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Sunday 4 December 2016

How to get the ideal seedbed

Water infiltration and a layer of fine aggregates helps

Published 24/08/2010 | 05:00

Soil with good natural structure acts like a sponge and can hold up to 40pc water by volume. The coarser pores that exist give drainable porosity
Soil with good natural structure acts like a sponge and can hold up to 40pc water by volume. The coarser pores that exist give drainable porosity

AVERAGE good tillage land offers the best pre-conditions for min till but sandy clays and loams are also ideal. Heavy soils are more critical as they are harder to handle. Mostly these soils distinguish themselves by the short periods you have to cultivate them.

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The target -- an ideal seedbed



  • It must allow water infiltration to avoid crusting and erosion. It has to have an upper layer of coarse aggregates and organic matter that does not crust (it has to be able to soak).
  • It must have a layer beneath this of finer aggregates that will stop evaporation of water and prevent the seed from drying out and give good seed to soil contact.
  • It must have capillarity from beneath to transport water from the soil up to the seed.


Trying to operate min

till in wet autumns

The preservation of the soil structure is the important target at soil cultivation.

On soils that have a weakened or compacted structure, for example on sandy soils which tend to compact, or in areas prone to temporary stagnant water, this will require the top soil loosening down to 20-30cm(8-12inches). This is required to ensure sufficient oxygen supply down to the roots.

Compaction causes changes in the vertical wetness gradient. Normally soils get wetter the deeper you go. When you have compaction it will be wetter at the surface and drier further down.

Soil with a good natural structure acts like a sponge and can hold up to 40pc water by volume. The coarse pores that exist give drainable porosity as these are nearly always free of water. They are very important and you lose these when you have compaction.

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There is a fine line between compaction and consolidation.

Around 50pc of the soil consists of solid material and 50pc is the pore space. Within this pore space, half should be water and the other air. This ratio can vary depending on soil structure and tillage management.

Optimum consolidation is a good seed to soil contact to supply water by capillarity and enough big pores for air. If there are abundant pores you will have good drainage and aeration. If there is not enough soil firmness in the seedbed the soil around the seed can be too dry due to a lack of capillary movement as the pores are too big.

Too much firming and the larger pores become compressed and less effective at draining away water and transporting oxygen and carbon dioxide. The ideal is where there is good seed to soil contact to supply water by capillarity and enough other larger pores to supply air.

Examining soils during or just after rainfall is a good time to see signs of compacted soil, as it will pond on the surface.

Slaking is the process of breaking down the soil aggregates. The rain breaks down the soil into fine particles which are then washed into pores and a cap or crust is formed. Capped soils reduce the ability of rain to soak into the soil. If you examine the top you will identify the surface cap. Anyone who grew sugar beet will be familiar with this process.

After heavy rain the pores of the soil may be entirely filled with water. If a soil has good drainage about half this water moves quickly and drains out of the larger pores because of gravity. When drainage continues to the point where the where half the pores are full of air and the other half full of water it is said to be at field capacity, a stage ideal for plant growth as water is loosely held on the larger pores and is available for plant growth.

This is a drain designed to take away excess water from an area prone to ponding and subsequent slaking and the crop dying out. The design and the positioning of the drain was achieved by inspecting the land after heavy rainfall.

It is then critical that land be well-drained for min till, as it is very important to work the land in dry conditions and if field drains are not correctly positioned or meet the right design for the purpose or are in generally poor upkeep it will increase the water holding capacity of the land and then increase the risk of compaction.

Irish Independent