Category Archives: Soil Management

Green Manures for Late Summer Sowing

Winter Mix

Leaving soil bare over the winter period can have detrimental effects on its structure and fertility.  Rain leaches nitrates from the soil and weeds can gain a foothold creating extra work for the gardener.  Green manure plants can help in a number of ways.  Members of the legume family can fix nitrogen in the soil improving fertility for the following crop, all of them help to suppress weeds and when dug in add organic matter.  Some, like the broad bean, Vicia faba can also provide food during the growing period.  Here are a few of the most beneficial plants for sowing now (late summer/autumn) and overwintering.

The Nitrogen Fixers

Alfalfa

Alfalfa (Lucerne) Medicago Sativa

This crop is a tall perennial with deep roots which can occupy a plot for a whole season. It provides plenty of organic matter when dug in, is a good weed suppressant and is a nitrogen fixer coming from the legume family.  It can be sown in late summer for digging in come spring.

Broad Bean

Broad (or fava) Bean Vicia faba

This crop is highly versatile withstanding the coldest winters, providing organic matter when dug in and again is a nitrogen fixer.  Additionally, if sown in rows 30cm apart with 10cm between seeds then you will get a crop of beans too.  Seeds can be harvested for the following year’s crop of green manure or beans if desired.  Sow in autumn or early summer.

Red Clover

Red Clover Trifolium pratense

Red clover is low growing with a wide spreading root system.  It fixes nitrogen and provides plenty of organic matter when dug in.  It needs to be sown before autumn, preferably in spring or late summer.  Sow in rows 15cm apart scattering the seed at 30g per sq metre.  When the land is needed just dig it in.

Winter Tare

Winter Tare Vicia villosa

This plant is extremely useful as an overwintering crop where land is unoccupied.   It has an extensive root system, fixes nitrogen and supplies a good return of organic matter to the soil.  Seeds are sown in late summer, in rows 15cm apart and about 7cm between each seed.  It can also be sown in spring and summer if needed.

 

Non Nitrogen Fixers

Rye

Rye Secale cereale

This agricultural crop is a perennial with an extensive root system.  It doesn’t fix nitrogen but adds a good amount of organic matter to the soil when incorporated in spring.  The perennial variety can be sown in late summer or autumn, scattered in rows 23cm apart.  It can also be broadcast sown at 30g per sq metre.  If you want seeds for the following year, allow some plants to set seed and save.

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Introduction to Green Manures

Green manure plants are grown specifically to improve soil structure and fertility. Knowing what soil type you have is key to understanding how to improve it and what plants to grow, or not as the case may be.

In clay soils, green manures can help by breaking up the soil.  In silty soils they can be used to protect the soil surface from inclement weather during the cooler seasons and help to improve the structure.  On sandy soils where sharp drainage and erosion are a problem, bulky green manures can provide organic matter helping to retain nutrients in the soil.

As I am gardening on clay I have sown Phacelia tanacetifolia in some areas of the garden to help improve structure.

Here are some other benefits of using Green Manures:-

  • They can help protect soil life from extremes of temperature and moisture;
  • They attract beneficial insects to the garden;
  • They take up nutrients that might otherwise be washed away, these nutrients are then returned to the soil when digging in;
  • They can act as ground cover and a weed suppressant;
  • Some, such as clover, take up Nitrogen from the air and fix it in their roots, in turn adding fertility to the soil, (Nitrogen is an important plant nutrient);
  • Others, such as grazing rye, are particularly good at improving the structure of the soil;
  • Some can look attractive in the garden in their own right.

There is a green manure to suit most sites, some can be grown very quickly in a matter of weeks whereas others are left for a year or more.  Most green manures are actually agricultural crops such as winter field beans, Vicia faba or grazing rye, Secale cereale and as such will compete well with weeds.  When using green manures from the legume or brassica families, careful consideration should be given to the crop rotation cycle to avoid encouraging related plant disorders.

Over the coming weeks I will be covering the different plants available as green manures, but if you have any questions in the meantime please don’t hesitate to ask.

Image

Phacelia tanacetifolia

Phacelia tanacetifolia

P. tanacetifolia is a ‘green manure’ and is used to improve the condition of the soil. The bees love it and it grows easily from seed sown directly where it is to flower. This image shows the flowers fully open having extended over the last two to three weeks, they are about to set seed and this is the right time to chop up the plants and dig them in.  After a couple of weeks the ground should be ready to plant or sow.

I have also seen P. tanacetifolia used in roadside wildflower plantings by my local council and it looks a picture.