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Healthy soil has just the right nutrients in the right amounts

April 30, 2016
James Campbell

When my family has guests over, our motto has always been, “It’s better to have too much food than not enough.” Apply this same spirit of generosity in your garden and you may end up with too many unwelcome guests — of the pest and disease varieties.

Healthy soil contains just the right nutrients in just the right amounts. Nutrient levels that are above optimum do not improve plant growth. Randomly adding fertilizers and amendments can create big problems.

Question is, how do you know if your soil is healthy? And how do you know if and when you should amend or add fertilizers?

If your garden is growing and thriving, your soil is probably fine. But if you’re the science type — or if you want to know for sure — then test your soil. Soil testing kits are widely available at nurseries. They’re inexpensive and easy to use. The optimum nutrient levels listed on soil test results represent the range at which at which nutrients are balanced and plant growth is maximized. This will tell you definitively if your soil is lacking any nutrients —and what you can add to bring it up to snuff.

Whether or not you opt for testing, let’s look at the two categories of what you can add to soil: amendments and fertilizers.

FERTILIZERS

Fertilizers, either synthetic or organic, provide essential nutrients to plants in the form of soluble salts compounds that directly affect plant growth. These are the bags or boxes on nursery shelves that usually have N-P-K percentages front and center (these stand for nitrogen, phosphorus and potassium, the three main ingredients for healthy plant growth). If you do use these products, be sure to follow directions closely.

Excessive fertilization, especially when high in nitrogen, encourages rapid plant and leaf growth. Although this sounds like a great thing, it actually stresses the plants. Rapid growth causes plant bark to crack, providing entry for pest and fungal invasions. The slower-growing roots may be unable to adequately supply water and nutrients to the speedy new flush of leaves and branches. The succulent, bright green new growth is tasty to sap-feeding insects like aphids, scale, spider mites and psyllids. These pests seek out young foliage and quickly multiply. This increases the plant stress even further and can even lead to disease. The end result? Overfertilized plants often look great for a while, then look worse than their unfertilized kin.

AMENDMENTS

Amendments, such as compost, indirectly affect plant growth by improving the physical properties of the soil. It may be hard to believe that you can add too much compost. The nutrient content of compost and other organic amendments is lower than that of most fertilizers. However, compost increases the nutrients in the soil not only by adding them directly, but also by boosting the activity of microbes that release nutrients already in the soil. A little is good, but too much and you’ve got yourself an issue.

Here’s what happens. Once organic matter levels surpass 8 percent the excessive nutrient availability changes the important nutrient balance in the soil that allows plants to absorb necessary nutrients. You end up with stressed plants that are more likely to be attacked by insects and suffer from more serious damage.

Excessively amended soil can also cause an undesirable build up of salts, raising the salinity and changing the pH of your garden soil. High or low soil pH makes soil nutrients less available. This causes a number of nutritional deficiencies that stress the plant and lead to issues with pests and disease. While we can head to the gym after an overindulgent meal, there is not much you can do to correct overfertilization except wait. Nutrient levels will come down over time through plant uptake and weathering. Adding too much amendment can be corrected by working the excess into deeper layers of the soil.

You can find more information on ways to improve your soil on the Marin Master Gardener website at www.marinmg.org. Search for “How To Build Healthy Soil.”

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