There appears to have been an increase in the number of soil samples taken over the last few months, judging by how busy soil-sampling labs have been. While this is good news as it shows there is renewed interest in soil fertility, the acid test is whether the results will be acted upon, or left on the shelf.
Acidity
Speaking of acid, this is the first thing that you should look at when you get the results back.
The water pH figure tells you the acidity of the soil. Soils will be either acid or basic. The optimum soil pH for grassland is 6.3. Soils that are below this are acidic and soils that are higher than this are basic and you don’t want either.
Bacteria thrive at optimal pH. When the pH is high bacteria can’t survive –that’s why we apply lime on cubicles beds as it raises the pH and kills bacteria that could cause mastitis.
Likewise, if you stuck your finger into a drum of acid descaler used for cleaning the milking machine, it would burn your skin and acid does the same in the soil to earthworms and other soil biota and organisms which are essential for healthy living soils.
While most soils in Ireland suffer from low pH or acidity, there are parts of the midlands that have high soil pH, so these soils don’t need any lime.
Most soil sample results give recommendations for the amount of lime needed to correct the pH. In general, each tonne of lime per acre raises soil pH by 0.3, but this varies depending on soil type. The lift is greater in light soils with pH rising by 0.4 to 0.5 and lesser in heavy soils with pH rising by around 0.2 for every tonne of lime spread per acre.
The most lime that should be spread in any one application is 3t/acre. Spreading more than 2t/acre is not advised on heavy land.
Spreading any less than 2t/acre is difficult so if fields need less than 2t/acre group them together and spread them once every couple of years. At prices of around €20/t to €22/t spread, lime is the cheapest fertiliser available.
Target fields that need high application rates first and spread 2t/acre to 3t/acre there. Lime can be spread all year round but the best time to spread is after grazing or cutting, so it won’t stick to the grass.
You should wait a week after spreading slurry or urea before applying lime and wait six weeks or two months before spreading urea on land that got limed. You can spread slurry after lime but you risk losing the nitrogen value of the slurry, but the nitrogen content of slurry is low anyway when spread in summer.
Some farmers in high molybdenum areas don’t spread lime for fear of locking up copper and selenium in grazing animals. While the risk is there, going below pH of 6 is critical and should be avoided, even in high molybdenum areas.
Phosphorus and potassium
The soil sample results will say what the phosphorus (P) and potassium (K) level is in mg/l and also what index the sample is. The soil index is a simple way of looking at the mg/l level. Soil index 1 and 2 is below optimal, index 3 is on target while index 4 is above target and no extra P or K is required.
The range within an index is broad, so it is better to look at mg/l when deciding on how much P and K you are going to spread rather than going entirely on the index.
There are two ways to look at P and K requirements. Firstly, soils at index 1, 2 and 3 need maintenance levels just to keep them at that level. Soils at index 4 do not require any P or K application as there is sufficient soil reserves to cover maintenance requirements. However, these soils should be sampled annually to make sure that levels do not drop below index 3 – a common mistake.
How much maintenance P and K requires depends on offtake, as in how much is being taken off the farm in the form of animal liveweight, milk solids or silage. This can be determined by the stocking rate.
Then you need to look at buildup – how much P and K is required to increase soil reserves to bring them up to optimum. See Table 1 for a breakdown of the maintenance and buildup requirements for soils at various stocking rates and soil indices.
So if you are stocked greater than 2.63 cows/ha and have soils with index 1 for P and K, you need to spread 43kg and 100kg per hectare of P and K, respectively.
Multiplying kg/ha by 0.8 gives you units per acre.
Teagasc soil fertility researcher David Wall recommends spreading out the application of P and K across the year. He recommends spreading about 50% of the P requirement in the spring and then splitting the remainder in two to three applications later in the year. Maintenance levels of K should be spread during the main growing season, with buildup rates spread in autumn to prevent issues with luxurious K uptake in animals.
Of course, slurry is a valuable source of P and K on your farm. Typically, every 1,000 gallons of cattle slurry contains five units of nitrogen, five units of P and 30 units of K. Care should be taken when spreading slurry not to apply more than 3,000 gallons per acre. The risk with this is luxury uptake of K in the grass and when grazed or eaten as silage it can cause metabolic disorders. A high K diet reduces a cow’s ability to mobilise magnesium so it is a risk factor in milk fever and grass tetany.
The most amount of K that should be spread in any one application is 90 units/acre. So while the K requirement for a one-cut silage system is 96 units/acre and for a two-cut silage system is 124 units/acre, these rates include what was spread before closing, what was spread at closing and what was spread after cutting.
Some farms are quickly becoming depleted in K. One possible reason is the increased cutting of surplus grass paddocks for bale silage. David Wall says that for every tonne of silage dry matter, 5kg P/ha and 25kg K/ha is taken up.
Remember, nitrogen and phosphorus are the only nutrients governed by the Nitrates Directive. There are no restrictions on lime or potash use. How much nitrogen and phosphorus you can spread and remain compliant depends on stocking rate, meal feeding levels and soil sample results.
Optimum soil pH is 6.3, spread 2t to 3t/acre of lime to increase soil pH.Work out how much P and K you need to spread in each field and spread out the applications over the year.Don’t forget the value of slurry.Don’t spread more than 90 units of K per acre.
There appears to have been an increase in the number of soil samples taken over the last few months, judging by how busy soil-sampling labs have been. While this is good news as it shows there is renewed interest in soil fertility, the acid test is whether the results will be acted upon, or left on the shelf.
Acidity
Speaking of acid, this is the first thing that you should look at when you get the results back.
The water pH figure tells you the acidity of the soil. Soils will be either acid or basic. The optimum soil pH for grassland is 6.3. Soils that are below this are acidic and soils that are higher than this are basic and you don’t want either.
Bacteria thrive at optimal pH. When the pH is high bacteria can’t survive –that’s why we apply lime on cubicles beds as it raises the pH and kills bacteria that could cause mastitis.
Likewise, if you stuck your finger into a drum of acid descaler used for cleaning the milking machine, it would burn your skin and acid does the same in the soil to earthworms and other soil biota and organisms which are essential for healthy living soils.
While most soils in Ireland suffer from low pH or acidity, there are parts of the midlands that have high soil pH, so these soils don’t need any lime.
Most soil sample results give recommendations for the amount of lime needed to correct the pH. In general, each tonne of lime per acre raises soil pH by 0.3, but this varies depending on soil type. The lift is greater in light soils with pH rising by 0.4 to 0.5 and lesser in heavy soils with pH rising by around 0.2 for every tonne of lime spread per acre.
The most lime that should be spread in any one application is 3t/acre. Spreading more than 2t/acre is not advised on heavy land.
Spreading any less than 2t/acre is difficult so if fields need less than 2t/acre group them together and spread them once every couple of years. At prices of around €20/t to €22/t spread, lime is the cheapest fertiliser available.
Target fields that need high application rates first and spread 2t/acre to 3t/acre there. Lime can be spread all year round but the best time to spread is after grazing or cutting, so it won’t stick to the grass.
You should wait a week after spreading slurry or urea before applying lime and wait six weeks or two months before spreading urea on land that got limed. You can spread slurry after lime but you risk losing the nitrogen value of the slurry, but the nitrogen content of slurry is low anyway when spread in summer.
Some farmers in high molybdenum areas don’t spread lime for fear of locking up copper and selenium in grazing animals. While the risk is there, going below pH of 6 is critical and should be avoided, even in high molybdenum areas.
Phosphorus and potassium
The soil sample results will say what the phosphorus (P) and potassium (K) level is in mg/l and also what index the sample is. The soil index is a simple way of looking at the mg/l level. Soil index 1 and 2 is below optimal, index 3 is on target while index 4 is above target and no extra P or K is required.
The range within an index is broad, so it is better to look at mg/l when deciding on how much P and K you are going to spread rather than going entirely on the index.
There are two ways to look at P and K requirements. Firstly, soils at index 1, 2 and 3 need maintenance levels just to keep them at that level. Soils at index 4 do not require any P or K application as there is sufficient soil reserves to cover maintenance requirements. However, these soils should be sampled annually to make sure that levels do not drop below index 3 – a common mistake.
How much maintenance P and K requires depends on offtake, as in how much is being taken off the farm in the form of animal liveweight, milk solids or silage. This can be determined by the stocking rate.
Then you need to look at buildup – how much P and K is required to increase soil reserves to bring them up to optimum. See Table 1 for a breakdown of the maintenance and buildup requirements for soils at various stocking rates and soil indices.
So if you are stocked greater than 2.63 cows/ha and have soils with index 1 for P and K, you need to spread 43kg and 100kg per hectare of P and K, respectively.
Multiplying kg/ha by 0.8 gives you units per acre.
Teagasc soil fertility researcher David Wall recommends spreading out the application of P and K across the year. He recommends spreading about 50% of the P requirement in the spring and then splitting the remainder in two to three applications later in the year. Maintenance levels of K should be spread during the main growing season, with buildup rates spread in autumn to prevent issues with luxurious K uptake in animals.
Of course, slurry is a valuable source of P and K on your farm. Typically, every 1,000 gallons of cattle slurry contains five units of nitrogen, five units of P and 30 units of K. Care should be taken when spreading slurry not to apply more than 3,000 gallons per acre. The risk with this is luxury uptake of K in the grass and when grazed or eaten as silage it can cause metabolic disorders. A high K diet reduces a cow’s ability to mobilise magnesium so it is a risk factor in milk fever and grass tetany.
The most amount of K that should be spread in any one application is 90 units/acre. So while the K requirement for a one-cut silage system is 96 units/acre and for a two-cut silage system is 124 units/acre, these rates include what was spread before closing, what was spread at closing and what was spread after cutting.
Some farms are quickly becoming depleted in K. One possible reason is the increased cutting of surplus grass paddocks for bale silage. David Wall says that for every tonne of silage dry matter, 5kg P/ha and 25kg K/ha is taken up.
Remember, nitrogen and phosphorus are the only nutrients governed by the Nitrates Directive. There are no restrictions on lime or potash use. How much nitrogen and phosphorus you can spread and remain compliant depends on stocking rate, meal feeding levels and soil sample results.
Optimum soil pH is 6.3, spread 2t to 3t/acre of lime to increase soil pH.Work out how much P and K you need to spread in each field and spread out the applications over the year.Don’t forget the value of slurry.Don’t spread more than 90 units of K per acre.
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