Rice Production: Chapter 9 - Fertilizer sources and management

Introduction

Many rice varieties, particularly the high-yielding improved varieties currently being introduced into many traditional farming systems throughout the world, respond markedly to fertilization. When used in conjunction with good management practices (thorough land preparation, controlled irrigation, timely weeding) fertilizers can increase yields many times over. On the other hand, if used improperly fertilizers can damage crops, waste money, or possibly lead to a dependence on scarce chemical inputs. In many developing areas fertilizers are still unknown or poorly understood, and the extension agent represents the farmer's only source of information about safe, economical, and effective fertilization practices.

This chapter describes the most common sources of fertilizer and outlines a recommended application schedule for irrigated rice. Included are practical suggestions for effective handling, application, and storage of chemical fertilizers.

I. Fertilizer Sources 

A. Organic Fertilizers

Organic fertilizers are decomposed remains of plants and animals. In the natural ecosystem, elements absorbed from the soil by living organisms are returned to the soil through decay and decomposition following the death of the organisms. Organic fertilizers traditionally have provided all of the nutrients in shifting agriculture systems where periods of cultivation alternate with fallow periods (during which the natural regenerative process restores organic fertilizers to the soil). Organic fertilizers also have traditionally provided most nutrients to swamp farming systems, since swamps serve as natural catchments for organically derived nutrients which wash down off the surrounding uplands.

The introduction of improved rice varieties into a traditional swamp farming system tends to create a situation here the nutrient demands of the crop outstrip the natural ability of the ecosystem to replenish its organic resources. Although knowledgable swamp farmers can exploit the natural regenerative properties of many so-called "waste" products to help maintain soil fertility (e.g. by incorporating organic material back into the soil), and although they can replenish nutrients by planting nitrogen-fixing legumes during part of the year, repeated high yields will inevitably extract greater amounts of some nutrients particularly nitrogen, phosphorus, and potassium - than can rapidly be returned to the soil through the use of organic fertilizers or alternate cropping practices. Thus, the swamp farmer is faced with a decision. Either s/he opts for a balanced farming system which will be able to sustain medium yields over a long period with minimal chemical inputs. Or s/he elect to try for the highest possible yields - which in most cases means relying on inorganic fertilizers.

B. Inorganic Fertilizers

Inorganic fertilizers are chemical compounds (either synthesized natural) which are added to the soil to improve fertility. The most important characteristics of inorganic fertilizers are that they can be extremely economical can vastly increase yields land can result in significant profits), and they are fast-acting (since the nutrients they contain are immediately available to the crop and do not have to be processed by microbes in the soil).

Depending on their chemical composition, inorganic fertilizers may contain only one nutrient or several:

Single-element fertilizers contain only one of the primary nutrient elements (N,P, or K)

Incomplete fertilizers contain two of the three primary nutrient elements (N and 1, N and K, or P and K) Complete fertilizers, contain all three of the primary nutrient elements (N,P, and K)

It is important to remember that inorganic fertilizers always co:., + of the nutrient element (s) bonded to can inert "carrier'. Consequently the total weight of the fertilizer does not correspond exactly to the weight of the nutrient it contains: the weight of the nutrient comprises only a part of the total fertilizer weight and varies according ding to the chemical composition of the fertilizer. For example: one 100 lb bag of ammonium sulphate ( 20% N) contain 20 lbs nitrogen and 80 lbs of inert material, while one 100 lbs bag of urea 45% N) contains 45 lbs nitrogen and 55 lbs of inert material. In terms of its ability to supply nitrogen en, urea is therefore more than twice as "strong;" as ammonium sulphate because it contains more than twice as much nitrogen of by weight.

Listed below are the fertilizers most commonly used in Sierra, a Leone ( fill in current prices ):

1 Single-element fertilizers

Ammonium sulphate 20% N)	Price:
Urea ( 45% N)	Price:
Single superphosphate ( 18% P2O5)	Price:
Basic Slag ( 14.5% P2O5 )	Price:
Muriate of Potash ( 60% K2O )	Price:
sulphate of Potash (50% K2))	Price:

2) Incomplete Fertilizers

N-P-K 20-20-0 (20% N, 20% P2O5, 0% K2O )	Price:
N-P-K 0-20-20 ( 0% N, 20% P2O5, 20% K2O )	Price:

3) Compete fertilizers

N-P-K 15-15-15 (15% N, 15% P2O5 15% K2O)

Price:

Notes on fertilizer storage:

Great care should be taken in handling, transporting, and storing chemical fertilizers. Although most inorganic fertilizers are fairly inert when kept dry, many undergo drastic chemical changes when exposed to rain or even moisture in the air. The resulting gases and liquids not only carry off valuable nutrierits, but they car cause considerable corrosive damage to cement floors and wall, metal tools, motorcycles, etc.

It best to store chemical fertilizers by themselves in a dry, well ventilated room. Be sure to keep the bags off the floor (use wooden pallets to elevate them) and stack them so that air can circulate between the bags. Avoid storing food, seed rice, or pesticides nearby.

Take the time to construct a proper fertilizer-tore. Improper storage can Present a safety hazard and decrease the power of the fertilizer.

II. Fertilizer Management

There is no single recommendation for rice fertilization which will fit all situations. Fertilizer application will vary considerably, depending on crop requirements, the availability of fertilizers, the financial resources of the farmer, and most importantly, the ability of the farmer to follow application schedules (some of which can be quite complicated).

Tests and field experience have shown that the application of 40 kg/ha (36 lbs/acre) each of nitrogen phosphorus, and potassium gives optimum results under most local conditions. (Important: that's 40 kg/ha of nutrient, not of fertilizer.) 40 kg/ ha is the generally recommended application rate. Of course many farmers will be either unwilling or unable to purchase this amount of fertilizer, arid they will end up fertilizing at a much lower rate (or not at all). Decreasing the amount of fertilizer will result in more modest yields, but keep in mind that any amount of fertilizer, no matter how small, will help. If a farmer deeds to fertilize but can afford only one bag of N-P-K 15-15-15 per acre, don't necessarily discourage him/her If applied properly, even this relatively small amount of fertilizer will affect favorable results.

Timing Fertilizer Applications

For optimum results, fertilizer should be applied to the soil at three points in the crop cycle:

1) The Basal Application

Just prior to transplanting, fertilizer should be broadcast and puddled into the sell to ansure an abundance of nutrients during the critical seedling establishment phase. The basal application facilitates the plant's rapid recovery from the shock of transplanting. If possible, all phosphorus should be applied basally, since root development is crucial at this time.

2) First Top Dressing

When the seedlings have fully recovered from the shock of transplanting and have entered into the active tillering stage (approximately two weeks after transplanting for most varieties), fertilizer should be broadcast and puddled into the soil. This operation can be combined with the first weeding. The first top dressing ensures rapid vegetative growth, promotes tillering, and helps strengthen the plants against disease attack. Nitrogen and potassium should be applied with the first top dressing.

3) Second Top Dressing

Immediately after panicle initiation (the date will vary according to the duration of the variety), fertilizer should once again be broadcast and puddled into the soil. The second top dressing ensures complete grain filling, increases the size and weight of the grains, and improves the quality of the crop by increasing the protein content. Nitrogen and potassium should be applied with the second top dressing.

An ideal fertilization schedule thus might look something like this:

Day of transplanting -	Basal Application
	40 kg/ha phosphorus
Active tillering -	First Top Dressing
	20 kg/ha nitrogen
	20 kg/ha potassium
Panicle initiation -	Second Top Dressing
	20 kg/ ha nitrogen
	20 kg/ha potassium

HOWEVER: it is possible that many farmers will have neither the means nor the ability to adhere closely to such a schedule. Often you will find yourself working with smaller amounts of fertilizer, or with only one incomplete or complete fertilizer instead of several single-element fertilizers. Expect to adjust this application schedule to the requirements of each situation - "go with the flow." You may want to devise a separate schedule for each farmer; do the best you can to get the nutrients to the rice when they will be needed most.

Some tips on applying fertilizers:

- It is sometimes difficult to broadcast small amounts of fertilizer over a relatively large area. Since uneven distribution of nutrients is ineffecient and in some cases actually harmful to the rice, remember that fertilizer car; always be stretched by the addition of an inert filler such as dirt or sand.

- Never broadcast nitrogenous fertilizers (ammonium sulphate, urea, N-P-K) onto standing water. Upon contact with water, the ammonium ions are converted to ammonia gas, and much nitrogen is lost into the air. Broadcast all top dressings onto drained plots and then puddle in the fertilizer by hand before re-flooding.

- Be sure that water does not flow out of recently fertilized plots. Water carries off nutrients!

- Always be aware of safety considerations. Try to avoid fertilizing in swamps from which water is used for drinking, washing, or laundering, unless an alternative water source is available. Helping dig a well or construct a water system is one of the most beneficial secondary projects you can undertake with the farmers in your community.

Additional thoughts on fertilizers:

Although it is tempting for all of us as agriculture extension agents to attempt to sell farmers on the use of fertilizers, we must always be careful not to present a distorted picture of nothing but higher yields. The potential benefits of fertilizer use should be described only in the context of the potential costs: possible dependency and possible safety hazard. Above all, if farmers are to be taught to use chemical fertilizers, they should be taught to use them efficiently, safely, and in moderation. It is unwise to encourage a heavy reliance on inorganic fertilizers in an era when the spiralling price of petroleum products (and many fertilizers are petroleum by-products) theatens to make widespread use of fertilizers uneconomical. When working with fertilizers, try always to remain aware of the fine line between the use of fertilizers to enhance yields and total reliance on them.

III. Fertilizer Calculations

There are a number of variables to consider when calculating the amount of fertilizer to be applied to a given area. These include:

- the percentage of nutrient contained in the fertilizer being used

- the desired rate of application

- the size of the area to be fertilized

In addition, because most farmers do not have access to an accurate scale or balance, it will often be necessary to devise a simple means of converting amounts of fertilizer into a common volume measurement (e.g the 8 oz. cup)

The extension worker can attack the problem of calculating fertilizer for farmers in a number of ways, depending on the degree of accuracy desired. Described below are two approaches to the problem which have proven useful in the past in Sierra Leone.

1) The Rough - and - Ready Method

The simplest method of determining a farmer's fertilizer needs is to calculate how many bags of fertilizer will be needed for the entire swamp and then to divide the swamp and the fertilizer into smaller units at the time of application to ensure a relatively even distribution. First, measure the entire swamp and calculate its area. Practiced field workers can sometimes make an approximate guess at a swamp's area, but make sure the practice at measuring precedes the guessing. Choose the fertilizer you will use - or in most cases, assess what is available. Using the recommended rate of 40 kg/ha, calculate the amount of fertilizer required with this formula:

W = (A x R) / P where

W = weight of the fertilizer to be applied (this is what you are calculating)

A = area to be fertilized (hectares or acres)

R = desired rate of application (in this case the recommended rate, or 40 kg/ha)

P = percentage of nutrient contained in the fertilizer

Example:

You have measured a farmer's swamp to be approximately 1.5 ha. How many bags of urea will be required to fertilize the entire swamp with nitrogen at the recommended rate?

Using the formula W = (A x R) / P where:

W = ?

A = 1.5 ha

R = 40 kg/ha

P = 45% or 45 (determined from the fertilizer chart)

then

W = 1.5 ha x 40 kg/ha / 0.45

W = 60 kg / 45

W = 133.33 kg

W = approximately 130 kg urea

Each bag of urea weighs 50 kg; therefore, to convert 130 kg into bags, divide 130 kg by the weight of one bag:

130 kg = 130 kg / 50 kg = 2.6 bags

The problem now is to ensure that the 2.6 bags of urea get distributed evenly over the 1.5 ha swamp. Try dividing the swamp into ten small sections. Then get together with the farmer and, using a local unit of measure, divide the fertilizer into ten equal piles. Make certain that the farmer understands exactly what you are doing together, because in the future s/he will be doing the same operation without your assistance, Above all, keep things simple.

2) The Fertilizer Tables

Developed swamps are divided into plots smaller than a hectare, acre, or even half acre. A more exact method of calculating fertilizer applications is by the individual plot. This method lends itself well to swamps in which management practices have reached a more sophisticated level, or in which only a few plots are planted at a time.

Then simply multiply this figure - the multiple of the standard area - by the figure appearing in column 6 (since we are working in English measurements here) to determine the cups of basic slag needed:

2.5 x 4.8 = 12 cups of basic slag

Now suppose the farmer checks the storeroom and discovers that there is no more basic slag. But there is a quantity of another phosphoric fertilizer - single superphosphate. How many cups will be needed to fertilize the plot at the recommended rate with single superphosphate?

Since you have already measured the area of the plot and calculated the multiple of the standard area - and recorded this information on a map of the swamp - a quick reference to the fertilizer tables will enable you to determine how many cups of single superphosphate will be needed.

Multiply 2.5 (the multiple of the standard area) by the figure appearing in column 6 opposite the line marked "single superphosphate":

2.5 x 7.4 = 18.5 cups of single superphosphate

The use of the fertilizer tables requires a detailed area survey and a fair amount of math. However, the survey need be performed only once, since the measurements and plot area calculations can be preserved for future reference on a sketch map. The same map can later be used for a number of different purposes: fertilizer calculations, seed requirement calculations, pesticide calculations, and yield comparison.

How to use the tables The columns in the table are as follows:

column 1 - name of the fertilizer

column 2 - number of 8 ounce cups (filled to overflowing) in one 50 kg bag

column 3 - weight of the fertilizer in one cup in grams/(ounces)

column 4 - weight of the nutrient in one cup in grams/(ounces)

column 5 - number of cups needed to fertilize 100 m2 at the recommended rate of 40 kg /ha column 6 - number of cups needed to fertilize 1000 f² at the recommended rate of 36 lbs/acre

If the area of the plot to be fertilized is known, columns 5 and 6 will enable you to calculate rapidly the amount of fertilizer necessary to fertilize at the recommended rate. Simply divide the known area of the plot by 100 m² (if you are using metric measurements) or by 1000 f² (if you are using English measurements) to get a multiple of the standard area used in column 5 and column 6. (Be careful to keep to the same system metric or English, throughout each calculation.) Then multiply this figure - the multiple of the standard area -by the figure appearing in either column 5 or column 6 (whichever 1; appropriate for the units of measurement you are using) to determine the number of cups of fertilizer needed to fertilize at the recommended rate.

Example:

You have measured a plot to equal 2500 f². You want to make a basal application of phosphorus using basic slag as the source of F. The desired rate of application is 36 lbs/acre. How many 8 ounce cups of fertilizer are needed?

First, divide the plot area (2500 f²) by 1000 f² to get a multiple of the standard area:

2500 f² / 1000 f²= 2.5

FERTILIZER TABLES

Note: one cup = 8 ounces

Figure