Maximising output from soil takes more than just lime, phosphate and potash, according to a recent study by Farm Stock (Scotland).

The Knowledge Transfer project found that regardless of their chemical profile, soils needed to be physically well structured with no compaction and biologically full of life. Through studying 40 different fields across Scotland last winter, the project found that careful management of livestock, especially cattle, is as important as planning machinery use, to minimise compaction.

Over the 40 sites, acidity was not as low as expected but of all the main nutrients, phosphate (P) was most below recommended levels. Potash (K) levels were generally on target, but where it was below, target silage crops were especially vulnerable to yielding less.

A major conclusion was that soil testing at the field level can give misleading results. While most P and K is recycled through the animal, they typically distribute it unevenly across a field via their dung and urine. Livestock farmers may therefore need to adopt a similar approach to soil testing as crop farmers (ie, testing at greater detail than field level) to get a true picture of the lime, P and K needed across a field.

Slurry and dung (FYM) are particularly precious sources of P and K. Careful use of organic manures through well planned application and use of new technologies like 'trailing shoe application' is essential.

The participants in the project found that on most of the farms, the 'grazing only' ground received a single annual application in the spring (typically 30-40kg N/ha). For fields used for silage (and grazing) an annual application of 120-150kg N/ha was more typical (multiply by 0.8 for units/ac).

In this project the average for the 20 poorest producing fields had slightly higher carbon soil levels than the best ones at 11.4% compared to 10.31%. With little cultivation of fields, especially the worse ones, organic matter levels often reflect the inherent nature and drainage of the soils with a notably wet, difficult field recording a high for the project of 25.9%.

This is sequestered soil carbon, the focus of much current climate debate. This comes from a small proportion of organic matter binding to soil particles and associated aggregates to help form the crumbly texture of topsoil. Measuring soil carbon to the standard recognised by the Intergovernmental Panel on Climate Change (IPCC) is technically demanding and therefore expensive. So, while this project tested the fields with the recognised test, the limited number of cores taken meant the results can only be indicative of soil carbon levels.

Still, the results were broadly consistent with the poorer fields scoring slightly higher amounts of sequestered carbon at 122t/ha than the good fields which had 112t/ha. There are no recognised benchmarks as to what are low, moderate and high soil carbon levels for Scottish grassland soils. Sandy soils, for instance, will naturally hold far less carbon than clay soils. However, to give participating farmers some idea of scale, the following ranges were noted on the scorecard – low <60t/ha; 60-120t/ha moderate; >120t/ha high.

Read more: Farm Carbon Toolkit – how best to understand soil health

The tentative conclusion is that the soil carbon levels of many of the fields tested are at, or near, equilibrium. That is, the opportunity to sequester and store more carbon may be limited. However, the project concluded that more detailed research is needed to test and confirm this important conclusion.

Farm consultant Kev Beven who ran the project said: “Based on the scorecard piloted in this project, the soil health of a representative group of livestock farmers was generally good. However, the project also found plenty of scope to improve both farm profitability and environmental footprint through closer attention to soil management.”