Stone Content and Available Nitrogen Supply
In certain areas of the UK soil type characteristics are dominated by soil depth and inherent soil content. Crops grown on these soil types may benefit from making an adjustment to the total Nitrogen reservoir reported via the N-min analysis.
It is important to make an adjustment to the reported N-min total reservoir if you consider the field sampled to contain a high percentage of stone particles as the values measured are based upon soil volume.
The adjustment must be based upon local knowledge of the area and the field in question; an incorrect adjustment may result in an under/over application of fertiliser Nitrogen.
Stone abundance can be estimated two ways, one being a laboratory method the second being a field assessment where an estimation is carried out by eye.
|Very slightly stony (bouldery)||1-5||Few|
|Slightly stony (bouldery)||6-15||Common stones|
|Moderately stony (bouldery)||16-35||Many stones|
|Very stony (bouldery)||36-70||Abundant stones|
|Extremely stony (stony)||>70||Extremely abundant stones|
The above table should be used when assessing the scale of stone abundance. Very large volumes of soil are needed for meaningful estimates of the percentage volume occupied by larger stones and although the volume can be estimated byeye, visual assessments via a small profile pit will be inappropriate and provide an inaccurate judgement.
The following diagram illustrates a visual indication of stone content, taken from the soil survey field estimating stones above assumes each quarter of any one square has the same area of black.
When adjusting your Nitrogen requirement for stone content the calculation involves reducing the total N reservoir proportionate to the % stone content assessed. For example, 30% stone content equivalent N rate adjusted by 70% of the total N reported. If you estimate that your submitted sample contains ≥30% stone then it is advised that you make an adjustment to the reported N-Min result. If the sample is taken from a field which is shallow <40cm then it may be sensible to disregard the Est. N value reported in the 30-60cm soil depth. For assistance with this advice please contact your GrowHow Adviser, the GrowHow Agronomist or Hill Court Farm Research.