Canberra Organic Growers Society

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All organic gardeners should be building up a healthy soil full of nutrients available in a balanced form, well aerated and of a good crumbly texture. Plants grown in such a soil will grow strong and healthy.

However, it can take time to create this soil, and in the meantime problems can occur due to nutrient deficiencies in the soil and subsequently in the plants grown in that soil. This article gives guidelines on how to identify the common deficiencies that can occur in some of our favourite vegetable plants and offers some advice on how to remedy them.

The remedy in the longterm is often to add compost regularly to the soil, and the importance of this fertiliser cannot be over emphasised. Remember to make plenty of this wonderful stuff from a variety of ingredients and problems will rarely arise. There are many articles in Canberra Organic on making compost, and compost making features in almost all books on organic growing.

Check your soil pH

If you think there is a possibility of a nutrient deficiency in your soil, first a word of warning: before you start adding amendments to correct the supposed imbalance test your soil pH.

If the pH is outside a range of about 6.0 to 7.0, which the best range for growing most vegetables, certain nutrients although present in the soil will not be present in a form that is accessible to your plants. Adding more is unnecessary. You need to correct the pH.

pH is a measure of the acidity or alkalinity of a substance. The scale ranges from 0 to 14, with a pH of 7 as neutral i.e. neither acid or alkaline. pH values below 7.0 are acidic, above 7.0 are alkaline. It is important to remember that pH is not a linear measure each unit change in pH corresponds to a ten fold increase in acidity or alkalinity e.g. a pH of 6.0 is 10 times as acidic than pH 7.0, and pH 5.0 is 100 times more acidic than 7.0.

If your soil is too acid (as it often is in this region, and many parts of Australia) add some Dolomite (contains calcium carbonate and magnesium carbonate) in Autumn. It is a slow release substance which will raise the pH over winter ready for next Spring’s planting.

Usually you will only have to correct your soil pH every 3 or 4 years. Never apply fertilisers at the same time as dolomite, as, unfavourable chemical reactions can occur. Wait a few weeks or a month if possible.

The Big Three: N:P:K

Nitrogen (N):

Nitrogen is an essential component of chlorophyll which gives leaves their green colour. It improves the quality of leaf crops and is very influential in determining the size of the fruit. Deficiencies in this nutrient show up clearly in the overall slow growth of the plant and the colour of the leaves which are typically light green to yellowish.

See Table 1 for possible solutions. Be careful however correcting any imbalance as an excess in nitrogen can lead to lush foliage but little or no fruit on your plants.

Particular symptoms, in addition to leaf colour and slow growth, for common vegetables are:

Tomatoes: Flower buds turn yellow and drop off. Fruits small.

Corn: leaves have yellowish centre streaks. Lower leaves affected first, gradually become brown and drop off.

Potatoes: Young top leaves may curl upward. Tubers will be small.

Cucumbers: Fruit light in colour. and pointy at blossom end i.e. part furthest from the vine.

Phosphorus (P):

Phosphorus is important for the hardy growth of the plant, playing an important role in root development and cell activity.

When a deficiency in phosphorus exists, particular symptoms can include:

Tomatoes: Leaves dark green, purple underneath. Very slow growth.

Corn: Stalks small. Leaves purple at the tips and along the margins.

Potatoes: Stunted plants. Leaves may curl upward. Tubers may have brown specks inside, often radiating out from the core (note extremes of temperature or viruses can cause similar effects).

See Table 1 for ways to overcome this deficiency.

Potassium (K):

Potassium is an essential element for plants, and is involved in a wide range of physiological and biochemical functions. Some of these are:

• strengthening stems and stalks controlling the turgor pressure within plants to prevent wilting;
• contributing to a thicker cuticle which guards against water loss and disease;
• starch formation and sugar transport from the leaves to the roots.

Specific symptoms of a deficiency in this nutrient are:

Tomatoes: Leaves dark green, leaves bunched together. Older leaves become yellow, then brown at the edges. Fruits fall off soon after ripening, and often have hard white “core”.

Corn: Lower leaf tips become scorched and brown. Poor root system. Stems can become so weak that they snap off, Potatoes: Leaves very dark green, may turn brown at edges and die.

Other Macronutrients:

Calcium:

Calcium is a component of cell walls, and stimulates root and leaf development and activates enzymes involved in plant metabolism.

In tomatoes, calcium deficiency is exhibited as “blossom end rot. This is also associated with uneven watering, which means that calcium does not reach the flower in time for fruit set. Tips of new growth are stunted.

Calcium is not usually deficient in soil as most gardeners apply dolomite to raise the pH. If the pH does not need raising and a deficiency exists it can , be corrected by adding gypsum, calcium sulphate. Note that natural gypsum is usually grey in colour, while synthetic gypsum is usually white and may contain impurities.

Calcium can also be added by crushing eggs shells and adding to the compost.

Magnesium:

This element is needed for photosynthesis to occur. It also stimulates the uptake of phosphorus.

In plants such as tomatoes and beans a deficiency causes older leaves to turn yellow while the veins stay a bright green. They eventually turn brown and die. Only a few flowers and fruit form.

The fastest solution to a deficiency is a foliar spray of kelp or other seaweed extract. In the longterm a deficiency is usually corrected by adding dolomite. If the soil pH does not need to be raised however add Epsom Salts (magnesium sulphate).

Trace Elements:

Generally to know if you have a deficiency in any of the trace elements you will need to have, your soil tested, as deficiencies cause a wide range of problems which vary from plant to plant.

Two deficiencies which can be easily identified by visual means are boron and molybdenum.

A deficiency of boron shows clearly as a brown discolouration of the heads in broccoli and cauliflower (and the stem of broccoli too). Use a liquid seaweed foliar spray for immediate results. Be cautious about adding boron to the soil as it can be toxic to plants. Use precise measurements after a soil test.

A molybdenum deficiency in cauliflowers shows as stunted growth and yellow colouring. Spray with a liquid seaweed product. In the longterm add compost, organic mulch or green manure. Make sure the soil pH is right.

Liquid manures or foliar sprays from seaweed products are extremely useful for correcting deficiencies in micronutrients e.g. deficiencies in iron, manganese, zinc and copper in tomatoes.

Make sure your soil is at the right pH as the supply of micronutrients can be restricted if the soil is too acid or alkaline.

The visual descriptions given above can be used fairly reliably to detect obvious deficiencies of the macronutrients in your soil. For most organic gardeners who regularly use compost and other organic fertilisers it would not be necessary to worry further. However, if you want be sure a soil test would be necessary for these as well as the trace elements.

References

Rodale’s Chemical Free Yard and Garden: the ultimate authority on successful organic gardening, Rodale Press 1991.

Poncavage, Joanna., Visual Guide to Nutrient Deficiencies,  “Organic Gardening” May/June 1995.

LaMotte Soil Handbook

Cundall, Peter., Organic Gardening, Gardening Australia Collector’s Series No. 1

 

TABLE 1: SOLUTIONS TO NUTRIENT DEFICIENCIES

 Nitrogen

Short term:  Use water with fish emulsion made up to a weak “tea”, or another high nitrogen liquid manure e.g. one based on animal manures.

Long term:  Add plenty of compost, well rotted animal manures or Blood and Bone. Grow a green manure crop high in nitrogen e.g. legumes such as hairy vetch.

Phosphorus

 Short term:   Use a seaweed foliar spray or liquid manure made from fish extracts for immediate results. These products stimulate the enzyme system of plants, making them more efficient at absorbing phosphorus from the soil. Alternatively, work Blood and Bone or rock phosphate into the soil where it can be reached by the roots. Water well.

Long term:  Remember phosphorus in the soil may be unavailable to plants if the soil pH is not between 6.0 and 7.0. Add blood and bone or rock phosphate to your soil. Peter Cundall advises that in cold climates there is no point in adding blood and bone after the end of April and before the end of July as it cannot be used. Adding rock phosphate to your compost in Autumn and spreading the finished compost in Spring will make the nutrient available faster as the microbial activity converts some of the phosphorus to soluble form readily absorbed by plants.

Potassium

 Short term:   Use a foliar spray based on a kelp extract. Apply wood ash around the plants, water in well. Comfrey liquid manure can be very beneficial.

 Long term: Add compost regularly to the soil, or aged manure or ground kelp.

 

Note:

• Seaweed can be applied as a liquid manure to the soil surrounding the plant, or sprayed as a foliar spray. Foliar sprays can be beneficial if the roots of the plant are stressed and cannot take up nutrients well. However, applied to the soil it can also stimulate soil bacteria which will indirectly benefit the plant.
• Other liquid manures are best used carefully to water the soil (after it has been watered well with plain water). Foliar spraying can burn the foliage although this should not be a problem if the “tea” has been diluted enough. Remember the important words with liquid manures are DILUTE and FREQUENT.
• Although this article is discussing nutrient deficiencies, it is worth keeping in mind that liquid manures can be useful applied at certain critical stages of plants growth to give it a boost e.g. tomatoes at fruit set with a comfrey liquid manure.
• Be careful with the blood and bone product used. Some contain impurities. To be sure that the product is not contaminated use a certified brand.
• There are a number of certified fertilisers coming on to the market now e.g. some Dynamic Lifter, Alroc. Check the relative composition of the products to see which nutrients they supply and then use as directed.