Iain Tolhurst outlines field scale methods that can also be adapted to smaller areas.

For many years now the organic standards have stated that any arable rotation must include a legume as a green manure – a fertility-building phase.

We all know that this makes sense, helping to increase organic matter and provide a ready fix of nitrogen for following crops. For arable rotations this is fairly easy to implement and in some cases it may well be utilised by grazing livestock thereby reducing the amount of time that land is considered to be unproductive. This however is not an option for the vegan grower.

With intensive horticultural rotations on some soil types this fertility-building phase may occupy as much as 50% of the land area. With the steadily increasing area of organic production growers have to fine-tune their fertility building policy. There are insufficient animal manures of suitable quality to go around and their handling and composting requires expensive machinery or hired contractors. The increased production of municipal composts will help in some cases but better management of green manures will always be of better value than brought in bulky organic materials, utilising ones own fertility and reducing the amount coming onto the farm has to be the priority of organic production in the future.

The use of under sown green manure (ugm) techniques that reduce the amount of time needed in the fertility-building phase or allow an increase in fertility and therefore higher yields whilst still being able to grow a crop offers an attractive option to long term green manuring. However it should not be seen as a way of eliminating the need for a fertility break within a rotation. The fertility break is essential not just for nitrogen accumulation but also to allow a resting period when the soil is not being cultivated and hence allow populations of earthworms to flourish.

Under-sowing of green manures has some very definite advantages

Increase in fertility and organic matter accumulation Ugm 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 off-take. 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.

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.

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 dependent on pre-ugm tillage and suitability of the whole rotation, providing that this has been well carried out then an improvement in weed populations will be experienced. Timely establishment of the ugm is essential to smother any potential weeds and choosing the right crop to apply the technique is important.

Reduced tillage in some situations This is due to reduced weed control techniques in the crop post sowing of ugm 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 ugm. 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.

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

High degree of management required The incorporation of ugm 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 bio-diversity will mean higher beetle and other predator populations to control the slugs. 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 ugm, or you may miss the window of opportunity for sowing the ugm.

Crop competition With some crops the ugm 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 will need additional items of machinery to establish and subsequently manage the green manures. In some cases additional labour may be required.

The choice of rotation is very important

This will need to be designed to take into account the needs of the crop and the ugm. There may well be some compromises to make. The following is an example of how I integrate ugm into field vegetable production on 17 acres:

Year 1+2 Long-term red clover and rye, ploughed in early March.

Year 3 Potatoes. As soon as the earlies are cleared the land is sown with a red clover green manure. The maincrop are harvested early in September and red clover is sown here also. If the harvest is delayed then cereal rye is sown.

Year 4 Sprouts planted after early potato, cauliflower planted June and late autumn, and winter cabbage planted after maincrop potatoes in July. This allows some fertility building prior to planting. Land is ploughed 14 days prior to planting. I have tried ugm with various clover and cereal types but have run into problems of establishment and seed falling into the heart of brassacaes and taking root there!

Year 5 Leeks and onions. The leeks are ugm with cereal rye in early autumn and the tractor wheelings between the onion beds are sown with red clover at the same time as planting. Once the onions are cleared I use cereal to winter cover the bare land.

Year 6 Parsnip are sown after onions, and carrots after leeks. The rye in the leeks is allowed to grow up to point of flowering before cutting down; it provides a bulk of organic matter.

Year 7 Squashes and sweet corn in separate blocks are under sown with red clover, sowing date is critical for squash, it has to be as soon as the plants are established but before they go rampant with growth, usually mid–end June. This red clover will then be left in for a further two years afterwards, being regularly topped.

This rotation should not be considered as a model for any farm as each farm has to be considered individually, taking into account a range of factors for rotation design such as soil type, climate, incidence of pests, disease and weeds, cropping and labour. Careful rotation planning is the key to successful production and is the keystone of the “whole systems approach” to organic production.

Other suitable crops for under-sowing are runner beans with red clover sown when the beans are ½ m tall. Courgettes when 6-8 leaves developed with red clover. Tomatoes and cucumbers in glasshouses are easy to do, use Kent Wild White clover when plants are ½ m tall. I have not found low growing crops such as lettuce suitable for ugm techniques.

Techniques for establishing the ugm 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. 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.

Table of green manures for under sowing techniques

ugm Suitable crops Sowing rate/ha Dates
Red clover Most field crops 7-12kg[1] Apr-early Sep
White clover Most field crops 6-10kg Apr-early Sep
Kent W. White Clover Tom, Cues, Aub, C.F.beans 1-2g/sq[2] Apr-early Sep
Cereal Rye or Oats[4] Late Aut sowings field veg 120-250kg[3] Sep-Nov
Lucerne[5] Sweet corn/maize 12kg Apr-Jul

[1] The higher rate should be used in difficult situations such as late sowings or advanced crop stage. For white clover use stong growing varieties.

[2] Can be left over winter, good for shade tolerance.

[3] Use higher rate for late sowings under leeks. Excellent for weed control in following spring.

[4] Be sure to use winter hardy cereals.

[5] Needs high pH and well drained soil. Excellent for 2-3 years after crop.

Other crops There must be potentially 100s of other suitable plants for under-sowing we desperately need Research and Development into this important area of organic production.

This article appeared in Growing Green International magazine Num 9 (Summer 2002), p22. We are grateful to Peter Munday, editor of the Soil Association’s “Organic Farming” magazine, for his permission to use this article, which has been amended and updated for VON by the author Iain.