Think last year’s garden soil is good enough? Think again.

Soil is soil, right? In some years, the dirt in your garden yields decent tomatoes and pretty marigolds while in other years nothing much grows. (Maybe squirrels are to blame.)

Not so much. There are many things going on with Colorado soil and we’ve got some ways to get your ordinary dirt into superstar shape.

What’s in soil

Soil is made up of small bits of different-sized mineral particles, rocks and other stuff. The mineral particle sizes and how much of each your soil contains hold the keys as to how well your spade goes into the soil and how well it drains after being watered.

If your soil has too many small clay particles, then water and nutrients can’t move well down and through the soil. Think of a stack of pennies in a jar and how poorly water would pass through.

Sandy particles, the largest in soil, let everything pass through quickly, especially water and nutrients. Compare sandy particles to a jar of tennis balls and how fast water gets through and around each ball.

Too much or too little of any of these particles can mess with your soil’s main job, which is to grow nice plants.

Soil has more than just mineral particles; important “extras” make it a living medium. There are all sorts of organic elements like living and dying things including plants, leaves, fungi, bacteria, worms, insects and tiny creatures (even spiders). All this great organic matter acts like veins to plants. As they break down, they carry nutrients that plants need along with the other key ingredients of soil: air and water.

Soil tilth

When the soil extras are in harmony (or getting close), then the soil has what is called good tilth, or a nice loamy consistency. In other words, its physical condition is dark in color, crumbly but not overly to the point of not holding together. Good soil tilth supports plant and root growth with enough pore space for air and water to supply plant roots. Seeds emerge easily and our shovels can dig and turn the soil without too much effort.

Colorado soil has a mixture of tilth qualities, much of it related to the varying percentage differences of the mineral particles clay and sand. Most of our landscapes along the Front Range are afflicted with excessive clay, and we know how challenging it is to deal with. When clay soil is wet, it often stays wet longer. When too much water is added to clay soil from sprinklers or from hand watering, it simply runs off without absorbing into the soil. When clay soil is dry, it tends to be lumpy and hard. It also warms up slowly in the spring. Digging into moist clay soil feels like slicing through sticky, dense holiday fudge. No fun.

Conversely, sandy soils are porous, so they tend to be quick-draining and need more frequent watering and added fertilizers. Soil in Colorado can also have both clay and sand or somewhere in between. Sigh.

Some gardeners don’t mind their clay soil conditions for its water- and nutrient-holding abilities. If you have plants that are growing well in clay soil, you’re fortunate. Today, I’m mainly talking about improving soil conditions for growing vegetables, herbs and common ornamental plants.

Whether you have overly sandy or clay soil, one of the best ways to improve it is to add a soil amendment, which acts like a glue to bind soil particles. Keep in mind that soil amendment is a catch-all description and used interchangeably with other products such as compost, organic matter, manure and other decomposing materials like leaves or grass.

Soil amendments are sold in bags, in bulk or can be free if you make compost at home or have access to well-aged animal manure.

Added soil amendment improves the mineral particle balance for better or reduced drainage, air and water movement depending on whether you have soil that is overly clay or sandy.

Adding soil amendments to clay reduces the number of small clay particles, resulting in improved soil drainage and air getting to plant roots. Sandy soil particles are reduced, which improves the soil’s ability to hold moisture and hold on to nutrients.

Many gardeners got the memo about adding soil amendment to their vegetables and herbs and around established ornamental plants for soil improvement. Fertilizers are also often added in the hopes of improving plant growth and health.

But here’s a question we should be asking ourselves: Do we need to add these materials year after year? If so, how much, how often and how should it be incorporated into our existing soil?

Too much of anything, including soil amendment, is not good for the soil or plants. Too much amendment can interfere with air, water and nutrient penetration to the soil or lead to groundwater contamination. Too much soil amendment can rob plants of nutrients during the decomposition process. Excessive (or not well-aged) animal manure can “burn” plant roots. Animal-based compost can contain high levels of salt. Bagged products may have added excessive nutrients.

Too much of one fertilizer nutrient can interfere or create imbalances with other nutrients. Too much of the micronutrients like zinc, boron, copper, and manganese added to the soil can be toxic to plants.

Soil testing

Before indiscriminately guessing and adding “stuff” to your soil every year, run a soil test to see what your soil needs in the form of soil amendment and nutrients. Consider doing the soil test in the fall after the growing season or in the spring before planting and before adding any soil amendment and fertilizer (or anytime of the year when the soil is not frozen).

A soil test is recommended for brand-new planting beds or new lawns where it is important to establish a baseline on salt levels, soil pH, soil texture, organic matter and key macronutrients: nitrogen, phosphorus and potassium.

If additional soil amendment is recommended based on soil test results, or if it’s a brand-new lawn or perennial area and a soil test isn’t done, the rule of thumb is to dig in 1 inch of animal-based soil amendment, or 2 inches of plant-based soil amendment and mix well into the top 6 to 8 inches of existing soil or deeper if possible.

If the soil is super difficult to turn and mix down that far, use proportionally less amendment. Avoid going beyond 25 percent compost to 75 percent existing soil.

Run a soil test every few years as an ongoing guide so you’re not regularly adding too much soil amendment and fertilizer.

Some options to get away from dealing with heavy clay or sandy soils are to build berms, or use containers or raised beds for growing plants. Purchase a planter’s type soil by the bag or in bulk to fill raised beds or build berms. Potting soil mixes work well in containers. Be aware that these amendments can be high in nutrients or salts.

If a soil test reveals excess salts and nutrients, try running water (leach) over the soil or berm several times to wash them away. Adequate drainage is important under a berm or raised bed so any leached nutrients aren’t being absorbed back into the upper soil profile.