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Posted on Jul 28, 2013 in Soil management | 0 comments

Soil Nutrition

Soil Nutrition

Treatments for amending and improving soil health. Includes detailed information on identifying and correcting nutrition deficiencies, sources of common garden amendments and using mulch to improve soil health.

NPK overview

Nitrogen, phosphorus and potassium (as well as trace elements) are required by all plants for them to be healthy and productive. NPK is found both in natural soil conditioners such as manure and compost as well as in manufactured fertilisers.

Fertiliser labelling

An NPK fertiliser with a label reading: 10-2-4 means that the fertiliser contains 10% nitrogen, 2% phosphorus and 4% potassium. Any trace elements included, should be labeled separately. The remaining 84% is made up of bulking agents that make it easier to apply, or delay the release of the fertiliser.

Approximate NPK values of organic sources can be found under the ‘NPK Sources’ tab.

About the elements

Nitrogen (N)

Nitrogen promotes healthy leaves and new shoots in shrubs, trees and leafy vegetables and greens up lawns. Basically, the more leaf a plant produces, the higher its nitrogen requirement. e.g lawn fertiliser is high in nitrogen. Plants require the most nitrogen during periods of quick growth.

Nitrogenous fertilisers are quickly washed out of the soil by rain and need to be renewed annually. For crops that require a lot of nitrogen during their growing season e.g corn, adding nitrogen incrementally through the growing season is the most efficient application method. Citrus are heavy nitrogen feeders and should be feed 3-4 times per year.

Identifying deficiency & oversupply

Plants lacking nitrogen will show stunted growth, weak stems and yellowed or discoloured leaves. An excess of nitrogen during flowering or fruit set will limit fruit production as plants divert energy away from fruiting back into producing green growth.

Phosphorus (P)

Phosphorus is essential for seed germination and good root development. It is needed particularly by young plants forming their root systems and by fruit crops. Root vegetables such as carrots, swedes and turnips need phosphorus to develop well.

Phosphorus remains in the soil for two or three years after application so the amount in a general fertilizer is probably enough. Add just before planting or top dress during growth periods.

Identifying deficiency & oversupply

Without ample phosphorus you will see stunted growth, possibly a purple tinge to leaves and low fruit yields.

Potassium (K) (sold as potash)

Potassium is applied just before flower budding to encourage the plant to set plentiful flowers and to develop fruit. It also controls the water content and transportation of other plant foods from roots to shoots. Ideal for fruit bushes/trees, roses, tomatoes and hanging baskets. Potassium/potash increases the effectiveness of other nutrients on the plant and helps reduce the loss of nitrogen from the soil.

Potassium is naturally found in wood ash which is where it its name potash is derived from.

Potash usually last for two or three years in the soil but for vegetable and fruit production (tomatoes, capsicum, potatoes especially) additional will be required.

Identifying deficiency & oversupply

Plants that are short of potash may have low resistance to disease and leaf scorching. Fruit yield, size and flavour may be poor.

Trace elements

Trace elements such as boron (B), calcium (Ca), chlorine (Cl), copper (Cu), iron (Fe), magnesium (Mg), molybdenum (Mo), manganese (Mn), nickel (Ni), sulphur (S) and zinc are also required by plants, but in smaller quantities.

Factors affecting nutrient uptake

Weather, correct water levels, soil pH, good undamaged root system,

Nutrient mobility

Nutrients such as nitrogen, phosphorus, potassium and magnesium are readily translocated within the plant and are referred to as “mobile”. Symptoms first appear on older leaves as the nutrients are moved to new growth. Other nutrients such as iron, zinc, calcium and boron are immobile and are not moved around the plant, with symptoms generally occurring on the growing tips. Manganese mobility is complex and depends on the species and the age of the plant; manganese deficiency may appear on either old or new leaves.

Soil pH

When the pH of the soil is too low or high, essential nutrients become unavailable. At high pH values, phosphorus, iron, copper, zinc, boron and manganese become less available. Soil pH

Nitrogen (N)
Necessary for all phases of plant growth
Little new growth, yellow leaves: this being more pronounced in older leaves.
Earlier autumn leaf drop.
New shoots may be red to red-brown.
Quick fix: Weekly foliar applications of fish emulsion or manure tea.
Long term: In spring, add aged compost, manure or seaweed extract to soil.
Phosphorous (P)
Necessary for strong stems, fruiting, rooting and seed making.
Overall dark green with purple, blue or reddish cast to leaves particularly on underside.
Veins and stems of some plants respond to lack of P with yellowing.
Foliage may be sparse, small and distorted becoming mottled and bronzy with maturity.
Excess foliage with no flowers can also indicate lack of (P).
Quick fix: Spray plant weekly with fish emulsion until symptoms subside. Apply a light dressing of wood ash. Incorporate aged compost.
Long term: Mix rock phosphate or aged manure into the soil in autumn.
Potassium (K)
aka Potash
Necessary for strong root systems and for forming starch, protein and sugar.
Sickly looking plants, undersized fruits, leaves showing marginal and interveinal yellowing.
Yellowing starts on older leaves and progresses upwards.
Leaves may crinkle, turn brown and roll upwards.
Blossoms may be distorted and small.
Plant has little resistance to heat, cold and disease.
Potash deficiency mostly occurs in the upper levels of soil.
Quick fix: Spray plant weekly with fish emulsion until symptoms subside.
Long term: Apply seaweed, manure, granite dust or greens to the soil in autumn.
Hardwood ashes may be applied anytime.
Calcium (Ca)Young leaves are small and distorted with curled back leaf tips.
Shoots may be stunted and show dieback, roots will be stunted.
Unusual. Poor calcium uptake is usually related to incorrect pH.
Magnesium (Mg)A lack of magnesium looks very similar to iron deficiency but the older leaves, generally at the bottom of the plant, show marginal and interveinal reddening or yellowing with leaf base and midrib staying green. Later in the season interveinal necrosis may occur.
Leaves may be brittle and thin with leaf curling and stunted growth.
In the autumn as temperatures cool, plants are unable to take up Mg and leaves will turn a purple color. Raspberries, tomatoes and capsicum are particularly susceptible.
Epsom salts (magnesium sulfate) can be used for magnesium deficiency. Dissolve 1-2 teaspoons of Epsom salts in 4.5 litres of water. Water into soil or use as a foliar spray. Make 3 applications 6 weeks apart. Other treatments include adding fish meal, greens and or dolomite.
Sulfur (S)Leaves are pale yellow-green at any stage of development.
Shoots are stunted. Similar to chlorosis.
Add sulfur or potassium sulfate. Use caution when applying sulfur compounds. Too much sulfur (“sulfur toxicity”) appears as veinal chlorosis followed by rapid defoliation of the lower leaves.
Boron (B)
The quantity of Boron in soil is directly proportional to the quantity of its organic matter.
Youngest leaves may be red, bronze or scorched also small, thick or brittle.
New shoot tips may form what is called a witches broom. Stems stiff; terminal buds die and growths die back; lateral shoots developed, giving plant flat top; leaves highly tinted purple, brown and yellow.
Fruit and vegetables may have heart rot.
Fruits pitted and corky areas in skin; ripening is uneven.
Boron deficiencies are found mainly in acid, sandy soils in regions of high rainfall, and those with low soil organic matter.
Borate ions are mobile in soil and can be leached from the root zone.
Boron deficiencies are more pronounced during drought periods when root activity is restricted.
Apply household borax at a rate 1 tbls borax to 12 litres of water. This amount will treat a 30 metre row of vegetables or 10 square feet of soil. Apply two times 2-3 weeks apart. Be careful, plants can suffer boron toxicity which is very difficult to correct. Only add if you are 100% sure the soil is deficient.
Copper (Cu)Copper deficiencies are mainly seen on sandy soils which are low in organic matter.
Copper uptake decreases as soil pH increases. Increased phosphorus and iron availability in soils decreases copper uptake by plants.
Small leaves with necrotic (dead) spots and brown areas near the leaf tips. Rosetting of the leaves and dieback of terminal shoots.
Iron (Fe)New leaves are the most symptomatic and when condition is most severe they can be all yellow or white but still have green veins. Overall you see yellow leaves with green veins leading to marginal scorching or browning of leaf tips. Tip leaves, especially basal areas of leaflets, intense chlorotic mottling; stem near tip also yellow. Fruits have poor color. Shoot diameter is small. Iron deficit often occurs when the soil pH is higher than 7.5 meaning it is more alkaline. Lack of Fe is common in plants living next to concrete walls, foundations etc.In iron-deficient soils, add bone meal or blood meal organic amendments, or add iron sulfate or chelated iron liquid or granular inorganic amendments.
Quick fix: Apply chelated iron directly to soil or as a foliar spray. Reduce soil pH to at least 7.0 which is considered neutral.
Long term: Improve the soil by adding 1-2 inches of compost in the spring every year.
Manganese (Mn)Similar to N deficiency.
Leaves display marginal scorching, rolling and reduced width. Yellowing may also occur between leaf veins or total yellowing on youngest leaves.
Correct to 6.5 or lower.In deficient soils, add millorganite or houorganite treated sludge organic amendments, or add manganese sulfate inorganic amendments.
Molybdenum (Mo)Only a problem with brassicas like broccoli, cauliflower etc in acid soil. Heads can fail to form,  leaves will become thin, elongated and rippled.Add lime to soil before planting or sowing seeds.
Zinc (Zn)Zinc deficiencies are mainly found on sandy soils low in organic matter and on organic soils. Zinc deficiencies occur more often during cold, wet spring weather. New and intermediate leaves are small, yellow, sometimes with a grayish cast. Narrow and older leaves may drop. Small shoots may show rosetting followed by die back.Ensure that the pH is between 5.8 and 6.2. A pH imbalance can inhibit the absorption of zinc and other nutrients. Use fertilizers that generate acidity. Organic compounds such as zinc chelates (zinc EDTA and zinc NTA) are about 5 times more effective than inorganic salts with equivalent amounts of zinc. Apply aged organic manure.


Diagnosing deficiency in tomatoes (with images)

Organic sources of NPK

NPK ratios below are approximate and vary considerably depending on source. More info:

Animal manure (cow, dry) :  2 – 1 – 2.4

Best when composted. Its slow release of nutrients makes it valuable as a soil conditioner and good for all-purpose garden uses.

Animal manure  (poultry, dry) :  4 – 3 – 1

Rapidly supplies nitrogen and phosphorous. It should be composted first as fresh manure will burn plants. Good for bulbs, root crops, flowering , fruiting plants and trees.

Fish emulsion / hydrolized fish fertiliser :  4 – 4 – 1

More of an all round plant tonic, it can be applied as a foliar spray for quick results. When used as a liquid drench, results are quicker than with other organics. It is a concentrated liquid food containing a wide range of trace elements that will green foliage, grow vigorous roots and big flowers whilst also enriching the soil. Fish emulsion however, is heat processed meaning that a lot of the nutrients and amino acids are destroyed during processing. If there is a choice, use hydrolized fish fertilizer instead of emulsified.

Liquid seaweed :  4 – 2 – 3

Provides a slow release of potassium and is a good source of micronutrients. Contains plant growth hormones that stimulate plant and root growth. Gives a quick boost for greening foliage, and when mixed with fish emulsion or hydrolized fish, it can’t be beat because the two benefit one another in so many ways. Its growth promoting substances enhance plant development, color and vigor. Seaweed has also been found to increase plant hardiness and resistance to adverse environmental conditions, such as early frost, extreme heat. and lack of moisture. Used as a seed inoculant, seaweed fertilizer increases and accelerates germination, and enhances the rapid development of a healthy root system. Good for all plants as a general energizer/tonic.

Worm castings :  0.5 – 0.5 – 0.3

Well balanced soil additive used mostly to improve soil structure. The castings slowly release nutrients needed for healthy plant growth and increased production rates for fruits and vegetables. Good for herbs, fruits, or vegetables.


Lawn clippings:  1.2 – 0.3 – 2.0, Hair: 12 – 0 – 0, Sawdust: 0.2 – 0 – 0.2, Coffee grounds: 2.1 – 0.3 – 0.3, Ashes: 0 – 2.0 – 6.0

NPK and trace element sources


Mulching soil using organic materials provides the following benefits: maintains an even soil moisture and temperature, provides protection from wind erosion, encourages earth worms and other organisms beneficial to soil and plant health, builds organic organic matter in the soil as it breaks down.

Anything you place on the soil to cover it is essentially a mulch. Common organic materials used as mulch include: legumes such as alfalfa/lucerne, grass and leaves.

Recommended mulches

Leguminous mulches are ideal for vegetable patches as they feed the soil on decomposition. They are readily available from garden centres or you can grow your own. My favourites are alfalfa/lucerne followed by pea straw.  Clovers (sold as cover crops or green manure) can be sown in autumn to grow over winter. If left, they die back naturally in late Spring forming a mulch on the soil. Simply part the clover to sow summer seeds.

Leaves and weed free grass clippings work well and are free and easily available. Be sure to only use light applications of grass clippings as they are very high in nitrogen and can also mat together, preventing rain from filtering through to the soil. Mixing leaves with grass clippings is a very good option. Collect leaves in Autumn when they are plentiful and stockpile to use as needed.

What to avoid

Avoid woody materials such as bark and wood chips on vegetable gardens as they tie up nitrogen when decomposing, causing nutrient imbalances in the soil.

Whether or not to use eucalyptus leaves on vegetable gardens is still up for debate and as such, they’re probably best avoided. The waxy coating on eucalyptus leaves can lead to poor water penetration and there is a chemical property contained in some eucalyptus leaves that inhibits plant growth and seed germination. They can also tie up nitrogen during decomposition.

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