Chapter 1 Basic Principles

What do Roots Require to Give Good Production?

In this chapter we will look at the basic principles behind all of the rooftop gardening methods.

First we look at what the roots require to give good production. Then we will discuss how to decide between the various method. Finally we will discuss at some length options for providing fertility to the rooftop garden.

The roots themselves require a constant supply of only three things. I am assuming that other conditions for plant growth are met, such as sunlight, appropriate temperatures, and a means of support. The three things roots require are:

a constant supply of air

a constant supply of water

a constant supply of nutrients

It is also necessary to provide a means to

keep the sun and wind off of the roots

provide a space where the roots can grow

support the growing plant (in some cases)

That's it. Note in particular that the roots of most vegetables do not require soil, potting mix or a deep container. In fact, the place where the plants grow can be so shallow that it is almost a two-dimensional "container," e.g. a piece of cloth. Really large plants like trees and shrubs would of course require a larger volume in which to grow and support the plant.

Criteria for Selecting Gardening Methods

I use seven criteria in selecting gardening methods for those with limited financial resources. Here I am thinking of the exceptionally poor, living primarily in urban environments. However, the simplicity of these low-technology methods, the reduced risk of sudden failure if a piece of equipment stops working, and low cost are attractive to most of us even in the United States. Here are the seven criteria:

1. Very low weight per area. This will not be a factor if the garden is not located on top of an unsubstantial roof or if it is placed on some other solid, study structure, e.g. if it is on a large flat rock or in a container sitting on the ground.

2. Inexpensive, almost no cost, preferably based on recycled materials. Because gardens made from recycled materials are often free or nearly free, this has obvious merit for any of us. Even if money is no object, it is satisfying to know that our container gardens have made minimal negative impact on the environment. Naturally a person with greater income may put more value on appearance. For example, s/he may choose to paint a tire or purchase and paint a child's wading pool for the container.

3. Satisfactory production with minimal inputs. Our goal is to get good production that meets these six criteria, not to try to match the high producing commercial hydroponic gardens.

4. No energy or moving parts needed. Electricity in some situations in developing countries may not be available, or power outages are frequent. Even in Florida we have occasionally lost our electric power long enough that the disruption would have caused us to lose our plants if their survival depended on near-continuous operation of some higher technology system.

5. Made from local materials (not imported). This is very important in economically developing countries where imported things are almost always expensive. It is not as important a factor in economically developed countries.

6. No instruments or analyses needed. Commercial hydroponic systems are incredibly productive, but such high levels of production require monitoring and adjusting nutrient levels. Neither the poor nor the North American homeowner wants to bother with this level of technical detail.

If There Is No Soil The Plant Is Like a Baby: You Must Provide Everything It Needs for Health.

Plants require several essential elements that they obtain from the soil. If any one of these is absent the plant may fail to thrive or reproduce and may die. The elements are grouped into three types, based on how much of each element is needed.

The "big three" nutrient elements are nitrogen, phosphorous and potassium-the N, P, and K found on all general-purpose fertilizer labels. Two more elements, calcium and magnesium, are needed in considerable quantities, but less than N, P, and K. These five are called "macro-nutrients." In addition, several "minor" or "trace" elements are needed, but in extremely small quantities compared to the major. These include elements such as iron, manganese, and zinc.

If the garden medium has all of these in approximately the right amounts throughout the growing season, results should be good. But it is up to the gardener to make sure this happens. Adding well-rotted manure or compost to the bed may be all that is necessary. Don't assume though that organic material that appears black and rotted has just the right mix of nutrients. There is an art to making really good compost, not to mention the work.

If these are not available, chemical fertilizers can be used to provide the nutrients. The N, P and K can come from almost any fertilizer made for farmers or gardeners. (A series of three numbers on the fertilizer label refers to these three in sequence, e.g. 10-5-8 means that there is 10% nitrogen, 5% phosphorous and 8% potassium.) A higher priced fertilizer may contain N, P and K plus micronutrients, which will be listed individually by name or chemical symbol. I usually go to a local feed store and ask for so many bags of "10-10-10 with micro nutrients." Fertilizers that have been manufactured in a special way so that the nutrients dissolve slowly over many weeks are called "slow release fertilizers." This is an advantage where there are frequent rains that may dissolve and wash away the nutrients. You can also buy the N, P and K fertilizer and supplement it with a micronutrient mix that contains only the micronutrients. Finally, farmers and gardeners sometimes use specially formulated fertilizers that are designed to dissolve quickly in water for use in a hydroponic system or to sprinkle over the leaves of plants. I will refer to them as hydroponic fertilizers.

Now there is one complication. I know that most readers will not want to hear that it is a bit more complicated but you will fail if it is not understood. When gardening without the usual soil or compost, the fertility problem you are most likely to run into is that of an insufficient amount of the lesser two of the five macronutrients, calcium and magnesium.

ECHO once set up a series of experiments with a soilless growing method we wanted to evaluate. We purchased a bag of fertilizer that said, "Everything needed for a complete soilless mix." Halfway into the experiment, all the plants started having problems. Belatedly, I looked at the fertilizer label more closely and noticed that it had N, P, K and all the micronutrients, but it did not contain calcium or magnesium. Apparently, they assumed that we would buy some company's potting mix called "a complete soilless mix" and that "soilless mixes" contained inert ingredients plus magnesium and calcium! We had to cancel the experiment.

A homeowner growing plants in any soilless medium that has not had nutrients added might think that s/he could purchase a soluble fertilizer (in the United States the most common one is called Miracle Grow) and it would provide everything that is needed. Beware though, for if you read the label you will likely not find magnesium or calcium on the list of nutrients. Fortunately, it is not very difficult to provide these two nutrients.

In any soilless gardening method, I like to add a special kind of limestone called dolomitic limestone, or simply dolomite. All limestone contains calcium carbonate, and so is a source of calcium. But dolomitic limestone also contains magnesium carbonate. Both kinds of limestone can be purchased in any gardening store in the USA and I suspect in many countries. Farmers usually think of adding lime (calcium carbonate) to reduce the acidity of the soil, but it is also a great source of calcium. This finely ground stone does not appear to dissolve, but over time it slowly releases the elements in the soil.

This may be all that you need to do to provide for the calcium and magnesium, but it could become depleted or may not dissolve quickly enough to support rapidly growing plants. So in my gardens at home I water container gardens about once a week with soluble fertilizers that do not contain calcium or magnesium, using about one tablespoon per gallon. Our well water apparently contains enough calcium. But every other time I add about half a tablespoon of Epsom salts (magnesium sulfate) that instantly dissolves and makes up for any magnesium deficiency. I don't wait until I see a problem; I just add it to make sure no problem arises. It is inexpensive and can be purchased at any pharmacy (ask for Epsom salts) or garden center.

"Manure tea" is made by soaking animal manure in water (usually in a permeable bag such as burlap) for several days. With the aid of microbial organisms, nutrients are released and dissolve the water. The dark, smelly liquid is used to fertilize the growing medium. It may be necessary to dilute the tea first. Manure tea should never make direct contact with the edible part of the plant because it may contain disease organisms.

Whether you use fertilizer, manure or manure tea depends on your location (and philosophy). In many urban situations it is easier to obtain fertilizer than manure and it is much less smelly. Remember that your goal is to have a foolproof system that can be taught to and duplicated by dozens of gardeners. You can develop very precise instructions for a system designed to use a particular commercial fertilizer, but it will be much more difficult with manure tea. The response of plants to manure depends upon the age of the manure, the animal's diet, how much bedding is included in the manure, and on the kind of animal. (Goat manure is reportedly one of the best manures for hydroponic systems. This may be because goats are browsers, eating a little from many kinds of plants each day. Consequently their manure has a composite of nutrients found in a wide variety of plants.) Do not just assume that people cannot afford fertilizers, especially if the produce is to be sold.

Why do we rely so much on fertilizers? When I give presentations on rooftop gardening methods I am often asked, "Why do you promote methods that rely so heavily on commercial fertilizers?" ECHO exists to help the exceptionally poor to grow food plants for their own use and possibly to provide them with an income. Our intent is to provide development workers as well as more affluent gardeners with alternative methods for many scenarios. At the risk of sounding trite, let me say that the world is a very big and diverse place! If one of ECHO's low-tech and extremely light weight hydroponic systems is the best for your situation and makes economic sense, that may be the one you will choose. If weight is no problem and there is plenty of rich soil or compost, a bed at least six inches deep might be ideal and you would not need fertilizer.

Many have suggested making a hydroponic garden with manure tea as the source of nutrients. The basic method of making manure tea is to place a burlap bag of manure in a barrel of water and let it sit for a few weeks. Nutrients slowly leach from the bag turning the water into a dark "tea." We have not found manure tea by itself to be sufficient to provide every nutrient in the right amounts. If plants show signs of nutrient deficiency you can try fertilizing with a manure tea as a supplement. We often use manure tea (or a hydroponic fertilizer) at ECHO as a nutrient boost to a bed that is already relatively fertile.

If you were going to rely heavily on manure tea, then you would need a more precise, repeatable way of making it. You could develop your own precise formula, e.g. so much manure of a certain age from a certain kind of animal allowed to stand in a certain amount of water for a specific time.

It is important to make sure that manure tea does not come into direct contact with the edible parts of the plants because the manure may contain disease-causing microorganisms. In general disease-causing microbes are more likely to dominate a system where organic matter is decaying under anaerobic conditions and are not so prominent in well-aerated systems. I suppose one could avoid this by vigorously bubbling air into the barrel, but that is a higher level of technology than is suitable for most of those whom ECHO exists to serve.

Our first attempt at a soil-less rooftop garden. When I returned from my first trip to Haiti I began considering what kind of garden would be suitable for the roof of that orphanage. It is hard to beat the productivity of a good raised-bed garden made from fertile soil and compost as long as weight is not a problem. I ruled out finding enough quality soil or making enough compost to create significant gardening space on a Port-au-Prince rooftop (to say nothing of the work of getting it onto the roof). And the weight of many beds of deep soil was definitely a problem.

I decided to base my first rooftop garden on the principle behind the successful tomatoes I was growing in woodchips in half-bushel bean hampers. Wood chips are lighter than soil and meet the criteria that beds should be made of materials that are recycled and at little or no cost in the community where the gardens are being grown. They are free in my community because tree-trimming companies produce countless truckloads of woodchips every day and welcome a place to dump them where they do not need to pay a landfill charge. The nature of the materials will differ from community to community and especially in different climatic zones.

Buildings at ECHO do not have flat cement rooftops, but there was a rectangular cement slab on the farm that measured about 5 ft by 8 feet. I decided to pretend that it was a rooftop and build a woodchip garden on it. I found some old roofing material and made sides to hold a three-foot deep garden. That was a bit deeper than the woodchips in the bean hamper basket system that had give good production of tomatoes.

I filled the garden with woodchips, added some garden fertilizer with micronutrients and some dolomitic limestone, and then thoroughly wet the garden. I quickly realized that this would be way too heavy for a rooftop. But it was built, so I planted a variety of vegetables and some flowers. Every day I poured some soluble fertilizer over the top of the plants. Amazingly it was a productive garden. But it used too much material, was too heavy and required a lot of fertilizer.

Here is a very important point when growing in organic materials that have not yet been decayed into compost. The reason it required so much fertilizer is that I was both feeding the plants and the microbes that were silently at work decaying the woodchips. After about a year of continuous production in a warm climate the woodchips in such a garden turn into beautiful dark compost. So most of the fertilizer is not really wasted after all and you end up with beautiful compost.

Few if any roots extended to the bottom of the bed, so it was clearly much deeper than we needed. What would happen if we made the woodchip garden shallower-much more shallow? We made some 2x4 foot trays about 3 inches deep, filled them with woodchips, planted some vegetables and watered with a soluble fertilizer that contained all the required nutrients. To our surprise these shallow gardens did as well or better than the deep woodchip garden and used much less fertilizer. We called it "a shallow bed garden." A welcome extra benefit of these very lightweight shallow bed gardens is that when a rare southern Florida freeze came we could easily carry the gardens into a garage for the night.

This led to many trials with various depths and using various materials for the growing medium. This will be discussed in the following chapter, where we will take a closer look at just how to make the various kinds of gardens in unlikely places.