'Review the past season'

REGULAR soil testing has always been a part of good farm practice and was recently included as legislation in the Farming Rules for Water. Taking this further and analysing soils in conjunction with plant tissue, harvested crops, and inputs, can help growers and advisors to better understand their nutrient status and benchmark against other farms, Sajjad advised in a recent submission for PDA (Potash Development Association).

When it comes to soil nutrient analysis, the starting point must be to understand the pH of the soil, he stressed. 

"Incorrect soil pH can have a big impact on nutrient availability, particularly phosphate, however it is surprising how many soils still show up as being too acidic and in need of rectifying. In 2021/22, the samples analysed by NRM showed that soils ranged from pH 4.8 right up to pH 9, although the median of all results was close to pH 7. 25% of soils were below the optimum of 6.5 for arable soils and therefore in need of lime," he said.

The Professional Agricultural Analysis Group (PAAG) data has regularly shown the proportion of UK soils that fall below the target levels for phosphate and potash, and NRM’s data from last season is no different. 

For phosphate, whilst the median result came in at the top of index 2 (23.2mg/l), the spread ranged from just 4mg/l (low index 0) right up to 56.6mg/l (index 4). Nearly a quarter of samples came in below an index 2 therefore below the target for optimal uptake by potato crops.

"As with phosphate, the potash samples showed that the median result was at the target level of index 2- (154.7mg/l). However, unlike the P samples, a greater proportion were measured at, or below, index 1 (29%) and therefore at risk of potassium deficiency. This could reduce crops’ yield potential, especially with the recent history of dry weather patterns throughout spring and summer," said Sajjad.

For magnesium, the median value was 2, which is the target level, and although 27% of samples were measured above the target index, there were still nearly a quarter of the results that were at index 1 or below which would benefit from magnesium applications for most crops.

This is not surprising, econsidering the large increase in the cost of fertiliser last season. More concerning could be the results of future soil samples, where applications may have been reduced or omitted, Sajjad said.

"Soil analysis is one piece of the puzzle but is not the only measure that is worth considering. The basic principles of P&K nutrition in the UK revolve around ensuring that the soils contain sufficient nutrient (and the correct pH to ensure optimal availability of these nutrients) to provide crops with adequate daily quantities through the season to meet the peak uptake requirements. 

"Although this is the theory, every soil will work slightly differently, and no two seasons are the same. Just because a soil is at the target index, does not necessarily mean it is able to access sufficient quantities due to variables such as soil characteristics (texture, moisture content, temperature); root characteristics (depth, length, architecture, health, mycorrhizal associations) and crop species."

Source: PDA