Failed crop, that's good

18/07/2007 9:19:34 PM

Huck Shepherd, Ag Consultancy SA

Turn in a crop as green manure and the benefits can outweigh the loss

The down-side of turning in a crop as green manure is that you will lose a year’s income but the benefits can outweigh this loss, especially if you follow by growing high-value crops, either broad acre or horticultural.

Weedy paddocks can be areas where a green manure crop would be beneficial and this approach could save money you would otherwise spend on chemicals trying to control weeds.

Green manuring will also reduce the chance of chemical resistant weeds emerging.

The year of green manuring will also spell that paddock and give it a chance to regenerate soil microbial life and at the same time improve soil structure.

What will a green manure crop do for you?

It will:
• stimulate beneficial soil bacteria
• make nutrients more available to following crops
• build soil structure and help hold soil moisture
• increase humus levels

Before incorporating a green manure crop, a spray of beneficial microbes, a carbohydrate such as sugar and a nitrogen source will assist in the decomposition process.

Good crops to green manure are legumes such as vetch, alfalfa, and clovers and grass species such as rye and oats. Oats are particularly good for releasing phosphorus in the following year.

Alfalfa, beans, and clovers provide the most nitrogen—up to five per cent.If you have excess soluble potash levels then a green manure crop will help to reduce these excesses.

Different soil types may need different green manure crops.

Sandy loams soil with good structure will benefit from growing a legume which is incorporated well before it matures. This will add nitrogen and stimulate soil microbes.

Fresh plant tissue of a green manure crop contains more available sugars, amino acids, enzymes, vitamins, and other cell materials than dried residues.

Clay soils on the other hand, especially those with a hard pan, or light sandy soils with poor structure would both benefit from a green manure crop that will build humus levels. Use grassy crops or well-matured legumes for this purpose.

Mature coarse plant material will increase humus levels but will invariably deplete nitrogen levels as they decompose so add extra nitrogen if necessary.

Humus will loosen heavy dense soils and hardpans and will also improve soil structure and fertility of sandy soils.

On those light soils, incorporation near the surface will help to keep nutrients near the root zone.

However deep incorporation on heavy soils will help to alleviate hardpan problems. Deep incorporation means working in only as deep as your fence posts rot off as this indicates your aerobic zone.

Incorporating deeper than this zone will not get you the results you will be looking for and possibly the reverse—incorporation too deep into anaerobic conditions, which invariably will be detrimental.

Always remember though that for any material that is incorporated, you need adequate moisture for the proper decomposition to take place.

Help hold nutrients in soil

Another important point is that growing a green manure crop can help to hold nutrients in the soil that would otherwise be leached through the profile. This is especially true for nitrogen.

On Mullinger Park we have grown fenugreek as a green manure and found that it was quite successful. The following year we grew a 6 ton/Ha triticale crop. Although this wasn’t exceptional for the year, the quality was excellent and the weed control we were looking for was achieved.

Another aspect that hasn’t been mentioned is that of “opportunity” green manuring. If your growing crop has an influx of undesirable weeds or you assess mid-season that it wont be worth harvesting, then just incorporate it and get all the benefits of the green manure crop. You might be surprised with the results the following year.

We incorporated a safflower crop last year that wasn’t doing well and which also had a small amount of radish growing in it.

A week or two after incorporation we also put sheep on to graze any plants that hadn’t been completely incorporated to prevent any weeds from seeding down. We expect increased yields from the paddock this year as well as helping our weed management.

Good luck the next time you try a green manure crop. I am sure you won’t be disappointed.