A discussion of the following topics:

Summary

  • Soil is about half empty space and most of the rest was originally rock. Only about 2-5% is “organic,” meaning that it is composed of decomposing plants and animals. Both the rock portion and the organic portion provide needed nutrients to plants. Most soils that are “good” for gardening can provide the nutrients for many years any fertilization.
  • Plants need many elements from the soil. Nitrogen (N), phosphorus (P), and potassium (K) are needed in the largest amounts by your garden plants.
  • Fertilizers provide substantial amounts of N, P, and/or K. They are typically sold on the basis of their N, P, and/or K content, with a 20-10-15 fertilizer containing approximately 20 kg of N, 10 kg of P, and 15 kg of K per 100 kg of fertilizer.
  • Leaves, compost, and composted manure contain too little N, P, and K to be considered fertilizers.
  • Fertilizers used at Eagle Heights Gardens should be “organic,” meaning that they must be made from animal or plant products (bone and blood meal, cottonseed meal, manure, etc.). Fertilizers used at University Houses Gardens can be either organic or “conventional” — conventional fertilizers are not made from animal or plant products and usually have higher N, P, and/or K content than organic fertilizers.
  • In most cases, it is difficult to know just by visual inspection of your plants what nutrient levels are like in your soil. If you wish, you can have soil tests done on your plot for a small fee.
  • For historical reasons, Eagle Heights and University Houses garden plots have high levels of potassium. Of the major three nutrients plants need, you should only fertilize with nitrogen and potassium on a regular basis.
  • For plants like tomatoes, fertilizer can be placed in each hole at planting time, scratched into the soil around the base of the plant after the plant has grown for a couple of weeks, or both. For crops like lettuce, work fertilizer into the top few cm of soil in the entire bed where the crop will be planted.
  • See the Garden Manual for more detailed recommendations on how to take care of your soil and fertilize particular crops.

What is soil?

When you walk on soil, it’s reasonable to think that you’re walking on something solid. After all, it holds you up. However, you might be surprised to learn that about half of the volume in a typical soil is empty space.

As it turns out, this empty space is critical to the function of soils, especially where plant growth is concerned. This empty space holds water after a rain (or after you water), and because water and soil interact in complex ways, at least some water stays in the soil and available to plants long after the force of gravity would otherwise have forced it to move down and out.

The water-holding capacity of soil is important not only because plants need water, but also because plants “drink” their nutrients. Plants rely on nitrogen, calcium, iron, and other elements to move from soil into water, which then carries the nutrients to special, element-specific gates in the cells of plants’ roots. Without water, plants are both thirsty and hungry.

The empty space in soil is also important because it allows air to move through the soil. Plant roots need to take up oxygen and get rid of carbon dioxide just like you and I do, and if there’s no air movement through the soil, roots die. Soil conditions are ideal for most plants to grow when half of the empty space in the soil is filled with water and the other half with air. Walking or driving on top of soil can compress the soil such that there is less empty space per unit volume – this is called “compaction,” and compaction reduces a soil’s potential for supporting plant growth.

So if half of the soil is empty space, what’s the other half? Well, basically, the earth is a big ball of rock. Over millions of years, water and wind and freezing and thawing have broken the surface of this big ball into small pieces. Soil scientists group these pieces into three categories by size. Sand particles are the largest, ranging from 2.1 millimeters (1/12 in) down to 0.05 millimeters (1/500 inch) in diameter. Silt particles are next smallest, and clay particles are even smaller – so small that individual particles cannot be distinguished even under a fairly powerful microscope. Together, the sand, silt, and clay accumulated in a particular place may or may not be abundant. Some parts of the world have no soil at all (they’re just bare rock), while other places have soil that’s hundreds of feet deep (usually because it’s washed or blown there from elsewhere). Most of the world, including southern Wisconsin, is somewhere in between, with soils 30-200 cm (1-6 feet) deep.

Theoretically, a soil could be all sand, all silt, or all clay, but most soils are made of at least some of each of these three particles. Soil scientists group soils into types depending on how much of each size of particle they contain. The soils of Eagle Heights Gardens and University Houses Gardens consist largely of what’s called “silt loam.” This means that these soils have all three sizes of particles, but that silt is the most abundant. Silt loam soils tend to be good for agriculture, though they can be easily eroded.

Between them, sand, silt, and clay make up 90 to 99 percent of the half of the soil that’s not empty space. The last little bit – between 1 and 10 percent of the total weight of the soil – is made of what’s called “organic matter.” Organic matter is made of formerly living things (stems, roots, worms, bacteria, etc.) that are in various stages of breaking apart. When these creatures were alive, they concentrated the nutrients that living things need in order to function. As these creatures start to come apart after death, the nutrients their bodies contain are released, and released much faster than the same nutrients are from sand, silt, and clay. As a result, even though organic matter is scarce, it’s critically important to a soil’s ability to support life. The silt loam in our gardens is 2 to 5 percent organic matter.
Back to top

What do plants take out of the soil?

A single tomato seed weighs less than 1 gram (3/100 of an ounce), but the tomato plant it grows into can weigh more than 20 kg (45 lbs). Where does all that additional weight come from? For thousands of years, even as humans came to depend almost entirely on agriculture for food, no one had any idea or even thought to ask the question.

Then, in the early 1600s, an early Flemish researcher named Jan Baptiste van Helmont experimented with growing plants in pots of carefully weighed soil. In one trial, he grew a tree from a seedling to a weight of 86 kg (189 lbs) in 90 kg (200 lbs) of soil, watering the tree with rainwater. Carefully re-weighing the soil, he found only 57 g (2 oz) missing. While van Helmont’s conclusion – that the material making up the tree had come from the water – was not exactly correct, he had made a very important discovery, which was that almost none of it had come from the soil. (In case you’re confused, I should note that most of the weight of a living plant is water, but if you put the plant in an oven and force the water out, very little of the dry material that’s left came from water).

Four hundred years after van Helmont, we know that most of a plant’s tissue is literally built from air, through the process of photosynthesis (photo = light, synthesis = making). That is, plants use light energy from the sun to accumulate, or “fix,” gaseous carbon dioxide, which they build it into sugars. At the same time, we have also learned that plants do get at least tiny amounts of a number of different elements from the soil, and without these elements the plants are unable to grow or reproduce.

In case you’re curious, here’s the list of 14 elements plants typically get from the soil, together with the symbols chemists use for them: nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), phosphorus (P), sulfur (S), chlorine (Cl), iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), and nickel (Ni). We say “typically,” because while plants do almost always get these nutrients from the soil, it’s possible to make a liquid solution that contains these elements and grow plants in that. Growing plants in water is called “hydroponics” (hydro = “water”, “ponos” = work), and because it allowed researchers to create solutions missing a particular element, hydroponics was critical in helping to figure out which nutrients plants needed and which they could live without.

Now, none of the elements mentioned above are really abundant in plants, but they are listed above in order of decreasing abundance in dried plant tissue. This means that an average plant contains a relatively large amount of nitrogen (if you took 100 kg or 100 lbs of dried plant tissue, about 1.5 kg or 1.5 lbs would be nitrogen) and an absolutely tiny amount of nickel – so little nickel that a giant redwood tree weighing more than 450,000 kg (1 million lbs) would contain only a few grams (much less than one ounce) of the element.

As in every living thing, the relative abundance of each chemical element in a plant reflects what the plant does with it. Nitrogen is relatively abundant because proteins contain nitrogen, and every plant cell contains many different proteins performing many different functions all the time. Nickel, in contrast, is rare because plants only use nickel to do a few things, and only at certain points in their lives. You could perhaps say that this means nickel is less essential to plants than nitrogen, but plants do die (or at least fail to reproduce) if they don’t have nickel, so even the rare elements are important in their way.
Back to top

How do plants move nutrients from soil into their roots?

For starters, it’s important to remember what soil is. About 95 to 98 percent of the soil beneath our feet is basically tiny chunks of rock. These chunks of rock contain many different chemical elements, but the vast majority of the weight of the rock is made up of oxygen (which plants need but don’t get from the soil) along with aluminum and silicon (neither of which plants need, though some plants do accumulate silicon). While some of the nutrients plants need are present in rock, those nutrients are present at higher concentrations in the organic matter (pieces of formerly living things, like plant stems and insect bodies) that makes up the other 2 to 5 percent of the weight of soil.

Unfortunately, none of the nutrients present in either rock or organic matter are directly available to plants. Jethro Tull (1674-1741), an early agricultural researcher, thought that plant roots had tiny mouths that they used to “eat” soil. This turned out not to be true, and plants have no way to take actual soil (rock or organic matter) into their tissue. Instead, they have to wait for nutrients to be released from the rock or organic matter in very small, chemically simple forms.

In the case of nitrogen, for example, nitrogen atoms that in a living animal are built into protein molecules containing hundreds or thousands of atoms must be broken off of these molecules (usually by the feeding of bacteria or fungi) such that they are present in the soil in very small, simple forms – most often the molecules NH4+ (ammonium) or NO3- (nitrate). With minor exceptions, these are the only forms in which plants can take up nitrogen. The story is pretty much the same for each of the other elements plants get from soil. That is, big molecules in rock or organic matter must get broken down by physical, chemical, or biological processes so that just the right small molecules or even atoms are present in the soil. For some elements like phosphorus, there’s only a very narrow range of soil conditions under which this happens, so these elements are ones that are most likely to be in short supply for plants.

Unfortunately (again!) having the right small nutrient molecules present in the soil isn’t enough. Just as a soccer ball must be moved by a soccer player to make a goal, a significant amount of water must be present in soil to carry nutrient molecules to (and then into) roots. Plants drink their nutrients – they don’t eat them!

Even if nutrients are present in the right forms with plenty of water, we still haven’t covered exactly how those nutrients get into plants. At one level, that’s really complicated – it involves elaborate protein “machines” built into the walls of the cells that make up plant roots. It also involves some complex chemistry and physics.

At another level, the way plants take up nutrients is simple, resembling the workings of a common children’s toy. Many small children I know have boxes with different shapes cut into the sides. A box like this comes with a set of blocks whose shapes match the shapes cut in the box. The protein “machines” that transport nutrients through the surface of a root are like the specially shaped holes in the box, and the nutrients are like the blocks – all the child (or plant) has to do is find the right block (or molecule) to go through the right hole.

One difference between children and plants is that while a child might struggle for a minute to find the right hole for one block, plants can move thousands or even millions of nutrient atoms or molecules through a single transporter in seconds (they also have millions of transporters). Another difference I’ve already mentioned is that while a child uses its fingers to bring the block to the hole and push it through, plants rely on water to carry nutrient atoms or molecules up to and through nutrient transporters.

The analogy to the child’s toy may make it sound like plants don’t make mistakes (most toy boxes will only allow each block through a single hole), but some nutrient transporters are actually not all that specific. A few, like the one that moves the essential element zinc, also unintentionally pick up elements like cadmium, which is quite toxic to humans. This is one of many reasons to keep lead, cadmium, and other metals out of our gardens and farm fields.
Back to top

How plants get enough nutrients from the soil to grow well

The previous section talked about the mechanisms that plants use to transport mineral nutrients like nitrogen, sulfur, and iron into their roots from the soil that surrounds them. Understanding the workings of the tiny protein “machines” that plants use for this tells you how plants get individual atoms or molecules of nutrients from the soil, but it doesn’t explain how plants get enough nutrients to grow well.

Why is this an issue? Well, some plants, including most popular vegetables, grow quickly. Others, like trees, grow slowly but get very big. In either case, plants have no choice but to try to get the nutrients they need from the soil they start growing in. If they find themselves in an infertile spot, they can’t get up and look for a better one the way you or I would go off to find a neighborhood with more restaurants or grocery stores.

So how do plants find enough nutrients? There are three basic ways. The first of these, which scientists call “root interception,” is the closest plants get to finding a new place to grow. If a plant cannot get enough nutrients where it already has roots, but it has enough minerals and energy stored to grow at least some new tissue, it can grow roots either down or out, away from the roots it already has. These new roots can then pick up, or “intercept,” nutrients from the “new” soil they’ve moved into.

Even when a root has moved into a new bit of soil, it must still wait for nutrients to come to it from that soil. As explained in the previous section, plants “drink” their nutrients, taking them in with water, and it’s the movement of nutrient-containing water to the surface of the root that ultimately gives the root access to nutrients.

The second mechanism plants rely on get more nutrients is one scientists call “mass flow.” Mass flow is closely related to a process called “transpiration,” the movement of water into, through, and out of plants (from the Latin transpirare meaning “to breathe through”). Transpiration is happening all the time in every living plant as the plant loses water from tiny holes in its leaves. This departing water pulls more water up the stem of the plant, and this in turn pulls water out of the soil and into the roots of the plant. The process then extends one step further, as water farther away from the plant gets carried toward the plant. This water has nutrients dissolved in it that are then available to the plant.

The final way in which plants get access to nutrients other than those that immediately surround them is called “diffusion.” You may remember a diffusion experiment from high school or college in which you put a drop of blue or red food coloring into a large container of clear water. Even if you didn’t stir the water at all, the food coloring gradually moved out from the area of high concentration (the original location of the drop) to all of the areas of low concentration, and this continued until the water was uniformly blue or red.

If, in the same experiment, you could somehow take some of the food coloring out of a corner of the container, the rest of the food coloring would “diffuse” through the container, again redistributing itself from areas of high concentration to areas of low concentration until it was (again) uniformly distributed. This is similar to what plants are able to do, removing nutrients from the water immediately next to their roots. This then creates an area of low concentration into which nutrients move from farther away, and that area is then depleted of nutrients, which move in from still farther away, etc., etc.

For some nutrients, a single one of these mechanisms – either interception, mass flow, or diffusion – is sufficient to meet a plant’s needs throughout its life. For other nutrients, two or three mechanisms must act together to get enough nutrient to the plant. If a plant is growing particularly fast, even all three mechanisms combined may not be sufficient to provide adequate nutrients unless you place fertilizer close to the roots of the plant. Because all three mechanisms require water, fertilizer alone isn’t enough, either – you must provide enough water for the fertilizer to dissolve and move toward the plant.

The next section talks about what happens to plants when they don’t get enough of one or more nutrients to grow well, and discusses how you can in some cases “read” a nutrient-deficient plant to learn what it needs.
Back to top

Identifying and preventing nutrient deficiencies in plants

Eagle Heights Gardens was established in 1962, making it one of the oldest community gardens in the United States. That’s great, because that means a lot of people have been gardening here for a long time, but it’s also a problem, because for decades many gardeners have grown plants without using fertilizers. As a result, if fertilizers are not used now, the plants tended by today’s gardeners can suffer nutrient deficiencies that lead to poor growth and reduced yield. Fruits and leaves of nutrient-deficient plants may also be more vulnerable to disease, less nutritious and store less well than produce from plants that have everything they need.

Before we go farther, it’s good to emphasize that fertilizers should be used every year even in plots that have been fertilized regularly in the past (see the garden manual and previous Garden Plot columns available under “Gardening Tips” at http://www.eagleheightsgardens.org/ for advice on how to use fertilizers; keep in mind that leaves and compost, while useful in the garden, are NOT fertilizers!). If you garden intensively, you take a large weight of produce out of your plot each year. While most of that weight is water, and much of the rest is carbon, hydrogen, and oxygen (chemical elements that plants get from either air or water), you’re still taking away many kilograms (or pounds) of the 14 essential elements that plants can only get from the soil (see previous section for a list). Some of these 14 elements are abundant in the soil, but others are not and must be replaced regularly. It’s also possible for a nutrient to be present – even abundant — in the soil but locked away chemically in a form that’s unavailable to plants.

For those who know what to look for, nutrient-deficient plants may display one or more symptoms or visual signs indicating that something’s wrong. In some crops and for some nutrients, these symptoms may be obvious. Corn is one of the easiest crops to “read” this way. In corn, a lack of nitrogen (the most commonly deficient nutrient for all crop plants) can make a plant’s leaves turn yellow, with the yellowness moving in a V-pattern down the center of older leaves. Potassium deficiency in corn also causes yellowing of older leaves, but the yellowness moves in from the edges of the leaves. A lack of phosphorus in corn causes reddening, again beginning with the oldest leaves. In each case, a severe, prolonged deficiency causes tissue death, once again starting with the oldest (bottommost) leaves on the plant.

Why should the oldest leaves on a plant be affected by nutrient deficiencies and not the younger ones? The answer has to do with the mobility of certain nutrients within plants. Some elements, including nitrogen, phosphorus, and potassium (the three that plants need from the soil in the largest quantities) are “phloem-mobile.” This means that if a part of a plant like a new leaf or a rapidly growing fruit needs one of these elements, the plant can take it from some old tissue and move it through fluid-filled tubes called phloem to the new place.

Other nutrients, including calcium, are not phloem-mobile. As a result, plant parts like tomato fruits that need a lot of calcium can end up without enough of it. In tomato plants, this can result in a condition called “blossom end rot,” in which the bottom of each affected fruit turns black and soft. Blossom end rot is common in Eagle Heights garden plots, in part because of failure to use fertilizers containing calcium (or any fertilizers at all). Related conditions are also common in squash, cucumbers, and watermelons, all of which are fast-growing fruits that need a lot of calcium.

Unfortunately, most nutrient deficiencies are not easy to diagnose visually, and some are easily confused with diseases. If you suspect a nutrient deficiency, you can submit some tissue from your plants to the UW Soil and Plant Analysis Laboratory (go to http://uwlab.soils.wisc.edu/madison/ and look for the section titled “Plant analysis… for diagnosis of nutrient problems”; this service is not free but is not very expensive). On the other hand, you could also just try providing your plants with what’s called a “complete” fertilizer – one that provides all of the nutrients plants need. This won’t necessarily fix the problem within the current gardening season (once a plant has visible deficiency symptoms, it’s often too late to help it), but should help to prevent the same problem the following year.
Back to top

Fertilizers: what they are; why to use them; synthetic vs. natural fertilizers

You MUST fertilize to get the most out of your garden. If you go year after year without fertilizing, your soils will be depleted of nutrients, your yields will drop, and your plants may show symptoms of either nutrient deficiency or disease. Fertilizers can be purchased at many stores, and if you use them properly you will certainly get your money back in the form of green, growing things.

If you aren’t familiar with the word, a “fertilizer” is anything that’s added to soil to help plants grow. Fertilizers can be “natural” (some use the word “organic”) in origin, meaning they are made from manure, animal parts (blood or bones, for example), plant parts (sugarcane waste or ground seaweed, for example), or naturally occurring rocks or rock-like materials (sulfur, for example, from ancient volcanoes). Fertilizers can also be “synthetic,” meaning that their production somehow involves the use of industrial chemicals like acids and (often) a source of energy like natural gas.

As far as we know, plants don’t care where their nutrients come from. To a plant, nitrogen is nitrogen, whether it comes from cow manure or from a giant factory that pulled it out of the air (more than 70% of the air we breathe is nitrogen, but in a form that is unavailable to plants).

To both people and plants, however, there are some important differences between natural and synthetic fertilizers. Some of these differences have to do with opinion. There are people who think that fertilizer that comes from plants or animals is “good,” somehow healthful, while fertilizer made using industrial processes is “bad,” somehow damaging.

Other differences between natural and synthetic fertilizers are less a matter of opinion and have more to do with what’s in the fertilizer than with where it came from. Synthetic fertilizers are certainly much more concentrated than natural fertilizers. A bag of synthetic fertilizer might be labeled 18-51-20, numbers that represent the percentages in the fertilizer of the three nutrients that plants need in the largest amounts (in order, these are nitrogen, or N, phosphorus, or P, and potassium, or K). This is supposed to mean that a 100 kg (220 lb) bag of this fertilizer would contain 18 kg (40 lbs) of N, 51 kg (112 lbs) P, and 20 kg (44 lbs) K. This is not exactly true, but it does show you that the fertilizer contains a lot of N, P, and K.

By comparison, natural fertilizers contain much lower of amounts of nutrients. For example, a fertilizer called Milorganite (made in Milwaukee from sewage) says on its bag that it is 5-2-0 (5 percent N, 2 percent P, and no potassium, or K, at all).

You might think that using a synthetic fertilizer would be the obvious choice – why not give plants lots and lots of nutrients? – but the nutrients in synthetic fertilizers can actually be too abundant and too available. A small, growing plant can’t take up a bunch of fertilizer all at once, so there’s the possibility that much of the synthetic fertilizer you apply will be washed away or otherwise lost. Natural fertilizers contain fewer nutrients, but release what nutrients they do hold over a longer period of time such that plants are always getting at least some of what they need.

Many people do not have strong preferences for natural or synthetic fertilizers. If used properly, both can yield healthy, productive plants without damage to the environment. There do appear to be compounds in some natural fertilizers that are not actually nutrients but still perform some important function, like controlling fungal diseases that would otherwise infect your plants. Scientists are just beginning to do research on this, and it could be a reason to use natural rather than synthetic fertilizers in some situations.

At Eagle Heights Gardens, the choice is made for us: all gardeners there are requested to use organic methods. That means (among other things) that gardeners there should use natural fertilizers rather than synthetic ones, and must also avoid synthetic chemicals that kill weeds and insects. At University Houses Gardens this is not the case – you can use whatever fertilizers and pest controls you like, though you are asked to be considerate of your neighbors.

In general, most plants benefit from nitrogen fertilization every year, while phosphorus and potassium need be added only every 2-3 years, or even less often. As shown below, many garden plots in Eagle Heights have high levels of phosphorus. As a result, of the three elements plants need in large quantities, only nitrogen fertilizer and potassium are probably required here on a regular basis.
Back to top

What tests are commonly done on soils to determine if they are “good” for gardening?

When you send a soil sample to a lab, there are many, many possible tests you can request. Unless you have a good reason to request a particular test (for example, you are concerned about the possibility of lead contamination from old pipes or paint, which are fortunately not a problem in Eagle Heights or University Houses), there are really only a few very useful things to know about. Descriptions of the most important tests are below, together with a brief description of what your results might tell you.

  • Soil pH is a measure of the acidity of a soil. This is possibly the most important single soil test because the pH influences the availability of all the essential elements that plants obtain from the soil. It also affects the activity of soil microorganisms. pH values above 7 represent “basic” or “alkaline” soils, while pH values below 7 represent “acidic” soils. Most plants grow best at a pH of between 6.6 and 6.8, though many species can tolerate soils that are as acidic as pH 6 or as alkaline as pH 8. A few plants such as blueberries do best in soils that are strongly acidic. If a soil’s pH is at or below 6, adding agricultural lime (calcium carbonate or calcium magnesium carbonate) is a good idea. If the pH is above 7, this is generally not a problem unless you want to grow plants like bluberries.
  • Organic Matter influences the moisture-holding capacity of the soil (more organic matter means the soil can hold more water), serves as a supply of nutrients, feeds microorganisms, insects, earthworms, etc., and makes it easier to dig in the soil. The amount of organic matter depends on temperature, rainfall, aeration and other factors. It tends to be a characteristic of a soil type in a given place and is not easily changed. Rototilling and even frequent hand tilling can result in loss of organic matter through oxidation (soil-dwelling organisms eat it up) and erosion. It is possible to increase a soil’s percentage of organic matter, but this requires large additions of crop residue, compost, mulch, or manure (NOT wood chips, sawdust, or other wood products!), and is usually possible only on the scale of a garden. There is no ideal target number of organic matter when testing – generally, more is better, and a soil that has less than 2% organic matter is likely to be less productive and harder to work in than a soil containing 5% organic matter.
  • Phosphorus is usually reported as parts per million, or ppm, where 1 ppm is approximately equal to 0.1 pounds of available phosphorus per 1,000 ft2 of vegetable garden (0.05 kg per 100 m2). In general, levels of phosphorus should be about 60-80 ppm, though higher or lower levels may be better for particular plant species, and different methods of soil testing can give very different results for this element. Shallow-rooted plants such as onions, carrots and radishes require higher phosphorus levels than do deep-rooted plants like sweet corn and legumes (beans and peas). This element is needed for energy transfer reactions in plant cells and stimulates root development and flowering in plants.
  • Potassium is usually reported as parts per million, or ppm, where 1 ppm is approximately equal to 0.06 pounds of available potassium per 1,000 ft2 of vegetable garden (0.03 kg per 100 m2). In general, levels of potassium should be greater than 180 ppm, though higher or lower levels may be better for particular plant species. is an activator of many plant enzyme systems. It is involved in carbohydrate and protein formation and movement of many other nutrients within the plant. Abundant potassium promotes disease resistance and, in perennials, some degree of winter hardiness. Root crops (radishes, carrots, potatoes) have particularly high requirements for this element. Low potassium levels result in weak stems in many plants. Fertile garden soils contain 250-350 lb. of available potassium per acre.

Given the importance of nitrogen in plant growth (and its presence in many fertilizers), you might think that you would want to have a test done for nitrogen levels in your soil. However, unlike phosphorus and potassium, nitrogen does not accumulate in the soil in forms that are available to plants. If you attempt to build up high levels of these forms (nitrate or ammonia), they are either taken up by soil-dwelling organisms other you’re your target plants, or they leach (wash away) in rainwater, potentially contaminating nearby lakes and streams. While there are several different soil nitrogen tests, these are generally only useful for farmers and researchers. Gardeners trying to decide how much nitrogen fertilizer (or manure, or other nitrogen-rich material) to use should look at recommendations for how much is needed by a particular crop, and apply accordingly each season. Many other tests can be done on soil, and some of these (for example, tests for elements like zinc and boron that plants need in very small amounts) may be useful in some circumstances. However, doing these tests makes getting soil tested more expensive. More importantly, fertilizing with a either an organic fertilizer (manure, bone and blood meal, etc.) or a “complete” synthetic fertilizer (that is, one that contains nutrients other than just nitrogen, phosphorus, and potassium) is likely to provide these elements in sufficient quantities such that you don’t need to test for them.
Back to top

What do soil tests say about the soils in Eagle Heights and University Houses Gardens?

The most recent tests done on our soils were performed by UW-Madison’s Soil and Plant Analysis Laboratory in November 2009. We tested soils from several “average” plots (plots that we think have been fertilized at least a bit over the years), as well as samples from plots that have been either fertilized well or not (so far as we know) fertilized at all in recent years. We also tested soil from the forested area south of the Eagle Heights garden shed, soil from the blueberry plot near the entrance to Eagle Heights (which has been deliberately acidified), and the soil-like compost from the community compost pile at Eagle Heights. Results for selected tests are shown in the table below, together with average values for Eagle Heights soil from 1993 (the last time extensive soil tests were conducted). See the previous section for an explanation of why we performed the tests shown here and not other ones.

Some comments on these test results are provided after the table. If you are looking at this table for information about what to do in your own plot with respect to fertilization, keep in mind that all of these soil properties vary naturally from place to place, and that any two adjacent plots in our gardens may have very different histories. To get the soil from your own plot tested, you can submit a sample to the UW Soil and Plant Analysis Laboratory (go to http://uwlab.soils.wisc.edu/madison/ and look for the section titled “Lawn and Garden”). This service is not free but is not very expensive for a single sample. If you do have your soil tested, you could do other gardeners a favor by sending the results of your soil tests to the garden registrar. With your permission, she or he may share them with the rest of the gardens.

Sample description Organic matter % pH Phosphorus (P) parts per million (ppm) Potassium (K) parts per million (ppm)
Average Eagle Heights soil, 1993 4.6 7.5 190 215
Average plot in the 200s at Eagle Heights, 2009 8.0 7.3 209 95
Plot in the 600s at Eagle Heights, 2009 (believed to be unfertilized for years) 6.0 7.2 176 86
Plot in the 700s at Eagle Heights, 2009 (known to be well fertilized for years) 8.0 7.3 215 130
Plot in the 1100s at Eagle Heights, 2009 (known to be well fertilized for years) 9.2 7.2 199 116
Plot in the 1300s at Eagle Heights, 2009 (believed to be unfertilized for years) 5.1 7.1 142 84
Soil from forest south of Eagle Heights garden shed, 2009 3.5 6.9 56 90
Soil-like material from the community compost pile at Eagle Heights, 2009 6.9 7.4 104 109
Plot in the C row at University Houses, 2009 5.5 7.3 185 150
Soil from blueberry planting near Eagle Heights entrance 6.5 3.1 146 109

Except for the blueberry planting (which is much too acidic for most plants, and perhaps even a little too acidic for blueberries), all of the soil tests shown in the above table indicate soils with pH and organic matter suitable for growing crops. The soil from the forest near Eagle Heights has a lower organic matter content than soil from nearby garden plots that probably used to be forested as well. Forest soils naturally have relatively low organic matter content, and most gardeners have added leaves, compost, and other materials to their plots over the years such that their soils’ organic matter levels are higher than they would have been before gardening began here. Soil tests in 1993 showed very high phosphorus and potassium levels throughout the gardens. This presumably reflected the fact that for many years, all of the garden plots at both Eagle Heights and University Houses were tilled annually using large machinery and manure from UW’s dairy barns was applied heavily during the process. Manure contains large amounts of phosphorus and potassium relative to nitrogen, and unlike nitrogen, both elements (but particularly phosphorus) can be held in the soil for long periods. As a result, even though this manuring stopped almost two decades ago, gardeners today are still “spending” the extra phosphorus and should be able to do so for years or even decades to come. In 1993, levels of potassium were also still very high, but they have declined since then such that potassium fertilization would make sense in most plots.

For gardeners who are interested, two soil samples (one from a plot that has been fertilized occasionally and lightly over the past five years, one of which has been fertilized annually and heavily over the past five years) were also analyzed in fall of 2009 for their micronutrient content and their levels of exchangeable cations. These results are too complex to explain in depth here, but the results of the tests are shown in the following table. Besides pointing out that levels of most nutrients are adequate in both plots for most crops, one of the main take-home messages from these results is that fertilization with nitrogen, phosphorus, and potassium-rich fertilizers (the main purpose of the fertilization done to one of these plots) has not substantially altered the levels of micronutrients relative to those found in the less intensively fertilized plot. To do that, fertilization with special micronutrient-rich fertilizers would probably be required.

Average plot in the 200s at Eagle Heights, 2009 Plot in the 1100s at Eagle Heights, 2009 (known to be well fertilized for years)
P % (10,000 ppm = 1 %) 0.12 0.14
K % (10,000 ppm = 1 %) 0.34 0.34
Ca % (10,000 ppm = 1 %) 1.73 2.09
Mg % (10,000 ppm = 1 %) 0.63 0.63
S % (10,000 ppm = 1 %) 0.06 0.07
Zn ppm 89 86
B ppm 11 13
Mn ppm 1006 789
Fe ppm 13,591 12,466
Cu ppm 12 14
Al ppm 16,081 14,282
Na ppm 180 156
Exchangeable Ca 3013 3628
Exchangeable Mg 620 542
Exchangeable K 382 625
Exchangeable Na 38 32

Back to top

Formal descriptions of soils in Eagle Heights and University Houses Gardens

Compared to soils in much of the world, the soil in Eagle Heights and University Houses gardens is rich and well-suited to vegetable gardening (with the exception that much of Eagle Heights is sloped enough that erosion is a significant concern when soil is left exposed).

For those who are interested, the United States government’s Geological Survey (USGS) has for many decades provided maps and descriptions of the country’s soils. An aerial photograph of Eagle Heights with the main types of soil present is shown below. Following the map are formal descriptions of each of the major soils present.

 


Soil Map of Eagle Heights


Soil Map of University Heights

Kidder loam (KdD2), 12 to 20 percent slopes, eroded

This soil is on lower side slopes. The Kidder series consists of deep, well-drained, gently sloping to very steep soils on glaciated uplands. The depth to calcareous glacial till is 24 to 40 inches.

In a representative profile the surface layer is very dark grayish-brown loam about 3 inches thick. The subsurface layer is brown loam about 6 inches thick. In cultivated areas all or most of the subsurface layer commonly is incorporated into the surface layer and the surface layer is lighter in color.

Soils have medium fertility, medium water capacity, and moderate permeability.

These soils are suited to all crops grown in the county. If these soils are cultivated, control of erosion and maintenance of tilth and organic matter content are helpful conservation practices. The hazard of erosion is very severe.

This soil is on lower side slopes. Areas of this soil are elongated tracts 20 to 120 acres in size. These areas are characterized by a few narrow drainage ways. The Kidder series consists of deep, well-drained, gently sloping to very steep soils on glaciated uplands. These soils formed in glacial till under mixed hardwoods. The depth to calcareous glacial till is 24 to 40 inches. In a representative profile the surface layer is very dark grayish-brown loam about 3 inches thick. The subsurface layer is brown loam about 6 inches thick. In cultivated areas all or most of the subsurface layer commonly is incorporated into the surface layer and the surface layer is lighter in color. The subsoil is 29 inches thick. The upper 21 inches is brown sandy clay loam, and the lower 8 inches is strong-brown sandy loam. The underlying material is clay platy, calcareous sandy loam till.

These soils have medium fertility. The available water capacity is medium, and permeability is moderate. These soils are suited to all crops commonly grown in the county. The main crops are corn, oats, and alfalfa. The soils are also suited to pasture, woodland, and wildlife habitat. If these soils are cultivated, control of erosion and maintenance of tilth and organic matter content are helpful conservation practices. The hazard of erosion is very severe. The major concerns of management are improving organic-matter content, maintaining tilth, raising the level of fertility, and controlling erosion. Capability unit IVe-1; woodland suitability group 2r2.

Dodge silt loam (DnC2), 6 to 12 percent slopes, eroded

The Dodge series consists of deep, well-drained, gently sloping and sloping soils on glaciated uplands.

Formed under mixed hardwoods in 26 to 36 inches of loess over sandy loam glacial till. In a representative profile the surface layer is dark grayish-brown silt loam about 6 inches thick. The subsurface layer is brown silt loam 3 inches thick. These soils have high fertility, high available water capacity, and mod. permeability. The seasonal high water table is deeper than 5 feet.

Suited to all crops commonly grown in the county. The main crops are corn, oats, and alfalfa. Also suited to pasture woodland, and wildlife habitat. If these soils are used for crops, the control of erosion and maintenance of tilth and organic-matter content are useful conservation practices. The Dodge series consists of deep, well-drained, gently sloping and sloping soils on glaciated uplands. These soils formed under mixed hardwoods in 26 to 36 inches of loess over sandy loam glacial till. In a representative profile the surface layer is dark grayish-brown silt loam about 6 inches thick. The subsurface layer is brown silt loam 3 inches thick. The subsoil is 31 inches thick. The upper 20 inches of the subsoil is brown silty clay loam, and the lower 11 inches is brown, firm clay loam and sandy clay loam. The underlying material is calcareous, yellowish-brown sandy loam till. These soils have high fertility. The available water capacity is high, and permeability is moderate. The seasonal high water table is at a depth of more than 5 feet. These soils are suited to all crops commonly grown in the county. The main crops are corn, oats, and alfalfa. These soils are also suited to pasture woodland, and wildlife habitat. If these soils are used for crops, the control of erosion and maintenance of tilth and organic-matter content are useful conservation practices.

This soil has a profile similar to the one described as representative for the series, but it is shallower to sandy loam glacial till. If properly managed this soil is suited to all the crops commonly grown in the county. The only limitation of this soil is a severe hazard of erosion. The major concerns of management are control of erosion and improvement of the organic-matter content, tilth of the surface layer, and fertility. Capability unit IIIe-1: woodland suitability group 201.

St. Charles silt loam (ScB), 2 to 6 percent slopes

The St. Charles series consists of deep, nearly level to moderately steep, well drained and moderately well drained soils on glaciated uplands.

Formed in deep loess and loamy glacial till under mixed hardwoods. In a representative profile the surface layer is dark grayish-brown silt loam about 6 inches thick. These soils have high fertility. The available water capacity is high, and permeability is moderate. The seasonal high water table is below a depth of 3 feet, and it usually is below a depth of 5 feet.

These soils are suited to all crops commonly grown in the county. The main crops are corn, oats, and alfalfa. If these soils are cultivated, controlling erosion and maintaining tilth and organic matter content are helpful conservation practices. The St. Charles series consists of deep, nearly level to moderately steep, well drained and moderately well drained soils on glaciated uplands. These soils formed in deep loess and loamy glacial till under mixed hardwoods. In a representative profile the surface layer is dark grayish-brown silt loam about 6 inches thick. It dries to a distinctive gray color. The subsurface layer is brown, friable silt loam about 3 inches thick. The subsoil is about 41 inches thick. The upper 32 inches is firm, yellowish-brown silt loam and silty clay loam; and the lower 9 inches is friable, brown loam. The underlying material is massive, calcareous, brown sandy loam till. These soils have high fertility. The available water capacity is high, and permeability is moderate. The seasonal high water table is below a depth of 3 feet, and it usually is below a depth of 5 feet. These soils are suited to all crops commonly grown in the county. The main crops are corn, oats, and alfalfa. These soils are also suited to pasture, woodland, and wildlife habitat. If these soils are cultivated, controlling erosion and maintaining tilth and organic matter content are helpful conservation practices.

St. Charles silt loam (ScC2), 6 to 12 percent slopes, eroded

Present on nearly uniformly shaped middle side slopes. Areas of this soil are ribbonlike tracts 100 to 150 acres in size. These areas are characterized by a few narrow drainageways. Slopes are slightly convex. Otherwise it is similar to other soils in the St. Charles series.

Tilth is poorer and fertility is lower than in uneroded areas. If this soil is properly managed, it is suited to all crops commonly grown in the county. The major concerns of management are controlling erosion, improving organic-matter content and tilth of the surface layer, and increasing fertility.

This soil is on nearly uniformly shaped middle side slopes. Areas of this soil are ribbonlike tracts 100 to 150 acres in size. These areas are characterized by a few narrow drainageways. Slopes are slightly convex. Tilth is poorer and fertility is lower than in uneroded areas. If this soil is properly managed, it is suited to all crops commonly grown in the county. The only limitation is a severe hazard of erosion caused by slope. The major concerns of management are controlling erosion, improving organic-matter content and tilth of the surface layer, and increasing fertility.

McHenry silt loam (MdC2), 6 to 12 percent slopes, eroded

The McHenry series consist of very deep, well drained soils formed in loess or other silty material and in the underlying loamy till on moraines and till plains. Some areas are hundreds of acres in size, but on moraine slopes areas may be long and narrow.

Soils in these series have a large water-holding capacity. Most areas are cropped. Corn, soybeans, and small grain are the principal crops. Some areas are used for meadow or are still in woods. Native vegetation is mixed hardwood forest. The major concern of management is controlling erosion.

Dodge silt loam (DnB), 2 to 6 percent slopes

The Dodge series consists of very deep well-drained soils formed in loess and in the underlying till on ground moraines, end moraines, and drumlins.

Most areas of the soil are used for cropland. Common crops are corn, small grain, legumes, and canning crops. Some areas are used for pastureland or woodland. Native vegetation is primarily deciduous forest with maple-basswood and oak-hickory predominating.
Back to top