There is an increasing awareness across all farming sectors of the importance of soil fertility, but few realise its important in plant production. In this article, we look at the principles behind soil fertility and plant fertilisation. This requires a basic understanding of feeding a growing plant and of what soil fertility means. Let’s begin with the latter.
Soil fertility
Soil and especially clay provides the backdrop for a range of chemical processes and it also acts as a store for nutrients. Given that clay is the smallest of the particles that make up your soil, there is an awful lot of clay in an acre, even where the percentage clay in the texture may be low. A soil test measures the amount of readily available nutrient held in this pool of clay and the more that is present, the higher nutrient availability will be.
Higher nutrient availability means a higher test reading and a higher soil index. Look at it as nutrient density rather than quantity. The greater the density, the easier it is for a plant root to find and access.
No matter how much nutrient you apply to a soil, the density of applied granules in the soil bulk is very low compared with normal soil distribution. I have no idea what the relative odds are of a root accessing a nutrient molecule from the clay versus a fertiliser granule might be but the magnitude of the difference is likely to be hundreds if not thousands in favour of availability from the soil.
This is one of the reasons why soil fertility is so important and why it cannot be improved via a single fertiliser application, regardless of the quantity applied.
Having the fertility in the soil rather than coming solely from the fertiliser provides a far greater chance that the plant will always have access to what it needs and that is important to build the highest potential yield possible.
While fertility is not the only characteristic of a soil that matters, it is important. Having your soil in good biological condition with the help of adequate organic matter is another very useful way to enhance the availability of the soil’s nutrient pool. But again this is a long-term scenario with no instant fixes.
Balancing off-take
Modern crop fertilisation means supplying what is needed by the crop for optimum yield plus a little extra to help build soil fertility, where necessary. This is the principle behind recommendations for P and K in particular.
As crops grow, they take up nutrients from applied fertiliser and the soil. The efficiency of this process can be influenced by where the fertiliser is applied, eg combine drilling versus surface application for a spring crop. Crops also need additional nutrient to come from the soil as they generally need much more for optimum growth than they actually remove.
The amount of offtake is governed by the crop grown, its yield and whether or not straw is removed. But even if straw is left on the land it can take quite a number of years for the nutrient returned to impact on soil availability.
One other important factor is that the level of fertility in the soil to begin with can influence the yield level and so soil fertility itself can have a direct bearing on offtake. In general, winter wheat and barley remove less nutrient per tonne than their spring counterparts and oat crops take off even more.
Crops need more nutrient for optimum growth than they remove. They get this from the soil (if it is available) and then they return it back to the soil as the crop matures and is available for the following crop. It is estimated that an 8t/ha cereal crop will take up around 190kg N/ha, 35kg P/ha and 240kg K/ha.
In all instances, part of the uptake of all nutrients comes from soil sources, while the reminder comes from applied fertiliser. In the case of potash, the total crop requirement is up to 2.5 times higher than the offtake and if the balance is not available from the soil, yield potential may be reduced.
It is important to remember that soil fertility can only be improved when the applied nutrient level is higher than the offtake. So, knowing your soil fertility level is important, and this makes a soil test essential.
Soil testing
The accuracy of a soil test is only as good as the quality of the sample, so take them correctly and make them representative. Soils should be tested every three to four years, with a sample taken from every four hectares (or less) if the area is uniform. Sometimes more intensive testing could be very useful.
Nutrient and trace element application rates are determined in part by the quantity of the element in the soil and this is determined by soil analysis for most nutrients except nitrogen. A nutrient management plan is based on a whole-farm nutrient balance system as opposed to crop by crop or field by field.
A nutrient plan must also comply with restrictions on organic nitrogen from livestock manures (170kg/ha) or phosphorus (crop allowances, soil level and yield history).
The N requirements of any crop are met by nitrogen from the soil, mineralisation of organic matter and fertiliser (organic or chemical).
For tillage crops, soil N is to be deduced from previous cropping and organic manure history. An index system is used to categorise the likely supply of N from the soil, which is high following permanent pasture or low following continuous cereals.
Phosphorus (P), potassium (K) and other nutrient and trace element use are determined from the levels available in the soil. Available levels of soil nutrients and trace elements are grouped into classes, otherwise referred to as a soil index, where index one is low and index four is high availability.
Once the levels in the soil are known, standard recommendations are used to determine requirements for each crop. These are published in Teagasc’s Major and Micro Nutrient Advice for Productive Agricultural Crops 2016. The recommended total nutrition must include all nutrient sources, both organic and inorganic.
Adequate balanced nutrition is an important backdrop to plant production.Having adequate nutrient available from the soil reserves (index three or four) is important for establishing high yield potential.Soil levels can only be increased when the nutrient needs of the crop are exceeded.Index systems are used to indicate likely nutrient supply from the soil.
There is an increasing awareness across all farming sectors of the importance of soil fertility, but few realise its important in plant production. In this article, we look at the principles behind soil fertility and plant fertilisation. This requires a basic understanding of feeding a growing plant and of what soil fertility means. Let’s begin with the latter.
Soil fertility
Soil and especially clay provides the backdrop for a range of chemical processes and it also acts as a store for nutrients. Given that clay is the smallest of the particles that make up your soil, there is an awful lot of clay in an acre, even where the percentage clay in the texture may be low. A soil test measures the amount of readily available nutrient held in this pool of clay and the more that is present, the higher nutrient availability will be.
Higher nutrient availability means a higher test reading and a higher soil index. Look at it as nutrient density rather than quantity. The greater the density, the easier it is for a plant root to find and access.
No matter how much nutrient you apply to a soil, the density of applied granules in the soil bulk is very low compared with normal soil distribution. I have no idea what the relative odds are of a root accessing a nutrient molecule from the clay versus a fertiliser granule might be but the magnitude of the difference is likely to be hundreds if not thousands in favour of availability from the soil.
This is one of the reasons why soil fertility is so important and why it cannot be improved via a single fertiliser application, regardless of the quantity applied.
Having the fertility in the soil rather than coming solely from the fertiliser provides a far greater chance that the plant will always have access to what it needs and that is important to build the highest potential yield possible.
While fertility is not the only characteristic of a soil that matters, it is important. Having your soil in good biological condition with the help of adequate organic matter is another very useful way to enhance the availability of the soil’s nutrient pool. But again this is a long-term scenario with no instant fixes.
Balancing off-take
Modern crop fertilisation means supplying what is needed by the crop for optimum yield plus a little extra to help build soil fertility, where necessary. This is the principle behind recommendations for P and K in particular.
As crops grow, they take up nutrients from applied fertiliser and the soil. The efficiency of this process can be influenced by where the fertiliser is applied, eg combine drilling versus surface application for a spring crop. Crops also need additional nutrient to come from the soil as they generally need much more for optimum growth than they actually remove.
The amount of offtake is governed by the crop grown, its yield and whether or not straw is removed. But even if straw is left on the land it can take quite a number of years for the nutrient returned to impact on soil availability.
One other important factor is that the level of fertility in the soil to begin with can influence the yield level and so soil fertility itself can have a direct bearing on offtake. In general, winter wheat and barley remove less nutrient per tonne than their spring counterparts and oat crops take off even more.
Crops need more nutrient for optimum growth than they remove. They get this from the soil (if it is available) and then they return it back to the soil as the crop matures and is available for the following crop. It is estimated that an 8t/ha cereal crop will take up around 190kg N/ha, 35kg P/ha and 240kg K/ha.
In all instances, part of the uptake of all nutrients comes from soil sources, while the reminder comes from applied fertiliser. In the case of potash, the total crop requirement is up to 2.5 times higher than the offtake and if the balance is not available from the soil, yield potential may be reduced.
It is important to remember that soil fertility can only be improved when the applied nutrient level is higher than the offtake. So, knowing your soil fertility level is important, and this makes a soil test essential.
Soil testing
The accuracy of a soil test is only as good as the quality of the sample, so take them correctly and make them representative. Soils should be tested every three to four years, with a sample taken from every four hectares (or less) if the area is uniform. Sometimes more intensive testing could be very useful.
Nutrient and trace element application rates are determined in part by the quantity of the element in the soil and this is determined by soil analysis for most nutrients except nitrogen. A nutrient management plan is based on a whole-farm nutrient balance system as opposed to crop by crop or field by field.
A nutrient plan must also comply with restrictions on organic nitrogen from livestock manures (170kg/ha) or phosphorus (crop allowances, soil level and yield history).
The N requirements of any crop are met by nitrogen from the soil, mineralisation of organic matter and fertiliser (organic or chemical).
For tillage crops, soil N is to be deduced from previous cropping and organic manure history. An index system is used to categorise the likely supply of N from the soil, which is high following permanent pasture or low following continuous cereals.
Phosphorus (P), potassium (K) and other nutrient and trace element use are determined from the levels available in the soil. Available levels of soil nutrients and trace elements are grouped into classes, otherwise referred to as a soil index, where index one is low and index four is high availability.
Once the levels in the soil are known, standard recommendations are used to determine requirements for each crop. These are published in Teagasc’s Major and Micro Nutrient Advice for Productive Agricultural Crops 2016. The recommended total nutrition must include all nutrient sources, both organic and inorganic.
Adequate balanced nutrition is an important backdrop to plant production.Having adequate nutrient available from the soil reserves (index three or four) is important for establishing high yield potential.Soil levels can only be increased when the nutrient needs of the crop are exceeded.Index systems are used to indicate likely nutrient supply from the soil. 
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