Soil Fertility: Module 3

Module 3. Identifying Soil Nutrient Problems & Opportunities – Making a Research DATE

Module Overview

You should now be well on your way to designing an effective ISFM program. You are familiar with a conceptual and operational framework to implement ISFM and have learned about key principles and concepts of soil fertility. You have also selected and characterized a target area for your efforts. In the last module you have taken a close look at the soils you expect to be dealing with and described their predominant characteristics. This information will provide a good basis for the work you are expected to do in this module – identifying soil fertility problems you will need to deal with in the area you have selected. Identifying problems is an important first step in developing an integrated soil fertility management plan or program. Identification helps you to set priorities for interventions and managing soil fertility.

In order to make the diagnosis as valuable as possible, we will suggest and describe a participatory learning and action research (PLA/R) approach – an iterative cycle of working with farmers and other soil fertility stakeholders to highlight soil fertility problems and take informed action. The framework for this will be the Research DATE described earlier. As you remember, DATE consists of four phases:D(iagnosis), A(ction planning), T(rying things out) and E(valuation). This is a bottom-up approach aiming at strengthening farmers’ capacity in observing and analyzing soil fertility management practices, and taking decisions leading to improvements. The focus of the Research-DATE is on developing answers to site-specific nutrient problems, exploiting opportunities, making the best use of locally available resources and knowledge and decision making in combination with research-based understanding and analysis of the underlying principles.

This module will focus primarily on the D of DATE. In the following lessons you will find descriptions of a range of diagnostic techniques including testing, field observations, mapping and experimentation. The potential contribution of computer-based decision support tools in the diagnostic process will also be highlighted. We will then go on to talk about the role of participatory action planning, research and evaluation in the validation of the diagnosis leading to preliminary recommendations for a limited area.

Lesson 3.1: Diagnosis

The diagnostic phase of the Research-DATE approach aims to get a common understanding of the local landscape, how soil fertility has been transformed over time, and what initiatives farmers have taken in the past. An important aspect of the diagnostic phase is to identify different ‘types’ of farming systems. Village-level and beyond-village level factors that have influenced farmers’ soil fertility strategies should be looked for and analyzed. Such beyond-village level factors typically include infrastructure (roads), market development (inputs, credits), national- and regional-level policies related to land tenure, and access to credits and inputs, the presence of rural development projects, strategies and focus of research and extension institutions etc..

The diagnostic phase should lay down the first ideas and options that can be used in the ‘action-planning’ phase to come later on. A number of learning and decision-support tools (DSTs) can be used in this phase. These tools range from simple rules of thumb (expert knowledge), to complex, crop growth simulation models and the table below gives an overview of decision support tools that can be used during this phase.

While these tools can help to improve understanding of biophysical processes and interactions between soil, climate and animal and plant production systems they mainly deal with nutrient aspects of soil fertility, mostly ignoring physical and biological aspects of ISFM. These non-nutritional effects are especially important when using organic amendments, and in combination with inorganic fertilizer use, they may lead to important gains in fertilizer use efficiency. Also, these diagnostic tools presented tend to give only ‘pictures’ of today’s reality and do not give an idea of the evolution and changes that have occurred in the farming-system(s). Knowing what changes have occurred over a longer time period can give insights into how knowledge is generated, which group of farmers have been most successful in adapting to changing circumstances and why. A comprehensive discussion about analyzing diversity can be accessed in the Supplementary Reading section below.

These tools are also currently weak in terms of capturing the socio-economic aspects of a plot or farm. Farmers may have much more opportunities off-farm for improving livelihoods and this often means that adopting more labor intensive agricultural technologies (as many ISFM options are) is counter productive.

The following lessons will provide additional information on the individual tools.

Lesson 3.1.1: Yield Gap Analysis

Farmers often achieve far less than 50% of the climatic and genetic yield potential for a given sowing date, cultivar choice and site. Figure 1 illustrates factors that define yield gaps at different levels. The potential yield or maximum yield (Ymax) is limited by climate and crop cultivar only, all other factors being optimal. Under irrigated conditions, water is assumed not to be limiting, but under rainfed conditions this assumption is often not true. Ymax is not constant but fluctuates from year to year and with sowing date because of climatic variability. The attainable yield (Ya) is the ‘nutrient-limited’ yield that farmers can achieve with current soil fertility management practices, but with optimal water and crop management. The maximum Ya is often about 80% of Ymax. This is often referred to as the economic yield target (Ytarget) as it is often not economical to close the remaining gap of about 20% of Ymax. In reality actual farmer yields (Yf) are much lower because of a range of constraints to crop growth, including weed pressure, pests and diseases and sub-optimal soil fertility and water management practices.

A first approach to try to understand causes of low yields is to compare average yields in the village with the yields best farmers obtain. Discussions with farmers may give hints about what ‘best farmers’ do differently. This will help to identify the causes of the differences, e.g. weeds, pests or diseases (reducing factors), and will also provide the scope for short term improvement (yield gap 1 = best farmer yield – average yield).

Crop growth simulation models can be applied to determine the attainable yield ceiling under given growth conditions (yield gap 2 = attainable yield ceiling – best farmer yield). This ceiling is limited by nutrients and / or water (the limiting factors). Finally these models can also be used to determine potential yield, i.e. when sufficient water and nutrients are available. It should be realized that these yield gaps give indications about what is agronomically possible, not what would be economically optimal.

Crop growth simulation models may also be helpful to analyze farmer management practices, and identify areas for improvement.

Figure 1: Effect of crop management on potential or maximum yield, attainable yield, best farmer yield and actual average farmer yield.

When analyzing growth reducing and limiting factors, soil fertility will often be one of them. It should be realized, however, that crop growth in farmers’ fields may also suffer from other factors, such as drought or excessive flooding or from incidence of pests, diseases and weeds. Current management practices may prevent the farmer from obtaining better yields, such as choice of variety, plant population, sowing data and the type of fertilizer applied. In the latter case, crop response to fertilizer application may be disappointing due to the fact that the type of fertilizer applied does not match the requirements of the soil, e.g. soils that are low in K will not respond to large doses of N or P.

Supplementary Reading

Lesson 3.1.2: Soil Testing

Soil testing is any chemical or physical measurement that is made on a soil. Soil tests are done to:

  • Determine the relative ability of a soil to supply crop nutrients during a particular growing season,
  • Predict the probability of obtaining a profitable response to fertilizer application,
  • Determine the need to adjust soil pH,
  • Diagnose problems such as excessive salinity or alkalinity,
  • Provide a basis for fertilizer recommendations for a given crop,
  • Evaluate the fertility status of the soil as the basis for planning a nutrient management program.

A soil test report will give basic information about deficiencies and problems and suggest measures that should be taken to correct problems and specific nutrients that are needed to obtain better yields. Soil tests are considered to be a helpful diagnostic tool but do not provide absolute recommendations. The information they provide must be interpreted using common sense and consider the goals and circumstances of the grower. A key point to remember is that the test only provides information about the fertility level and chemical properties of the soil. Correcting these is only one part of a growers crop management program. There are many other factors that may result in low yields even when nutrients are adequate.

Remember, a soil analysis is only as good as the soil sample taken. If the sample submitted for testing is not representative of the actual status of the field, the results and recommendations will not be very valuable and will probably be misleading. It is therefore important that you know proper procedures to follow when collecting soil samples.

Participants should review the supplementary readings below for more information on soil testing and soil sampling. For some examples of calculating and interpreting N, P, K soil test results in terms of nutrient availability for the crop see the insets below.

See:

Nitrogen

Nitrogen is vital for plant growth, and is an important component of plant proteins. It is a very mobile nutrient and can move relatively quickly by infiltrating rainwater or by changing from a soluble form in soil solution to gaseous forms that eventually escape into the air. Because it is so mobile nitrogen moves rapidly to the growing part of the plant, and often produces a green ‘flush’ at the start of the wet season. After a dry period soil life regains momentum, and rapidly decomposing organic material generates a sudden increase in available N. If there are no roots to capture the flush, significant quantities may be leached and eventually lost. Plants lacking in nitrogen turn a light green/yellow, especially the older leaves.
Nitrogen is unstable, and when it is applied in mineral fertilisers it can easily be lost through leaching and N-carrying gases. Such losses can be reduced by applying nitrogen fertilisers at several intervals during the wet season, particularly after rain or during rainy periods, when there is less risk of it burning the crop. Sources of organic matter which increase nitrogen levels in the soil include animal manure, compost and green manure.
For nitrogen, the fertility status of a soil can be estimated by observing its surface colour, texture and structure. A dark, clayey and well-structured soil with plenty of active soil fauna (especially worms) indicates good levels of nitrogen.
Table 3.4 gives an idea of the quantities of nitrogen in different kinds of soil. The total reserve of nitrogen in the soil can be estimated by measuring the percentage of soil nitrogen. About half of this reserve will always be available as it is stored in the relatively active form of organic matter. This part is called the dynamic reserve, and it will give an indication of the length of time that crop production is potentially possible. As reserves diminish it becomes increasingly difficult for a crop’s roots to find the nitrogen they need, and consequently yields decline. The other half of the total nitrogen reserve, a fraction of organic matter that does not easily release its nutrients is called the inert reserve. It should be noted that only about 1-4% of the dynamic reserve is directly available for crop production, and this is subject to losses.

How to estimate soil’s nitrogen reserves?
For light, sandy soils one can assume that 1 litre of soil weighs 1.5 kg. Soils with a higher clay content tend to be heavier, weighing about 1.7 kg/l.

If we take the top 20cm of soil the volume of one hectare will be:
10,000 m2 x 0.2 m depth = 2,000 m3 of soil; or
2,000 x 1,000 litres = 2,000,000 litres.

This equals 2,000,000 x 1.5 kg of soil = 3,000,000 kg.

A soil with a good supply of nitrogen contains about 0.1% nitrogen (see Table above), so one hectare will contain 3,000,000 kg x 0.1/100 = 3,000 kg of nitrogen. Of this, 1500 kg (50%) represents the ‘dynamic reserve’. Only about 1-4% of this is directly available for crop production, and is subject to losses of between 15-90 kg.

This estimate of available N shows the relative importance of nutrient recycling during crop production. The ratio of nitrogen exported (through removing crops) : the dynamic reserve indicates how long a farmer can afford to continue extracting N without replacing it. If, for example, the dynamic reserve is less than 750 kg/ha and a crop annually exports 75 kg/ha, crop yields will soon drop.

Phosphorus 

Phosphorus is a basic nutrient which, like nitrogen, contributes to essential proteins in the plant. As it is not a mobile nutrient and cannot easily be lost through leaching, applying P-fertiliser can be a good investment that will bring returns over many years. However, when organic matter is added to the soil phosphorus may become more mobile and erosion can also cause substantial losses when it removes the more fertile topsoil.

Plants that suffer from phosphorus deficiency tend to be stunted and often have dark green leaves and reddish-purple leaf tips.

Phosphorus can be applied in less soluble forms such as rock phosphate (which is especially recommended for acid soils), and is also present in organic fertilisers such as animal manure. Farmers using phosphorus fertiliser need to take account of the soil’s nitrogen status, as it is only economical to apply large quantities of phosphorus when there is enough N in the soil. Red soils tend to contain significant amounts of iron, and when phosphorous is added to such soils it may become fixed to the iron compounds. This means that the farmer will have to use large quantities of phosphorus fertiliser to ensure some phosphorous remains available for the plants.

In general there is not much phosphorus available in soils unless they are regularly fertilised. The P-reserve is mainly found in organic matter, and is fixed in barely soluble aluminum- and iron-compounds in the soil. In areas where the soil is aluminum-saturated, large amounts of phosphorus may slowly become available, keeping the P-reserve well supplied. In other areas, such as the Sahel, P reserves are more reliant on the SOM content, as is the case with nitrogen.

There are several ways of extracting phosphorus. The Bray method is used for more acid soils, while the Olsen method is more generally applicable. P-Bray gives an estimate of the phosphorus that is immediately available for crop growth. As a rule of the thumb, 6 or more mg/kg P-Bray shows that there is enough phosphorous for the coming cropping season; less than 6 mg/kg indicates a possible deficiency.

P-Olsen gives an indication of the total P reserve. A concentration of less than 200 mg/kg P-Olson shows a poor phosphorus reserve, while more than 800 mg/kg indicates a good P reserve. Sandy soils usually contain less phosphorus than soils with a high clay content, and a pH of less than 5.0 (acid soils) often means that very limited amounts of P are available.

The dynamic reserve of phosphorous is roughly 80% of the total reserve, of which 50% is found in the SOM. So a soil with a total P reserve of 2,500 kg/ha will contain a dynamic phosphorus reserve of about 2,000 kg/ha, of which 1,000 kg/ha is found in the SOM.

How to estimate soil’s phosphorus reserves?

This estimate assumes that we are looking at a soil whose top layer weighs 3,000,000 kg per hectare. In that weight of soil we could expect to find about 6 ppm (parts per million or mg/kg) P-Bray, which amounts to 18,000,000 mg or 18 kg of available P per hectare (see Table above). To gauge the significance of P extraction we compare the amount of available P per hectare with the amount of P exported in crops and residues removed from the field. A cotton crop yielding 2,000kg of fibre and 3,000kg of straw exports about 11 kg of P, which means that more than half of the immediately available P in the soil is removed with the crop.

Potassium

Potassium is essential to plants for the formation and transfer of carbohydrates in photosynthesis, and also for protein synthesis. It is particularly important for fruits, leaves and stems, and is needed to strengthen the plant’s structure. Potassium deficiency in plants can be quite difficult to detect, but indicators are yellowing leaf tips and margins, and increased lodging.

Potassium promotes high crop yields, particularly in root and tuber crops. Farmers aiming to maintain maize yields of over 2 tons per hectare will need to apply extra potassium, as well as large amounts of nitrogen and phosphorus. Crop residues often contain considerable quantities of this nutrient, so it is important to recycle them to maintain the soil’s potassium levels. Mineral fertilisers do not always contain potassium, so that mining can take place.

Potassium reserves are largely dependent on the type of soil minerals present. Soil potassium may be classified according to its availability to plants, and falls into three categories: (1) the inert reserve or slowly available K; (2) the dynamic reserve and (3) the readily available reserve.

The inert reserve or slowly available potassiumconstitutes about 95% of the soil reserve, and is mainly contained in primary minerals and clays such as vermiculiteor illite. These minerals release K very slowly, and the amount released over a single growing season is negligible.

The readily available potassium is measured by the exchangeable K (expressed as cmol(+)/kg) absorbed in the clay-SOM complex and found in the soil solution. Normal (kaolinitic) soils contain about twice as much K in theirdynamic reserve as in their readily available reserve.

When plants take up potassium the equilibrium between the dynamic and readily available reserves is temporarily disrupted. Some of the exchangeable K must then be immediately released into the soil solution to re-establish this equilibrium. Clayey soils contain more potassium than sandy soils, so that a soil containing about 40 % clay has four times more exchangeable K available than a soil with only 10 % clay.

How to estimate the availability of potassium
This estimate assumes that we are looking at a soil whose top layer weighs 3,000,000 kg/ha.
Exchangeable potassium is expressed in cmol(+)/kg, which originates from the atomic weight of the element, 39. One cmol(+) corresponds to 39 ¸ 100= 0.39g. This weight is always expressed per kg soil.
So, if a soil contains 0.10 cmol(+)/kg soil of exchangeable K, the total amount of readily available potassium for the given soil layer will be:

3,000,000 kg x 0.1 x 0.39/1,000 = 117 kg/ha.

The ratio of potassium exported through crop removal : exchangeable K indicates how long a farmer can afford to continue extract K without adding fertiliser.

Supplementary Reading

Lesson 3.1.3: Plant Tissue Analysis

Plant tissue analysis is a way to measure the nutrients actually taken up by the plant and is another aid in diagnosing crop nutritional problems. Plant analysis is often used to confirm soil test results and can indicate when the cause of the problem is something other than a nutrient deficiency in the soil. For example, if the soil test level is adequate but the plants are deficient, some other factor is limiting the plant’s ability to take up available nutrients. Possible explanations include the effects of crop management practices like tillage or pesticide use, pest injury, varietal characteristics and soil physical conditions. Plant nutrient content represents the effects soil nutrient status and all the other factors controlling plant growth.

Just as in soil testing, sample collection is very important. The nutrient concentration in a plant varies with the plant’s age and the part of the plant sampled. If plant analyses are to be meaningful, the appropriate plant part must be collected for the age of the plant, and a number of plants must be included to obtain a representative sample. Samples should be taken from the problem area and a nearby “normal” area for comparison. Specific directions on plant sampling generally are available with each sampling kit from the plant analysis laboratory.

For more information on plant tissue analysis and tissue sampling, please read through the supplementary references below.

Supplementary Reading

Lesson 3.1.4: Field Observation

Of course, it is realized that laboratory testing of soil and plant tissue samples are not generally economic or even possible options for most farmers in developing countries. But there are many other tools. Making timely and focused observations in the field has been a valuable way to diagnose problems since the dawn of agriculture and continues to be the most common and valuable way to identify deficiencies and the basis for interventions.

Probably the simplest approach in observation is the transect walk. A grower or agriculturalist can acquire a tremendous amount of information just by walking through a field or production area and noting what looks good and what doesn’t. In a transect walk, the diagnostician walks from one edge of the field or area to the other and makes notes on what is observed including the types of landscape, plants and animals and obvious differences that may be problems that need addressing. Many find it helpful to draw the walk on paper in what is known as a transect map. Such maps show a topographical cross-sections of the territory and are made more useful if notes are made below each portion of the cross-section. See below for an example of a transect map:

A bit more resource intensive but also more valuable is to prepare a more detailedresource map of the entire area in which you are interested. Resource maps are physical maps that identify land use systems and help to graphically illustrate the spatial relationships between different land use systems. Resource maps can be an aid in assessing (potential) conflict between land-use systems and available resources.

Lesson 3.1.5: Diagnostic Keys

Of course, it helps if you can recognize problems as you observe fields and areas.Keys are an excellent tool for helping you do this. Diagnostic keys provide a systematic approach to observing plant and crop systems and help to narrow down the possibilities. In order to use them effectively, however, you will need to be familiar with a few fundamental terms used to describe observed symptoms.

Some of the most common nutrient deficiency symptoms used in keys are listed and defined in the table below:

Term Definition and comments
Chlorosis General yellowing of the leaf tissue. A very common deficiency symptom, since many nutrients affect the photosynthesis process directly or indirectly.
Firing Yellowing, followed by rapid death of lower leaves, moving up the plant and giving the same appearance as if someone touched the bottom of the plants
Interveinal Chlorosis Yellowing in between leaf veins, but with the veins themselves remaining green. In grasses, this is called striping.
Necrosis Severe deficiencies result in death of the entire plant or parts of the plant first affected by the deficiency. The plant tissue browns and dies. The tissue which has already died on a still living plant is called necrotic tissue.
Stunting Many deficiencies result in decreased growth. This can result in shorter height of the affected plants.
Abnormal coloration Red, purple, brown colors caused by pigments

See our guide to Plant Nutrient-Deficiency Symptoms

I. Effects general on whole plant or localized on older, lower leaves 2
2. Leaves light green. Uniform chlorosis of older leaves, which may die and turn brown. Abnormal production of anthocyanins in stems and leaves. Stems with greatly reduced terminal growth Nitrogen
2 Leaves dark green. Stunted growth. Abnormal production of anthocyanins resulting in red and purple colors. Death of older leaves. Stems weak and spindly Phosphorus
II. Effects mostly localized on older, lower leaves 3
3. Older leaves chlorotic, initially interveinal, beginning at tips of leaves. Margins and tips of leaves may turn or cup upward. If severe, all leaves become yellow or white. Older leaves may drop off. Magnesium
3. Older leaves mottled, with necrosis of leaf tips and margins. Leaves may curl and crinkle. Internodes abnormally short and stems weak, sometimes with brown streaks. Potassium
III. Effects localized on new leaves 4
4. Terminal bud dies. Tips and margins of youngest leaves necrotic and then buds. Initially young leaves pale green with hooked tips, as well as being deformed Calcium
4. Terminal bud remains alive 5
5. Leaves light green (never yellow or white), beginning with younger ones. Veins lighter than interveinal areas. Necrotic spots may appear but not common. Sulfur
5. Leaves chlorotic, beginning with younger ones. Veins remain green, except in case of prolonged, extreme deficiency. Iron

Source: http://scidiv.bcc.ctc.edu/rkr/Botany110/labs/pdfs/MineralNutrition.pdf

When using the keys you will notice that many of them start by asking where the symptoms are most evident on the plant. This is because different nutrients exhibit different patterns of nutrient mobility.

Mobile nutrients can be translocated from old tissue (bottom of the plant) to new tissue (top of the plant). Nutrients such as potassium and magnesium, which are highly mobile in the plant, show deficiency symptoms in the older leaves. Nutrients such as calcium boron, copper, iron, manganese, molybdenum, and zinc, which have a low mobility in the plant, show deficiency symptoms in the younger leaves. Nutrients such as nitrogen, phosphorus and sulphur, which have a medium mobility in the plant, show deficiency symptoms evenly spread over the plant.

Another important factor to keep in mind when using keys or when observing symptoms is that deficiency symptoms can often be confused with other complex field events, such as high water tables, salt damage, disease, drought, herbicide stress and varietal differences. The appearance of a growth disorder based on visual symptoms does not absolutely mean a nutritional deficiency exists. The observation of a symptom could also be a result of nutrient unavailability or other environmental factors and not to the absence of a particular nutrient in the soil. If more than one deficiency is present, one can be more dominant in its symptoms, obscuring the symptoms of the other element.

For more information on using keys and some examples, we suggest you visit the sites and resources listed below.

Supplementary Reading

Lesson 3.1.6: Photographs

We are sure that you have all heard the old saying, “A picture is worth a thousand words.” Many people find it much more useful to be able to see what a particular deficiency symptom looks like rather than just reading a description. A good source for pictures of common local nutrient deficiency symptoms is your local extension office. Also, as more and more organizations take advantage of the Internet for disseminating information, it is becoming easier to access quality pictures online.

Lesson 3.1.7: History and Record Keeping

Photographs of and keys to nutrient deficiencies are useful in diagnosis, but field experience and knowledge of field history based on local experience is the best diagnostic aid. Good records can provide valuable insights into potential nutrient deficiency problems even when there are no obvious symptoms. Probably more common than acute deficiencies associated with a particular nutrient is the phenomenon of sub-clinical deficiency. Sub-clinical deficiency is said to occur when there is a reduction in yield or yield potential without the visual symptoms of deficiency being seen. Many crops fail to live up to expectations without obvious cause, and a high proportion of these cases can be put down to sub-clinical nutrient deficiencies. Accurate accounting of nutrient removal and replacement, crop production statistics, and soil analysis results will help the producer manage fertilizer applications.

Lesson 3.1.8: Nutrient Flow Analysis

Accurate historical records can be valuable but keeping such records is not all that common, particularly for developing country farmers. One way to get a handle on what is happening to the nutrient status of a field over time is to analyze and map nutrient flows.

Nutrient flow analysis can be used to give insight into the impact of farmer management decisions on soil fertility in his or her farm. Farmers transport material that contains nutrients – be it harvested products, manure, fertilizer or straw that is used to build roofs. Some processes may lead to a loss in nutrients, e.g. burning of straw will result in complete loss of carbon and nitrogen. Estimating nutrient flows is a useful way to find out if farmers’ crop management practices are sustainable, i.e. are outputs of nutrients balanced by a sufficient level of inputs.

To compare flows, there is a need to express them in the same unit, e.g. kg of nitrogen, phosphorus or potassium. This means that one needs to know the concentration of nitrogen in e.g. manure, millet grains and millet straw, etc. and the amount of dry matter (at 0% moisture) that is produced, transformed or transported. Nutrient flow analysis should enable a farmer to answer questions such as: ‘ What is happening to my soil if I do not apply any fertilizer to my rice field, and I sell both rice grain and rice straw?’ It is important to realize that such analyses try to model a complex reality and should, therefore, used with care. Boundaries of the farming system that is analyzed, and boundaries of its subsystems (e.g. rice production system, vegetable production system, animal production system, and household system) should be clearly defined. A wealth of literature is now available demonstrating the nutrient budgeting approach and there are links to some good references in the Supplementary Reading section below.

The nutrient balances include, on one hand, major nutrient inflows from rainfall, organic manure, mineral fertilizers, symbiotic N-fixation and sedimentation; on the other hand, nutrient outflows through harvested produce and losses due to erosion, leaching etc. For a given soil nutrient (usually N, P or K) the equation reads:

Balance = [IN1 + IN2 + IN3+ IN4 + IN5 + IN6] – [OUT1 + OUT2 + OUT3 + OUT4 + OUT5 + OUT6]

where:

IN1 = mineral fertilizers; IN2 = animal manure; IN3 = atmospheric deposition; IN4 = biological nitrogen fixation; IN5 = sedimentation; IN6 = uptake by deep-rooted plants; and OUT1 = harvested production; OUT2 = crop residues; OUT3 = leaching; OUT4 = gaseous losses; OUT5 = soil erosion; OUT6 = losses in deep pit latrines.

Clearly some of these parameters are easier to measure or estimate than others. Nutrient inflows from atmospheric deposition, or losses as gases are invisible and not easy to comprehend by farmers. Often, estimations are combined with actual measurements, which may lead to considerable errors. A simple method to get an idea of nutrient flows associated with a farm or larger area is to develop resource maps.

If nutrient flow analyses are done with farmers it is important to realize that farmers do not think in terms of kg per hectare, but rather in terms of head loads, bags, cans, acres, carrés, etc. and one should as much as possible use these terms as tools of analysis. Such discussions will, therefore, often be more qualitative than quantitative, but can still give important insights, pinpointing e.g. at ‘ leaks’ in the system (e.g. unused animal manure, burning of straw).

Supplementary Reading

Lesson 3.1.9: Resource Flow Mapping

Resource flow mapping consists of making a simplified picture (map) of the farm system and its resource flow pattern, including elements that are crucial in soil fertility management. To make a resource flow map, first draw farm fields and other farm elements such as buildings, grazing areas and compost pits. Then for each field, both present and previous crops are noted and arrows are drawn representing resource flows between fields and other farm elements. Arrows indicate the use of crop products and residues leaving the fields and organic fertilizers produced on-farm, entering the fields. They are also used to show resources leaving and entering the farm such as products sold and mineral fertilizers purchased.

The resulting picture presents an overview of how the farmer actually manages the fertility of his lands, and depicts interactions (or absence of interaction) between farm elements and elements outside the farm. In this process, elements that initially were ‘invisible’ to the farmer are thus made more explicit and ‘visible’. Only the essential elements of the complex farm system are presented within an overall picture that is drawn on a single sheet of paper. This picture permits the analysis of strong and weak points in management, in view of identifying possible improvements.

Below are examples of resource flow maps prepared by farmers.

http://www.nutmon.org/files/6d2413cec969e35b850c104fe40deb9e.pdf

Supplementary Reading

Lesson 3.1.10: Computer-based Diagnostic Tools

No discussion of diagnostic decision support tools would be complete without some reference to the now available computer based tools. Various software models and applications can help to quantify, calculate and visualize nutrient flows (NUTMON), calculate optimal fertilizer doses / ratio’s (NuMaSS, QUEFTS), simulate important aspects of an agricultural system (e.g. a model that simulates the development of soil carbon over a number of years: the Rothamsted Carbon model), and dynamic models that mimic the most important processes of the system of interest (e,g, models that simulate the effects of weather, soil, crop characteristics and crop management on yield, such as DSSAT, COTONS, APSIM and RIDEV)

To learn more about these tools, click on their names below to popup a window containing additional information. Below you will also find a link to an article describing how one of these tools was used in an ISFM program.

  • DSSAT – Decision Support for Agrotechnology Transfer
  • APSIM – Agricultural Production Systems Simulator (APSIM)
  • RothC-26.3 – THE ROTHAMSTED CARBON MODEL
  • NUTMON – Monitoring nutrient flows and economic performance in tropical farming systems
  • COTONS
  • NuMaSS – NUTRIENT MANAGEMENT SUPPORT SYSTEM

Lesson 3.2: Action Planning

Once promising improved soil fertility management options are identified, joint experimentation can be planned with farmers and change agents to test and verify potential interventions. During this phase, farmers should be encouraged to come up with their own ideas. Planning should take place during one or several joint meetings between farmers and change agents where the outcome of the Diagnosis phase is discussed and topics for experimentation of different ISFM options are debated. The results from the learning and decision-support tools developed in the Diagnosis phase can be used to guide the discussions. The outcome of this phase is a timetable for the next growing season(s). This Action plan calendar shows when certain experiments or training sessions will be conducted, when field visits or monitoring tours are planned, and may also include scheduled meetings with local input dealers or credit providers for certain ISFM options. The Action plan calendar will also clearly highlight the division in responsibilities between farmers and change agents.

Lesson 3.3: Trying Things Out

Once the Action plan calendar is developed, ISFM ‘learning plots’ (i.e. fields that are proposed by farmers to be used for joint experimentation and learning) can be established around certain ISFM options. The ISFM learning plots should be followed frequently throughout the season. Field observations and participatory analysis (learning processes instead of comparing just one or two options) are key here. Farmers should be encouraged to make observations and take notes. Ideally farmers keep records of ISFM management practices, i.e. how things were done in practice and keep records of ‘observation indicators’ (e.g. plant height, weed infestation, quality of land preparation, etc.). Such forms need to be developed with farmers, and should be easy to fill in. Visual aids, like drawings and photographs can be useful. Such forms become important learning tools, give a record of cropping history and can be used in farmer discussions.

Farmers should try out new things for themselves. Successful ideas spread rapidly and never more so than when the ideas are developed by farmers themselves. ISFM learning plots will usually focus on a restricted set of management interventions and are farmer-led. Experimentation may deal only with soil fertility related issues, like a certain combination of mineral fertilizer and organic amendments, but may also address other issues that reduce the efficiency of external inputs, such as water and weed management.

ISFM learning plots can be complemented by more detailed analysis of what nutrient is limiting growth. In systems where farmers have the possibility and the means to apply fertilizer, nutrient-omission trials can be installed. Such trials deliberately omit one nutrient to investigate its importance. Through the yield obtained on the plot you get an idea of the supplying capacity of the soil for the nutrient that was omitted. Such trials are very useful, as soil tests are beyond the means of the average farmer and results of soil tests do not always correspond to crop performance, especially for N. The trick is to place the trials at representative sites, on different major soil types, and on sites with different cropping history, such as close to a village, far away etc. Nutrient omission trials are not repeated in one farmers’ field, but each participating farmer is one repetition. Good management of such nutrient omission trials is important, to ensure that nutrients are determining crop growth, and not other factors, such as weeds, diseases, pests, water shortage etc.

Some trials take the form of a fertilizer strip test. In very general terms, this involves alternating strips of a specific fertilization rate or application method with the normal practice. It is important that only one fertilizer variable be changed when comparing two treatments so that valid yield comparisons can be made.

The objective of a strip test is to measure crop yield performances with and without the additive or amendment. Some of the key things to keep in mind when doing such a comparison include:

  • Applying the test material or practice in several strips across the full length of the field. The treated strips should be alternated with untreated strips of the same size. The strips should be wide enough to allow for easy segregation and measurement of crop yield from each field strip, both treated and untreated.
  • The soil fertility and cropping history should be uniform in the field to be used for the trial.
  • The field should have a uniform crop stand and been uniformly fertilized in the previous and current season. If the field is irrigated the frequency and amount of irrigation should be uniform during the test period.
  • Yield data should be collected from each strip. If feasible, it is desirable to take yield subsamples within the treated and untreated strips to obtain an estimate of field variability.

Supplementary Reading

Lesson 3.4: Evaluation

Evaluation is a continuous process during the cropping cycle. ISFM learning plots should be regularly visited (ideally at least weekly), and compared with farmer practice. Farmer meetings and wrap-up sessions at the end of the season allow discussion of what worked and what didn’t. If monitoring of the experiments was done well (i.e. frequent good observations and sound analysis) recommendation domains can be established for each ISFM option. This will allow certain ISFM options to be fed forward to the DATE-extension cycle. Gradually key villages may become knowledge centers in soil fertility management and may even take a lead role in farmer-to-farmer training.