7a: SOIL FERTILITY MANAGEMENT

Determining Fertility Status and Fertilizer Needs

• The available nutrient supply of most soils is seldom adequate to support the requirements of crops.

• Soils become depleted of nutrients by:

  • – crop removal

  • – leaching losses

  • – volatilization (particularly N)

  • – erosion of topsoil

  • – fixation by clays

  • – immobilization

• Thus, fertilizers must be applied to supplement the soil supply of nutrients.

1. Soil analysis

2. Field fertilizer experiments

3. Plant tissue analysis

4. Greenhouse tests

5. Evaluation of symptoms of nutrient deficiencies

The kind and amount of fertilizer to be added may be determined by one/a combination of the following: (5)

sound fertilizer recommendation

A __ is usually arrived by:

• – a combination of the above approaches

• – making correlations among plant response and soil and plant nutrient status

• – consideration of type of crops

• – physical properties of soils

• – environmental conditions

Soil analysis

• relatively rapid method of determining the fertilizer needs of crops.

• The method consists of:

  • – taking soil samples properly

  • – subjecting the soil samples to chemical analysis

  • – interpretation of results of analysis

• Assumes that the chemical extractants remove the potentially available forms of nutrients from the soil.

Soil Analysis and Fertilizer Recommendation

• These available forms of nutrients are rapidly absorbed by the plant and are quickly replaced by the reserve forms of nutrients;

  • so that the same level of the nutrient taken is supplied and maintain at the time of analysis.

1. – pH

2. – OM content

3. – available P

4. – exchangeable K

5. – lime requirement (if pH < 5.0)

For a simple soil fertility test, the following soil properties are determined: (5)

Soil pH

• important in interpreting results of a soil analysis and helps in predicting other nutritional problems of the soil.

• When OM content is determined;

  • N supply can be estimated by assuming that SOM contains 5% N.

Soil Analysis and Fertilizer Recommendation

• Thus, if the OM = 3%, the total N content (organic) would be about 0.15%.

• Assuming that one hectare-furrow-slice (HFS) weighs 2,000,000 kg

  • the potential supply of N in the soil = 3,000 kg.

Soil Analysis and Fertilizer Recommendation

• P is determined by any of the following methods: Bray 2, Olsen and Truog.

• The method to be used depends on the predominant forms of P present in the soil.

  • these methods differ in the extracting solution used.

Soil Analysis and Fertilizer Recommendation

• K is likewise determined by different methods.

  • – ammonium acetate

  • – sulfuric acid as extractant (the method used must be taken into account because the nutrient sufficiency value for one method is different from that of another)

Soil Analysis and Fertilizer Recommendation

• Lime requirement of acidic soils is analyzed by incubating soil samples with increasing amounts of lime (e.g. CaCO3) and then plotting the pH change with lime increments.

  • the required amount of lime to reach a desired pH value is then found from the curve.

Soil test results

• are compared with known values of deficiency/sufficiency.

• These values are derived from previously calibrated data of correlation between soil tests and field fertilizer experiments.

1. Good representative sample.

2. Adequate laboratory tests that determine the amount of nutrients the plant can remove from the soil.

3. Considerable experimental work to correlate the soil test results with fertilizer recommendations and actual crop yields.

Good soil testing requires 3 components:

How Soil Samples are Collected for Soil Analysis?

• The farm for soil fertility evaluation by soil analysis is first delineated on a rough map to group similar areas in terms of visible soil properties and management.

• Homogeneous areas are designated as sampling units.

• Some of the similarities of each sampling unit are:

  • soil texture, soil color, topography, previous cropping and management and position in the landscape.

How Soil Samples are Collected for Soil Analysis?

• Samples are then taken at random all over the sampling area;

  • by shovel or auger

• The depth of holes depends on whether shallow-rooted/deep-rooted crops are to be planted.

• The samples from several holes are mixed in a container and then from the soil sample mixture;

  • only one composite sample of about one kg is submitted for analysis in the laboratory.

How Soil Samples are Collected for Soil Analysis?

• The composite sample is supposed to represent a big area of thousands of kg of soil, hence the need for careful sampling.

  • soil analysis has no value, no matter how accurate, if samples are taken improperly.

How field fertilizer experiment is used as basis of fertilizer recommendation

• A simple fertilizer experiment to determine the optimum amount of fertilizer for a particular crop;

  • consists of treatment plots starting from zero and increasing at regular increments.

• Example: to determine the rate of N for a specific soil and other environmental conditions, the fertilizer treatments could be:

  • 0, 30, 60, 90, and 120 kg N/ha

How field fertilizer experiment is used as basis of fertilizer recommendation

• The plot size is about 20 m2 and the treatments are replicated at least three times.

• Agronomic data (e.g. corn as test crop) such as: stand count, plant height, and yield are collected and the results are plotted against the increasing amount of N added.

How field fertilizer experiment is used as basis of fertilizer recommendation

• The field experiment is repeated for the next season;

  • may be conducted simultaneously in other locations.

• One of the most sound bases for formulating fertilizer recommendations;

  • plants are grown under the natural conditions of their environment.

Greenhouse studies

• primarily preliminary approaches to determine what nutrient is insufficient in specific soils.

• Cannot be used as a sole basis for fertilizer recommendation because of the highly artificial conditions;

  • volume of soil explored in a pot by plant roots is limited by the size of the pot.

1. the simultaneous testing of several kinds of soils under similar conditions

2. cheaper to establish and maintain

Some advantages of a pot experiment include: (2)

Plant analysis

• usually done to verify what nutrient is deficient in the soil.

• May also be used to estimate the subsequent nutrient needs of long growth duration crops like:

  • – sugarcane

  • – banana

  • – fruit trees

1. Nutrient stress may occur before the fertilizer can be applied.

2. It is difficult to determine how much fertilizer to apply.

3. Can be affected by the weather

Problems with tissue testing are: (3)

Visual Deficiency Symptoms

• Useful to aid in identifying when plant is deficient in a nutrient.

  • it is usually complemented by plant analysis and/or by soil analysis.

• They are often difficult to interpret;

  • many symptoms look similar/may look like disease/insect damage.

• When we see deficiency symptoms - often too late to apply additional fertilizer to aid the plants’ future growth.

1. seasons of the year

2. economic value of the crops

3. nutrient preference of particular types of crops

4. soil pH

5. soil moisture conditions

6. target yield levels

Aside from native fertility of the soil, the other considerations in the formulation of fertilizer recommendations are the following: (6)

Other Factors Considered in Formulating Fertilizer Recommendations

• In general, crops require higher rates of fertilizer in the dry season than in the wet season.

  • greater solar energy available

  • more vigorous plant metabolism —plants have higher demand for nutrients

• For high value crops:

  • high fertilizer rates justify their high cost

• For low value crops:

  • the value of returns at high fertilizer rates may not offset the cost

Other Factors Considered in Formulating Fertilizer Recommendations

• High fertilizer rates are applied when high yielding crop varieties and intensive management practices are used.

• Different crops have different demands for N, P, and K;

  • grain crops demand for high N

  • legumes for high P

  • sugar, fiber, tuber and oil crops for high K

Other Factors Considered in Formulating Fertilizer Recommendations

• Sufficiency level of a nutrient for a certain crop may not be the sufficiency level for another crop.

• Soil pH affects the behavior and availability of applied nutrients.

  • Example: acidic soils = have greater P fixing ability.

  • Thus, P fertilizer application must take into account the proportion of P that will be immobilized in the soil.

Fertilizers

• any organic/inorganic material which contains one/more of the essential nutrients for plant growth.

• Organic fertilizers - derived from plants and/or animals.

• Inorganic fertilizers – synthesized/ processed from mineral deposits.

  • usually called chemical fertilizers.

• Fertilizers, whether organic/inorganic may be:

  • solid, liquid, or gaseous form

Fertilizers

• Examples of organic fertilizers are:

  • – animal manures

  • – compost

  • – crop residues

  • – green manures

  • – certain industrial by-products

• Inorganic fertilizers contain one/combination of the three primary elements: N, P or K

Fertilizers

• Fertilizers which contain only one of those elements are called = Single element fertilizers.

• Those which contain two/more are called = Compound fertilizers.

• Materials containing all three N, P, and K are called = Complete fertilizers.

Fertilizer grade

• The numbers on a bag of fertilizer –> “14-14-14” = guaranteed chemical analysis.

• These numbers indicate the bag of fertilizer contains:

  • 14% N, 14% P2O5, and 14% K2O.

• These numbers –> “14-14-14” = are referred to as the __.

Fertilizer grade

• For P and K, the chemical analysis is given in the oxide form.

• This is the way the nutrients were first thought to be absorbed by the plant and is still used today to express the analysis of fertilizer.

• For a 14-14-14, elemental analysis:

  • 14 – 6.2 - 11.6

P2O5 — P & K2O — K

• P=31;O=16; K=39

• %P in P2O5 =

  • (31x2) ÷ (31x2) + (16x5) x 100

  • 62÷142x100= 43.66% or 44%

• %K in K2O =

  • (39x2) ÷ (39x2) + 16 x 100

  • 78÷94x100 = 82.98% or 83%

To convert from the chemical analysis à the elemental analysis for P and K fertilizers, use this formula:

Nitrogen fertilizers

• Ammonium sulfate = 21-0-0

• Urea = 46-0-0

• Anhydrous ammonia = 82-0-0

• Others

  • Ammonium nitrate = NH4NO3 (32% N)

  • Ammonium chloride = NH4Cl (26% N)

  • Calcium cyanamide = CaCN2 (20% N)

Ammonium sulfate

• Contains sulfur

• Recommended in S-deficient soils

• Hygroscopic

• Nearly 100% soluble

• Crystalline and white

• In aerobic soils, NH4+ is quickly converted to NO3-

• Residual acidity can be neutralized by the application of 5 kg lime/kg N

Urea

• Hygroscopic and 100% soluble

• Crystalline and white

• When applied to soil, it is acted upon by microbial enzyme urease to form ammonium carbonate

Anhydrous ammonia

• Basic, pungent, and colorless

• Less acidifying

• Residual acidity equivalent to 1.8 kg lime/kg N.

Phosphorus fertilizers

• Ordinary Superphosphate = 20% P2O5

• Triple Superphosphate = 45-50% P2O5

• Ammonium Phosphate = 16-20-0 or 11-48-0

Ordinary superphosphate

• Pelleted as grayish granules

• About 85% of P is water soluble

• Contains traces of other nutrient elements such as: Mg, Fe, Cu, Zn, Mn, and Cl

Potassium fertilizers

• Muriate of Potash = 60% K20 and 47% Cl

  • Highly soluble

  • Contains traces of S, Mg, Ca, Fe, B, and Na.

• Wt of Fertilizer Needed = Wt. Nutrient Needed ÷ PNIF

• Wt of Fertilizer = Recommended rate ÷ PNIF

Fertilizer computations:

Percentage of Nutrient in Fertilizer

PNIF

Band placement

• __ of fertilizer is done in several ways:

  • applied on the row below the seed level

  • slightly on the side of the seeds along the row

• The roots after germination will have to grow through the fertilizer.

• Especially important for P fertilizers.

  • the negative effect of the immobility of P are reduced by the close association of roots with the banded fertilizer.

Broadcast application

• the fertilizer is spread evenly on the soil surface.

• Frequently done after the crop has been harvested, in preparation for the next crop.

• These materials should then be incorporated by plowing to avoid water pollution by runoff.

• Generally, more P and K will be needed when broadcasting than if the fertilizer was banded.

Top dressing

• refers to adding fertilizer to the surface of a crop already growing.

• Fertilizing established turfgrass is also done with this

Side dressing

• is often done with anhydrous ammonia.

• It must be accomplished before the crop is too high for the implements.

• Ammonia is injected into the soil at least 3 inches deep.

  • It is rapidly changed to NH4+ and can be taken up by roots, attached to cation exchange sites, or converted to NO3-.

• Urea can also be side dressed with cultivators.

Foliar application

• of fertilizers is made when quick action of nutrients is desired or when certain micronutrients are needed to be supplied to the crop.

• Only small amounts of nutrients can be absorbed by the leaves.

  • if large amount of nutrient is needed, soil application is recommended.

• Generally, micronutrients such as iron/zinc are applied in a foliar spray.