Don't waste money giving plants more nutrient supplements than they can absorb.

The major elements are those listed on plant food labels as N (nitrogen), P (phosphorus), and K (potassium, available as potash or K2O). Many soils are naturally high or low in any of these, but nitrogen is the most water soluble, and therefore the most likely to leach away (and into ground-water supplies). Adding more nitrogen than plants can take up is a waste of money.

The elements plants use less of are called secondary elements. They are no less critical to healthy growth than the major elements; plants just require smaller doses of them. They are calcium (Ca), magnesium (Mg), and sulfur (S). Don't waste money by adding these to your soil on a yearly basis, because plants take them in slowly, and they don't leach away. A single application usually lasts several years.
Trace or micronutrients are those that plants require only small amounts of. They include boron (B), manganese (Mn), copper (Cu), zinc (Zn), iron (Fe), molybdenum (Mo), and chlorine (Cl). Short of a soil test, the best way to tell if you need to add any of these is by plant response.

Is Your Soil Nutritious?

So you agree humus is good stuff. But will your soil contain all the nutrients necessary for healthy plant growth if all you add to it is humus? You already know incorporating organic matter that becomes humus means you will probably have to add nitrogen. Plants also need many other nutrients.
With the exception of carbon, which plants take from carbon dioxide in the air, as well as hydrogen and oxygen, which they derive from water, all other elements plants use must come directly from the soil. So what do they need?

Start with healthy soil for healthier, longer-lived plants

Thou shalt not skimp on soil preparation! The more you put into your soil, the more you will get out of it. What, you may ask, is the big deal? Dirt is still dirt, right? The truth is your soil health predicts the health of your entire garden. Rich, healthy soil sustains healthy plants that are more productive and give you the best possible return for your gardening investment. They also cost little or nothing in pesticides, fertilizers, and replacement.

For a better understanding, look at the functions of soil in your garden. Soil provides physical support for the plants and a reservoir for

nutrients, water, and oxygen. Some soils do the second part better than others. Sandy soils drain quickly, providing plenty of oxygen, but they also lose water, and dissolve nutrients too quickly. Clay holds minerals and moisture well, but drains poorly. The magic ingredient for improving either type of soil, or any type in between, is humus.

Humus is organic matter that has gone through a degrading experience. Organic matter is the remains of previously living things, such as plants, micro-organisms, bugs, and us. When added to sandy soils, humus improves water retention by attracting water molecules. When added to clay, humus improves drainage by breaking up the clay particles that naturally cling together. Humus also provides fuel for millions of micro-organisms that reside in the soil. As they break it down, the micro-organisms release elements necessary for plant growth.

High levels of humus are a sign of well-managed soil. It is best to add small amounts of humus each year and build it up, rather than dump it all on in one application. This is important because in the process of decaying organic matter, micro-organisms tie up precious soil nitrogen, which must be replenished. A good rule of thumb is to add not more than 4 inches of organic matter each year to any garden area.

Incorporating humus into your garden can be as cheap or expensive as you make it. Not surprisingly, the cheaper ways are more work than the expensive ones. The ideal way to incorporate humus is to compost. Barnyard manures also provide plenty of partially processed organic matter. Of course, garden centers offer bags of dried organic matter in the form of peat moss, processed compost, and steer manure. The advantage of compost, manures, or commercial products containing fertilizers is they also contain soil nutrients, usually including necessary nitrogen. So while it is less expensive to compost your own, the good news about buying bags of fertilizer is your money will be well-spent.