Why use organic fertilisers?

Why are manure, compost and worm castings not enough to keep our organic food garden healthy and productive over the years? What else do we need and what is the best way to add that to our food garden?

Why compost, manure and worm castings are not enough

If we lived in a river delta situated in the food plains of a major river or we lived on rich volcanic soils, we would need very little besides manure, compost and worm castings. However, Australia is a very old continent with soils and minerals have leached out of them. As a result, Australian soils no longer have the full range of minerals that more recently formed soils have. 

If your soil and the soils in your region are lacking in certain minerals, then you can compost and mulch until the cows come home, but you will never add those minerals to your soil, simply because they are not present in all the materials that you add to your soil or compost heap. They won't be in any of the plant material that comes from your garden. They won't be in any mulches or manures that you buy and that come from your region. 

The only way to make sure that these elements are present in the soil in your organic foodgarden and therefore in the produce that you eat, is by getting organic fertilisers that come from elsewhere and that contain the elements.

When is a fertiliser considered organic? 

A fertiliser is considered organic if it was harvested from natural sources and processed without using chemicals. Fertilisers made from plant material, fish, meat, rock or other materials naturally occurring in the landscape are examples. 

Extraction and processing of these fertilisers and soil additives without chemicals mean no residual chemicals in your food garden soil and in the crop that you harvest. 

Let’s look at four important elements

For best results in the food garden our plants we need nutrient-rich soil with a wide variety of elements. The table below shows four of the most important ones - nitrogen, phosphorus, potassium and magnesium, what plants use them for, and which organic fertilisers contain that element.

More about these organic fertilisers

Banana skins are a rich source of phosphorus and potassium. Cut two banana skins into small pieces. Soak them in a litre of water for 24 hours. Strain the liquid, so its banana-skin-free. Once a fortnight water the base of plants with this liquid. 

Blood & bone is a very effective organic fertiliser made from abattoir byproducts.

Bone meal is finely ground animal bones. It is used in situations where the nitrogen that blood & bone contains is not needed or wanted. Bone meal is not commonly available, but can be found via an internet search. 

Epsom Salts are magnesium sulfates. They are a mineral that occurs naturally and that is generally allowed in organic gardening and farming. 

Fish emulsion is an organic fertiliser fluid made from fish byproducts. It is for sale at most nurseries and hardware stores under names such as PowerFeed and Charlie Carp. 

Kelp meal is pulverised dried kelp seaweed. It's a rich source of nutrients like nitrogen, potassium, phosphorus, and a wide range of trace minerals. Bags of kelp meal are available at agricultural supply shops.  

Legumes are vegetables like broad bean, pea and bean, that fix nitrogen out of the air in root nodules. Legumes are a very useful part of crop rotation because when the plants die, the nitrogen is mineralised by microbes and made available for use by the next crop grown where the Legumes were IF you cut of stems at soil level rather than pulling plants out. You can find more about this in The Importance of Rotating Crops on this blog.

Manure is a great fertiliser for organic food gardens IF the animals the manure comes from didn’t receive any chemical innoculants and their food was not contaminated with chemical pesticides. Purchase of manure from an organic farm is the ideal solution. Find out which manures are the best for food gardens and how to add them to your garden in Marvelous Manure on this blog.

Rock Phosphate is a naturally occurring rock rich in phosphate minerals. The product in the photo below is called Guano Gold, but the content of the bag is rock phosphate, not the bird droppings called guano  by the Incas, who harvested bird droppings rich in phosphate. Bags of rock phosphate can be bought at agricultural suppliers. 

Seed Meal is a good source of nitrogen made from pulverised seeds. Seed meal is mostly for sale at agricultural suppliers. In Australia seed meal is often made from canola or rape seeds. The seeds are pulverised so you don't run the risk that they will sprout. 

Sulphate of Potash (SOP) can be made in two ways: one method allows SOP to be an allowable input for organic farming operations. This is where natural potassium-containing minerals are mined and carefully rinsed with water and salt solutions to remove by-products. The result is more expensive than SOP produced using chemicals. If you want organic, choose the dearer SOP if you have a choice.

Wood ash made from untreated wood is a good source of potassium, phosphorus and calcium. Wood ash is ‘strong stuff’, so don’t use it directly on your food garden. Wood ash is best added in only small quantities to a compost heap where it will be mineralised and lose its intensity.

How to find out what the mineral levels are in your soil

Here are three suggestions (from no-cost to expensive) :

• Be alert to what your food-plants tell you: are your carrots forked? Are your cabbage plants not forming proper cabbages? Are your fruiting vegetables (tomatoes, capsicums, pumpkins etc.) not producing much? Use column 2 in the table above to guide you to what element might be missing in your soil and what fertilisers can help solve the problem.

• Buy a basic soil test kit and test Nitrogen, Phosphorus and Potassium levels in your soil yourself. There are basic soil test kits for sale online that allow you to  in soil samples. You will find an example here. 

• Have your soil tested by a company that offers this service. In Tasmania comprehensive soil testing is for instance offered by Longley Organic Farm. If you are aware of soil testing services that are not mentioned here and that are affordable for home-gardeners, please email your info to foodgardengroup@gmail.com and they will be added to this blog post. Thank you!

What is Complete Organic Fertiliser (COF)?

Steve Solomon, once a seed farmer in the United States, settled in Northern Tasmania some decades ago and began to gain experience with Tasmania’s often-not-brilliant soils. Based on his extensive knowledge of what makes soils fertile, he began to experiment with what he called Complete Organic Fertiliser (COF), a mix of organic substances, some in the form of pulverised seeds, others pulverised rock, in order to provide his vegetables and fruit trees with all the minerals they need over and above local manure and compost, all with the aim to optimise the nutritional value in them. 

Steve Solomon wrote several books on the subject. His best-known work is Growing Vegetables South of Australia. In this book Steve discusses the need for COF in Tasmanian food gardens and his recipe finetuned for average Tasmanian soils. The book is for sale via Southern Harvest, Dave’s Seed and Fullers Bookshop.

Ready-to-use COF using Steve Solomon’s recipe is sold at various nurseries around the state. Some of them are mentioned in Complete Organic Fertiliser – Suppliers on this blog.

A basic version of COF

I opted for a version of COF (let’s call it ‘basic COF’) that is not as complete as the one recommended by Steve in later versions of Growing Vegetables South of Australia, but one that I found easier to achieve in terms of getting the ingredients. 

I buy my COF-ingredients in large bags from agricultural supply companies like Hollander Imports (near the Brooker Roundabout in Hobart). The bags are much bigger than ones you buy at nurseries or hardware stores (if they have the ingredients), and per kilo it works out much cheaper.

Column one and two below show the basic-COF recipe that I use: 

 

I have used this recipe for over a decade and have found it to be good or adequate to keep my food garden soil fertile in the long-term. 

Organic fertilisers are not substances you use wherever you like at any time you like. I use my basic-COF mostly in my compost heaps, and also in the ‘leafy greens’ bed of my crop rotation – see The Importance of Rotating Crops on this blog.

 

Ag lime (mentioned above) (also known as Garden Lime) is a soil-conditioner, not a fertiliser. Most food garden crops prefer an acidity-level (pH) of between 6 and 7. Ag lime helps to increase the pH if it is below that level. It is good to add ag lime to Tasmanian soils as they tend to have a low pH. There is more info about lime in article Acid or Alkaline? On this blog.

Dolomite lime is a soil-conditioner, not a fertiliser. Dolomite is a naturally occurring rock composed of calcium carbonate and magnesium carbonate, making it a good way to add both magnesium and calcium to the soil, while increasing pH. However, many Tasmanian soils already contain quite a bit of magnesium and regular use of dolomite may actually compact your soil. This is why I don't use it in my basic-COF.

Gypsum is a soil-conditioner, not a fertiliser. Gypsum is a common natural mineral that is used to loosen up compact soils and thereby improve soil structure. It doesn’t alter the pH of soils. 

Don’t expect your plants to respond instantly

Most organic fertilisers are complex natural substances that need to be broken down (‘mineralised’) by soil microbes before they can be absorbed by plants. Therefore, their effect on plants is never instant when you add them to soil. Their effect, as the mineralisation process progresses, is gradual and often long-lasting. Gradual mineralisation is what happens in nature all the time and it is what the whole biosphere in and around plants is built for. 

The healthier your soil, the more complete the gradual mineralisation. Healthy soil, healthy food. The willing-worker-soil-microbes that perform the mineralisation process are found in soil that contains compost, manure and worm castings. 

How long before plants can use these fertilisers?

How long it takes before microbes have mineralised these organic fertilisers and converted them into components that plants can absorb and use, depends on a number of factors such as how healthy the microbial biosphere in your soil is, what soil type you have, the amount of rain that falls over this period and temperature.

The table above has a third column with the header 'Available to plants after'.  The weeks mentioned in that column are meant as an indication based on the scientific testing that I found. The point of including this info is to make it clear that organic fertilisers will not create an instant response in your plants. 

That need not be a problem! Do what soil experts like Letetia Ware (see Twelve simple food-gardening practices on this blog) recommend and it becomes far less important how many weeks mineralisation takes: 

Make your compost heap central in growing nutrient-rich food

  

As you add garden clippings, kitchen scraps, manure, grass clippings etc. etc. to your compost heap, also regularly add fertilisers like COF and mix them all well together. The COF ingredients will be broken down by the microbes in your heap and mineralised without you having to worry about how many weeks it takes. 

When, at the end of the composting process, you add the compost to your soil, the plants will be able to take up all the mineralised organic fertilisers immediately. The microbes in your soil will be happy too because they don’t have to deal with any un-natural concentrations of fertiliser that might upset them.

May your harvests be bountyful,

Max Bee