Making your soil analysis help you manage fertiliser costs

The last 12 months have seen fertilisers rocket in price, with nitrogen typically tripling in value with potash and phosphate doubling over the same period. This has meant that farmers and growers are faced with significantly increased production costs for this year’s crops. With the concerns over gas supplies and prices combined with political flashpoints in key production areas, there is no sign of respite in the short-term meaning farmers need to get used to these price levels for the foreseeable future.

How can farmers adapt to these high fertiliser prices? The easiest option is to buy and use less, however this can be a false economy if reductions in yield and quality exceed any savings made- so while it is a valid strategy care needs to be taken.

Why do agronomists and advisers bang on about the importance of soil analysis?

Make sure soil analysis is up to date for each field- if not then there is still time for analysis to be turned round to be of use to this year’s crop. With the right interpretation it could identify where savings could be made or identify problems that may be having a detrimental effect on both your crop and fertiliser utilisation.  Soil analysis is not expensive- typically around £20 to advise you of pH, Phosphate, Potash, Magnesium and even soil organic matter and will stay valid for 4 to 5 years.

Why is it so important?

Making sure pH is within recommended levels allows the optimum uptake of nutrients from the soil. Remember the soil is full of nutrients- but everything needs to be right for plants to take as much of these up as possible with one of the key factors being pH as shown in Figure 1.

Figure 1. Chart showing nutrient availability relative to pH (redrawn by PDA  from Troug. E. 1946)

If the pH is too high or as is more common in Scotland, too low, this can cause nutrients to become less available or even locked up. If pH is allowed to slip too far, toxicities can occur as too much potentially harmful nutrients become more available. Choice of product can also help with other deficiencies – where Magnesium is deficient the use of a Magnesium Lime product can address this. Application rates required will vary depending on product and soil type and care needs to be taken when applying larger amounts to ensure the lime can be well mixed through the soil profile.

Your soil analysis will also tell you what levels of phosphate and potash are in the soil- if you don’t know what you already have how can you know what you actually need. Where soils indices are found to be high then savings can be made although low indices need to be built up. Phosphate and Potash are vital for crop development and yield as well as playing a key role in nitrogen utilisation.

How much P & K is needed is determined by the following factors-

• The current nutrient status of the soil using soil analysis results
• The soil phosphate sorption capacity (PSC) of the soil
• The expected offtake based on anticipated yield

These factors will vary from site to site- with soil phosphate capacity (PSC) helping further refine Phosphate recommendations. Soils vary in how they bind with applied phosphate and as such how much becomes temporarily unavailable to the crop will vary from soil to soil. This means that recommendations need to be fine tuned to take this into account. Soil associations have been rated as PSC 1- 3 and are listed in FAS TN 715-8. These Technical Notes also include the relevant adjustments for each rating relative to soil analysis results. Information on soil associations can be found here.

Basing recommendations on offtake helps ensure soil indices are maintained- higher yielding crops will take more nutrients from the soil and P & K levels can take many years to rebuild. Crops vary in terms of offtake and while Phosphate offtake doesn’t vary too much from crop to crop, Potash offtakes can vary significantly depending on the crop grown as shown below.

By taking your soil analysis, your PSC and your projected offtakes based on yields together, a recommendation can be drawn up, with an example shown below.

Pulling everything together to provide recommendations

A farm in the North East expects to grow a spring barley crop yielding 7t/ha. A soil analysis has been undertaken recently with Phosphate coming back as Moderate (+) and Potash as High. The soil association has been found to be rated as PSC 2 so no further adjustment is required. The table below shows the recommendation and potential saving when soil analysis and PSC are considered.

This example shows the benefit of making recommendations bespoke to each field or even smaller area if necessary, for example where soil types differ within field or where historically there was more than one field. Blanket applications may keep things simple but taking the time to work out individual recommendations can pay dividends- if the example above was even for a small 4 hectare (10 acre) field, the saving would be £336, not a bad return on a £20 sample and few minutes with a calculator. Taking it a stage further, the use of Farmyard Manures, slurries, digestates and composts offers further scope for savings.

Soil analysis is only part of the nutrition jigsaw

An up-to-date soil analysis can be a powerful tool when it comes to planning crop nutrient requirements and is more important than ever this year. This article has only looked at how to calculate requirements of Phosphate and Potash Making sure these nutrients can both be accessed and utilised effectively is also crucial. For example, spring crops should always receive at least 25kg/ha of phosphate down the spout unless soil phosphate levels are low. Compaction, the hidden yield robber, should be avoided at all costs and machinery should be calibrated and the correct settings used.

Further information

Understanding Soil pH | Helping farmers in Scotland | Farm Advisory Service (fas.scot)

Technical Note (TN715):Phosphate and potash recommendations for crops grown in Highland and Islands | Information helping farmers in Scotland | Farm Advisory Service (fas.scot)

Technical Note (TN716 ):Phosphate and potash recommendations for crops grown in South West Scotland | Information helping farmers in Scotland | Farm Advisory Service (fas.scot)

https://www.fas.scot/downloads/tn717-phosphate-and-potash-recommendations-for-crops-grown-in-north-east-scotland-and-tayside/

https://www.fas.scot/publication/technical-note-tn718-phosphate-and-potash-recommendations-for-crops-grown-in-fife-lothian-and-scottish-borders