Getting the most from arable crops means understanding soils and their water supply capabilities. ADAS soil scientist Selwyn Richardson goes back to basics.
RECENT rains may have seemed never-ending but drought was the talk in the first few months of 1997.
This rapid transition has focussed attention on both soils and water.
Besides having somewhere to anchor themselves when it gets windy, crops need soil for three major reasons:
• To supply food – they get
most of their nutrients from
the top foot of soil
• To supply air – roots have
to grow in well-aerated soil
• To supply water – and
they need lots of it
The most important of these is water and to achieve high yields, crops need to use as much water as possible from reserves stored in the soil over winter. They can only do this if their roots are exploring as deeply as possible.
This year, very little rain fell in the first four months over most of England and Wales and by the end of April, cereal crops on sandy soils were almost dead in some areas.
Two months later, after one of the wettest Junes this century, the soil was wet and crop growth had leapt forward.
On the other hand, 1995 and 1996 were very dry during summer and heavy soils were deeply cracked. In dry seasons, crops with shallow roots can suffer badly.
For a wheat crop to produce 10t/ha (80cwt/acre) it needs 40cm (16in) of water. It uses little of this over winter so has to find this 40cm in five months between early March and early August. Most of it has to come from the soil.
For example, annual rainfall at the Cereals 97 site near Huntingdon is about 58cm (23in). Thus, in five months only 24cm (9.5in) falls from the sky on average. This leaves the crop to find another 10.7cm (6.5in) from somewhere – and it comes from the soil.
The amount of available water which a soil holds depends on the soil type.
This will vary from the 8.1cm (3.2in) held by wheat in 30.5cm (12in) of peat soil to just 2.03cm (0.8in) in a light sand. It means that if a wheat crop is to find 16.5cm (6.5in) of water from a heavy soil, its roots must travel to 1.22m (4ft).
Crops vary in their ability to root deeply in soil. As long as the soil environment is suitable they will easily achieve depths shown in the table (right).
Potatoes are particularly shallow rooted so they respond well to irrigation. Clearly you must do everything you can to encourage crop roots to travel as deeply as possible.
On heavy soils an efficient underdrainage system is essential to produce high yielding crops. Water has to be able to reach the drains and, since heavy soil is only slowly permeable, this means using gravel fill above the drains followed by moling.
Good drainage allows the soil to warm up faster in spring so that roots can start travelling quickly at the end of March. It also allows the first bag of nitrogen to be applied at the end of February to strengthen the roots.
In recent years, dry seedbeds in autumn have been a problem. The most important requirement is to conserve moisture.
Keep cultivations to a minimum to restrict moisture loss and use a furrow press or roll after each cultivation and after drilling to keep in the moisture.
Even in a very dry year, cereal yields can be high on silts because they hold large amounts of available water. Furthermore, their subsoils contain large numbers of fissures and pores so it is not unusual for cereal roots to travel to 1.83m (6ft).
However, silt soils tend to have a weak structure and are easily damaged in wet conditions. Compaction just below plough depth (plough pan) can be caused on any soil but silts are particularly prone.
A tractor wheel travelling at 25.3cm (8in) in the furrow bottom in wet soil can cause compaction down to 33-35.5cm (13-14in). In a run of wet autumns, a plough pan builds up across the field which can seriously restrict growth. If only a few roots penetrate below 30.5cm (12in), crop yield can easily be reduced by 2.5t/ha (1t/acre). Soil compaction is the most serious barrier to roots using soil water.
In theory, crops growing on very sandy soils such as those at ADAS Gleadthorpe need to reach more than 2.44m (8ft) because they are so droughty. In practice, subsoil rooting is often poor.
The reason is not fully understood but Tom Batey in his book Soil Husbandry suggests it is because sand particles interlock and cannot readily move apart. Roots then have difficulty pushing their way through. This theory is supported by the fact that the last couple of inches of roots growing in sandy soils are often stubby.
Therefore sandy soils can be low yielding both because of low water holding capacity and restriction to root depth.
Chalk and limestone
In some fields, these soils can have subsoils which are effectively solid rock. If this is the case, at 45.7cm (18in), roots cannot travel further and crop yield will always be low.
Root exploration in chalk is usually mediocre yet yields in dry years are better than expected. The reason is capillary attraction.
Pore sizes in chalk are such that water will travel upwards as crop roots dry out the subsoil. So, although roots may not be deeper than 0.9m (3ft) they actually suck water up from 1.83m (6ft) or more.
Perhaps surprisingly, cereal roots can travel very deeply in limestone soils where the limestone is shattered. They seem to travel round the plates of rock with ease and a curtain of roots can be found below 1.52m (5ft).
Code of practice
The MAFF Code of Good Agricultural Practice for the Protection of Soil (PB0617) is a readable source of information on soil management.
It is available from MAFF Publications, London. Tel: 0645 556000.