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Recommendations

Recommendations
C. Owen Plank
Associate Professor, University of Georgia

The last phase of a soil test program is the development of a recommendation.  This can be a complex process because the recommendation phase encompasses the interpretation phase as well as incorporating research results and adjustments for different climatic conditions and production practices. This is the phase that most turfgrass managers are most interested because when they submit a soil sample to a laboratory they want to determine: what do I need to apply; when do I need to apply it; and how much do I need to apply.

 Depending upon soil and growing conditions soil test recommendations can be very comprehensive.  The magnitude of the recommendations given depends largely upon the needs of the turf manager and the laboratory performing the soil test. For most turfgrass managers, recommendations for limestone, nitrogen, phosphorus, potassium, and magnesium will suffice.  However, certain situations may dictate that additional tests be conducted.  For example, they may include analyses for micronutrients, total soluble salts, or sodium (Na) for assessing the sodium saturation percentage. The attached figures illustrate two soil test reports generated by state and private laboratories.

Turfgrass managers sometimes encounter discrepancies in recommendations between laboratories.  However, this is not surprising since differences exist relative to the manner in which individuals may interpret data. In addition, and probably the area of greatest disparity in laboratory recommendations are differences in philosophies used to interpret the results and formulate the recommendation.  As noted earlier, some laboratories use the basic cation saturation ratio (BCSR) approach to interpret exchangeable base-forming cations, many use the sufficiency level of available nutrient (SLAN) concept, and some use a combination of the two. There are also differences in philosophies as to what liming and fertilization should accomplish.  For example, suppose the soil test data show that soil phosphorus (P) is high.  Some laboratories will not recommend phosphorus fertilizers at a high-test level, whereas some laboratories will recommend a maintenance application. This is why it is  important for turfgrass managers to be well trained in soil fertility, and plant nutrition.  The ultimate decision as to what will be applied to the turf area rests with the turf manager.  He will need to incorporate his knowledge relative to local soil conditions, climatic conditions, how the turfgrass responded to previous nutrient applications and potential environmental hazards into his final decision.  In addition he must consider the conditions for which the turf is to be used.  Fertilizer application rates for golf courses and recreational sites are often higher than for general-purpose grounds or home lawn areas due to higher-quality expectations and growth needed to recover from traffic.


Nitrogen Recommendations

Nitrogen recommendation are developed in a slightly different manner than recommendations for other nutrients.  Whereas, P and K recommendations are based in part on the amount of each nutrient extracted from a soil, N recommendations are not.  Because of the many factors that influence the amount of N present in a soil at any given time, soil test laboratories do not test samples for N for the purpose of making recommendations.  Nitrogen recommendations are developed through a series of field experiments that are conducted over several years in which variable rates of nitrogen rates are evaluated relative to selected turf properties.  Some laboratories will then modify the recommendation based on the amount of organic matter in the soil.  Therefore, N recommendations for turf can vary considerably depending upon how it is to be used, the rate of growth and color desired, and the geographical region in which it is being grown.


Phosphorus (P) Recommendations

Phosphorus recommendations for turf are based on soil test P levels, the intended use and management practices that will be used. For most turf, phosphorus fertilizers are recommended for very low, low and medium testing soils.  However, very few laboratories in the southeast recommend phosphorus fertilizers for maintenance purposes on soils with high or very high soil test P levels.  The laboratories that do recommend phosphorus at high P test levels recommend grades with low amounts of P2O5 relative to the amount of N or K2O. Examples would be 25-2-5, 32-5-10, 24-5-11 and 31-3-4. Research has shown that phosphorus does not build up in high P testing soils when these grades or similar ones are used to supply the recommended amount of nitrogen.  Several laboratories recommend phosphorus applications on high testing P soils for turf establishment.  This is due in part to the critical role that P plays in promoting early root and vegetative development during the initial phases of turf establishment (Wood and Duble, 1976; Juska et al., 1965).

Except for very coarse-texture acid soils P does not leach through soils in a manner similar to N or K.  However, in fine-textured soils phosphorus becomes bound to different soil components and with continued indiscriminate applications can build up to excessively high levels.  In some cases it may become a potential environmental hazard.  As developments encroach upon tributaries and other environmentally sensitive areas, turf managers will need to rely much more on soil testing in the future to guide fertilization practices than they have in the past.


Potassium (K) Recommendations

Potassium recommendations are based on K soil test levels and the management practices that will be followed in maintaining the turf.  Potassium is generally recommended on very low, low, medium, and high testing soils.  However, very few laboratories recommend potassium fertilization on very high testing soils.  In some cases recommendations are modified for sandy soils to allow for the higher rate of leaching compared to finer textured soils.  Consequently, it is generally recommended that the annual rate of K2O be divided into split applications to help prevent K from leaching below the root zone and preventing potential K deficiencies.  The K2O applications can be made in combination with the N applications not to exceed 1.5 lb. K2O per 1000 ft2 per application.  The rates may be adjusted depending upon the total amount of N recommended for the growing season.  Carrow (1995) suggests the following:

  • Use a 1.1:5 (N:K2O) ratio for annual N rates of 1 to 3 lbs. N per 1000 ft2.
  • Use a 1:1 (N:K2O) ratio when annual N rates are 3 to 6 lbs. N per 1000 ft2.
  • Use a 1:0.75 or 1:0.5 (N:K2O) ratios at annual N rates above 6 lbs. N per 1000 ft2 to avoid potential excessive salt buildup
  • In the summer, apply K at 0.25 to 0.5 lb. K2O per 1000 ft2 spaced at two- to six-week intervals.

Maintaining adequate K in the root zone for turfgrasses is very important. Potassium contributes to total cell solutes and, therefore, cell turgidity. Potassium-deficient turfgrass tends to lose stomatal control; whereby, the stomata remain open, transpirational loss of water is high, turgidity decreases, and wilt is more prevalent. Thus, adequate K is necessary, not only as a cell solute for turgidity, but for stomatal control of transpiration. However, when K levels in the soil become excessive, high soil salinity limits plant water uptake, induces wilt, and reduces wear tolerance.  Saline conditions can easily build up in sandy soils if high levels of salts are added (via fertilization, irrigation) and leaching is limited (Carrow, 1995).


Calcium (Ca) and Magnesium (Mg) Recommendations

Most laboratories determine Ca and Mg on soil samples used for turf.  However, few have specific recommendations for these nutrients if they are found to be low.  Generally, low levels of these nutrients are associated with low soil pH and are corrected by the application of dolomitic limestone (Ca·Mg(CO3)2) to increase pH.

While Ca deficiency is very rare, Mg deficiency can occur on very acidic and on low CEC soils subject to frequent leaching.  Also, Mg deficiency may be induced by large applications of calcitic limestone (CaCO3) or large applications of potash (K2O). To help prevent this problem from occurring use dolomitic limestone as the liming source (Carrow, 1995).

If a Mg deficiency is suspected a plant analysis will aid in confirming the diagnosis.  Contact your local County Extension Office of Private laboratory for sampling instructions.

An alternative method is to apply a foliar application of Mg, using 1 lb. magnesium sulfate (MgSO4·7H2O) (i.e. 0.1 lb. Mg) per 1000 ft2 in 2 to 3 gallons of water.  If Mg is deficient the turf will start to green up in a matter of a few days (e.g. generally less than 5 days). In severe cases higher rates need to be applied.  Apply 2 to 4 lbs. MgSO4·7H2O per 1000 ft2 with immediate irrigation to avoid burn (Carrow, 1995).  Another alternative is to apply Mg in the regular fertilizer program.  Apply at the rate of 0.5 lb. Mg (5 lbs MgSO4·7H2O) per 1000 ft2 (Plank, 1988).


Sulfur (S-SO4) Recommendations

Although procedures are available to determine sulfate-sulfur (SO4-S) levels in the soil, because of its mobility, particularly on coarse-textured soils, few soil test laboratories in the southeast determine SO4-S on a routine basis.  Instead some laboratories make a routine recommendation.  For example, Auburn University routinely recommends 10 lb of S per acre (0.23 lb per 1000 ft2) per year for all turf types.


Micronutrient Recommendations

Until recently, few research studies have been conducted relating turfgrass performance to micronutrient deficiencies (Turner and Hummel, 1992).  Consequently, with the exception of Fe and Mn, extractants used for assessing the availability of micronutrients have not been evaluated extensively on turf.  Although extractants for Fe and Mn are best correlated, they are not very reliable for predicting maintenance or corrective treatments.  This may be due in part to the fact that most turfgrasses are very efficient in utilizing micronutrients in the soil (Carrow, 1995).

Iron (Fe) deficiencies are readily apparent from plant symptoms - chlorotic turf with yellowing being most apparent on the new leaves; spindly, thin leaves; chlorosis often occurs in irregular patches rather than uniformly across the turf.  Application of foliar Fe at 1 lb Fe/acre will result in rapid greening of the turf.  Grasses often respond to foliar Fe even when there isn't a Fe deficiency; thus, a true deficiency is present only if real Fe-deficiency visual symptoms are present.

Manganese deficiencies are most prevalent on near neutral to alkaline pH soils. The solubility of Mn in soils is strongly influenced by soil pH and deficiencies are rarely encountered in turfgrass growing on soils with a pH less than 6.5. Therefore, for a Mn soil test to be meaningful, soil pH must be included in the interpretations. Researchers take this into consideration and include pH along with extractable soil Mn levels in regression equations when developing the interpretative guidelines. If soil pH is not taken into account with the extractable soil Mn level, the soil test may indicate a low soil level, even though the level is sufficient for good growth. If a Mn deficiency is suspected it can generally be confirmed by plant analysis. Another method for confirming a suspected Mn deficiency is to apply a foliar application of Mn, using 1 lb. of Mn/A in sufficient water to thoroughly wet the foliage. If Mn is deficient, a greening response will occur in a few days

The following table lists some examples of spray rates that can be used to spot check for micronutrient deficiencies.

Suggested solutions to spot treat for micronutrient deficiencies (Adapted from McCarty, et al., 1993)


Element

Fertilizer source

Product
oz. per gal

Lb. element per 1000 ft.2

Lb. element
per acre

Fe

Iron sulfate

0.75

0.025

1.1

Mn

Manganese sulfate

0.50

0.025

1.1

Zn

Zinc sulfate

0.50

0.010

0.44

Cu

Copper sulfate

0.50

0.003

0.13

B

Sodium borate

0.05

0.001

0.04

Mo

Sodium molybdate

0.01

0.001

0.04

 

 

In This Section

Introduction
Sampling
Plant Analysis
Extraction
Interpretation
Recommendations
Soil pH
Reference

 

   

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