Evaluating, Preparing and Amending Lawn and Garden Soil

One of the biggest steps to establishing and growing turf, vegetables, ornamentals or flowers successfully is understanding the soil that provides their physical support and supplies them with water and nutrients.

Sometimes, the original soil will serve as an excellent growth medium, while other soils may need to be amended or even replaced with more adequate topsoil. This publication will help serve as a guide to evaluating soils and suggest amendments that will improve the success of lawn and garden plantings.

Soil evaluation should be conducted physically and chemically. Physical characteristics affect the ability of roots to explore the soil for water and nutrients, and the capacity of the soil to hold water as a reserve for plants between rains or irrigation. They also affect how easily water moves through the soil, preventing ponding.

Aeration – the process of using equipment to extract soil cores or to pierce the soil to allow more oxygen to enter the soil and carbon dioxide to exit, thereby enhancing root health

Bulk density – the weight of dry soil per unit of volume. Well-aerated soil has a relatively low bulk density; compacted soil has a high bulk density.

Permeability – the ability to allow water to move freely through soil

Porosity – the percent of soil volume devoted to pore space. Compacted soils have lower pore space and higher bulk density.

Texture – the percent of sand, silt and clay that makes up the mineral portion of soil. Texture can be estimated by feeling for sand grains between your fingers and by the ability to produce a clay ribbon between your thumb and index finger (Figure 1, Page 2). If necessary, texture can be measured accurately with a soil evaluation.

Fine textures – soil with more clay than sand or silt

Medium textures – soil with no dominance of sand, silt or clay

Coarse textures – soil with more sand than silt or clay

Water-holding capacity – the amount of water a soil can hold. Soil can hold two types: water that is loosely held and available to plants and water that is tightly held by soil particles and is unavailable for plant use. A silt loam has the most plant-available water but clay soil holds the most total water. Medium-textured soil holds more plant-available water than coarse-textured soil.

Photo Credit:

Photo by Esther McGinnis

Determining clay content in soil via the ribbon test. The ribbon length indicates the soil is fine-textured.

While not a physical characteristic, the topography of the soil surface also will be an influence on water movement into the soil.

Soils in nature develop in layers, called horizons (Figure 2). The sum of horizons from top to bottom is called the soil profile.

A slice of soil showing the various layers or horizons that have been formed through time.

The uppermost layer of soil in most of North Dakota developed under prairie grasses and forbs, and tends to be dark colored due to the resulting organic matter accumulation. This layer is the most productive in the soil.

The organic matter and biological activity in the zone help soil particles clump together into aggregates.

These aggregates tend to resist compaction and contribute greatly to permeability, favorable bulk density and porosity. The surface layer, rich in organic matter, also is a storehouse of slow-release nutrients that are helpful for plant growth.

The underlying layers are not as productive as the surface layer due to lower organic matter content. Sometimes the subsurface layers have been altered with an accumulation of soluble salts and carbonates.

These accumulations generally are not desirable and may limit the cultivation of some types of plants.

With ornamentals, having a subsoil chemical analysis before selecting adapted species for planting is particularly important. Subsoils cannot be improved easily.

Soil evaluation should begin with physical observations. Physical problems or limitations are most difficult to remedy, so knowing if the soil has any restrictions on use is important to determine.

A soil test is the best way to evaluate nutrient needs in plants, regardless of whether you plan to use an organic or chemical approach to nutrition. Home test kits provide general guidelines.

North Dakota recommendations are based on locally calibrated laboratory tests to get more reliable and repeatable results. Sending a soil sample to local labs across the state or to NDSU’s soil testing lab on campus in Fargo will assure more accurate results with more reliable recommendations.

Collect soil samples from three to six spots in the area to be tested. The depth of each sample for lawns and vegetable gardens should be 6 inches and can be collected using a trowel, shovel or soil probe. If sampling for the purpose of planting a tree or shrub, collect to a depth of 12 inches. When coring below 6 inches, call the One-Call 800 number in your area to avoid contacting or cutting utility cables. If these obstacles are in the intended garden area, consider another site for the garden.

Do not include organic matter such as turf, the thatch layer or leaves with the soil sample. Break up clumps and mix the three to six samples using a clean bucket and send in 1 pint of soil following the lab’s instructions for submitting samples.

Soil testing bags for the NDSU soil testing lab are available from your local Extension agent. Alternatively, a quart sandwich bag can be used.

A separate set of samples should be collected for each location to be tested: backyard, front yard, vegetable garden, etc. For more detailed instructions, consult NDSU Extension’s video titled “How to Take a Soil Sample in Your Garden” (https://tinyurl.com/vxvb9j62).

All fertilizers registered for sale are required to be analyzed for their plant nutrient content and display the results on the bag: for example, 30-5-10. In this instance, the numbers refer to the amount of nitrogen, phosphorus pentoxide and potassium oxide, but simply are expressed as N, P and K.

A 35-pound bag of fertilizer with this analysis would have 30% N (10.5 pounds of N), and 5% and 10% of the other two materials, respectively. Often the single element of nitrogen is sold as Ureaform, with an analysis of 46% nitrogen, and would show up on the bag as 46-0-0.

In North Dakota, the soil often is quite alkaline (high pH), resulting in the tie-up of a particular element, iron (Fe). When the usual fertilization practices don’t green up the lawn to full potential, the problem may be an iron deficiency, characterized by
a yellowing (chlorosis) of the turf.

In that case, look for a fourth number on the bag following potassium to indicate iron content, generally 3% to 5%, and usually combined with sulfur (S) as iron sulfate.

Although Earth has more than 100 elements, only 14 mineral nutrients are required universally for plants to grow and produce fruit or seed. They are listed below with their chemical symbols:

nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), zinc (Zn), copper (Cu), manganese (Mn), boron (B), molybdenum (Mo), iron (Fe), chloride (Cl), nickel (Ni).

A soil sample should be used to guide amendments. Generally, organic gardening practices with the heavy use of plant and animal waste from composting are not in need of any drastic additions. The soil nutrient levels are usually in good balance
for optimal plant growth.

Crop rotation and cover crops are common practices with organic gardeners. As the growing season comes to an end, sow an oat or cereal rye crop when the seasonal vegetables have been removed to help build the soil and mitigate the common home gardener habit of planting the same vegetables in the same spot each year.

Compaction reduces soil porosity, which means air and water have more trouble penetrating and moving through the soil. Air movement and root penetration are restricted in compacted soils. Roots move readily in larger pores but may not have enough strength to part soil particles between smaller pores.

Spots in the yard or garden where water ponds may indicate areas of sodium accumulation. Soils dry hard and, when watered, water tends to pond very quickly. Sodium prevents soil particles from aggregating, instead forming large monoliths with soil particles so tightly bound that roots can explore only the surface. Plants growing in these areas require frequent watering and grow poorly, if at all.

Plants develop better in deep, homogenous soil layers that are similar in texture. A few inches of good topsoil over some not-so-good soil will result in plant roots growing mostly in just the good soil. This restricts the amount of soil used for water and nutrient uptake, which will require more frequent watering and fertilization.

Salty soils are a natural, but undesirable, result of relatively young soils. They are high in nutrient content, have poor drainage and are often in a semiarid climate (Figure 3). Some plants have a higher salt tolerance than others. Most garden plants and many desirable lawn grasses have a low tolerance for salt.

When the water table is high (the water table is a fluctuating zone under the soil where the soil is saturated with groundwater for a long period of time), salts are brought to the surface by capillary action. The water is pulled toward the soil surface and evaporates, leaving the salts behind.

During times of continuous dry weather, any rainfall received tends to drive the salts deeper. In wetter periods, salty areas expand as water tables move closer to the soil surface.

Soil cracking is common in dry soils with high clay content. These cracks accelerate the drying of subsoil and limit soil water-holding capacity during hot summer months.

Crusts may form following a hard rain or intensive irrigation on bare soil. Crusts are most harmful when establishing a lawn from seed or when a small-seeded crop, such as lettuce or carrots, has just been seeded into the garden.

Around a home, soil temperatures can vary as much as 10 degrees at any given time. The north side of a house shades the soil for as much as two spring months more than the south side.

A thermometer can guide a gardener toward plants more adapted to cooler or warmer soil environments.

If the daytime temperature is too hot for some plants, mulching the soil after emergence or planting will help reduce soil temperature fluctuations and produce a cooler soil on average.

If soils are too cool, warming them is difficult. However, you can select plants that tolerate or require cooler temperatures and shade to thrive.

Take a small amount of soil in your palm and wet it slightly. Roll it into a small ball in your palm. Next, try to make a flat ribbon between your thumb and your first two fingers (Figure 1). Push the soil into your fingers with your thumb.

If you can make something that looks like a ribbon about one-quarter inch thick, the soil is at least medium-textured and may be fine-textured. If the ribbon breaks off readily before it reaches ¾ inch in length, the soil is medium-textured.

If you can make a ribbon longer than that, the soil is fine-textured. If the soil doesn’t make a good ribbon and you can feel the sand grains easily, it is coarse-textured.

Thatch is a surface layer of undecomposed organic material in established lawns that can be good for the turfgrass if it is ½ inch or less in thickness. If it is more than ½ inch thick, it becomes restrictive to water, air and nutrient movement into the root zone.

Excess thatch can develop hydrophobic properties that repel water. It also can harbor insects and diseases, and because of the restrictions of air and water movement, it limits rooting.

Thatch can be controlled through regular core aeration; power-raking; adjusting cultural practices, including fertilization, mowing and pesticide use; and selecting nonaggressive turf cultivars. It is not caused by clippings being returned to the surface when mowing.

In certain parts of North Dakota, primarily along the northern tier of counties, the shale bedrock that underlies most of the state is fairly close to the surface. In these areas, glaciation mixed the shale more abundantly in the soil. These shales often contain relatively high levels of cadmium, selenium and arsenic.

If in the course of tillage, small pieces of gray, flat rock (shale) are evident in the soil, the chances are greater than normal that higher levels of these undesirable minerals will find their way into leafy green garden plants.

These elements tend to accumulate in green, leafy vegetables, particularly broccoli, cauliflower, Brussels sprouts, kale and Swiss chard. They are much less concentrated in grain and fruit crops such as sweet corn and tomatoes.

If you are in a region prone to high levels of undesirable minerals, growing leafy crops in raised beds with imported soil would reduce exposure to these minerals.

The water-holding capacity is the amount of plant-available water possible if the soil is wetted to a point called field capacity – approximately one-third of atmosphere suction. This is almost totally dependent on soil texture, but the amount of aggregation and organic matter extends a soil’s capacity, while sodium decreases it.

• Coarse-textured soils – 1 inch per foot in depth
• Medium-textured soils – about 1½ to 2 inches per foot in depth
• Fine-textured soils – 2½ inches per foot in depth

Checklist for Soil Evaluation – Garden Soil

Site location ___________________________________________________

Date ____________ Plant species _______________________________

Site location ___________________________________________________

Date ___________ Turfgrass species _____________________________

Consult the NDSU Soil Testing Lab’s website for more information on taking and submitting a soil sample for testing: www.ndsu.edu/snrs/services/soil_testing_lab/

For more information on understanding soil issues that affect turfgrass:

Zuk, A. and E. McGinnis. 2017. Interpreting the NDSU Soil Test Analysis for Managing Turfgrass. NDSU Extension publication H1824. www.ag.ndsu.edu/publications/lawns-gardens-trees/interpreting-the-ndsu-…

This publication was authored by Ron Smith, retired NDSU Extension horticulturist; and Dave Franzen, Extension soil science specialist.