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Plant analysis – what does it mean for turf growers?
You sent your turf samples off to the lab for analysis of their nutrient content and the results have come back. Now what? How do you interpret the results and decide whether anything needs to be done?
Although the relationship between nutrient content and growth is well known, interpretation of plant analyses for turf species can be very complex. Factors that make interpretation so complex include cultivar or hybrid, nematodes, and environmental factors such as soil moisture, temperature, and light quality and intensity. And reliable data is lacking for a number of turf species, particularly at different stages of growth and for nutrient concentrations near or at toxicity levels.
Several different methods are commonly used to interpret plant analysis data. Single-concentration values have been used to identify nutrient sufficiency, but research shows that ranges better describe the nutrient status of a plant. Another method is based on critical values—the concentration below which growth can drop by 10%. A more useful method is based on sufficiency ranges: the optimum concentration range below which a nutrient is low or deficiency occurs, and above which a nutrient is excessive or toxicity occurs. Sufficiency range is the focus of this article.
Ideally, sufficiency ranges are developed by plotting yield or plant growth against nutrient concentration. They can also be based on survey data from large populations of normal-appearing plants and establishing the upper and lower boundaries of sufficiency from the population mean plus or minus one standard deviation. Figure 1 illustrates the theoretical relationship between growth and nutrient concentration in plants.
Figure 1. The relationship between nutrient concentration in plants and yield (plant growth).
The curve typifies one for macronutrients. The curve for micronutrients tends to be much steeper through the deficiency/marginal zone. This difference illustrates the importance of accurate analyses and interpretations for micronutrients, because at the low end of the adequate zone (sufficiency range), the difference in nutrient concentration between adequate and deficient is small. Fortunately, with most turf species, micronutrient deficiencies are not common.
A defined sufficiency range may not apply to all situations or environments. Plant nutrient concentrations are not absolute with respect to sufficiency or deficiency, because several factors can affect the nutrient concentrations in plant tissues: nutrient uptake and internal mobility, nutrient ratios and dry-matter changes. Consequently, nutrient concentrations are not static: they change during the growing season in response to environmental and management conditions.
Differences between plant growth and nutrient absorption along with movement of nutrients around the plant cause nutrient concentration and dilution. Under normal growing conditions, nutrient absorption and plant growth closely parallel each other during most of the vegetative growth period. However, if the normal rate of growth is interrupted, nutrient accumulation (higher values than expected) or dilution (lower values than expected) can occur.
Various factors can result in nutrient accumulation:
- Extremes in temperature.
- Heat or moisture stress.
- Stress due to traffic or cultural practices.
- Stunting due to a soil deficiency of a particular nutrient or to nematode infestations.
Nematodes can produce nutrient deficiencies similar to those resulting from low soil nutrient levels. When elements such as calcium and phosphorus are deficient in the plant tissue, but soil pH and soil phosphorus and calcium are adequate, this is a good indication that nematodes are the likely cause of the problem.
Various factors can result in dilution, too, by stimulating rapid growth:
- Highly favourable weather.
- Rapid growth response to nitrogen applications.
Numerous factors can affect a plant analysis result. Therefore, it is important to supply historical information when submitting samples to the lab. Likewise, it is essential to have an expert practitioner interpret plant analysis results.
It is a good policy to maintain a record of soil tests and plant and water analyses and refer to them each time you prepare a lime and fertiliser program. Look for upward or downward trends in soil pH and nutrient levels in both soil and plants. With this information, visual observations of the turf and knowledge of field conditions, you can adjust lime and fertiliser applications to maintain the nutrient content of the soil and turf within the sufficiency range for the majority of elements tested.
The following table is provided as a guide for interpreting plant analysis results for some turf species. The values have been taken from various published sources.
| Creeping bent | Tifgreen couch | Perennial rye | Emerald zoysia | |||||||||
| Low | Sufficient | High | Low | Sufficient | High | Low | Sufficient | High | Low | Sufficient | High | |
| Macronutrients (%) | ||||||||||||
| Nitrogen (N) | <4.00 | 4.00–5.00 | >5.00 | <3.00 | 3.00–4.30 | >4.30 | 3.34–5.10 | 2.04–2.36 | ||||
| Phosphorus (P) | <0.30 | 0.30–0.60 | >0.60 | <0.20 | 0.20–0.40 | >0.40 | 0.35–0.55 | 0.19–0.22 | ||||
| Potassium (K) | <2.20 | 2.20–3.50 | >3.50 | <1.60 | 1.60–2.25 | >2.25 | 2.00–3.42 | 1.05–1.27 | ||||
| Calcium (Ca) | <0.25 | 0.25–0.75 | >0.75 | <0.25 | 0.25–0.50 | >0.50 | 0.25–0.51 | 0.44–0.56 | ||||
| Magnesium (Mg) | <0.20 | 0.20–0.40 | >0.40 | <0.15 | 0.15–0.30 | >0.30 | 0.16–0.32 | 0.13–0.15 | ||||
| Sulphur (S) | <0.25 | 0.25–0.75 | >0.75 | <0.15 | 0.15–0.65 | >0.65 | 0.27–0.56 | 0.32–0.37 | ||||
| Micronutrients (mg/kg) | ||||||||||||
| Boron (B) | <3 | 3–20 | >20 | <5 | 5–60 | >60 | 5–17 | 6–11 | ||||
| Copper (Cu) | <5 | 5–15 | >15 | <5 | 5–20 | >20 | 6–38 | 2–4 | ||||
| Iron (Fe) | <50 | 50–300 | >300 | <50 | 50–500 | >500 | 50–500 | 188–318 | ||||
| Manganese (Mn) | <25 | 25–300 | >300 | <20 | 20–300 | >300 | 30–250 | 25–34 | ||||
| Zinc (Zn) | <20 | 20–70 | >70 | <15 | 15–200 | >200 | 14–64 | 36–55 | ||||
Because of the numerous ways in which different turf species are utilized and managed, it is impossible to formulate specific recommendations for corrective treatments when a nutrient level falls outside the sufficiency range. Good recommendations depend on:
- the integration of soil, plant and water analysis results
- historical information relative to treatments
- visual observations.
Water quality analysis for irrigation suitability is becoming increasingly important as part of the overall nutrient assessment for turf in many applications. This is because more turf managers are irrigating with non-potable water, which can contain various nutrients and salts at various levels.
The nutrients in the table above are discussed below:
Nitrogen (N)
N is the nutrient most commonly found to be low in turf. Low levels are generally due to inadequate fertilisation, heavy leaching rain, over-irrigation or root damage. In golf greens, low N may be due to management practices implemented to obtain the desired putting surface characteristics. N deficiency may be manifested as a light green colour, slow growth or excessive seed head production. If a deficiency is detected, base your N applications on soil test recommendations, being sure to split applications or use a controlled-release form of N where leaching may be a problem.
Phosphorus (P)
Deficiency is usually due to low soil P, cool-wet growing conditions, nematodes or excessively low soil pH. If deficiency is detected, apply P based on soil test recommendations. High levels of P generally pose more problems for intensively managed turf than do deficiencies. Excessive P levels in the leaves can cause deficiencies of other nutrients, particularly Fe. High P-to-K ratios in leaf tissue increases winterkill in couch. When high P is detected, omit P from the fertilisation program until it is within acceptable limits. In most instances, 3 or more years may be required. When low P is detected in the tissue but soil pH and soil test P are adequate, check for nematodes.
Potassium (K)
Low tissue K is generally due to low soil K levels, inadequate K fertilisation or coarse-textured sandy soil that is subject to leaching. Low K may also be associated with low N fertilisation. When soil K is adequate, N fertilisation increases the uptake of K. When low tissue K is detected, apply potash and N based on soil test recommendations. When tissue K drops below 1.0%, deficiency symptoms appear and are characterized by spindly growth (narrow leaves, thin turf), leaf tip burn, reduced tolerance to wear, cold and disease, and reduced growth rate. Excessive K levels may induce Mg deficiency and suppress the uptake of Ca and Mn. If high tissue K levels are detected, reduce the K fertilisation or omit K from the program until K is within the sufficiency range.
Calcium (Ca)
Grasses are able to take up Ca under a wide range of soil conditions, so Ca is rarely deficient. If low levels are detected, check for low soil pH and correct if required. If soil pH and soil test Ca levels are adequate, check for nematodes. A high Ca level may indicate some other nutrient deficiency or disorder.
Magnesium (Mg)
Low levels may occur on sandy soils; on soils with low pH and low Mg; where high rates of NH4-N and K fertilisers have been applied; and where clippings are continuously removed. If low levels are detected, include Mg in the fertilisation program. If soil pH is low and pH adjustment is required, apply dolomite (rather than lime) according to soil test recommendations. Excessively high tissue Mg is not common.
Sulphur (S)
Low S may occur on sandy soils low in organic matter where S-free fertilisers have been used; following extensive periods of heavy rainfall where grass has been over-irrigated; and where high rates of N have been applied. The N-to-S ratio is as important as the S level and should not exceed 18:1. Ideally it should be approximately 14:1 for optimum growth and turf quality. If S is low or the ratio exceeds 20:1, include S in the fertilisation program. S may be supplied as gypsum, elemental sulphur or S-based fertilisers (e.g. superphosphate, sulphate of ammonia).
Boron (B)
Deficiency is very rare; however, toxicity is possible with some sources of irrigation water, particularly in coastal areas. The B content of irrigation water should be less than 0.5 mg/L to guard against the possible development of B toxicity in soil.
Copper (Cu)
Deficiency is not likely to occur except on organic soils or sandy soils with a pH above 7.0. Excessive Cu levels can result from application of soil amendments containing high concentrations of Cu or repeated applications of Cu-based fungicides.
Iron (Fe)
Fe determinations are invalid unless samples are properly washed to remove soil from the plants. Generally if Fe and aluminium levels are both high, it is due to contamination rather than inherent levels in the plant tissue. Fe deficiency can occur on high pH soils (pH > 7.0), during periods with cool-moist growing conditions, and where soil P levels are excessively high. Fe deficiency is best controlled by applying a foliar spray as Fe sulphate or Fe chelates, repeated as necessary to prevent recurrence of the deficiency. Do not apply foliar spray to grasses in the heat of the day. Soil applications of Fe materials are not recommended for correcting Fe deficiencies.
Manganese (Mn)
Deficiencies are rare but may occur occasionally on sandy soils that are low in Mn, high in organic matter and with a soil pH above 7.0. Frequent use of foliar Fe may contribute to deficiencies by suppressing Mn uptake. Turf grown on areas receiving high sodium inputs may also be more susceptible to Mn deficiency. Mn deficiencies can be corrected by applying a foliar spray of Mn sulphate or Mn chelates. Repeated applications will be required to prevent recurrence of the deficiency. Excessive Mn levels can occur when the soil pH is less than 5.5 or where soils are consistently over-watered. High Mn levels may be corrected by liming, proper irrigation practices and improving drainage.
Zinc (Zn)
Deficiencies are not common unless the soil is alkaline. In some cases, low Zn levels will be detected in grass grown on soils that are excessively high in P or that are compacted and waterlogged. Deficiency symptoms do not show up unless the Zn content is less than 10 mg/kg. Zn deficiencies can be corrected with foliar spray of Zn sulphate or Zn chelates. High Zn levels may occur when soil amendments containing Zn have been applied over several years. Most turf species can tolerate higher Zn concentrations than most agronomic crops.