Grow Your Own

by Angus Stewart

  Domestic compost is likely to be richer in nutrients than commercial compost because it has less wood and more leaf and vegetable matter.

  Choose a practical composting system that suits your lifestyle and space, and you can readily generate virtually all the fertiliser you need for free.

  The bottom line is that the various organics we generate in our households every day (particularly kitchen scraps) usually contain the right balance of nutrients that need to go back into the soil to grow all of our own food. Choose a practical composting system that suits your lifestyle and space, and you can readily generate virtually all the fertiliser you need for free. Having said that, we have no problem with using a mineral fertiliser to correct a specific nutrient deficiency when required, as long as it is part of a holistic soil-management program that involves regular applications of organic matter to keep a good balance of overall soil fertility.

  Wood mulches often cause nitrogen drawdown, so plants growing in that soil need a simple fertiliser to fix the imbalance.

  Yellowing of foliage and early leaf drop are both signs of nitrogen deficiency. Urea fertiliser will quickly address this issue.

  COMPOSITION OF FERTILISERS

  There are two different types of fertilisers: simple (those that contain only one fertiliser salt) and complete (those that incorporate a deliberate or natural mixture of one or more mineral fertilisers). There are no organic fertilisers in the simple category, as all composts and organic-based fertilisers are complete – they contain most, and sometimes all, the essential plant elements.

  Simple vs complete

  Simple fertilisers have one fertiliser salt only. However, they may contain one plant nutrient (for example, urea has only nitrogen) or two (for example, sulphate of potash has both potassium and sulphur). Simple fertilisers are useful when a single nutrient deficiency occurs. A common example is when a woody mulch or carbon-rich compost is used, and it causes nitrogen deficiency with obvious yellowing of plants. An application of urea fertiliser to supply some extra nitrogen is all that is needed.

  Another example is adding sulphate of potash to blood and bone (which is a more or less complete organic fertiliser but is quite low in potassium). By adding 20 per cent by weight sulphate of potash to the blood and bone, a well-balanced and complete fertiliser results. Another common deficiency, that of iron and manganese in alkaline soil, can be solved by spraying plants with a simple solution of iron or manganese sulphate (about 1 gram in a litre of water).

  A fertiliser that contains more than two elements is known as a ‘compound’ or complete fertiliser. All organic fertilisers (including domestic composts and worm castings) are complete fertilisers. Composts that are really low in plant nutrients are not used for their fertiliser value but for the organic matter they contribute to the soil.

  Properties of complete organic fertilisers are discussed in the Types of Organic Fertilisers table. You will appreciate that not all of them are as well balanced as we need them to be. The more sophisticated packaged organic fertilisers will usually provide information on the nutrient contents in the product. Bulk manures and composts rarely do so, and you are left guessing. Use the table as a rough guide to what might be in these products.

  COMPARING FERTILISERS

  By law, manufactured mineral or synthetic fertilisers must have a nutrient analysis on the package in which they are sold. This is not the case with all organic products, as an analysis is only mandatory for those claiming to have fertiliser value, such as pelletised, fortified manures. Look on the back of any fertiliser package, and you will see a label that shows the nitrogen (N), phosphorus (P) and potassium (K) content (and sometimes other nutrients), given as a percentage.

  FERTILISER LABEL

  Nitrogen (N) 4.2%

  Phosphorus (P) 5.4%

  Potassium (K) 6.7%

  Sulphur (S) 9.6%

  Calcium (Ca) 10.5%

  Magnesium (Mg) 1.02%

  Boron (B) 0.1%

  Manganese (Mn) 0.08%

  Copper (Cu) 0.04%

  Zinc (Zn) 0.02%

  Iron (Fe) 0.01%

  Molybdenum (Mo) 0.03%

  The purpose of the label is twofold:

  Just like a label on packaged food in the supermarket, it allows you to compare the ingredients and value of one product with another. This is important for both professional farmers and gardeners, so they can get the best value for money.

  It allows you to calculate how much of the product to use, so you don’t over-apply it. This is vital for the productivity of plants in the urban farm and also to prevent pollution caused by the run-off of nutrients.

  Before using a mineral fertiliser, check the nutrient analysis – does it have the correct ratio of elements you need for your garden bed?

  ORGANIC, MINERAL – OR BOTH?

  We should strive to supply nutrients in a sustainable, cost-effective way that also provides plants with the right balance of nutrients for strong growth and a high crop yield. Our first preference is to use recycled organic fertilisers, as these can not only supply a plant’s nutritional needs, but also add valuable humus and possibly beneficial microbes to the soil at the same time. However, organic forms of fertiliser, such as compost and manures, are sometimes not well balanced and need to be corrected. This is where a dual approach comes in handy, where you take advantage of the best of both organic and mineral worlds. For example, blood and bone and poultry-based manures are notoriously low in potassium and other essential elements for fruit and leaf production in fruit and vegetable crops. By adding 20 per cent by weight of sulphate of potash to the blood and bone or manure, a fertiliser that is beautifully balanced for plant growth results.

  FERTILISER VALUE FOR MONEY

  PRODUCT COST OF FERTILISER

  Controlled-release, plastic-coated, mineral NPK $43.70

  Pre-prepared liquid feed, mineral NPK $42.50

  Pre-prepared liquid feed, organic plus mineral NPK $27.80

  Solid soluble liquid feed $26.50

  Fortified blood and bone (organic plus mineral additives) $17.10

  Pelletised poultry manure with mineral additives $16.80

  Pelletised poultry manure, standard $16.00

  Commercial green-waste ‘soil improver’ $13.10

  Complete mineral NPK plant food $5.50

  Synthetic urea $2.20

  Keep in mind, however, that efficiency of use, convenience and labour saving should also be considered when assessing the value for money of the various fertiliser products.

  Working out the value

  With a bit of simple maths, you can compare the value for money of various fertilisers. The Fertiliser Value for Money table on the opposite page reveals the cost of assorted fertiliser types that we purchased from a local retail outlet. We simply worked out how many dollars per kilogram the product is, and then divided by the percentage of total fertiliser content.

  As you can see, the highly manufactured, coated, controlled-release fertilisers are very expensive. Next worst in value is pre-prepared liquids (liquid concentrates that are diluted for use). Even the soluble forms of powdered fertiliser that you simply dissolve in water are comparatively expensive. The solid value-added pelletised poultry manure products represent much better value. They rank with commercial compost made from green garden waste, which we normally think of as a soil improver rather than a fertiliser. Far and away the fertilisers with the lowest cost are ordinary solid complete plant foods made from a mixture of mineral and synthetic fertilisers.

  Keep in mind, however, that efficiency of use, convenience and labour saving should also be considered when assessing the value for money of the various fertiliser products. Controlled-release fertiliser is really made for use in pots, where adding solid soluble fertiliser would cause burning, and liquid feeding would need to be done at least twice a week. Thus, using a controlled-release product saves an enormous amount of labour for commercial nurseries, and this may also be a big incentive for the time-poor ur
ban farmer. Having said that, various organic fertilisers – such as blood and bone or manure – also release their nutrients slowly and can play the same role as a controlled-release fertiliser at a much lower cost, albeit with less precision on the nutrient content.

  The lesson to learn from this comparison exercise is that the price you pay for nutrients varies dramatically from product to product. We need to work out whether an expensive fertiliser has significant advantages (such as a controlled release rate, or adding humus as well as nutrients) that may justify the price tag. The bottom line, as always, is that there are abundant sources of nutrients available to you for free if you can devote the time and energy to recycling them.

  There are numerous pre-packaged fertilisers available to buy – but collecting your own garden waste and kitchen scraps to make compost is the cheapest way to obtain fertiliser.

  BEWARE OF ‘SNAKE OILS’

  There are many products on the market that make all sorts of claims about their ability to boost the growth of plants. Many, if not most, carefully avoid the label ‘fertiliser’, as that would bring them under the relevant fertiliser laws and force them to state the analysis of the product. Most use alternative wording and do not print the product’s composition.

  Seaweed is widely touted as a marvellous tonic for plants, but there is very little evidence to back up this claim – especially when it is used in small amounts. If applied at high rates, in the same way that you would use garden compost with significant amounts of nutrients and organic matter added, then you may have some success. Liquid seaweeds, in our experience, are usually recommended at rates that do not demonstrate any significant benefit to plant growth and represent poor value for money. Unless you live near the sea where seaweed is free, and you use it at high rates as mulch or in composted form, save your money and spend it on fertilisers and soil conditioners that have a far more quantifiable benefit to plants and soil. Most of the claims made in regard to seaweed are entirely unproven, and some seaweeds – for example, seagrass – contain toxic levels of boron and should never be used.

  Various seaweed products have plant hormones, such as auxins, that stimulate the formation of roots in plants. If your plants are healthy and growing normally, we see no benefit from applying seaweed products – and this could actually upset the fine balance of naturally occurring hormones in mature plants. The only time such hormones might be beneficial is when rooting cuttings or when transplanting, because the auxins found in seaweed products can improve propagation strike rates and assist with root recovery following the inevitable damage caused by transplanting.

  FIGURING OUT FERTILISER NEEDS

  There is no escaping the fact that growing food crops results in the removal of substantial quantities of nutrients from the soil. Our job as urban farmers is to make sure that there are enough nutrients for our plants from start to finish to ensure that our crops reach their full genetic potential.

  We always approach the subject of feeding soil from two directions: ‘getting it right’ and ‘keeping it right’. The former often means taking a low-fertility soil and rapidly bringing it up to the correct level and balance of fertility for the intended crop. It can also mean rebalancing a soil that has been abused or mistreated or is simply out of balance because it has too much of one thing, such as poultry manure. Before we start to understand how to get the best out of fertilisers, we need to distinguish between soil conditioners (also known as ameliorants) and fertilisers.

  Liquid fertilisers are usually sold as concentrates, so they must be diluted before they are applied to plants.

  Check the pH of your soil at least every two years, and use a soil conditioner to adjust the acidity or alkalinity if necessary.

  Soil conditioners (ameliorants) These are things we add, often once only (or at least only rarely), to correct basic soil properties like acid or alkaline pH, low calcium, poor soil structure, low organic-matter levels or grossly deficient nutrient levels. Some ameliorants have a fertiliser value as well, for example gypsum (calcium sulphate) – it contains the important minerals calcium and sulphur, but we usually add gypsum to the soil because of its ability to improve the soil’s physical properties (calcium binds clay particles together).

  We do not use ameliorants regularly, as they will end up unbalancing the soil in the other direction. For example, Simon once had a rose-growing client who was told he had acid soil, so he applied lime to his garden every year for about 15 years – he ended up with an alkaline soil and severe iron deficiency in his roses! ‘Flying blind,’ Simon calls it. Never use conditioners without first testing whether or not they are needed. Regular testing of soil pH and gypsum requirements is vital so you know when to use soil conditioners.

  Fertilisers On the other hand, fertilisers are things we apply to our garden on a regular basis to improve growth and maintain soil nutrient levels (they replace nutrients lost by the removal of produce and through the leaching process). There is usually an emphasis on nitrogen and potassium, the two elements that are lost in the largest amounts. Relatively smaller amounts of phosphorus as well as the other macronutrients and micronutrients are usually also required.

  Note that there can be a fine line between a conditioner and a fertiliser. For example, if a soil is so alkaline that we are seeing iron deficiency in plants, we use iron sulphate both for its ability to neutralise the alkalinity and for the fact that it adds iron to the soil.

  Using a pre-prepared liquid feed is one of the most expensive fertiliser options, but it is very convenient for pots on rooftops and balconies.

  GETTING IT RIGHT

  Using soil conditioners is all about getting the pH and calcium levels right. In many unimproved soils, however, other nutrients may also be required to allow high-quality produce to be grown. Phosphorus and potassium may be so grossly deficient that, in order to bring a soil up to a general level of fertility consistent with the needs of intensive farming, we might need to add more of these nutrients than we would to simply maintain a soil. For example, the natural soils of an area may be known to be deficient in phosphorus (a common reality in Australian soils), and a one-off application of up to 50 grams of phosphorus per square metre may be required for good crop growth where the maintenance level would be 5 grams per square metre.

  The same applies to organic matter. Initially, we might need to add a high proportion of organic matter because it improves soil structure and texture, water- and nutrient-holding capacity and root penetration. Then we only need to use a fraction of that amount of organic matter every crop cycle.

  Because you can’t ‘see’ the nutrient content within a soil, other than via observations of plant growth, soil structure and organic-matter levels, you often need to test your soil to work out how to get it balanced in the first place. However, there are a few rules of thumb that can be used.

  Many – if not most – natural soils are too dense and heavy for horticulture.

  Dense soil is partly due to low levels of organic matter. The ideal organic-matter content is 5–10 per cent by weight, whereas most natural soils have less than 3 per cent organic matter (unless you are on some very special soils, which usually show dark colours and good structure). Otherwise, you will need compost and/or manures to quickly improve organic matter.

  Many low-fertility natural soils will also be acid and low in calcium – these need lime. As previously discussed, pH testing for acidity or alkalinity is a simple process (see Testing the pH of Your Soil).

  Most natural soils in Australia, as well as in many other parts of the world, will be too low in phosphorus for intense use in urban farming.

  Soils with a low clay content (sands and sandy loams) will often be low in potassium.

  Potassium is very important for all fruiting crops, such as chillies, as it aids fruit set. Sandy soils are often low in potassium.

  Dense soils will benefit from having composted organic matter forked into the topsoil to improve both structure and texture.

 
; Using mineral fertilisers to get it right

  There are many different soil conditioners available. The Common Mineral-based Soil Conditioners table below reveals the mineral-based additives that are most frequently used to improve a new soil.

  COMMON MINERAL-BASED SOIL CONDITIONERS

  SOIL CONDITIONER EFFECT INDICATIVE RATE

  Lime Neutralises pH and adds calcium 50–500 g/m²

  Gypsum Adds calcium as a ‘clay breaker’ to improve structure 100–1000 g/m²

  Superphosphate Rapidly builds phosphorus levels to those needed for horticulture in deficient soils 20–100 g/m²

  Sulphate of potash Rapidly builds potassium levels to those needed for horticulture in deficient soils 20–100 g/m²

  Composted organic matter* Raises organic-matter level of topsoil to around 5–10 per cent; in larger-scale orchards, it is more cost effective to do this by growing green manure in the winter months 20–50 litres/m²

  *Note: All organic matter contains mineral nutrients, so it can be thought of in one respect as a low-nutrient source of mineral fertiliser.

  Using organics to get it right

  Interestingly, there is one rather specialised product that will do most of this job for you, and that is mushroom compost. It contains organic matter, lime, phosphorus and gypsum, and is almost a ‘one-hit wonder’ for improving acid, low-phosphorus soils. A high level of nitrogen gives it fertiliser value, but its potassium is not so high. Apply 10–50 litres (1 to 5 bucketfuls) per square metre, as well as 50 grams per square metre of sulphate of potash – and stand back!