequate supply of N.
Grass growth will begin one the soil temperature reaches about 5C and a low rate (28kg/ha (23 units/acre)) N application will help stimulate growth at this point. However, care should be taken where a blanket application is being applied across a farm, as the utilisation of N applied may be poorer in fields with cold, wet soils. It may be between to hold off applications on these fields until ground conditions improve.
The next target for N fertiliser should be in March, between four and six weeks after the first application. The N application rate should be matched to the stocking rate at this point, with rates of 28kg to 49kg/ha (23-40 units/acre) required, depending on the stocking rate.
The next N fertiliser application for grazing should coincide with closing up for the first cut silage in April. Aim for 40kg to 50 kg/ha at this stage (32 to 40 units/acre).
Slurry can also be used to offset fertiliser N costs in spring. Slurry should be targeted towards fields that are low P and K.
This article, written by David Wall, Noeleen McDonald and James Humphreys, was first published in the Irish Farmers Journal on 8 February 2014.
For decades, farmers in Ireland have been able to have good information on their soil fertility regarding phosphorous (P), potassium (K), pH and lime status from soil test results. This estimate of nutrient levels is the foundation that is necessary for good fertiliser planning that will allow a farmer to:
Identify fields that have poor fertility and allow targeted fertiliser applications to address the issue.Identify fields that are rich in nutrients and that can be managed in a way that reduces the expenditure on fertilisers.Unfortunately, in the case of N, no soil test is available that can reliably predict the ability of the soil to supply N. Most of the N on earth is in the atmosphere as gaseous N. Gaseous N accounts for almost 80% of the air that we breathe. The manufacture of fertiliser or biological fixation by legumes involves the conversion of atmospheric N to plant available N (ammonium and nitrate). These compounds are highly volatile in the soil and are readily broken down to gaseous N. Indeed, much of the N applied in fertilisers ends up as gaseous N in the atmosphere, which completes what is known as the N cycle.
Difference between soils
Irish soils typically contain between two and 7.5t/ha of N in the top 10cm of soil. However, most of this N is in the soil organic matter, and is not available to plants for uptake until it has been broken down in the soil. This process releases the N in the soils for uptake by plant roots.
Soil organic matter is variable and consists of fractions with differeing composition and stability. Therefore, the rate at which soil organic matter is broken down to release N is also variable.
Figure 1 shows results from a study in Johnstown Castle that measured grass growth on 28 soils under optimised growth conditions in a lab. Over a five-week period, the grass growth potential of the different soils varied from less than 1t/ha to more than 3t/ha of grass DM.
The large range in DM yield highlights the differences between soils in potential soil N supply for grass growth. This equates to a difference in soil N supply of over 80kg/ha between the lowest and highest yielding soil. Previous studies conducted in Ireland show that the total annual N release to grass can range from 70kg/ha up to 270kg/ha. This difference of up to 200kg/ha could make a substantial difference to a fertiliser plan if it could be reliably predicted. It is potentially a difference of over €200/ha in the fertiliser bill on a farm.
Predicting N supply
Nitrogen is involved in a large number of interacting processes in the soil. It is constantly moving between available and unavailable forms in the soil. Available forms of N are also highly prone to being lost from the soil to the air, or in drainage water.
There are a number of tests available that can estimate the available pool of N present in the soil at the time of sampling, but because N can move so quickly in the soil and also change into different forms, and because it can be lost quickly out of the soil, these tests are of little value to predict the long term availability of soil N. This is particularly problematic in wetter climates such as Ireland. These tests are used in some countries where conditions are drier, resulting in greater stability of the forms of N in the soil.
Various soil N testing methods have been developed, all of which have had mixed levels of success in the field. There are a number of biological N techniques available, but they are slow to process and results can take too long to be returned to be useful for fertiliser planning.
More recently, the focus has been placed on identifying and quantifying the fractions of N in the soil organic matter that can act as an indicator of the N release from the soil over a longer period. One such test is the Illinois soil nitrogen test (ISNT), which has been used successfully to estimate soil N supply on arable soils in North America. Work is ongoing at Johnstown Castle to see if this test might be applicable for Irish grassland.
So far, an evaluation of a range of chemical N tests to predict soil N supply in 35 Irish grassland soils has shown that this test has a strong relationship with some of the standard biological incubation tests. In a follow-up study, the test was positively correlated to grass DM yield and N uptake on these soils over five-week growth intervals. This study indicates that the ISNT may be applicable in the future as a way of helping farmers better match fertiliser N applications to the N supply potential of their soils. This would be a major breakthrough in improving fertiliser efficiency on grassland farms in Ireland.
Timing of N applications
The timing of N application should be based on maintaining a constant supply of good quality grass for grazing livestock. It is important to plan ahead to have optimum grass covers at each stage of the grazing season.
A good start is important so plans for the first N application should be in place in advance of stock turnout. In the early spring, grass growth is dependent on soil temperature and an adequate supply of N.
Grass growth will begin one the soil temperature reaches about 5C and a low rate (28kg/ha (23 units/acre)) N application will help stimulate growth at this point. However, care should be taken where a blanket application is being applied across a farm, as the utilisation of N applied may be poorer in fields with cold, wet soils. It may be between to hold off applications on these fields until ground conditions improve.
The next target for N fertiliser should be in March, between four and six weeks after the first application. The N application rate should be matched to the stocking rate at this point, with rates of 28kg to 49kg/ha (23-40 units/acre) required, depending on the stocking rate.
The next N fertiliser application for grazing should coincide with closing up for the first cut silage in April. Aim for 40kg to 50 kg/ha at this stage (32 to 40 units/acre).
Slurry can also be used to offset fertiliser N costs in spring. Slurry should be targeted towards fields that are low P and K.
This article, written by David Wall, Noeleen McDonald and James Humphreys, was first published in the Irish Farmers Journal on 8 February 2014.
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