The economic value of analysing soils
NRM
Agriculture
With fertiliser prices so high, it is essential to plan applications as carefully as possible. This must start with good up-to-date knowledge of the nutrient status of your fields.
Standard soil analysis, which includes pH, P, K, & Mg, only costs around 60p/ha/yr/nutrient, based on a sampling area of 4ha. Broad spectrum analysis, on the other hand, costs around 75p/ha/yr/nutrient. Decisions to either increase or decrease fertiliser rates to fully optimise nutrient supply must be based on good data. This is to minimise the risk of reducing yield or overspending and over supplying inputs.
How much could I lose by misjudging soil nitrogen supply (SNS)?
Consider nitrogen planning for next spring. In this economic climate, a typical winter wheat crop would need to receive around 220kg/ha to optimise productivity. This rate is based on the principle that 5kgs of grain needs to be produced to pay for 1kg of nitrogen. With today’s prices of around £2/kg, we must produce twice as much grain to pay for 1kg of nitrogen, even though the current price for wheat is high. This change in the fertiliser price economics means we should be prepared to reduce the amount applied to wheat by 60-70kg N/ha.
Accounting for the supply coming from the soil is an effective way of substituting nitrogen from the bag. However, we need to interpret SNS carefully because under- or overestimating the SNS can jeopardise yield and crop quality, and the efficiency at which applied nitrogen is utilised. As stated before, fertiliser costs around £2/kg, and so the cost of misjudging the SNS by as little as 10kg N/ha will be between £18-20/ha.
The value of plant tissue analysis
To ensure the last few kilograms of expensive nitrogen fertiliser are economically justified, the optimal application rate will need to be reduced. However, these decisions should be constantly reviewed in season. As crop and fertiliser prices change, farmers and advisors should adapt their management plan, guided by the crops’ nutrient status, with tissue analysis.
Analysing the last expanded leaf on a winter wheat crop, for example, helps establish if the critical level of 3.5% nitrogen in the dry matter is being maintained. Keeping levels at this value will help prevent yield loss. So, checking crops at tillering (GS25-30) with tissue analysis can guide in-season management decisions.
Soil mapping and its benefits
The variation in soil status for both phosphates and potash can be considerable, with parts of many fields able to support the next crop with little or no additional fertiliser. However, it is highly likely that other parts of the same field will be below soil phosphate and potash index 2. These areas will require inputs to not only replace what the crop removes but also enough to build levels in the soil.
Mapping the soil nutrient status of fields means inputs can be managed in a cost-efficient way. It typically costs about £5/ha and identifies soil indices in the field by site specific analysis. Management plans can then be developed to ensure the fertiliser applied balances the supply in that part of the field and that nutrients are used as efficiently as possible by the crop.
Yield generally doesn’t increase in response to phosphate and potash if applied to crops above soil index 2. Growers should avoid applying fertiliser to fields that are super-optimal for phosphate and potash in favour of a more focussed approach.
Data from Rothamsted Research shows the potential yield penalties where crops are grown on soils at sub-optimal phosphate levels of 0.5 t/ha at index 1 and up to 1 t/ha at index 0. With recent grain prices as quoted in Agritrade last week, this yield reduction would amount to a financial loss of between £105 to £210/ha for feed wheat. A more strategic approach to the application of phosphate and potash will protect returns and optimise productivity. For phosphate, the additional risk of loss to water courses from high index soils is great, where even minute quantities will lead to eutrophication, decimation to aquatic life, and destruction of sensitive habitats.
Guidance in the AHDB Nutrient Management Guide (RB 209) recommends no additional phosphate is required for combinable crops grown in fields over index 2. However, there are recommendations for some crops that pose an exception to this, such as potatoes, vegetables, and first cut grass silage at index 3 where crop demand is higher and rooting potential lower. Phosphate availability will be enhanced on soils with good structure where roots can penetrate and easily explore the soil volume.
The value of grain analysis
Where there are reduced rates or no phosphate and/or potash applications planned, determining the actual crop nutrient removal is advised. This can be done either by referring to the average values published in RB 209 or by sending grain samples to the lab for testing after harvest. This retrospective analysis allows a review of how well a crop has been supplied with nutrients and if the management strategy needs adjusting for the following year. Producing higher yields than expected generally results in a higher removal rate. So, soil indices at the cusp of a change need to be reviewed carefully and managed accordingly.
Recent grain analysis data has suggested a decline in wheat grain phosphate levels over the last decade. This indicates either: sub optimal phosphate uptake due to under application, especially on low index soils; or that grain critical values are lower than recommendations have previously advised. As discussed, by regular mapping and site-specific nutrient analysis, soil nutrient status can be improved and evened up across the field. This approach helps to avoid yield penalties and, at the cost of £5/ha, can be recouped in just 25kg of grain per ha this year.
Overall, better placement of nutrients in fields helps to improve financial margins and increase resilience, enabling businesses to cope with the volatile swings in fertiliser and grain prices.
Planned strategic use of each and every nutrient is essential to control production costs without significantly jeopardising productivity. Talk to your agronomist or contact NRM to arrange for soil or plant tissue analysis to help you make appropriate crop and farming management decisions.
References
Nutrient Management Guide (RB209) | AHDB
Microsoft PowerPoint – The Rothamsted LTEs AMacdonald 2018-05-21
Cereals and Oilseeds markets | AHDB
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