Underground storage of grain

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Lining the pit

One WA farmer in a 325mm rainfall area achieved good results without any form of lining in the pit. Only a few centimetres of grain at the sides deteriorate and this does not warrant the time and small expense of lining the walls. He also advised against lining the pit floor, believing that any water that enters the grain should be allowed to drain away.

The trial pits dug by the department were fully lined with 0.1mm black polythene. If the pit is lined, all horizontal joints must overlap with the higher sheets outside the lower ones. Water will then flow down the outside of the sheets without entering the grain.

On suitable heavy soils there should be little contamination of grain by soil from the pit sides during emptying.

Filling the pit

It is important to ensure the pit and its contents are as dry as possible at this stage. The grain must hold less than 13% moisture. If the pit is dug just before harvest, there is usually little chance of the soil being damp in a high site. If rain wets the soil, or if an old pit contains water from the previous winter, the soil must be allowed to dry out. In eastern Australia, pits have been pumped out and the soil dried by burning wood in them.

An alternative is to refill unused pits with soil, opening them again when the need arises.

The pit is filled to ground level at the sides, sloping up to a ridge of grain along the centre line to aid formation of a water-shedding surface.

Covering the pit

Before covering with soil, polythene sheeting is laid across the ridged grain and for about 2m either side of the pit, as shown in Figure 3. Sheets should overlap at least 20cm at the edges and full sections should be used without cross joins. Medium grade 0.1mm polythene will resist breakage by lumpy overburden better than 0.05mm grade sheeting. Sheeting is generally not reused as the cost is small and damage during uncovering is inevitable.

The polythene cover provides a barrier to any water penetrating down the soil overlay, but the aim should be to shed most of the rain on impact with the soil. Care over forming a water-shedding cover is the most important aspect of underground storage after the selection of a dry site.

Quick run-off from a smooth covering mound is the main aim together with sufficient depth of soil to exclude air and protect the grain from large temperature changes. The soil cover needs to be about 30mm deep, sloping to ground level well clear of the pit.

Cross section of pit

Cross-section of pit showing polythene and soil cover in place

Peg the pit corners to allow easy location when emptying. It could be 10 years or more after filling before the grain is needed. A modified cover which will speed up the opening of a pit is achieved by attaching the polythene to sections of welded steel mesh. These are long enough to span the pit in one section and are overlapped in the reverse order of removal. Once the overburden has been reduced to about 20cm deep by blade or loader the mesh sections and remaining soil can be towed off with little soil contamination to grain.

Maintenance and safety

During the first winter the soil cover may need to be reformed and smoothed after settling to eliminate cavities and promote run-off. There will be virtually no settling of the grain. Little else is needed except to see that no depressions which hold water remain near the pit. Periodic checking for rats and mice may be necessary but they are generally deterred by a well-compacted soil cover.

Once open, the pit should be emptied out quickly to avoid the risk of rain damage and infestation by insects. If the grain is to be held for more than a month or so, transfer it to sealed silos where insects can be readily controlled by fumigation. In the shorter term, sufficient unsealed or temporary storage must be on hand when the pit is opened.

When emptying the pit with an auger operated from the ground surface there is a risk of injury to anyone falling into the cone of grain. This risk is increased by irregular edges to pits, deep grain and slippery polythene. Great care should be taken when moving near the pit.

Pits left open for any period are a hazard to people and stock. They should be fenced or refilled with soil.


The cost of underground storage consists of the earthmoving involved in excavation, covering with soil and later removing the cover: the soil is handled three times. The cost will depend on the farm equipment available and the contract price for work which cannot be done by the farmer.

Uses of grain

Provided care is taken over the siting and covering of the pit to prevent water entry, there will be no deterioration in grain quality for stock feed. One sample buried for six years still had 100% germination, but grain of this age should not be sown as seedling vigour may be reduced.

Delivery of aged, pit-stored grain to a bulk handling company may be subject to various conditions. Before delivery is considered, check with the company.


David Cousins