Soil & Rootzone Science For Sports Turf

Gypsum corrects sodium build-up or low calcium where there is a breakdown in soil structure, infiltration loss, surface instability or dispersion shows in the rootzone. It supplies soluble calcium with no increase in soil pH, displaces sodium from cation exchange sites, and restores aggregation and permeability.

It works best where soil tests or irrigation water confirm sodium is high relative to calcium and magnesium. This is common with recycled or saline water, and on sand-based sports fields and golf courses where the drainage drives surface performance.

Don’t apply it blindly. How effective gypsum is depends on SAR, ESP, drainage capacity, and whether the leaching fraction is enough to flush displaced sodium out of the rootzone. Without drainage, gypsum mobilises sodium but doesn’t remove it.

You calculate the use rate from soil test data (exchangeable Na, Ca, Mg) and water analysis (SAR, residual sodium carbonate). Fine-particle gypsum outperforms agricultural grade on established turf.

Organic matter build-up in the top 0 to 40 mm of a golf green is the single most common cause of poor drainage, surface softness and uneven moisture. As organic matter builds up, pore spaces shrink, capillary water retention rises, and oxygen diffusion drops. This results in shorter roots. If this build-up continues, a layer forms at the organic matter to sand interface, and that’s where a perched water table develops. Under irrigation or rainfall, this layer holds moisture, softens the surface, and creates conditions that favour root pathogens such as Pythium spp.

Sand-based USGA greens are particularly sensitive. Their physical properties (infiltration rate, air-filled porosity, capillary porosity) are calibrated to a specific particle size distribution. Above about 4% organic matter by weight in the top 25 mm, these properties start to break down. Proper management is based on dilution: coring, hollow-tine aeration, deep scarification and regular sand topdressing. Annual sand inputs of 1.0 to 1.5 m³/100m² are typical on intensively managed greens. LOI testing by depth should set the intensity, not the calendar.

Where organic matter accumulation drives water repellency, the interaction between thatch breakdown and the build-up of hydrophobic compounds becomes the main cause of localised dry spot.

Biological amendments can improve turf performance in some cases. However, the results are inconsistent, and they depend heavily on the rootzone. For example, products with mycorrhizae, plant growth-promoting rhizobacteria (PGPR), Trichoderma spp., humic substances and compost-derived stimulants are sold to boost root growth, nutrient cycling and stress tolerance.

In the field, though, the response varies. It shifts with the rootzone, with phosphorus availability (because high P suppresses mycorrhizae), with fungicide use, and with organic matter, irrigation and soil temperature. Moreover, in intensively managed turf, the limiting factor is usually physical or chemical rather than biological. On the physical side, that means compaction, layering or lost drainage. On the chemical side, it means sodium build-up, salinity or pH extremes. In other words, the size of the microbial population is rarely the problem.

Mycorrhizae show this clearly. As a rule, they establish poorly on mature, fertilised turf, since native populations already exist and high P inputs suppress the symbiosis. By contrast, they are far more likely to deliver a measurable benefit on new constructions, sterile sand profiles or low-input systems.

Therefore, you should evaluate biological products alongside physical and chemical management, not as a substitute for it. After all, where rootzone physics is the limit, no biological input will compensate. Ultimately, the only reliable test is a site trial: use control plots, and measure clear endpoints.

MLSN (Minimum Levels for Sustainable Nutrition) is a soil test interpretation framework developed by Woods, Stowell and Gelernter at PACE Turf and the Asian Turfgrass Center for intensively managed turfgrass. It identifies the minimum soil nutrient levels at which turf performance holds up, and then it bases recommendations on projected plant uptake plus a buffer that keeps soil reserves above that minimum.

In practical terms, the MLSN minimum guidelines (Mehlich-3 extraction) sit at roughly: phosphorus 21 ppm, potassium 37 ppm, calcium 331 ppm, magnesium 47 ppm, and sulphur 7 ppm. Importantly, these figures came from analysis of thousands of soil samples taken from healthy turf, which in turn pinpointed the limits where performance still stayed acceptable.

By contrast, Sufficiency Level of Available Nutrients (SLAN) interpretations call for nutrient additions to reach a target sufficiency range. However, this target often sits well above the level at which turf actually responds. As a result, on sand-based rootzones with little capacity to hold nutrients, SLAN can push overuse and lift leaching losses.

For that reason, MLSN is most relevant on sand-based golf greens and high-performance turf, where precision matters, leaching is real, and input cost is significant. Today, the framework is widely adopted across golf course agronomy and is also applied to other intensively managed turf systems. Even so, it does not replace soil testing; rather, it simply changes how the results are read and turned into recommendations.

For MLSN-aligned fertiliser products calibrated to sand-based rootzones, see the Gilba Solutions range.

Cation exchange capacity (CEC) measures how well a soil holds positively charged ions (calcium, magnesium, potassium, sodium and ammonium) on the negatively charged surfaces of clay and organic matter. In terms of units, you record the CEC in centimoles of charge per kg (cmolc/kg) or milliequivalents per 100 grams (meq/100g), and the two are identical.

Sand-based rootzones tend to sit below 5 cmolc/kg, and sometimes below 2. In contrast, loam and clay soils run from 10 to 40. When the CEC is low, the soil holds few nutrients, so any fertiliser that you apply is more likely to leach out of the rootzone. On top of that, the soil also resists pH change less well. For these reasons, sand-based greens need more frequent, lower-rate fertiliser applications, along with closer attention to pH, than push-up soil greens.

Base saturation, meanwhile, is the share of CEC taken up by the base cations (Ca, Mg, K, Na), and it is sometimes used to set fertiliser ratios. For instance, the Basic Cation Saturation Ratio (BCSR) approach targets specific proportions, such as 65% Ca and 10% Mg. However, research to support BCSR in turf is thin, and in practice it often leads to over-fertilising. By contrast, MLSN-based interpretation works from absolute nutrient levels rather than ratios, which makes it more defensible.

Finally, the most useful base-saturation number in turf is exchangeable sodium percentage (ESP), which is the share of CEC taken up by sodium. Above 15%, conditions are sodic, and the risks include damage to soil structure and reduced permeability.

Hydrophobicity, also called water repellency or localised dry spot, develops when waxy compounds coat sand particles and stop water from moving into the rootzone. As a result, you get patchy, irregular dry areas that resist both irrigation and rainfall, even when the ground next to them is waterlogged.

The main cause is organic compounds. These come from microbial decomposition, root exudates and thatch breakdown, and chemically they are long-chain fatty acids and aliphatic compounds. In addition, fungi play a major role. In particular, basidiomycetes in the order Agaricales, the fairy-ring fungi, produce waxy by-products that bind tightly to sand particles.

Several factors raise the risk:

• Sand-dominant rootzones, because their large pores dry out quickly.
• Warm temperatures, which drive microbial activity.
• Drought stress.
• High organic matter in the upper profile.

In practice, sand particles dry from the surface inward. This then exposes the waxy coatings, and once dry, those surfaces resist rewetting.

To manage the problem, three things work together:

• Surfactant programs, using soil wetting agents based on block copolymers, alkyl polyglucosides or related chemistries.
• Cultural practices that keep organic matter in check.
• Irrigation strategies that hold surface moisture without overwatering.

Curative wetting-agent applications can recover affected areas. However, preventive programs run through the season are more reliable. To quantify severity, meanwhile, use either the molarity of ethanol droplet (MED) test or the water drop penetration time (WDPT) test. Both measure how strongly a surface resists wetting. In turn, that result tells you how aggressive the program needs to be.

Compaction reduces total pore space, and especially air-filled pores. Heavy traffic squeezes soil aggregates and pores together, and as a result, you get slow water infiltration, short roots, low oxygen, weak growth and lower wear tolerance. On sports fields and high-traffic areas of golf courses, you can’t avoid compaction. Therefore, it has to be actively managed.

Soil core from a new green with a good agronomic program.

For diagnosis, combine penetrometer readings, infiltration rate testing (with an infiltrometer), bulk density samples, and a visual check of root depth and rootzone colour. As a guide, bulk density above 1.4 g/cm³ in sandy soils, or above 1.6 g/cm³ in loams, points to compaction that’s holding back root growth.

Mechanical aeration uses several methods:

• Hollow-tine coring, which removes cores so topdressing can backfill the holes
• Solid-tine aeration, which fractures the soil without removing a core
• Deep tine aeration at 200 to 300 mm depth, which targets deep seated compaction

How often you aerate depends on traffic. For instance, golf greens are often aerated two to four times a year. In contrast, sports fields are aerated after each season, and in the season when access allows.

Ultimately, prevention beats recovery. To help prevent this, traffic management (rotate practice areas, move goals, restrict access when the surface is wet), correct mowing, and encouraging deep roots all reduce how easily a surface compacts. Notably, sand-based rootzones resist compaction better than soil-based rootzones, which is the main reason high-performance surfaces are built on sand profiles.

For more detail on relief programs, see the science behind soil aeration.