Here’s a warning, though: don’t keep using mushroom compost year after year, as its very low levels of magnesium and potassium – and its excess of calcium – make it imbalanced as a fertiliser. A prominent rose grower learned this the hard way after applying mushroom compost to his extensive display of roses for years. Initially the mushroom compost was ideal for correcting gross problems in the natural soil of the rose garden, which is why the grower was so enthusiastic about it. However, the soil eventually became highly alkaline, it had far too much calcium and phosphorus, and it was deficient in potassium and magnesium. Simon got him onto a corrective fertiliser/ameliorant program, and he then instigated a long-term sustainable program using lucerne hay (a well-balanced product) as mulch; lucerne hay gradually breaks down to both feed and condition the soil with superb results, as it is naturally well balanced.
An alternative to mushroom compost for virgin soils is commercial chicken litter or manure (10–50 litres per square metre, supplemented with 50 grams per square metre of sulphate of potash and lime if required). Depending on the diet of the chickens, their litter usually has no liming value but it does contain calcium – so you don’t need gypsum, but still need some lime if the soil is acid. You can use a whole range of other organic products as well. The Sources of Organic Fertiliser table on the opposite page lists the most common organic sources of fertiliser and their nutritional properties.
Using a nutrient-rich blend of organic and mineral fertilisers is an excellent way to help leafy crops obtain rapid early growth.
SOURCES OF ORGANIC FERTILISER
ORGANIC NUTRIENT SOURCE PRINCIPAL ELEMENTS LOW OR MISSING ELEMENTS
Chicken and turkey litter and manures High nitrogen, phosphorus, calcium, trace elements Potassium and magnesium; usually acidifying because of the nitrogen
Duck litter and manure High nitrogen, phosphorus, calcium, magnesium, trace elements Potassium
Feedlot cow manure Moderate nitrogen, phosphorus, potassium, salt, zinc, copper; sometimes very high zinc Calcium and magnesium
Sheep manure All elements in moderate amounts and good balance; can be high in salt —
Horse manure All elements in moderate amounts and good balance; high nitrogen —
Mixed commercial or domestic green-waste compost All elements in low to moderate amounts and good balance Can be low in nitrogen if very woody
Mixed kitchen-scrap compost (vegetables only) Elements in moderate to high amounts depending on inputs; usually good balance —
Mixed kitchen-scrap compost (including meat scraps) All elements in good balance; high nitrogen —
Lucerne hay and compost All elements in excellent balance; high nitrogen —
Blood and bone, fishmeal Very high nitrogen, phosphorus and calcium if bone is included Magnesium and potassium
Mushroom compost High nitrogen, phosphorus, calcium, sulphur, zinc; very high in lime and gypsum (mushrooms love lime!) Potassium and magnesium
Rather than simply using mineral fertilisers or organics to improve an acid, low-fertility soil in its natural state, you could use a combination of the two. Here are two different mixtures that will achieve an excellent result.
Alternative 1:
Poultry litter or manure (10 litres per square metre)
Domestic kitchen compost or commercial green-waste compost (10 litres per square metre)
Sulphate of potash (50 grams per square metre)
Garden lime (200 grams per square metre)
Alternative 2:
Cow manure (10 litres per square metre)
Domestic kitchen compost (10 litres per square metre)
Sulphate of potash (100 grams per square metre)
Garden lime (100 grams per square metre)
Gypsum (100 grams per square metre)
If you compare these mixtures with the Common Mineral-based Soil Conditioners table as well as the Sources of Organic Fertiliser table, you will see that they are adding:
potassium from the sulphate of potash
nitrogen and phosphorus from the manures and mushroom compost
calcium from the poultry litter and lime
potassium and many other nutrients from the domestic and commercial green-waste compost.
Liquid feeding is a useful fertiliser strategy, but always shake organic liquid feeds well as they can have heavy residues.
Quality commercial composts add many important nutrients to fertiliser mixes, particularly potassium.
KEEPING IT RIGHT
This is really about fertilising and maintaining soil-nutrient levels to protect against the vast losses caused by the removal of produce and the leaching of nutrients during rainfall and irrigation. Ideally, we would feed plants with fertilisers that exactly parallel the ratios of elements found in their tissues. Disregarding the elements that plants obtain from the atmosphere, the Element Levels in Plants table in The Needs of Plants chapter shows the approximate concentration of each element in most plant leaves.
All plants need a balanced diet of essential nutrients, and there are various strategies that can be employed to ensure you are optimising the growth of your crops as well as keeping your soil or growing medium in good shape for future crops. The strategy you choose for your particular circumstance will depend on a variety of factors.
Availability and cost of fertilisers There may be a ready source of locally available recycled or free material, such as worm castings and liquid or stable manure. If you can utilise the nutrients that you generate from kitchen scraps each day, then it lessens your dependence on purchasing expensive packaged products.
Your growing system Growing in soil lends itself to using a solid fertiliser worked in prior to planting and supplemented by liquid feeding as necessary, whereas a hydroponic growing system will need to have a fairly precise and constant liquid-feeding program. Never use solid soluble fertilisers in potting mix, as they will burn plants. You must utilise slow-release or liquid feeds.
The nutrient-storage capacity of your soil or growing medium Fertile soils rich in clay and/or organic matter will have a higher capacity for nutrient storage than sandy soils or hydroponic media, meaning you don’t have to feed so often. Fertile soils will release nutrients to the plant on demand. A pre-plant fertiliser will often be sufficient, but look at the plants and decide if they need extra liquid or side dressings.
The amount of material being harvested Heavy-yielding plants, such as potatoes, will lead to the removal of significant quantities of nutrients every time you harvest. These nutrients will need to be replenished in the same quantities before the next crop is planted. Leguminous crops, such as beans, will tend to need less nitrogen because they fix their own nitrogenous fertilisers from the air around them, but they still need all the other nutrients.
There are generally two types of fertilisers used during a plant’s growing cycle: a pre-plant fertiliser, which is added to the soil or growing medium before the crop is planted and helps early growth, and a side dressing, which keeps the nutrient levels up towards harvest time.
HINTS FOR OPTIMISING YOUR FEEDING STRATEGIES
Green-manure crops are plants that are dug into the soil prior to the sowing of a crop. The idea is to turn in the green-manure plants while they are still at a very succulent stage of growth – at the latest when they have started to flower – as it is at this point they have the maximum nutrient content, which will quickly break down and be released back into the soil. The most common choice for a green-manure crop is some type of legume, such as beans and peas – whatever you can get hold of at a reasonable price (or you may save seeds specifically from previous crops). Members of this plant group all have the ability to convert atmospheric nitrogen into fertiliser, courtesy of a bacterium called Rhizobium that forms nodules on the plant’s roots. In return for the nitrogen that the bacterium passes on to the plant, it receives sugars from the plant’s photosynthetic activities – a true symbiotic relationship where both parties benefit.
Liquid fe
eding may be the sole means of delivering nutrients in systems such as hydroponics; however, in other systems it can be a very handy tool for supplementing the nutrients that you have previously added as solid fertilisers. The liquid (leachate) from a worm farm, or a liquid manure (prepared by placing manure into a porous bag and soaking it in a bucket of water for several hours), is a handy homemade liquid feed that is generally a well-balanced fertiliser, and can be used as a general boost for most crops on your urban farm.
Foliar feeding is a particularly useful form of liquid feeding in winter, when the soil is cold and it is more difficult for plant roots to take up nutrients from the soil. Liquid fertiliser is applied to the leaves, rather than the soil. It is important to ensure that your liquid feed is not too strong, as this could cause a salinity problem that might burn the foliage. It is also advisable not to foliar feed in the heat of the day.
Specific liquid fertilisers can be used to correct particular nutrient deficiencies that may show up on either a seasonal basis (for example, iron deficiencies through winter) or as a result of a lack of nutrients in the growing medium. Nutrient toxicities can sometimes also be addressed through liquid feeds – for instance, an application of dissolved iron sulphate can help to reduce the availability of excessive levels of phosphorus in soils.
Pre-plant fertiliser
These can be mineral, organic or a mixture of both, and they usually provide a broadly balanced spectrum of all the essential nutrients. If you know that your soil or growing medium has a particular deficiency, solid forms of a specific fertiliser (for example, potassium to encourage better fruiting) can be added. Pre-plant fertilisers are best dug into the garden bed as a whole; they can also be placed close to where the new plant will be located and dug in around the planting hole, or dug in along the planting row. Some ideal pre-plant fertilisers are:
mixed yard waste or garden compost with 20 per cent (by weight) blood and bone added
blood and bone with 20 per cent (by weight) sulphate of potash added
poultry manure with 10 per cent (by weight) sulphate of potash added
a ‘complete’ plant food with an NPK ratio around 10:3:8.
The perfect pre-plant fertiliser releases its nutrients slowly, so the nutrients are not all instantly soluble and prone to leaching during rainfall or irrigation. For this reason, organics such as blood and bone and the richer composts or pelletised manure products are ideal. Coated controlled-release products are also excellent because they are very restrained in their release, but they are the most expensive fertilisers weight for weight. They do, however, largely avoid the need to fertilise later in the progress of the crop.
Application rates vary, but the Application for Vegetable Production and Application for Orchard and Tree Production tables (see below and opposite) are useful guides to how much to apply per square metre for vegies and crop trees.
When using very rich poultry manures containing a lot of ammonium nitrogen (the smell of ammonia is often very obvious), it pays to either wait a week before planting or ensure the manures are not put into direct contact with the seed or seedling. Ammonium in high concentrations can be toxic to plants, but the soil will absorb it and metabolise it into the non-harmful nitrate form of nitrogen.
With trees, avoid putting the fertiliser close to the trunk, as there are few feeder roots here to take advantage of it. For trees in a row, place the fertiliser along the drip line of the foliage canopy, where the greatest concentration of feeder roots is likely to be found, and treat the entire row, not just the soil under the canopy.
Beds for seeds and small seedlings should have pre-plant fertilisers well worked in to the soil using a rake or shovel.
APPLICATION FOR VEGETABLE PRODUCTION
FERTILISER TYPE APPLICATION RATE (PER SQUARE METRE)
Garden compost 5–10 litres (half to one bucketful)
Animal manures and worm castings 2–5 litres
Fortified pelletised manures 0.3–0.5 litres
Compound mineral fertilisers 50–100 grams (2–4 tablespoons)
Coated controlled-release fertilisers 100–200 grams
Side dressing
This refers to putting fertiliser (either solid or liquid) alongside the crop to give the crop a mid-season boost of nutrition. The need for side dressing depends on the reliability of your initial pre-plant fertiliser (for example, how slowly it releases nutrients) and, of course, how fertile your soil is. If you are using sufficient amounts of either nutritious compost or coated synthetic fertiliser, you may not need to apply side dressing at all.
If plants are growing well – with dark green, glossy leaves and plenty of setting fruits – then the need for side dressing is reduced. However, if plants look spindly and pale, don’t hope they’ll get better – start side dressing straight away.
Any side dressing should comprise soluble and readily available forms of nutrients and should not contain high ammonium nitrogen levels. Raw poultry manures, for example, carry a risk of ammonia toxicity. Likewise, fertilisers such as urea must be utilised very carefully as they can cause root and leaf burn if they are overused. Apply only well-rotted composts and manures as side dressing. The nutrients will be in partial-soluble form, and the ammonium nitrogen will have dissipated.
The need for side dressing depends on the reliability of your initial pre-plant fertiliser … and, of course, how fertile your soil is.
Soluble fertilisers and liquid feeds are ideal for side dressing. Some people rely totally on a side dressing of liquid feed and don’t use a pre-plant fertiliser at all. Compost ‘tea’, wormcast ‘tea’ and purchased soluble liquid feeds all make excellent side dressing.
To apply a side dressing, use about half the amount utilised as pre-plant fertiliser. For greatest efficiency, place the fertiliser close to (but not touching) the plant stem, where the root system is most likely to be concentrated, and water well to ensure that the soluble nutrients are carried into the soil. Fortnightly to monthly applications are usually required when side dressing with composts. With mineral fertilisers, monthly applications are adequate. Because they are not very strong, liquid feeds need to be used between twice a week and once a fortnight, depending on how much pre-plant fertiliser you have used. If you didn’t apply a pre-plant fertiliser, then use liquid feeds twice a week for best results.
APPLICATION FOR ORCHARD AND TREE PRODUCTION
FERTILISER TYPE APPLICATION RATE ON ROW OR UNDER THE CANOPY (PER SQUARE METRE)
Garden compost 5–10 litres (half to one bucketful)
Animal manures 2–5 litres
Fortified pelletised manures 0.3–0.5 litres
Compound mineral fertilisers 50–100 grams (2–4 tablespoons)
When planting tree crops, such as papaya, it pays to put in the hard work and condition (ameliorate) the soil properly before planting, as this is very difficult to do later on.
COMPOSTING AND
MULCHING
RECYCLE YOUR ORGANIC MATERIAL
As we saw earlier in Soils and Soil Fertility, natural soil systems are replenished and improved via the cycling of nutrients and organic matter from the all-important litter layer on the soil surface. In an urban farm’s soil system, this litter cycling does not occur, and both soil fertility and organic-matter levels will decline with time if nutrients are not replaced.
You can run farming systems entirely on synthetic fertilisers, but even in such systems it is usual for organic matter – and hence soil structure and physical fertility – to decline with time. Broadacre farmers rotate crops, working some of those crops back into the soil as ‘green manure’ to build up the soil’s organic matter; while this can be done in intensive urban-farm situations, land area is usually limited so crop rotation is not feasible. This is where composting comes into play – transforming organic matter into a substance that is suitable for fertilising the soil. There are many benefits to composting:
It returns the farm’s nutrients back to the soil, reducing the need f
or outside fertiliser inputs.
It maintains and improves the soil’s organic matter by adding humus, which is a soil conditioner. Humus works like a sponge, soaking up water and nutrients, and then later releasing them back to the plants. It also acts as a glue between soil particles to improve soil structure.
It transforms the nutrients in organic waste from complex biochemicals, such as proteins, to simple soluble forms that can be taken up by plants.
It sanitises waste, killing weed seeds and soil-borne diseases.
It maintains a healthy soil fauna and flora.
It suppresses plant disease.
Given the enormity of these benefits for creating a sustainable farming system, composting of organic wastes – in our view – should be a substantial part of any urban farm. There are two main types of composts – hot (thermal) composts, which heat up enough to kill weeds and disease organisms, and cold (non-thermophilic) composts, which are ideal for weed-free kitchen scraps – and a multitude of composting techniques. It is important to choose a system that is appropriate for your particular urban-farm operation.
The natural cycle whereby leaf litter returns nutrients to the soil is mimicked on the urban farm by the regular addition of compost to garden beds.
GETTING STARTED
The process of transforming fresh organic matter into useful, clean, weed-free humus and available plant nutrients takes time. All composting should be aerobic – that is, enough air should be maintained in the pile so that it does not rot and develop a foul smell. Anaerobic conditions result in toxic by-products that can cause damage to crops.
Grow Your Own Page 18