Feeding your Vegetables – pH and micronutrients

Managing Micro-nutrients.

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. This is spelled out in more detail on the compost and nutrients page which covers the budget in terms of the big 3 Nitrogen, Phosphate and Potassium. Find out more here.

As we said, 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.

However there are also issues concerning the availability of other nutrients and the availability of these depend up to a point on how acid the soil is – that’s why it’s important to stay on top of the soil acidity which is measured by pH using a meter or soil test kit. Full details of the ones we recommend are here.

The soil tends to get more acidic with time and it needs to be checked periodically. In a traditional rotation its conventional to adjust the pH higher using lime at the time that the Cabbage tribe or brassicas are planted.  The overall significance of acidity is that it has a real effect on the availability of micronutrients in the soil. This is illustrated in the diagram

How soil acidity affects micronutrient availability

How soil acidity affects micronutrient availability

The diagram shows that the availability of nutrients in the soil depends on the acidity. In chemical terms the nutrients are bound to the soil particles and depending on how acid the soil is the nutrients are more or less happy to be released and so become available to the plant.

As you can see, most nutrients are readily available at pH 6 -6.5 which is what most varieties of plants seem to like.

Potatoes like it acid while cauliflower and other brassicas prefer it on the alkaline side. This is why in a traditional garden rotation you muck (which makes it more acid) before you put the spuds in and add lime before the brassicas which are then followed by the roots and the other vegetables by which time its ready to and manure and push the pH back to the acid side.

As you can see most nutrients are available at “normal” levels of acidity but many get more tightly bound as the soil gets more acid particularly Phosphate, calcium and magnesium and Potassium plus molybdenum.

However a gang of metals that are needed by the plant in small doses, Iron, Manganese, Zinc, Copper and Cobalt get more tightly bound as the soil gets more alkali and can be a problem if the soil gets very alkali.

Fortunately most of these are not needed in large quantities.

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