Soil health is of utmost importance to Scottish arable farmers, but mother nature continues to present huge challenges for growers across the country. The most significant challenge of recent months has been, as we all know, the relentless rainfall, with monthly totals higher than many have ever seen before.
The excessive rainfall is leading to many difficulties on farms across the country, from flooding through to soil capping and compaction. To make matters worse, the need to progress with spring work, such as fertiliser applications or ploughing, has forced many growers into saturated fields, creating deep tramlines or subsoil compaction.
Climate change is making rainfall events ever less predictable, with prolonged heavy rainfall often followed by periods of drought, which makes managing our soils more important than ever.
Scottish Agronomy recently ran a Healthy Soils Course to help growers take on the changing goalposts with confidence, looking at the complexities of soil and its functionality and what could work on their own farms.
Across the sessions, water management cropped up again and again as it influences so many aspects of crop production. Soil water dynamics is an area of soil science in its own right, due to the level of complexity involved within different profiles. Some of the key areas to consider when managing farm soils with water in mind are:
The Water Balance
Different soil types have different abilities to hold water, but the storage of water in all soil is a balance between water that is added through precipitation or irrigation, and water that is removed through percolation or evapotranspiration.
When additions exceed water removal, the storage capacity of the water is filled until the soil becomes saturated. In reverse, when more water is removed than the water being added, the soil will enter the solid phase, which is beyond the wilting point of most plants, commonly referred to as drought. The water that is available to plants is the water which is between the wilting point and field saturation and varies for different soil types.
Structure and Texture
The volume of stored water available to the plant within a soil is governed by a combination of structure and texture. Ideally on farm, we don’t want to exceed saturation, as this can cause runoff and erosion, but we also do not want to enter the solid phase, as this can cause yield loss. Generally speaking, sandy soils have a reduced storage capacity, as water moves through the coarse particles readily. However, a heavy clay soil, with a high ability to retain water can also drought easily as water binds closely to the clay particles making it unavailable for plants. Therefore, soils which are classified between sands and clays have the greatest water storage capacity.
Although soil texture cannot be altered on farm, understanding your soil type can help to influence how a soil is managed. For example, I suspect many farmers will be focusing on how to dry soils out as quickly as possible to establish spring crops, however, consideration for water availability to the growing crop should also be a priority.
This can be influenced by a soils structure, with a well-structured soil providing a high degree of porosity, which in turn can allow water to be stored for a crop to use. Care should therefore be taken to ensure that remedial work to dry out soil does not inadvertently cause soil compaction which will result in a reduced water storage capacity and make the soil more prone to droughting if we do end up with a spell of hot dry weather.
Drainage
Drainage is important to allow air into the soil, for plants to assimilate oxygen through their roots. Poorly drained soils generally have an absence of air, which causes microbes to decompose organic matter in anaerobic conditions.
The most common symptom of this is soluble iron or manganese deposits within the soil, displayed as rusty or blue pockets. When considering drainage on farm, I find it important to remember that stone, tile or plastic drainage schemes only form part of the picture.
Water must reach the drain before the drainage scheme can be utilised. Although gravel backfill can help this process, the most important factor which will allow the movement of water throughout the soil profile is soil structure. A plough pan or other layer of compaction can be enough to prevent water from reaching a drain, resulting in a build-up of water within a field. Recreating soil structure is a challenging objective, often requiring patience until the soil has dried out; a cultivation at the correct depth (slightly below a compaction line) and filling the soil fracture lines with growing roots is the best approach. Make sure that cultivated soils are not left bare, as these soils are most likely to slump and return to a waterlogged state very quickly.
Historically, we relied on soil water storage to supply water throughout the drier summer months, effectively utilising the excess water from the winter. However, periods of extreme weather are making this more challenging on farms. Farm soils reached capacity early in the winter, leading to mass soil erosion and compact anaerobic soil conditions. These soils are going to require remedial work throughout the summer to ensure they produce strong crops next year, however, less than ideal conditions may see many farms causing more damage, so patience will be key to remedy many waterlogged soils.
If you are in any doubt about how to fix a soil, the first step is to grab a spade and have a look at what is underneath.
