Back to basics: understanding soils, soil moisture, and soil tests
By Brad Jakubowski
Webster’s dictionary defines soil as the upper layer of earth that may be dug or plowed and a medium in which something takes hold and develops. As sports turf managers, that holds true for us. We dig in it, plow it, plant in it, roll it, and expect the soil to serve as a medium to exchange water, air, and nutrients for turfgrass, all to provide the best playing surfaces possible.
The type and quality of the soil influences everything we do as turfgrass managers. It can dictate which species of turfgrass we grow and maintain, how often and how much we fertilize and irrigate, and influence how we modify the soil through our various cultivation techniques.
Although there are days the soil appears to be one large “hunk of stuff” as we attempt to dig through it with a shovel, it is essentially one-half mineral and organic matter and another one-half pore space. Maintaining a balance between the two becomes crucial to the performance of the turf. The solid materials provide stability and a storehouse for nutrients, while the pore spaces provide air for root health and places for water to percolate and be stored.
Years ago I had a discussion with Mike Andresen, facilities & grounds director at Iowa State University. He stressed how important it was for him to maintain vertical air and water movement as a key to maximizing the performance of his turf. This is so true. Our key to success is to develop and maintain positive air and water movement throughout our soils. This includes water moving in all directions; up, down, and laterally throughout the soil. With native soil fields this may be a difficult task, however, the key is maximizing the potential each soil has.
Knowing your soil’s texture (the relative percentages of sand, silt and clay) becomes very important. How water moves throughout a clay textured soil will differ considerably from a sandy loam soil.
It is often stressed to water your turf deeply and infrequently, which may be interpreted as applying 1 inch of water or more per application, waiting a number of days and then doing it again. With most of our soils, this may not be possible. Interestingly, you may irrigate a clay soil and a sandy soil similarly, but for different reasons. As you irrigate a clay soil, the pores fill with water and seal the surface, allowing no more water to infiltrate and runoff like concrete. As you apply water to a sandy soil, a larger volume of water applied may percolate through and beyond the rootzone and not benefit the plant. So, in each case, you may be required to apply smaller amounts in repeat applications. If your goal was to apply 1 inch of water, you may be required to apply three 1/3 inch applications of water sequentially to maximize your irrigation efforts.
Soil texture also influences fertilizer application amounts and timings. Heavier soils like clays will have a higher cation exchange capacity (CEC) or greater capacity to hold onto nutrients. This means that you can apply larger amounts of fertilizers per application (1 lb of nutrient per 1000 sq ft or greater) without a higher risk of leaching nutrients beyond the plants rootzone as you would with lighter, sand-based soils. Lighter soils also have a higher degree of natural aeration because they have a larger percentage of macropores than heavier soils. Because of this, heavier soils require more cultivation events like slicing or aerification to maintain what macropores they have throughout a sporting season.
Soil structure is the arrangement of soil particles into clusters known as aggregates. Aggregates are formed in soils by mechanical means such as freezing and thawing, wetting and drying, or by the penetration of roots. Soil aggregates also form when microbes and plants release exudates and essentially “glue” the particles together. These glued particles have the ability to resist deformation better than particles that are simply pressed together and can help maintain valuable macropore space within the rootzone. Because of this, organic matter levels in native-soil fields become very important.
One of the best methods to make heavy soils more manageable can be to incorporate forms of organic matter, such as compost. The least disruptive method would be to topdress compost in concert with aerification events. Maintaining or improving soil structure will, in turn, aid in positive air and water movement.
Soils tests should be used as a regular tool just as any others in your toolbox. In addition to helping you diagnose turf problems, they will provide you with guidance on how to manage your fields over the long-term. It is especially important to test your soils if the fields you are managing are new to you. One of the first sections to review on your soil test would be the CEC or Cation Exchange Capacity section. In addition to telling you about your soils ability to hold nutrients, CEC (listed in meq/100g) will help describe your soil’s texture (figure 3). Remember, this influences irrigation, fertilization and cultivation practices.
Another important section would be soil pH. Sports turf species prefer a slightly acidic pH (6.0 to 6.5) because overall nutrient availability is at a maximum within this range. In most of the eastern US, the soils are acidic, while Midwest and Western soils tend to be neutral to alkaline. If your soils test pH results are far more acidic or alkaline than the preferred range you may be required to amend your soil with lime if the soil is too acidic or with sulfur if too alkaline.
Knowing your CEC and pH will give you an idea of the buffering capacity of a soil. Buffering capacity is essentially the resistance a soil will have to a change in pH. The higher the CEC the more resistant the soil will be to a pH change. A high soil buffering capacity may require multiple applications of an amendment or you may need to adopt a long-term management plan which includes amending soils on an annual basis.
Additional sections to review are the Soluble Salts and Sodium (Na). If either of these are in the high range this may indicate potential drainage problems in the areas tested. Poorly draining fields result in water sitting at or near the surface which can be “wicked” up through the soil and lost to the atmosphere via evaporation. When this happens, any dissolved materials in the soil water are deposited and concentrated on the soil surface. You may see white deposits developing in bare areas. If you have a sodium problem you will most likely have a high pH result of 8.5 or greater. If this occurs, establishing drainage by some method, including deep tine aerification or installing a drain tile system, may be required. Correcting sodium problems also requires amending your soil with gypsum to remove excess sodium from your soil system. Whenever you are in the process of amending soils, monitoring the soil status through soil testing will be very important. Once you start the amendment process, annual or semi-annual soil retesting will help you determine if your amendments are helping you reach your objective.
Once you get through those initial sections on your soil test, you can evaluate your organic matter levels and relative nutrient concentrations. The best thing to do with these results is to use them as your baseline, especially if your turfgrass appears fairly healthy. If you later experience problems with specific areas, you can make more informed choices as to what nutrients your soils may be lacking. If you are unsure of making an application throughout your facility, you can always do a “plywood” test. Apply a particular nutrient or active ingredient to a relatively small section of your turf, but have a piece of plywood covering a portion of the treated area. If you see a definite difference in the two areas, you know it would be a worthwhile application and expense. It is important to remember that any steps you can take to improve your soil characteristics, however insignificant, can result in significant differences with your turfgrass and your fields.
Brad Jakubowski is an adjunct professor for environmental sciences at Doane College in Lincoln, NE.