Green manures under sweet corn or tomatoes

Joyce Wilkie uses green manures to boost fertility on her very poor soils. She undersows sweet corn (maize) with soybeans as a green manure.

Joyce says the idea is to go for a balance of nitrogen, not just to sow legume green manures. Therefore it is a good idea to sow high volume crops that can catch a lot of moisture and turn these back into the soil. A plant's primary need is for moisture.

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The combinations are important.

She sees it as best to sow the green manure late to reduce competition between it and the crop. She sows the green manure when the corn is at least 15 cm/6 inches high. So she plants the corn and then around 5 weeks later, plants soybeans. This is the ideal stage because the corn is not too big and as a result is not swamping the soybeans, nor will the soybeans get too big for the corn. She has sprinkler irrigation set up so she doesn't need to get back through the corn again after sowing the soybeans.

When she picks the corn she is wading through soybeans up to her waist. The soybeans have kept the weeds down and it doesn't matter if they get damaged in the process of harvesting the corn. At corn harvest, the soybeans are flowering and once the corn has been harvested the lot is turned back into the soil. All that has been removed is corn cobs. What goes back into the soil is an enormous bulk of soybeans and corn stalks.

Michael Plane and Joyce Wilkie are innovative small-scale commercial vegetable farmers at Gundaroo, 50 km/30 miles north of Canberra, Australia.

Here is a variation on this that may be worth thinking about as you look at how you could apply this on your farm: In South America a common polycrop (several crops growing in the same space at the same time) is corn, soybeans and either squash or yams (sweet potato-type crops). These mixes have large areas of leaf that intercept most of the available sunlight. Their roots forage every skerrick of nutrient and moisture and as a result outcompete many weeds. So the problems are minimal and the advantages and the yields of corn, beans and squash or yams is high.

Joyce says tomatoes can also be undersown with soybeans, clover or vetches. They can also have oats grown between the rows of tomatoes, but only if the oats are mown regularly. Then the oat slashings are thrown onto the base of the tomato rows to provide nutrients and a mulch.

She spaces the tomato plants a bit wider than slasher width and sows enough rows of oats to fill the gap between them. When the oats are about 30 cm/12" tall, she or Michael runs a small slasher down the oat rows. This throws the oats onto the tomatoes to mulch them. This could be done with a lawnmower in a smaller patch. She gets about three cuts of oats during a summer tomato growing season. So all the time the tomatoes are growing, they grow their own mulch.

Replacing the oats with barley could increase the allelopathic effect and as a result the ability to inhibit weeds. Because the tomato plants are established, there is little chance of the barley having much impact, however it would be advisable to run a trial in a small area before using it on a whole tomato crop.

Related info:

Grazing shock explains some of what happens when a green manure is slashed.

Green manures

Green manures in orchards and vineyards

Choosing a green manure mix

Squeezing a green manure in tells how you can fit a green manure into a tight cropping program.

Return a third to the soil

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This page was updated on December 27, 2007