Compost and Nutrients

Professor Carrot Says

Professor Carrot Says

If you don’t put back what you take out you’ll be in trouble

Plant nutrition is the key to healthy plants.

They need to have enough to grow sturdily  without check – but not to have to much nitrogen too soon. Otherwise they grow lushly and sappily but the plant walls are too thin and weak and don’t resist the mouthparts of our insect and sluggy friends who fancy a treat at our expense.

Keeping up the fertility.

In a traditional rotation you need to add back the nutrients that the plants take out year by year.

We’ve worked out what the balance looks like for a large (10 rod) allotment. And that’s set out below. In general you will need about 1 tonne ( 2 cubic metres)  of compost per year per allotment.

Nitrogen and Phosphate movements in via compost and fixing - out via crops

Nitrogen and Phosphate movements in via compost and fixing - out via crops

The information in this table is taken from Francis Blake’s book Organic Farming and Growing but the figures have been recalculated to suit the scale of a 10 rod allotment.

It relates to field scale vegetable growing but it is useful to give us an idea of what we should do.  It illustrates the general principles together with how it might work.

We’re basically looking at a 3 year rotation here.

In year one we put on a load of muck and grow potatoes. The  Manure adds 3.75 Kilos of Nitrogen and 6.25 Kilos of Potassium. When the crops are harvested 2.5 Kilos of Nitrogen and 3 of Potassium leave with them.

The spuds are followed by Broad Beans – These take out a further 3.75 Kilos of Nitrogen and 2.5 of Potassium but the nitrogen fixing bacteria in the bean roots put 5 kilos back.  So at this point – end of May in Year 2 we’re still ahead on Nitrogen ( 3.75 -2.5 +5 -3.75 = +2.5)  while the Potassium budget is .75 Kilos up overall.

Our farmer then green manures the ground  to keep it covered in the winter to protect the soil structure and guard against erosion while he puts on another load of manure – half as much as the first time and proceeds to grow a crop of  Spring Cabbages followed by Spinach.  In the autumn. At this point the nitrogen budget is slightly negative (-6.2 kilos) while the Potassium is now down 3.5 kilos.  So our farmer tops up the fertility again by putting in some compost this time, taking out a crop of onions which aren’t so heavy duty in terms of their demand fro nutrients and follows up with an0ther green manure crop to cover the ground in the winter.  At the end of the rotation the  Nitrogen budget is about back to where it started but we’ve lost nearly 2 Kilos of  Potassium overall. This has to be replaced or otherwise made available to the plant by liberating some of it from the soil.

The trick is not to put too much Nitrogen on at once as this makes the plants grow lush and sappy and susceptible to pests.  Adding Phospate and Potassium neat can be a problem also as they get bound to the soil and become unavailable.  One way round this is to inoculate the plants with nycorhizza – a beneficial fungus which sets up a symbiotic relatioinship with the plant roots. The plant supplies the fungus with carbohydrates that its synthesised  – the fungus supplies the plant with phosphate which it extracts from being bound to the soil particles much more effectively than the plant root can. We have several mycorrhiza products available.

Adding compost and other organic material improves the soil structure by helping to get the right balance between large and small pores. The soil structure is really important. Too many larger pores and the plants are always on the edge of water stress – which checks growth and induces flowering – not so bad for tomatoes but terrible for lettuce. On the other hand too few and the fat roots of peas and beans can’t force their way through the soil.  If the soil structure is as you find in silty soils it’s prone to capping with too much rain and forms a crust that the plants can’t force their way through.

Maintaining the soil structure is a real journey. This has led to the no-dig approach of gardening.  The more you cultivate by digging, ploughing or rotavating  – the more likely you are to get degradation of the structure, In extreme cases you can end up with impervious  plough or rotavator pans that can lead to drainage problems.

A good approach when you take over a plot is to double dig it once to incorporate a good volume of organic matter as you go.  There after avoid digging as far as possible. A good approach is to operate in 4-5 foot wide beds as we do in our nursery. This means you can create paths to work from so that you don’t have to step on the soil that’s growing the plants. You can take this approach one step further by creating raised beds. This involves putting an additional 8-12 inches of soil above the original level of the ground. This can be mixed with compost and other organic matter to extend the depth of you topsoil. This can be a particularly valuable approach if the amount of original topsoil on the site is thin

We  supply a mycorrhizal pack suitable for activating compost heaps which

  • Speeds degradation of organic waste giving excellent compost in as little as 6-8 weeks.
  • Helps break down twigs, bark, hard leaves.
  • Increases disease resistance properties of finished product.
  • Produces a rich humus ideal for seedlings, pot plants, transplanting trees, shrubs, flowers and vegetables.

If leguminous or other nitrogen fixing plants are used to make the compost nitrogen content may be as high as 25kg per tonne

The activator itself contains a mixture of

  • bacteria to break down sugars, starches and cellulose,
  • fungi to break down cellulose and lignin
  • sugars and biostimulants to activate the existing microbes to speed up the process.

How to apply it

whattodo How to do it

Sprinkle over each new addition to the compost heap. Add enough water to keep moist. When compost heap gets warm (over 65oC) turn the heap to stop anaerobic conditions. If a lot of sticks are in the finished compost sieve the compost and return heavy undegraded matter to start the next compost heap.

Commercial composters apply it at the beginning and end of the composting process which usually takes between 35 and 50 days. The average gardener will probably need 10 weeks if they keep humidity at 50-60% and turn when the heap gets to 60 C If they keep adding on a weekly basis then complete composting will only take place after the last addition

We supply mixed with a bulking agent – you can buy it here

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