SOIL FERTILITY

Phosphorus deficiencyCrops need nutrients just like people do.  A fertile soil will contain all the major nutrients for basic plant nutrition (e.g., nitrogen, phosphorus, and potassium), as well as other nutrients needed in smaller quantities (e.g., calcium, magnesium, sulfur, iron, zinc, copper, boron, molybdenum, nickel).  Usually a fertile soil will also have some organic matter that improves soil structure, soil moisture retention, and also nutrient retention, and a pH between 6 and 7.  Unfortunately, many soils do not have adequate levels of all the necessary plant nutrients, or conditions in the soil are unfavorable for plant uptake of certain nutrients.

Soil scientists that focus on soil fertility are interested in managing nutrients to improve crop production.  They focus on using commercial fertilizers, manures, waste products, and composts to add nutrients and organic matter to the soil.  Sometime they also add chemicals that change the pH to a more optimum level for nutrient availability to plants.  Soil fertility experts must also be careful to ensure that practices are environmentally sustainable.  Inappropriate management of nutrients can lead to contamination of lakes, rivers, streams, and groundwater.  In addition, adding amendments to the soil is expensive and cuts into the profitability of farming operations, not to mention that toxic levels of nutrients can be as bad as or worse than too little nutrients for the plants.

Check out this fun interactive game on the Nitrogen Cycle.

Nutrient Deficiencies

soil pH and plants

There are 17 essential plant nutrients, three come from air and water (carbon, oxygen, and hydrogen) and 14 come from the soil.  The table below describes the essential and beneficial elements obtained from the soil.  Macronutrients are needed in high quantity, micronutrients are needed in small amounts, and beneficial elements are essential or beneficial to some plants, but not all.

 

 

 

Absorbed Form

Function

Deficiency

Macronutrients

 

 

 

 

Nitrogen

N

NO3-, NH4+

Protein and enzyme component

General yellowing of leaves, stunted growth, often older leaves affected first.

Phosphorus

P

HPO4-, HPO42-

Membranes, energy, DNA

Difficult to visualize until severe. Dwarfed or stunted plants. Older leaves turn dark green or reddish-purple.

Potassium

K

K+

Osmotic balance

Older leaves may wilt or look burned. Yellowing between veins begins at the base of leaf and goes inward from the leaf edges.

Calcium

Ca

Ca2+

Cell structure

Fruit/flower and new leaves are distorted or irregular. When severe, leaves will be necrotic near the base. Leaves can be cupped downward.

Occurs more often at low pH.

Magnesium

Mg

Mg2+

Chlorophyll, enzyme activation

Older leaves will turn yellow and brown around the edge of the leaf leaving a green center. May appear puckered.

Occurs more often at low pH.

Sulfur

S

SO42-

Protein and enzyme component

Yellowing leaves starts with younger leaves.

Micronutrients

 

 

 

 

Iron

Fe

Fe2+, Fe3+

Enzyme function, required for chlorophyll production

Yellowing between veins that start with younger leaves. Occurs more often at high pH.

Manganese

Mn

Mn2+

Enzyme component

Yellowing between veins that start with younger leaves. Pattern is not as distinct as with Fe deficiency, may appear in patches or freckled.  Occurs more often at high pH.

Zinc

Zn

Zn2+

Enzyme component

Yellowing between veins of younger leaves.  Terminal leaves may be rosette. Occurs more often at high pH.

Boron

B

H2BO3-

Cell wall

Terminal buds die.  Light general yellowing. B requirements are very plant specific.

Copper

Cu

Cu2+

Enzyme function

Dark green stunted leaves.  Curled leaves often bend downwards.  Sometimes wilted with light overall yellowing of leaves. Occurs more often at high pH.

Molybdenum

Mo

MoO42-

Enzyme function

Yellowing of older leaves and light green rest of the plant. It usually appears as N deficiency due to role in nitrate assimilation and in legumes in N-fixing bacteria.  Occurs more often at low pH.

Chlorine

Cl

Cl-

Osmotic balance, plant compounds

Almost never deficient.  Abnormally shaped leaves; Yellowing and wilting of young leaves.

Nickel

Ni

Ni2+

Enzyme component

Almost never deficient.

Beneficial

 

 

Benefit

Silicon

Si

 

increased pest and pathogen resistance, drought resistance, heavy metal tolerance, higher quality and yield of crop

Cobalt

Co

Co2+

Required for N-fixation by bacteria associated with legumes

Sodium

Na

Na+

Required for photosynthesis in C4 and CAM species adapted to warm climates

 

Soil Test Interpretations

 

Plant analysis samplingSoil testing evaluates “plant available” elements from the soil.  Some nutrients are in forms that are not readily available to plants, so when a soil test is performed only the available forms of the element are measured.  Due to the wide range of soils, it is difficult to have one method for all soils.  It is also difficult for any method to report the actual amount available to plants as all plants will take up elements differently.  Testing labs across the United States, and elsewhere, may use different test procedures for different soils.  However, most soil testing will involve a soil pH measurement.  An assessment of soil organic matter and lime requirement may also be performed.  Soil pH is very important for understanding nutrient availability to plants, and it affects the interpretation of the values reported in the soil test.

 

There are many factors to consider when producing a crop or growing a garden.  How much fertilizer to apply and when to apply it are some of the decisions that must be made.  These decisions depend on the crop to be grown, the soil type, and the environmental conditions under which it is grown.  Soil testing laboratories associated with universities have conducted years of field and greenhouse research with various crops and soils to determine how a particular crop responds to soil test levels of plant nutrients.  Most laboratories use a rating scale that includes “Low”, “Medium”, “High”, and “Very High” to describe the soil test level of a particular nutrient for a particular crop in a particular soil type.  When a nutrient level is low or very low level, a fertilizer containing that nutrient is usually recommended.  Once a soil test rating reaches “High” or “Very High”, then the grower can save money by not applying any more of that nutrient. By not applying when soil test levels are high and by creating rating scales that are specific to general soil types, the environment can be protected from excessive nutrients. 

Nutrient Management

nutrient response curveThe goal of soil nutrient management is to sustainably produce profitable crops.  This means that factors such as cost (amendments, fuel, and equipment) must be evaluated for their contribution to increased yields.  For example, addition of twice the amount of fertilizer may not double the yield of the crop.  So, a farmer must determine if the cost of additional fertilizer will be repaid by the predicted additional yield.  Furthermore, the farmer must always be thinking about how inadequate or excessive management practices will affect the soil over time.  One of the major causes of erosion or soil loss is due to destruction of soil structure, which can be attributable to practices such as intensive tillage (soil mixing), excessive vehicular traffic, excessive removal of plant material (fallow fields), and depletion of soil nutrients, especially nitrogen.