Mehlich 3 provides better results for soils with a pH of 7 or greater. More
information about Mehlich 3 is available in “Extraction of Soil Nutrients
Using Mehlich-3 Reagent for Acid-Mineral Soils of Florida” at < https://edis.
>. Based on such tests, the amount of fertilizer that is
needed to supplement the nutrition component of the native soil can be
calculated. The BMP program for vegetables requires a calibrated soil test.
More information about soil testing can be found in “Developing a Soil Test
Extractant: The Correlation and Calibration Processes” at <http://edis.ifas.
> and “Soil Testing for Plant-Available Nutrients—What Is It
and Why Do We Use It?” at <>.
PLANT TISSUE ANALYSIS
Analysis of plant tissues (e.g. leaves or petioles) for nutrient concentration
provides a good tool to monitor nutrient management programs. There
are basically two approaches to plant tissue testing: standard laboratory
analysis and the plant sap testing procedures. Standard laboratory analysis
involves analyzing the most-recently-matured leaf of the plant for an array
of nutrients. The resulting analyses are compared against published adequate
ranges for that particular crop. Laboratory results that fall outside the
toxicity (especially in the case of micronutrients). The most-recently matured
leaf serves well for routine crop monitoring and diagnostic procedures
for most nutrients. However, for the immobile nutrients such as Ca, B, and
certain other micronutrients, younger leaves are generally preferred.
The second approach is use of plant sap quick-test kits that have been
calibrated for N and K for several vegetables in Florida. These testing kits
analyze fresh plant sap for N and K. Quick tests can be a valuable tool
for on-the-spot monitoring of plant nutrient status. Diagnostic information
for leaf and petiole sap testing can be found in “Plant Tissue Analysis and
Interpretation for Vegetable Crops in Florida,” at <
ep081> and “Petiole Sap Testing for Vegetable Crops” <
edu/cv004>.
RIGHT SOURCE
N, P, K NUTRIENT RATES AND SOURCES
Nitrogen often is the most limiting nutrient in Florida’s sandy soils. The
amount of nitrogen required by vegetable plants must be applied each
growing season because it leaches rapidly. Therefore, crop nitrogen
requirements vary among crops and are not dependent on soil test results
(Table 5). Fertilizer rates of other nutrients must be applied based on soil
test results (see soil test above) to follow BMPs. The interpretations of Mehlich
1 (very low, low, medium, high, and very high) and Mehlich 3 (low, medium,
and high) are shown in Table 6. The soil test extractant used in UF/
IFAS recommendations recently has changed to Mehlich 3. UF recommendations
based on Mehlich 3 test include P2O5 and K2O (Table 7) and nutrient
management using fertigation (Table 8). More information on the change
to Mehlich-3 can be found in “Extraction of Soil Nutrients Using Mehlich-3
Reagent for Acid-Mineral Soils of Florida” at <.
Some private companies may use Mehlich 1 and recommendations include
P2O5 and K2O (Table 9) and micronutrients (Table 10).
The recommendations found in Tables 7 through 10 were determined
various soil pH levels. Crop plant development, crop yield, and vegetable
quality were considered in determining the optimum nutrient levels for UF/
IFAS recommendations.
Nitrogen (N) can be supplied in both nitrate and ammoniacal forms.
Nitrate-nitrogen is generally the preferred form for plant uptake in most
situations, but ammoniacal N can be absorbed directly or after conversion
to nitrate-N by soil microbes. Since this rate of conversion is reduced in
cold, fumigated, or strongly acidic soils, it is recommended that under such
conditions 25% to 50% of the N be supplied from nitrate sources. This ratio
is not critical for unfumigated or warm soils.
Phosphorus (P) can be supplied from several sources, including single
and triple superphosphate, diammonium phosphate (DAP) and monoammo-
Table 2.4. Essential plant nutrients supplemented by fertilizers and amendments.
Nutrient Occurrence
Macronutrients
Nitrogen (N) Stems thin, erect, hard. Leaves small, yellow; on some crops (tomatoes)
On sandy soils especially after heavy rain or after over-irrigation.
Also on organic soils during cool growing seasons.
Phosphorus (P) Stems thin and shortened. Leaves develop purple color. Older leaves
On acidic soils or very basic soils. Also when soils are cool and
wet.
Potassium (K) Older leaves develop gray or tan areas on leaf margins. Eventually a scorch
appears on the entire margin.
On sandy soils following leaching rains or over-irrigation.
Secondary nutrients
Calcium (Ca)
include blossom-end rot of tomato, pepper and watermelon; brown heart of
On strongly acidic soils, or during severe droughts.
Magnesium (Mg) Initially older leaves show yellowing between veins, followed by yellowing of
young leaves. Older leaves soon fall.
On strongly acidic soils, or on leached sandy soils.
Sulfur (S) General yellowing of younger leaves and growth. On very sandy soils, low in organic matter, reduced especially
following continued use of sulfur-free fertilizers and especially in
areas that receive little atmospheric sulfur.
Micronutrients
Boron (B)
of celery; blackheart of beet; and internal browning of turnip.
On soils with pH above 6.8, or on sandy, leached soils, or on
crops with very high demand such as cole crops.
Copper (Cu) Yellowing of young leaves, stunting of plants. Onion bulbs are soft with thin,
pale scales.
On organic soils or occasionally new mineral soils.
Chlorine (Cl) Usually only under laboratory conditions.
Iron (Fe) Distinct yellow or white areas between veins on youngest leaves. On soils with pH above 6.8.
Manganese (Mn) Yellow mottled areas between veins on youngest leaves, not as intense as
On soils with pH above 6.4.
Molybdenum (Mo) Pale, distorted, narrow leaves with some interveinal yellowing of older leaves,
On very acidic soils.
Nickel (Ni) Usually only under laboratory conditions.
Zinc (Zn) Small reddish spots on cotyledon leaves of beans; light areas (white bud) of
corn leaves.
On wet, cold soils in early spring or where excessive phosphorus
is present.
2019 Vegetable Production Handbook of Florida 5
/edis