Course: Soil Management | Last Updates: 7th October 2015
Soil is the basic building block of farming. Every farming operation depends on getting soil management right, and everything you do has an impact on the soil.
But good soil management is becoming more of a challenge. The hand of farmers is being forced into economies of scale as food production becomes more efficient. Farms are becoming specialised, losing the variety of enterprises and rotation on which soils thrive.
As fewer farmers and contractors farm wider tracts of land, the local knowledge and attention to detail which soil needs are being lost. Larger, more powerful machinery means less ability to respond to how it behaves.
Inspection and monitoring are therefore crucial to get the most out of the land you work.
What is a good soil?
There's no such thing as the ideal soil as they vary in nature so widely. But there are some key indicators you need to look out for, whatever type of soil you farm.
It needs to have good structure and good drainage – firmly bonded and stable, this allows free movement of air and water. It should provide enough support for roots, with no impediment to their growth. It should have a good balance of the right nutrients and minerals.
Importantly, a good soil should be full of life – earthworms, fungi and bacteria are essential for the processes that make a crop thrive. Organic matter is needed to feed these processes and maintain the soil in good physical condition.
What should you monitor?
The first sign of a potential problem is uneven crop establishment and growth. Start with yield maps. Identify areas where drainage is poor and establishment slow. Look for areas of necrosis, yellowing and early senescence. If these cannot easily be explained, they are likely to be soil-related.
Look also for signs of erosion – rills and gulleys in tramlines, soil washed into buffer strips or ditches and streams that run cloudy, collect sediment or show excessive or slimy green growth.
But it's not enough to walk and assess the surface. You need to take a spade to investigate areas you've highlighted. Dig a hole to a depth of around 20cm – in most cases this should be enough to assess the state of your topsoil. Then dig down further, to around 30-40cm, to determine how your subsoil is behaving.
Good soil structure1
How do you assess texture and structure?
Texture is the relative proportion of sand, clay and silt. Texture affects many properties, including drainage, workability, suitability for cropping and structure. Although there's little you can do to influence this, you still need to know what texture you have.
To do this there are easily available guides, such as the Environment Agency's Think Soils manual. This also has guidance on physical characteristics of different soil types.
Soil structure is how these basic building blocks have been put together. As you dig you need to assess the size of the ped, which is the naturally occurring lump or clod.
You should get a range of lump size from crumb to large block, and the distribution will depend on your texture – clay soils have a blockier structure, but you should still have some granular content.
You should have granular and sub-angular shapes – platey layers, where lines of weakness occur along the horizontal, or prismatic lumps, where the lines of weakness occur mostly in the vertical direction, are not a good sign.
Look for pores, fissures and a well-distributed root growth. If the roots reach down into the subsoil, that is a good sign. Colour is also a good indicator – darker brown is generally better. Good visual guides to help you assess structure include A Guide to Better Soil Structure by NSRI and Visual Soil Assessment from Väderstad.
How do you assess drainage and compaction?
The easiest problems to spot and resolve are surface capping, or poaching in livestock fields. Surface water that is slow to drain away is a sure sign of compaction. More often than not, the problem lies beneath the surface and is only identified through digging.
A plough pan is a compacted layer just below the level of normal cultivations, usually formed as a result of ploughing at the same depth every year. A wheeling pan is caused by field traffic. The structure will typically be platey at plough depth and you may even notice the soil is drier beneath, indicating water cannot drain through. Plough pans also impede root growth and can cause catastrophic erosion events.
Another indicator of compaction is a grey, mottled appearance or rusty colour, which is the iron oxidising as the soil changes from very wet to dry. If there's a bad smell of rotten eggs this may be rotting straw that cannot break down as it is stuck in a wet layer with no air.
What influence does organic matter and nutrient status have?
A soil isn't a soil without organic matter. It is part of the biota – everything that lives in the soil – and comes from broken-down leaf and straw. Organic matter binds the soil and keeps it from binding too tightly, retains moisture but keeps a soil from being too wet.
It also feeds the micro-organisms – some bacteria feed on it to produce polysaccharides that help bind the soil.
An organic soil will have more than 7% organic matter, while a good content is 3-5%, with grass leys usually at the top end, and arable with a lower content. The less you expose organic matter to the air, the less it is oxidised, so levels quickly drop through excessive cultivations. Raising it takes time, through incorporating straw, putting grass in the rotation, or applying compost, digestate, manure or sewage sludge.
Organic matter also helps a soil retain nutrients, especially nitrogen, which is easily leached, affecting salt marshes and coastal areas. Phosphates bind tightly to soil particles, so if eroded enrich streams and lakes, causing algal blooms. Soil samples every four to five years will check your nutrient status and organic matter content. A quick and easy way to improve nutrient availability is to check pH and correct if needed, aiming for 6-6.5. An acidic or alkaline soil will lock up nutrients.
How important are earthworms and other soil organisms?
Worms perform a multitude of functions. They break up the soil and improve drainage through their tunnelling. They graze on organic matter and take it to depth, breaking down straw to smaller particles which feed micro-organisms. They are also a good indicator of soil biota – in one teaspoon of soil there should be several billion micro-organisms. A soil with few worms will likely be low on life in general.
More than eight earthworms in a spit of soil you dig up is a good sign. A quick indicator of a healthy number is if your plough is followed by seagulls. Less tillage, the correct balance of organic matter and judicious use of pesticides, especially soil-acting herbicides and nematicides, will help retain earthworms.
Three golden rules
- Check your soil – use a spade to assess, sample regularly and act on your findings
- Cultivate wisely – keep tillage timely, to a minimum, and travel only when soil is fit
- Keep it living – maintain or increase organic matter and take care of worms and micro-organisms
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