How to be Stockfree Organic – Relay green manures

Iain Tolhurst

The under sowing of green manures is something that farmers have been doing for some time; under sowing with cereals mostly using clovers, but the technology within organic vegetable systems is something that is relatively new to organic growers. I prefer to call it “relay green manures”- the slotting in of a green crop within the productive cycle of cropping. There are several advantages to using this technique especially where the bringing in of bulky organic manures has become problematical due to lack of availability, costs or potential problems of contamination and concerns over importing “ghost acres”.

Increase in fertility and organic matter accumulation

Relay green manures (RGM) will, even in a growing crop, be able to add some nitrogen and organic matter to the soil. But its real value comes in ensuring that the soil is covered prior to the winter period when so many nutrients will be lost from the soil due to leaching. The greatest loss of nutrient is due to leaching not crop offtake. Horticulture makes heavy demands on the soil; it is pointless building fertility and then allowing it all to wash away during the winter. Far better to allow for modest and achievable inputs but ensuring that what you put in stays there. Deep rooting green manure will also help to recycle nutrients that may have been lost to lower soil levels during previous cropping. Most RGMs will benefit the following crop far more than its host crop. In some cases a very strong growing RGM may in fact get to overtake the main vegetable crop with resultant reduced yield.

Improved soil structure

Keeping the ground covered will aid the preservation and improvement of its structure. Some green manures will root very deeply in a short period and the resultant root structure will do much to improve soil profiles and improve nutrient availability. Improved earthworm and soil fauna populations. Earthworms just love to have a cover over their heads, happily munching away at plant debris and decaying green manure. Soil fauna multiply faster when soil is covered helping to process the organic material into available nutrients and helping to bind them to the soil.

Weed control

Good control of weeds is very much dependant on pre- RGM tillage and suitability of the whole rotation, providing that this has been well carried out then an improvement in reduced weed populations will be experienced. Timely establishment of the RGM is essential to smother any potential weeds and choosing the right crop to apply the technique is important. Land under green manure will enhance earthworm populations and earthworms are known to take weed seeds from the surface and bury them deep where they will not germinate.

Reduced tillage in some situations

This is due to reduced weed control techniques in the crop post sowing of RGM and also to easier tillage in subsequent years where soil structure is improved. Dramatic reductions in nutrient leaching over the winter period.

Reduction in pest attack

Increased bio-diversity is a good way to reduce pest problems, due to the increase in beneficial insects that will occupy the plants grown within the RGM. The effect of this tends to be accumulative increasing yearly as the system stabilises. Some insects may be discouraged or confused by a green cover between rows of crops. High ground beetle numbers love ground cover and they will predate slugs and the eggs of slugs.

As with all good things in life there are some disadvantages to be considered too: –

High degree of management required

The incorporation of RGM will mean that timeliness is very important there will usually be a very narrow
window of opportunity for pre-sowing weed control and subsequent sowing of the green manure.

Slug problems

In an ideal world the increased biodiversity will mean higher beetle and other predator populations to control the slugs and other soil pests. Unfortunately, this may not always be the case. If you have a slug problem choose your crops carefully. I have found that once the system of almost continuous green manures is established the slug population does decline to generally acceptable levels.

Weed problems

If you get the timing of cultivations and sowing wrong you can get in a mess! In some parts of the country on some soil types you may need irrigation to get seed established. In wetter areas it may not always be possible to generate a stale seed-bed prior to sowing RGM, or you may miss the window of opportunity for sowing the RGM.

Crop competition

With some crops the RGM may compete for nutrient and moisture to the detriment of the main crop. It may be acceptable to allow a yield reduction with some crops in order to build adequate fertility for future cropping.

Machinery/labour

You may need additional items of machinery to establish and subsequently manage the green manures. In some cases additional labour may be required.

You will need to redesign your rotation if you are going to be able to make the best use of the technique, ideally you will be able to have a green manure crop in just about every year of the rotation.

Combinations

In many cases it is preferred to mix several species of green manure together, for example white clover/trefoil or lucerne/red clover, there is an infinite range of possibilities. In our rotation at Hardwick the last crop is Squash, an easy one to relay sow, we use a mix of lucerne, red cover, white cover, trefoil, chicory and a selection of wildflowers. This is all sown together end of June/early July as the crop is about to grow frantically. It will stay in place for the two years after the squash have been harvested forming the main fertility building component of the seven-year rotation. We apply ramial chipped wood to this during those two years.

As a general rule, clovers, trefoils and lucerne can be sown at around 1-2 g square metre for small area crops. Other suitable crops for under-sowing are runner beans with red clover sown when the beans are½m tall. Courgettes when 6-8 leaves developed with red clover. Tomatoes and cucumbers in glasshouses are easy to do use Kent Wild White clover or trefoil when plants are½m tall, Persian clover also works really well here and has a wonderful aroma once flowering. I have not found low growing crops such as lettuce suitable for RGM techniques.

Techniques for establishing the RGM will vary depending on crop and soil type but generally seed will be broadcast into the established crop and either lightly cultivated in or irrigated. For small producers sowing the seed and hoeing-in to cover has the additional benefit of weed control. Best method is a hand made sower made from a jam jar with several small holes drilled in lid and used like a salt-shaker. You will need to experiment to get the seed rate right, blocking some holes with tape. For field scale use a seed fiddle or shanty barrow, the Earthway broadcaster will take 20 kg of seed and sow around 2ha in½ hour of walking.

Several stale seedbed passes should have been carried out within the growing crop prior to seed sowing. Once established there is nothing more to be done to the crop apart from possibly rogueing for perennials such as docks. Where row crops are grown on ridges or on raised beds inter row cultivations are needed to ensure freedom of weeds prior to sowing the RGM. Timing of sowing is critical; too early and the vegetable crop could become swamped by the green manure, too late and the green manure may not establish effectively. As a general rule vegetables need to be at the point of vigorous growth so that the green manure and crop can grow at the same rate. For example, squash which is very suitable for RGM needs to be just about to start throwing out its laterals, then the green manure will be just behind the crop. The window of opportunity is only about one week, so rather weather dependant, whereas sweet corn can be sown at any stage from about 15cm to½metre tall and still get good green manure cover without crop detriment. Brassicas have been problematical, due to lack of precision equipment for sowing, broadcasting is not very effective as the crop tends to get too large over the green manure and I have had problems with seed falling into the hearts and growing out of the centre of the cabbage!

Give it a try with an easy crop first such as squash or courgettes and gradually move on to other crops.

Table of green manures for under sowing techniques

Green Manure	Suitable crops	Sowing rate/ha	Dates
Red clover	Most field crops	7-12kg	Apr-E.Sept
White clover (1)	Most field crops	6-10kg	Apr-E. Sept
Kent W. White Clover (2)	Tom, Cucs, Aub, C.F.beans	1-2gm/sq m	Feb-Sept
Cereal Rye (3,4)	Late Aut. sowings field veg	120-250 kg	Sept-Nov
Oats. (3,4)	As above	120-250 kg	Sept-Nov
Lucerne (5)	Sweet corn/maize	12 kg	April-July