To be an organic gardener or grower, you need to know your soil intimately. Tests for soil NPK provide basic chemical information, but do not give a hands-on feel for what is happening beneath the ground. For that you have to train both your senses and your soil intuition.
The look and feel of your soil contains amazing amounts of information. With some simple procedures, you can easily track your soil’s fertility. All you need is basic equipment – a gardening trowel, measuring cup, newspaper, a one-pint glass jar and your senses of sight and touch.
First, collect three soil samples for comparison. Dig a cupful of damp soil from the top 3” layer of the most productive area of your garden. This is probably one you have cultivated for at least a season. Then take a cupful from the least fertile, barren section and a last one from some place that grows moderately well. Soil is dynamic, always changing. You want to get a good sense of the base-line nature of your soil. Then, as you amend and nurture, you can appreciate how it changes.
Lay out your samples on a large sheet of newspaper, making three small heaps. Look at each soil sample in turn, noting its color, composition and how it compares with the others. Take your time. Closely examined each heap, feel its texture and rub it gently between your fingers. Leave the samples for a couple of hours to dry out and again check the color and texture.
Now take your moderately fertile sample and shake it up with 2 cups of water in a pint jar, until it is well suspended. Leave it to settle for about 4 hours. You should be able to see different layers of particles with the larger ones at the bottom.
During the whole testing process, hold these questions in your mind: what differences can I notice between my fertile and infertile soils? What is the basic nature of my soil? If I were a plant, how would my roots experience this soil? From a more technical perspective, you want information about the five basic soil elements: humus, water, air, soil life and minerals.
First, notice the color differences between your samples. Rich fertile soil has a dark brown hue due to the presence of humus-based carbon. As you rub the soil between your fingers, it should feel slightly soft and leave a blackish film similar to charcoal or dark pencil on your skin. This tells you that humus is present. If not, your soil is mostly devoid of organic content.
Humus is an essential element of fertile soil and should ideally make up over 5% of its composition. Finer-grained humus/carbon is more biologically available to plants and soil life. Coarse fibrous humus is fresher and in the process of being broken down by bacteria and fungi; it is not so biologically active.
The proportion of humus in the soil correlates with the amount of organic nitrogen available. However, humus does much more than feed your plants — it holds both water and air and provides nutrition and living space for the microbial life in the soil. The more humus you have, the more likely you are to have a healthy soil food web.
Water and Air
Plants need a balance of both air and water in the soil. Loam soil has a sponge-like texture that contains about 25% air and 25% water. This is the ideal. If you squeeze a ball of your best soil in your hand, it should compress and hold together loosely but fall apart if dropped. If it does not cling together you probably have sandy soil that has plenty of air but little moisture. If it sticks together tightly, you probably have clay. This tends to exclude air when wet, and both air and water when dry.
Leave your three soil samples to dry out on the newspaper. Notice the changes in color as each sample loses moisture. Sand dries quickly to a lighter color and granular texture. Clay dries slowly, growing harder and cracking. Humus-rich soil stays dark the longest, letting go of moisture and taking in air at an even rate.
Both clay and sand need humus to hold water and provide nutrients and living space for the microbial life in the soil. Soil life is extraordinarily complex, forming a living ecology that includes everything from microbial bacteria and fungi to earthworms and the living tendrils of plant roots. Everything works together in a coordinated network that transforms minerals, humus, air and water into the most effective organic plant foods.
As soil life develops to include large colonies of worms, the soil changes in texture. It becomes gluey and tends to form small bobbles. Much of the soil has passed through the gut of earthworms and is infused with microbial life, producing sticky substances that bind the particles together. In my own very gritty soil, the life is so rich that the soil looks almost like clay, yet is mostly sand.
Most soil life is too small to see with the naked eye but shows up vividly under a microscope. If you find a worm or two in your soil samples, you probably have a rich and full soil ecology. To check for worm populations, choose a bare spot in the most fertile part of your garden. On a spring day, place a few layers of damp newspaper on your soil and cover with a 4” layer of organic mulch. After a week, lift the newspapers and check the number of wrigglers on top of the soil — the more the better.
Fertile soil is typically composed of about 45% ground up rock or minerals. Sandy soil has the gritty feel of larger granules, while clay soil is smooth and slightly slimy. The finer texture of clay means its minerals are potentially more plant-available. However, clay particles tend to cling together, excluding water and air. Wet clay is very sticky; it cracks and gets lumpy and hard when it dries. Sand, on the other hand, has less surface area to dissolve nutrients and larger air filled voids.
When you suspend your soil sample in water, you can see the proportions of gritty sand at the bottom, silt (very fine sand) in the middle and clay at the top. If you are lucky and have loam soil, you will have almost equal amounts of sand, silt and clay. My own sandy soil has almost zero clay. Many of my gardening friends have no sand, a little silt and lots of clay. Each kind of soil has its own strengths and weaknesses. Once you know your basic soil type, you can adjust your cultivation methods to bring out its best qualities.
Tending Your Soil
The table below gives a rough guide to the relative composition of humus, water, air, soil life and minerals in different types of soil. Only loam has the balance your plants need. Other types of soil require preparation and amendment in order to reach peak fertility.
Every soil asks to be nurtured and cultivated according to its own special nature. The more attention you pay to your soil, the more you develop soil intuition. Before long, without much thought, you can sense what each plot needs to foster extraordinary growth of particular crops. Then you truly know your soil from the inside out.