Preparing a site for planting in a larger scale production garden

During this practical session on April 30th, we worked in groups to prepare an area for autumn-winter planting of brassicas for the organic students at TAFE.

The soil in these areas are clay-based, (on feel somewhere between a sandy clay loam to a clay loam) and quite sodic.

Consequently, surfaces collapse easily during rain or irrigation, with the collapsed materials forming a dense surface crust. This impedes infiltration of water into the soil profile and inhibits emergence of seedlings. It can also influence drainage and removal of excess water from the profile as well as limiting the supply of oxygen to subsurface root systems and beneficial microorganisms.

The soil breaks into large clods which make for a very rough seedbed. Ideally, soil aggregates should be between 0.2 - 3mm in diameter.

Improving the structure of such a clay soil involves :

1. the addition of gypsum (Calcium sulphate) given that the soil is dispersive

2.addition of plenty of organic matter

3.judicious cultivation to break up hard and compacted ground (ie. only when soil conditions are right - not too wet and not too dry).

Dolomite (Calcium carbonate/Magnesium Carbonate) was added to raise pH of these areas.

In smaller garden beds, the double digging technique is a good way of mechanically relieving compaction, crusting in the top soil or hardpanning in the subsoil. For larger scale production areas such as the one in TAFE this is not necessarily practical. In this case, a rotary tiller and larger agricultural rotary tiller was used as shown below :

As always, there are benefits and disadvantages of using these machines.

Buried weed seed is brought up to the surface enabling them to germinate. Rotary tillers also can create a compacted layer at the base of the tilled soil. Aggregates in the tilled soil become pulverised. The organic matter that binds aggregates are exposed to attack by microbes. This is why repeated cultivation can steadily destroy a soil's structure. Mechanical tillage like this should only be done once and lightly - once a year probably at most, but with the maintenance of high levels of organic matter.

P.P.E. used during the operation of these machines includes eyewear, gloves, full sleeve shirt and long trousers, steel cap boots, ear muffs.

The smaller rotary tillers are propelled forward by the rotating tines and do not have powered wheels, although they may have small transport control wheels. An adjustable tine acts as a brake, slowing the forward moving machine due to friction as it passes through the deeper untilled soil. The operator can adjust the amount of friction/braking by raising and lowering the handlebars of the tiller. These machine can be pulled backwards or put in reverse to move over areas again, but care should be taken so that the operator does not stumble and pull the machine over himself. Both these machines had gear boxes to allow the forward speed to be adjusted whilst the rotation of the tines stays constant. The larger agricultural tiller has a dead man switch which when released stops the machine from moving forwards or in reverse. Unseen sub-surface objects, such as stones or buried garbage, are also potential hazards as they can abruptly and violently move in any direction.

The benefits of using these machines include efficiency and coverage of a large area, improving drainage and aeration which reduce potential for plant stress and the potential for pathogens.

Reference : Kevin Handreck, Gardening Down Under

Wikipedia - Rotary Tillers