The environmental benefits of analysing soils part 2: is phosphorus a useful nitrogen replacement?
NRM
Agriculture
Phosphorus is an essential, irreplaceable element. It plays a key role in crop development, particularly in rooting, cell walls, energy transfer, and the production of fruiting bodies. Unfortunately, phosphorus is the main cause of the eutrophication of freshwaters, resulting in many water bodies in England failing to reach ‘good’ status. It is therefore critical to plan and accurately target phosphate applications to fields where a need has been identified.
Phosphates, although generally applied at much lower rates than nitrogen, can be environmentally very damaging even when losses are minimal. Typically, an 8 t/ha wheat crop will remove about 30kg P/ha per year. This quantity compares with an amount of phosphorus almost 1000 times smaller of only 0.035 kg P/ha per year. If this amount is lost in 100mm of drainage water, typical for an east of England wheat field, it would increase the phosphorus concentration to a potentially eutrophic level of 35 µg P/l (35 parts per billion).
Ensuring responsible phosphate applications
Plant available phosphorus accumulates in the soil pool as a result of applying fertilisers and organic manures. For soils at the target maintenance index 2, phosphate recommendation rates are designed to match the phosphate removed by the crop. However, if soil indices are below index 2, then applying additional phosphate is recommended. Care needs to be taken to avoid building soil phosphate concentrations above index 2.
Where livestock and organic manures are applied, there is a greater risk of surface run off, particularly when applications are followed by heavy rain and/or fields are sloped. Planning based on soil analysis is the basic starting point. Progressing to site specific phosphate applications by soil mapping is a good way to target and deliver phosphate containing fertilisers to parts of the field where the crop need is the greatest. This avoids applying phosphate where soil indices are greater than 2.
Many of our heavier soils contain significant quantities of phosphate, but it is held in a form not readily available to the crop. Often, less than 1% of the total phosphate contained in clay soils is accessible to the crop at any one time. So, the challenge is how to best enable the crop to access to some of this legacy phosphate.
By providing the soil conditions that enable phosphate to be solubilised, such as improving soil structure to increase the oxygen flow within the soil, this in turn enables the microbes to solubilise precipitated or locked up phosphate. Increasing organic matter content is critical in supporting soil biology, as microbes need a food source, and organic matter provides the energy they require for a functioning soil system.
NRM offers services which measure not only standard nutrients, such as phosphorus, but also soil mineral nitrogen and organic matter with N-Check and N-Check Plus. For a broader picture of the benefits of organic matter, NRM also offers CarbonCheck and CarbonCheck Plus, enabling a better understanding of the quality and functionality of soil.
Contact your agronomist or NRM directly to arrange for samples to be tested today.
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