🌿 Soil Fertility and Weed Control

Every tonne of root that leaves your field takes soil with it. Not soil you can see going, but nitrogen, phosphorus and potassium that were in the ground before planting and are now in a truck. That is the whole of soil fertility in one sentence, and the rest of this lesson is about how much.

Here is where this course does something unusual, and you should understand why. The reference this course is built on contains two nutrient removal figures from the same source that do not agree with each other. This compilation could not reconcile them. So you are getting both, with the disagreement stated openly, rather than one confident number that might be wrong.

The first figure. An IAEA cassava production guide gives removal as 4.1 kg of nitrogen, 1 kg of phosphorus and 8.3 kg of potassium, per tonne of dry-weight-equivalent roots.

The second figure. The same guide, in its fertiliser recommendation form, gives 2 kg of nitrogen, 0.7 kg of P2O5 and 2 kg of K2O for every metric tonne of fresh root yield.

And the same guide gives a third shape of the answer, a recommended ratio of 3 N to 1 P2O5 to 3 K2O for a 15 tonne per hectare target yield.

Look at potassium in the first two. One says 8.3 kg, the other says 2 kg. That is a fourfold difference, and it is not a typing mistake in this course.

Part of the explanation is visible in the wording. The first figure is per tonne of dry-weight-equivalent roots. The second is per tonne of fresh root yield. Fresh cassava root is mostly water, so a tonne of fresh root contains far less dry matter than a tonne of dry-weight-equivalent, and figures on those two bases can never be swapped for one another. On top of that, the first is a removal measurement and the second is a fertiliser recommendation, which are different things in principle: a recommendation may already assume some soil supply.

But knowing why two numbers differ is not the same as knowing which one to plan with, and this compilation could not close that gap. The reference flags it explicitly: a single reconciled nutrient removal table for cassava was not retrieved, and the CIAT reference chapter on cassava mineral nutrition that would most likely settle it could not be obtained. That figure must be obtained from that chapter or an equivalent authoritative source. This course will not average two numbers on different bases and hand you the average as fact.

There is a third data point worth holding beside them. A potassium fertilisation field trial measured total potassium removal at 99 to 142 kg of potassium per hectare at 10 months, across a range of potassium fertiliser treatments. Notice what that does to the arithmetic. If you take the fresh-yield recommendation of 2 kg K2O per tonne and a 15 tonne per hectare crop, you get 30 kg K2O per hectare. The measured trial removal is several times larger. The numbers do not line up, and pretending otherwise would not help you.

So what can you actually rely on? One thing, and it is the thing every source agrees on. Potassium removal is large relative to nitrogen and phosphorus removal. Cassava is a heavy potassium feeder compared with a cereal crop on the same land, and a light nitrogen and phosphorus feeder compared with maize.

That single agreed direction is enough to change what you do, even without a settled number. It tells you which nutrient to worry about first, which is the opposite of the answer for maize. It tells you why continuous cassava on one block empties the soil of potassium in particular. And it tells you that a fertiliser bought out of cereal-farming habit, heavy on nitrogen, is aimed at the wrong nutrient for this crop.

Do the arithmetic for your own field, but do it on one stated basis at a time and write the basis down beside the answer. A calculation whose basis is not written down becomes a wrong number within a season, because nobody remembers which figure it came from.

Cassava's reputation is true. It really does produce a crop on land where maize gives you nothing. The mistake is in what people conclude from that.

Tolerating poor soil and maintaining poor soil are two different things. Cassava tolerates low fertility because of a deep, efficient root system and comparatively low nutrient demand per unit of biomass compared with cereals. It still removes nutrients. It just complains less while doing it. That is the double edge: cassava will produce where other staples fail, but continuous cassava cropping without fertiliser replacement can still mine the soil of potassium in particular. That is a documented finding, not folklore.

Think about what tolerance actually means for you as a business owner. A crop that shows you when it is hungry is giving you information. Maize on exhausted land goes yellow and short and you know. Cassava on exhausted land still gives you roots, fewer and smaller, and it gives them for several seasons while the potassium drains away. By the time the decline is obvious, several seasons of removal have already happened. Tolerance delays the warning; it does not cancel the bill.

Now the pH question, because it is the other half of the tolerance story. Cassava is repeatedly cited as tolerating an unusually wide soil pH band, from about pH 4.5, which is acidic, to about pH 8.0, which is alkaline. That is one of the widest pH tolerance ranges of any major African staple crop, and it is the practical reason cassava is recommended on acid, low-fertility soils where cereals struggle.

So if someone tells you your soil is too acid for maize, that is not automatically a reason to keep cassava off it. Within that band, acidity alone is not the deciding factor for this crop.

But a wide tolerance band is not a permission slip. The potassium trial in the next lesson was run on soil at pH 4.5, the very bottom of the tolerated band, and the crop still responded strongly to potassium fertiliser. Tolerating the pH and having enough nutrients are separate questions and you must not let one answer the other.

Which brings us to soil testing, and to a figure worth thinking about carefully. That same trial soil carried 164 mg of exchangeable potassium per kg of dry soil, and the study rated that as adequate by its own criteria. Read those last four words again. Adequate by the study's own criteria. Whether a soil potassium level is adequate is a relative, tested judgement made against stated criteria, not a number you can guess by looking at your land or by comparing with a neighbour.

So your own soil test, from your own national soil-testing service, is the only way to know your starting point. This course gives you removal rates and response evidence. It cannot substitute for a local test, and no honest source can.

If a soil test is out of reach this season, and for many farms it will be, you are not helpless. You have a history. Write down what this field has grown for as many seasons back as you can remember. Mark every season that was cassava. Mark every season that received manure, compost or fertiliser. A field on its fifth consecutive cassava crop with nothing ever put back is a field with a potassium problem, and you did not need a laboratory to work that out.

The sourced answer to that situation is not complicated. Continuous cassava monocropping without any nutrient replacement will progressively deplete soil potassium, and rotation, fallow or a maintained fertiliser programme is the sourced answer to it. Notice there are three answers there, not one. Rotation and fallow cost you land and time rather than cash, which on many farms is the only currency actually available. A maintained fertiliser programme costs cash. Which of the three you choose is a business decision about what you have, and Module 5 already put site selection at step one of the six-steps framework partly for this reason: choosing to rotate rather than replant the same block a fifth time is a fertility decision made before any land is cleared.

One last thing to be honest about. If your field has been mined for years, no single season of fertiliser turns it into good land. Restoring fertility is a programme measured in seasons. Start it, record it, and judge it over years rather than expecting one bag to settle the account.

Lesson 2 said the soil is being emptied. This lesson is about putting something back, and it starts with the strongest piece of evidence in the whole reference.

A potassium rate field trial ran five treatments, from 0 to 99.7 kg of potassium per hectare, and measured what happened.

At 0 kg of potassium per hectare, yields were 9.9 tonnes per hectare at early harvest and 15.9 tonnes per hectare at late harvest.

At 99.7 kg of potassium per hectare, yields were 14.6 tonnes per hectare at early harvest and 20.16 tonnes per hectare at late harvest.

Stated as a response, potassium fertilisation increased yield by up to 39 percent at early harvest and 21 percent at late harvest, compared with no potassium.

Read the two response figures against each other, because the gap between them is a planning fact and not a rounding difference. The relative benefit of potassium is smaller at the later harvest. Left in the ground longer, the unfertilised crop caught up somewhat on its own, from 9.9 to 15.9 tonnes. So the value of your fertiliser depends on when you intend to harvest. If you must harvest early, for cash or for a processing contract, potassium is doing more work for you than if you can afford to leave the crop standing.

Starch responded too. Starch content rose from 29.4 percent to 33.1 percent between early and late harvest in the same trial, which is a relative increase of about 12.5 percent, and that arithmetic is derived from the two sourced percentages rather than being a separately published figure. If you are selling into starch or flour processing, that matters as much as tonnage.

Now the warning inside the same trial, and it is the reason this lesson is not simply telling you to buy potassium. The highest potassium treatments showed reduced starch at late harvest. The source attributes that to possible magnesium deficiency caused by potassium and magnesium competing as cations in the soil. In plain terms, pushing one nutrient very hard can block another. So the lesson is not maximise potassium. The lesson is correct the shortage without unbalancing the rest.

What about nitrogen and phosphorus? The sourced position is that nitrogen and phosphorus needs per tonne of root produced are comparatively modest. Over-applying nitrogen to cassava is a common mistake carried over from cereal-farming habits, and it can encourage excessive leaf growth relative to root bulking. You should know the limit of that claim: a sourced quantification of that specific leaf-versus-root tradeoff was not retrieved in this compilation. So there is no number here for how much nitrogen tips a plant into leaf at the expense of root. That figure must be obtained from a source that measured it. What you have is a clear direction and no measurement, and you should treat it that way.

Now the options themselves, organic and inorganic, and here you have to be realistic about what a smallholder can obtain.

Inorganic fertiliser is bought, it is concentrated, and it acts on the nutrient you choose. Which products are available, at what price, under what subsidy scheme, and what the recommended rates are for your zone, are all market and authority questions. Ask your national extension service or agricultural ministry, not a seller with stock to move. This course cannot name a product or a rate for your country, and any source that does without knowing your country is guessing.

Organic options work differently. Manure, compost and crop residues return nutrients that were already in the system, and rotation and fallow let the soil recover without a purchase. Legumes deserve a specific mention here. Module 5 recommended cowpea and groundnut as companion crops partly because legumes fix their own nitrogen, so they answer the soil question rather than compete on it. On a farm without cash for fertiliser, the rotation and legume route is not a poor substitute for the real answer. It is one of the three sourced answers.

Finally, cost the decision instead of guessing at it. Take the price you were actually quoted, this month, in your own market, and write the date beside it. Work out what the fertiliser costs for your field, and what extra tonnage you would need to cover it at the price you actually get for roots. Any published price in any course is stale, and prices in this reference are flagged as unverified for exactly that reason. The method survives; the price does not.

There is one idea in weed science that is worth more than every technique put together, and it is this: weeds do not cost the same amount at every point in the crop's life. There is a window in which weed competition does irreversible damage to final yield, even if you remove the weeds afterwards. Weed inside that window and you keep your yield. Weed after it and you have a tidy field with the yield already lost.

For cassava, the sourced window is stated plainly. An IITA weed-management synthesis says cassava is most susceptible to weed encroachment in the first 10 to 16 weeks of cultivation, with active weed management needed until canopy closure at roughly 20 to 24 weeks after planting.

Read those as two different things, because farmers regularly collapse them into one and lose money doing it.

Weeks 1 to 16 are the critical period. Damage done here is not recoverable by later weeding.

Weeks 16 to 24 still need active management, but the crop is now helping. Once the canopy closes, weeds face real competition for light from the crop itself and they stop being the emergency.

Why is cassava so exposed in the first place? Not weakness. Structure. Cassava's own canopy takes a long time to close, so full ground shading arrives only at roughly 20 to 24 weeks. That leaves a long window in which weeds face no competition from the crop canopy for light. Maize covers its ground far sooner. Cassava spends months with sunlight reaching bare soil between the rows, and something will use that light.

Now line up the critical period with what else the plant is doing, because the overlap is the reason the damage is permanent. Storage roots begin bulking from around the 45th to 60th day after planting. Day 45 is week 6 or 7. So bulking begins right inside the critical weed period. The plant is trying to start filling roots at exactly the time weeds are taking water, nutrients and light from it, and roots it fails to start filling then are not recovered by a clean field in month eight.

What does that cost? A properly weeded cassava farm is reported to achieve 30 to 50 percent greater root yield than a poorly weeded farm. That is a very large sourced range, and it alone justifies the labour or herbicide cost of weeding in almost any market condition. There is no fertiliser decision, no variety decision and no spacing decision in this whole course with a bigger single reported effect than that.

The supporting figure should be read more carefully. Farmers trained under the same six-steps programme achieved more than 20 tonnes per hectare against a cited Nigerian national average of 9 tonnes per hectare. That is striking, but it compares a trained and supported cohort with a national average, and the trained cohort likely also received improved variety, spacing and fertiliser advice bundled into the same programme. Treat it as the ceiling good practice can reach, not as the guaranteed result of weeding alone.

Now turn all of this into three dates, because a window you have not written on a calendar is a window you will miss.

Take your planting date. Add ten weeks and write that date down. That is when you should already be well inside a weed-free field, not thinking about starting. Add sixteen weeks and write that date. That is the end of the period in which damage is permanent, and your field must have been kept clean up to it. Add twenty to twenty-four weeks and write that range. That is when the canopy is expected to close and the pressure comes off.

Those three dates are your weeding plan's skeleton, and the work has to be scheduled before them rather than at them.

One more thing, and it is a labour point rather than an agronomy point. The critical period lands 10 to 16 weeks after planting, which for most farms means it lands after the planting rush is over and everyone has moved on to other work. That is precisely why it gets missed. Module 5 told you to arrange planting labour and first-weeding labour at the same time. This is the lesson that explains why: the half of the job that decides your yield is the half nobody has diarised.

You know when to weed. This lesson is about how, and about what it costs, which is the part that decides whether the plan survives contact with the season.

Start with where weeding sits in the framework Module 5 introduced. The IITA six steps are site selection; slashing existing vegetation; land clearing mechanically or with herbicide; ploughing and ridging; planting with pre-emergence herbicide application where used; and post-emergence manual and/or chemical weed control.

Notice that weed control appears in three different places. Steps 2 and 3 remove what is standing before you plant. Step 5 is where a pre-emergence chemical programme, if you run one, is set up on planting day. Step 6 is the follow-through during the crop's life. A farm that only thinks about step 6 is trying to win the critical period with the hardest and most expensive tool, at the point where the crop is already in the ground and the options have narrowed.

That is the single most useful practical insight about method. The cheapest weeding you will ever do is the weeding you did before planting, because the field is open, nothing is in the way, and you cannot damage a crop that is not there yet. Every week you delay pushes work into a period where it is slower, more careful and more damaging.

Manual weeding is what most smallholders will use for step 6, and there are two cautions worth stating. First, work close to the plant with care. Cassava roots are the product, and a tool driven in hard beside the stem can cut into the very thing you are growing. Second, do not let the field be weeded once and declared finished. The critical period is 10 to 16 weeks long, not a single day, and a field cleaned at week four and left alone until week fourteen has spent most of the critical period under competition.

Chemical control is the other route, and here the course stops and hands you to an authority. Which herbicides are registered for use in cassava, at what dose, at what stage, and with what protective equipment, is set by your national plant-protection or agrochemical regulator. Ask your extension service or that regulator. Do not take the answer from a seller with stock to move, and do not take a product recommendation from a course written for a different country. The principle this course will state is only this: use a registered product, at the registered rate, with the required protection, or use hand labour instead.

Now the cost figures, and read the caveat before the numbers.

The same source cites manual weeding labour costs of US$28 to 46 per hectare, representing 30 to 50 percent of total labour costs, and herbicide application costing US$20 to 30 per hectare. These are flagged as unverified as current figures for any specific country today. They come from one programme's cost accounting at one point in time. Do not carry them into your own budget as prices.

What you should carry away is the structure, and the structure is genuinely useful. First, weeding is a very large share of total labour cost, reportedly 30 to 50 percent of it. Weeding is not a small line in your budget, it is one of the largest. Second, herbicide can be cost-competitive with hand labour rather than being automatically the expensive option. Whether it is cheaper on your farm depends on your local labour rate and your local product price, both of which you can find out and neither of which this course can tell you.

So cost your own weeding, and cost it in hours first. Money changes; hours are more stable and you can measure them yourself. Take one measured portion of your field, weed it, and time the work honestly, including rests. Scale that up to your whole field. Now you have the labour hours for one weeding pass. Multiply by the number of passes you expect across the 10 to 16 week critical period, and you have the season's weeding labour requirement.

Only then put money on it. Multiply hours by the wage you actually pay in your area this season, and write the date beside the wage. If you are considering herbicide, get a current quoted price for enough product to treat that same area, plus the cost of the protective equipment, and compare the two totals honestly.

And hold the comparison against the yield figure from Lesson 4. Poor weeding costs 30 to 50 percent of root yield. Whatever the two methods cost, doing neither properly is almost always the most expensive option on the table.

Everything in this module now has to fit on one page for one field, because a plan spread across five conversations is not a plan. This lesson assembles it.

Start with the field's history, from Lesson 2. Write the cropping history season by season, marking every cassava season and every season anything was put back. That single column tells you whether you are managing a fresh field or restoring a mined one, and the two need different plans.

Second, write your realistic target yield in fresh tonnes per hectare. Realistic means what this field has actually produced, not a published average and not the trained-cohort figure of more than 20 tonnes per hectare, which came with a bundle of other advice attached.

Third, do the removal arithmetic, and do it honestly. Take your target yield in fresh tonnes and multiply by the fresh-yield figure of 2 kg N, 0.7 kg P2O5 and 2 kg K2O per tonne. For a 10 tonne per hectare target that is 20 kg N, 7 kg P2O5 and 20 kg K2O per hectare. Then write underneath, in your own handwriting, that a second sourced figure from the same source gives 4.1 kg N, 1 kg P and 8.3 kg K per tonne on a dry-weight-equivalent basis, that the two are on different bases, and that this course could not reconcile them. Writing the uncertainty down is not weakness. It is what stops you next year from treating one calculation as settled fact.

Fourth, decide your fertility route from the three sourced answers: rotation, fallow, or a maintained fertiliser programme. Write which one you are choosing and why, in one sentence. If it is fertiliser, name what you can actually buy locally, at a price you were quoted this month, with the date. If it is rotation or fallow, name the crop or the resting period and the block it applies to. Remember the trial's caution: correct the shortage, do not simply maximise potassium, because the highest treatments showed reduced starch at late harvest.

Fifth, write the weeding calendar from Lesson 4. Planting date. Week 10. Week 16. Weeks 20 to 24. Beside each date, write who does the work and how many labour hours you estimated from your own timed pass in Lesson 5. This is the part farms skip, and it is the part with a reported 30 to 50 percent of root yield attached to it.

Sixth, add the two budgets together. Fertility cost plus weeding cost for this one field, as one total. Deciding them separately is how a farm ends up buying fertiliser in month one and then finding no money for labour in month three, which is the worst of both worlds: you have fed a crop you are about to let the weeds take.

If the total is more than you have, cut it deliberately rather than by accident. Given the reported figures, weeding during the critical period is the harder thing to sacrifice, because its reported yield effect is 30 to 50 percent and the crop cannot recover it later. Fertility can be partly addressed by rotation and fallow, which cost land and time rather than cash. That reasoning is yours to make with your own numbers, but make it on paper and in advance, not by default in week twelve.

Seventh, and this is what makes next year better than this year, record the outcome against the plan. At harvest write the actual yield beside the target. Write what you actually applied, not what you intended. Write when the field was actually weeded, not when you planned to weed it. Two seasons of that record on one field is worth more to you than any general figure in this course, because it is your soil, your rainfall and your labour.

And one honest closing point. Nothing in this module protects a crop from the two virus diseases that Module 7 covers. A well-fed, well-weeded field planted with infected cuttings is a well-fed, well-weeded diseased field. Fertility and weed control decide how much a healthy crop yields. They do not decide whether the crop is healthy in the first place, and that question is answered by what you plant, which is where this course goes next.