Southeastern U.S. Vegetable Crop Handbook - 2022 - 19

A large percentage of the vegetables grown in mineral soils
of the Coastal Plain are grown in soils with essentially no structure.
These sandy soils also have little intrinsic fertility; however,
they can be productive if managed properly. Adequate fertilization,
adjusting pH as needed, and use of irrigation water are
required.
Soil Management. In a good soil management program, proper
liming and fertilization, good tillage practices, crop rotation, annual
additions of organic matter with cover crops, and adequate
irrigation are all necessary to maintain high levels of production.
Winter cover crops, rotating with non-vegetable crops, utilizing
reduced tillage when possible, and periodically resting the
land with summer cover crops between vegetable plantings are
essential in preventing deterioration of the soil struc ture. In soil
management, soil structure is vital for maintaining highly productive
soils.
Nutrient Management and the Environment. The sandy soils
preferred for vegetable production in the southeastern US result
in an aerated root zone and enable timely tillage, planting, and
harvesting. The same drainage allows water and dissolved nutrients
to move through the soil profile. Even with loams or clays,
nutrients retained in surface soil may be carried with sediment or
as dissolved run-off to surface water. Nitrogen (N) and phosphorus
(P) remain the two agricultural nutrients of greatest environmental
concern. Small losses of N & P can impact water quality,
especially in eco-sensitive regions. Agronomically, other issues
of potential concern include K fertilizer losses and accumulation
of heavy metals such as copper, zinc, etc. supplied with organic
amendments.
Ongoing research has documented increased costs and reduced
profits, negative impacts on yield and quality, and human
health risks, due to over-fertilization. It is therefore critical that
both nutrients and irrigation are managed to optimize vegetable
production while minimizing impact on the environment. Careful
nutrient management includes consideration of the following
four issues: rate, timing, placement, and source. Land-grant university
recommendations are based on calibrated crop response
studies that can differ substantially across the region. Producers
should consult guidelines prepared specifically for their state for
the most appropriate nutrient management recommendations. A
well-balanced nutrient management plan represents good stewardship
and should satisfy any applicable environmental regulations.
Soil
Acidity and Liming. Many soils in the southeast are naturally
acidic, or become acidic with cropping, and need liming to
attain optimum production levels. Soil acidity is the term used to
express the quantity of hydrogen (H+
) and aluminum (Al3+
) cations
(positively charged ions) in soils. Soil pH is determined by
using a 1:1 soil-to-water solution. The pH of the solution is measured
by a pH meter (potentiometer). Soil pH is an indicator of
" soil acidity " . Combined, the use of the soil pH and soil textural
class determines the lime requirement. A pH of 7.0 is defined as
neutral, with values below 7.0 being acidic and above 7.0 being
basic or alkaline. Root growth and plant development may be severely
restricted if acidic cations, especially aluminum, occupy a
large percentage of the negatively charged soil cation exchange
capacity (CEC). This negative charge is due to the chemical
makeup of the soil clay and organic matter, and means that they
can attract positively charged ions. Acidification occurs when
H+
is added to soils by decomposition of plant residues and organic
matter, during the nitrification of ammonium when added
to soils as fertilizer (UAN solutions, urea, ammonium nitrate,
ammonium sulfate, anhydrous ammonia), manures, or plant residues
and from additions of acidic soil amendments. Declines of
one pH unit can occur even in properly fertilized beds. Other soil
processes, such as aluminum hydrolysis, will release H+ into the
soil system and thereby increasing acidity. Lime is applied to
neutralize soil acidity by releasing a base (HCO3
-, OH -
soil solution, which reacts with acid-forming ions (H+
) into the
). Increasing
soil pH reduces the concentration of dissolved aluminum, as
well as influencing the concentrations of other ions.
Lime recommendations must take into account differences
in acidity among soils as well as differences among various
crops' tolerance to acidity. Both the soil pH and some measure
of residual or exchangeable acidity are needed to calculate lime
recommendations. Although portable soil test kits determine pH
rapidly, it is not possible to make an accurate lime recommendation
based solely on a pH measurement made using water and
soil alone. Another issue to consider is that different soil laboratories
may use different testing methods developed for their
particular soil conditions. Due to these differences, producers
should consult with their local Extension office or consulting
laboratory about laboratory methods and target pH assumptions
used in determining lime recommendations.
If soil pH is too high for the desired crop, elemental sulfur
(S) is the most effective material to reduce soil pH. The amount
of acidity generated by 640 pounds of elemental S is the same
as that neutralized by 1 ton of lime. Soil pH can be lowered by
applying aluminum sulfate or iron sulfate. Whether trying to increase
or decrease the pH of your soil, always follow the manufacturer's
instructions for appropriate rates. A slight pH reduction
can be produced by using ammonium sulfate, ammonium
nitrate, or urea as a fertilizer source of nitrogen.
Liming materials containing only calcium carbonate (CaCO3),
calcium hydroxide [Ca(OH)2
], or calcium oxide (CaO) are
called calcitic limes. Pure calcium carbonate is used as the standard
for liming materials and is assigned a rating of 100 percent.
This rating is also known as the " calcium carbonate equivalent,
and is referred to as the CCE. All other liming materials are rated
in relationship to pure calcium carbonate. Liming materials with
significant amounts of magnesium carbonate (MgCO3
) are called
dolomitic limes. Dolomitic limes should be used on soils low
in magnesium, as indicated by the soil test report. It is possible
to use a magnesium fertilizer instead of dolomitic lime, but the
costs of this source of magnesium are almost always considerably
higher. Because lime dissolves very slowly, it must be finely
ground to effectively neutralize soil acidity. Lime laws in most
states describe standards for composition and particles sizes.
The most commonly used liming materials are finely
ground dolomitic or calcitic rock. Most agricultural lime is sold
in bulk by the ton. Additional liming materials include burnt
lime or hydrated lime, pelleted lime, liquid lime, wood ash,
ground seashells, and industrial slags. Lime pellets and lime sus
Southeastern U.S. Vegetable Crop Handbook - 2022

Table of Contents for the Digital Edition of Southeastern U.S. Vegetable Crop Handbook - 2022
