If you can measure it, you can manage it, goes the adage and soil fertility is one of those things. Regular monitoring of pH, P and K levels in soils can go a long way to driving farm productivity. A recent survey of farmers who were reseeding grassland showed that a large majority of fields were below the optimum levels for grass growth:
Nearly 60% of fields had a pH less than 6.0.Nearly 40% of fields had P levels below Index 2.Nearly 60% of fields had K levels below Index 2.Taking the pH in isolation, by getting it correct, grass production should rise by around one tonne of DM per hectare. This is worth around £50/ha. Optimising grassland pH will also increase the amount of nitrogen released from the soil organic matter by around 80kg/ha/yr. At £250/t for 34.5% nitrogen, this is worth around £58/ha. These two factors alone could potentially pay for three to four tonnes of lime per hectare to be applied.
Had these fields been reseeded without correcting the nutrient levels, they would revert to native weed grasses within a couple of years and demonstrate little return on the investment in reseeding.
Liming grassland is often an afterthought and yet work by Teagasc in Ireland would suggest that a significant level of lime equivalent is lost from grassland over the course of a year:
Drainage water removes between 250kg and 625kg/ha of lime equivalent, depending on rainfall levels.Cutting and grazing.– A 5t DM/ha silage crop removes around 75kg/ha of lime equivalent.
– A finished steer removes around 25kg/ha of lime equivalent.
Fertiliser: for every 1kg of nitrogen applied, there is a need for around 2kg lime.So, a field with two cuts of silage totalling 10t DM/ha and receiving moderate fertiliser levels, has a need for 570kg to 950kg of lime per hectare annually. Fields grazing store or fat cattle and receiving moderate amounts of fertiliser have a need for 350kg to 730kg of lime per hectare annually.
Soils that are low in pH also reduce the effectiveness of the fertiliser applied to them. Table 1 demonstrates the availability of fertiliser nutrients across the pH range.
If a field is at pH 5.5, for every 600kg bag of 20:10:10 applied, 28kg of N, 31kg of P and 14kg of K is being wasted. At today’s values, that is equivalent to the value of 1.5t of lime being wasted for every bag of fertiliser being applied.
Once pH has been corrected, building soil P and K indices to moderate status is the next part of the equation. Phosphorus is important for root development and early growth, especially in clover. Potash is the second most important nutrient for grassland after nitrogen. However, applications of potash to grassland require care to ensure that hypomagnesaemia (grass staggers) is not induced. Under grazing situations, the majority of both P and K are recycled and small maintenance doses are all that is required if soil indices are correct.
Cutting situations are very different with a much higher offtake of both nutrients. The best practice is to replace what you are taking off. However, if requirements are above 90kg/ha, then restricting the first fertiliser dose in the spring to 90kg/ha will reduce the risk of luxury uptake of K. The remainder can be applied later in the season.
Soils that are high in P or K can be run down over the course of a few years by applying a reduced quantity of fertiliser each year until the field reaches the Index M. This offers significant savings as adding more to soil indices at this point will not see an economical return. Low P and K status results in poorer grass yield with low index soils showing a yield penalty of 2t of DM/ha. Again, at £50/t DM, this shows a yield penalty worth £100/ha.
With the cost of a basic soil test coming in at around £30 per 10 hectares, the question has to be is it really cost effective not to test soils?
If you can measure it, you can manage it, goes the adage and soil fertility is one of those things. Regular monitoring of pH, P and K levels in soils can go a long way to driving farm productivity. A recent survey of farmers who were reseeding grassland showed that a large majority of fields were below the optimum levels for grass growth:
Nearly 60% of fields had a pH less than 6.0.Nearly 40% of fields had P levels below Index 2.Nearly 60% of fields had K levels below Index 2.Taking the pH in isolation, by getting it correct, grass production should rise by around one tonne of DM per hectare. This is worth around £50/ha. Optimising grassland pH will also increase the amount of nitrogen released from the soil organic matter by around 80kg/ha/yr. At £250/t for 34.5% nitrogen, this is worth around £58/ha. These two factors alone could potentially pay for three to four tonnes of lime per hectare to be applied.
Had these fields been reseeded without correcting the nutrient levels, they would revert to native weed grasses within a couple of years and demonstrate little return on the investment in reseeding.
Liming grassland is often an afterthought and yet work by Teagasc in Ireland would suggest that a significant level of lime equivalent is lost from grassland over the course of a year:
Drainage water removes between 250kg and 625kg/ha of lime equivalent, depending on rainfall levels.Cutting and grazing.– A 5t DM/ha silage crop removes around 75kg/ha of lime equivalent.
– A finished steer removes around 25kg/ha of lime equivalent.
Fertiliser: for every 1kg of nitrogen applied, there is a need for around 2kg lime.So, a field with two cuts of silage totalling 10t DM/ha and receiving moderate fertiliser levels, has a need for 570kg to 950kg of lime per hectare annually. Fields grazing store or fat cattle and receiving moderate amounts of fertiliser have a need for 350kg to 730kg of lime per hectare annually.
Soils that are low in pH also reduce the effectiveness of the fertiliser applied to them. Table 1 demonstrates the availability of fertiliser nutrients across the pH range.
If a field is at pH 5.5, for every 600kg bag of 20:10:10 applied, 28kg of N, 31kg of P and 14kg of K is being wasted. At today’s values, that is equivalent to the value of 1.5t of lime being wasted for every bag of fertiliser being applied.
Once pH has been corrected, building soil P and K indices to moderate status is the next part of the equation. Phosphorus is important for root development and early growth, especially in clover. Potash is the second most important nutrient for grassland after nitrogen. However, applications of potash to grassland require care to ensure that hypomagnesaemia (grass staggers) is not induced. Under grazing situations, the majority of both P and K are recycled and small maintenance doses are all that is required if soil indices are correct.
Cutting situations are very different with a much higher offtake of both nutrients. The best practice is to replace what you are taking off. However, if requirements are above 90kg/ha, then restricting the first fertiliser dose in the spring to 90kg/ha will reduce the risk of luxury uptake of K. The remainder can be applied later in the season.
Soils that are high in P or K can be run down over the course of a few years by applying a reduced quantity of fertiliser each year until the field reaches the Index M. This offers significant savings as adding more to soil indices at this point will not see an economical return. Low P and K status results in poorer grass yield with low index soils showing a yield penalty of 2t of DM/ha. Again, at £50/t DM, this shows a yield penalty worth £100/ha.
With the cost of a basic soil test coming in at around £30 per 10 hectares, the question has to be is it really cost effective not to test soils?
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