Spring is here, plants are coming out of winter dormancy, and it's time to resume a regular fertilization schedule.

As you check out the fertilizer display shelves in the next week or two, you'll see big changes in fertilizers. There are many new products with unfamiliar non-nutrient ingredients (e.g. mycorrhizae, humic acids) listed on their labels.

And many standard brands that formerly listed only the percentages of macronutrients (nitrogen, phosphorus and potassium) on their labels have reformulated their products and now include micronutrients (e.g. iron, sulfur, magnesium, calcium), as well as mycorrhizae and various bacillus.

Why all the changes? There has been a lot of research done on garden and agricultural soils in the last few years, and the research has shown that soil texture, structure and condition, microbial activity, and soil and water pH factors greatly affect the ability of plants' roots to draw up and absorb available nutrients.

For instance, our soil and water here in the Central Valley are highly alkaline with high pH factors. The high pH factors impede many plants' roots from drawing up sufficient iron, leading to a condition called chlorosis where the leaves turn yellow and show green veining (gardenias especially). Just adding iron isn't enough to correct chlorosis. We have to add sulfur, which lowers the pH factors, making the iron available to the plant.

For acid-loving plants that thrive with lower pH factors -- including azaleas, camellias, hydrangeas and gardenias -- we need to apply fertilizers that contain at least 5% sulfur.

Humic acids are found in humus, which is not a fertilizer but rather a soil conditioner. The texture and drainage in heavy clay soils and sandy soils are improved with the addition of humus.

Plants grown in soils with higher levels of humic acids show improved seed germination, nutrient uptake and root vigor.

Even if your soil has been well-amended, additional humic acid can produce healthier plants and improve yields.

Mycorrhizae are soil-dwelling beneficial fungi that attach to plants' roots and aid the roots in gathering nutrients and water. The fungi also extend long threads or filaments into the soil surrounding the roots.

The filaments break apart clay soils and bind sandy soils together, and they form a mass that holds moisture. Plants grown in soils deficient in mycorrhizae are more susceptible to disease and insect problems.

Mycorrhizae are credited with increasing drought and salt tolerance, increased root generation and reduced transplant shock.

I've been experimenting with some of the newer fertilizer formulations. For the last two years, I've used a lawn food that contains all the macronutrients as well as added iron and sulfur with good results, such as improved color and drought-tolerance.

I've found that vegetables produce as well or better on smaller amounts of an organic fertilizer that contains extra sulfur, calcium and humic acid and mycorrhizae. The improvements in the soil itself may not be visible, but the results are.