October 1995 - Volume II, Issue 10

Soil Characteristics and How They Affect Soil Moisture

David M. Kopec, Extension Turfgrass Specialist

The soil portion of a turf system represents a large fraction of the biological and physical activities necessary for turfgrass growth. It serves as a growth medium, and a source of nutrients and water. The interaction nature of soils and water together is the focus of this presentation. Basically the soil particle size, the soil particle size distribution, and the structure of the soil determines the moisture characteristics (soil water relationships) a particular turfgrass soil will have. Soil particles are basically composed of sands, silt, clays and organic matter. Sands include particle sizes which range from 0.05 mm to 2.0 mm in size. This is a very large range of particle sizes! Silt particles range in size from 0.05 mm to 0.002 mm, and clay particles are those particles less than 0.002 mm in size.

Soil textural fractions

Soil Diameter Surface area in
Separate mm 1 gram (cm2)
Very Coarse sand 2.00-1.00 11
Coarse sand 1.00-0.50 23
Medium sand 0.50-0.25 45
Fine sand 0.25-0.10 91
Very fine sand 0.10-0.05 227
Silt 0.05-0.002 454
Clay below 0.002 8,000,000

Generally speaking, the larger the soil particle size, the better the drainage will be. This is why golf greens and sports fields are ultimately designed with sand based soils. On the other hand, very fine particles (clay and silt) tend to drain poorly, but hold on to more water in the soil system. The distribution of soil particles (size and relative amounts) is used to determine the 'soil textural class'. There are basically twelve soil classes based on texture. From knowledge about the textural class of soil, information can be inferred about it's infiltration, water holding capacity, and how much water the turf can actually use.

This refers to the amount of water that passes through the soil surface in terms of depth (inches) in a given time period (usually 1 hour). Lighter textured soils (such as sands or sandy loams) have desirable infiltration rates. This is important in the fact that the turf can have water penetrate into the soil relatively quickly so no runoff or puddling occurs. 'Heavy' textured soils which have a lot of clay and/or silt often have poor water penetration (infiltration), because the space that the soil occupies is relatively dense. Organic matter helps infiltration because of the soil aggregation that occurs with organic matter (makes for larger soil particles)! The addition of turfgrass to the turf-system generally increases infiltration rates. For essentially all soil-types, the initial intake of water is greatest at the beginning of an irrigation or rainfall (when the surface is driest). After the first 1/2" becomes wet, the infiltrate rate will slow down.

[Practical Implications of Infiltration Rates of Soils on Irrigation]
(1) All dry soils will take up water rapidly at first, then slow down in water uptake.
(2) Soils which have slow infiltration rates need to be irrigated more frequently in order to meet the water requirements of the turfgrass.
(3) Soils which have slow infiltration rates should receive the desired amount of irrigation by applying the total amount in "shifts" or cycles so that the water can soak in.

Representative infiltration rates for different soil textures

Infiltration rate
Soil texture inches/hr.

Sand-coarse 1.00-8.00
Sand-very fine 0.50-3.10
Sandy loam 0.40-2.60
Loam 0.08-1.00
Clay loam 0.04-0.60
Clay 0.01-0.10

Depth of Water Penetration
The soil-type has a lot to do with how deep water will penetrate into the soil. Soils with large particle sizes (sands/sandy loams), or soils which have good aggregation (form larger soil particles by joining smaller soil units together) will have water penetrate the soil deeper than soils which have large amounts of smaller particles (clays, silts). On a typical "medium-sand" soil, an inch of water may penetrate to a depth of 12 inches. A 'clay' soil may have the same inch of water penetrate to only 4.0 inches. A 'loam' soil would be intermediate. Why? Look at you soil triangle for the answer. You will see that the loam contains about 60% sand, and up to about 30% silt, and 15% clay. The depth of water penetration is also dependent on how wet the soil is at the time of irrigation/rainfall. The wetter the soil, the deeper the irrigation/rainfall water will go.

[What are the implications of knowing how deep water will penetrate a particular soil type]
(1) If you sample for root depth, you can estimate the maximum irrigation amount for that soil type. so that water does not go way past the roots.
(2) Sandy or light textured soils have the greatest depth of penetration, 'heavy' textured soils have the most shallow depth of penetration.
(3) Applying a large amount of good quality water in a single irrigation on a sandy soil can be very wasteful.

Amount of Water Available to the Turf
The water holding capacity and the amount of water that the turf can actually use is also a function of the soil particles and soil texture classification. Fine textured soils as you now know, have small particle sizes, but actually have a large surface area around them. The surfacearea of the soil is what retains the moisture in a particular soil! Think of a 55 gallon drum filled with golf balls, and another 55 drum filled with seven or eight basketballs. Each drum has many holes in the bottom so water can drain out the bottom. If you poured 25 gallons of water into each drum, less water would come out of the bottom drains of which drum? Well, this would occur for the drum filled with the golf balls. The smaller sized particles (golf balls) kept more water around the balls, compared to that of the basketballs! Therefore, smaller sized soil particles will retain more water. This is also why water penetrates deepest in a coarse (basketball) textured soil.

The final consideration is that not all of the actual water held by the soil particles is available for the turfgrass roots. When the soil particles are very small (clays) the water can be held very tightly to surface areas around each clay particle. The smaller the particle(s), the less water there will be available for the plant roots to take up. This is true, even though the soil itself can hold a lot of water (see silt loam vs. clay loams below).

Total, plant available, and unavailable water-holding capacity of different soil texture classes.

Water-holding capacity
(inches per foot of soil)

Soil Texture Total Available Unavailable

Sand 0.6-1.8 0.4-1.0 0.2-0.8
Sandy loam 1.8-2.7 0.9-1.3 0.9-1.4
Loam 2.7-4.0 1.3-2.0 1.4-2.0
Silt loam 4.0-4.7 2.0-2.3 2.0-2.4
Clay loam 4.2-4.9 1.8-2.1 2.4-2.7
Clay 4.5-5.0 1.8-1.9 2.7-3.0

Soil moisture capacity and plant available water is usually expressed as how many inches of water there is in a twelve inch deep soil. Based on the water holding capacity table, which soil texture class has the greatest amount of water in inches per foot of soil? [clay] However, which soil texture class has the greatest amount of plant available water that the turf roots can extract? [silt loam]

[Practical implications of soil water holding capacity and amount of water available to the turfgrass.]
(1) Sandy soils need to be irrigated frequently, but with lesser amounts of water per irrigation event due to fact that sandy (basketball) soils do not store a lot of water.
(2) Sandy soils have about 50% of the soil water available to the turf.
(3) Turfgrasses grown on 'medium' textured soils (sandy loams/loams) have the most plant available water, even though they hold less total water in a foot of soil than clays. Therefore, turfgrass irrigation schedules on these soil types can go the longest between irrigations, and they can receive the greatest amounts of water per irrigation (other things equal). This is because water will not go excessively deep (as in a sand), and the plant available water amount is high. These soils also have about 50% of the soil water present available for root uptake.
(4) Heavy textured soils (clay loams, clay) hold the greatest amount of water (if it is capable of absorbing it). However, only about 30-35% of the total water in the soil is available. The finer the soil particles, the more resistance there is for the particle to 'release' the water to the turfgrass root. Heavy textured soils may 'feel' moist when sampled, or pass the 'screw driver test', but they may need the same amount of irrigation water applied as that of medium textured soils. Since these soils are slow to absorb water (slow infiltration) they need to be irrigated frequently with lesser amounts of water per irrigation. This particular consideration paints an irrigation picture similar to that of coarse sand, but for different reasons !!!

[Maximizing soil water uptake and use with turfgrass management techniques.]
(1) Almost all trafficked turfgrass surfaces get compacted. It is worse on heavy textured soils (high in silt/clay contents). Aerification, grooving and slicing are highly beneficial when done at the right time of the year. Deep-tine aerification is very beneficial. It is not necessary to refill the holes in aerifyed turf in areas other than golf course tees and greens.
(2) Do not make soil layers by surface amending soils. Sand topdressing on a loam, clay, or silty soil will eventually cause problems. Poor aerification and a man-made drainage barrier will result.
(3) Topical applications of organic matter will improve percolation through enhance soil structure formation. Light applications should be practiced.
(4) A drainage barrier or poor drainage within the soil profile is undesirable and sometimes detrimental. A barrier increases the water content between the soil particles at the expense of oxygen. Roots need oxygen to function properly. Only high profile areas have 'man--made' provisions for drainage (use of tile drains-sand rootzones, etc.). Deep tine aerification is the practical way to best manage drainage problems. 

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