Think Agronomy May

Think Agronomy Newsletter – May 2024

With good rain around there is a lot of potential in maize crops, so pushing them to reach their potential makes a lot of sense.

Topdressing is one of those inputs that can really make a big difference to yields, with the addition of nitrogen raising yields by another 30-60% typically in our trials. It varies enormously across the country however, with higher altitude and heavier texture soils with more clay often requiring slightly less nitrogen.

But topdressing comes at a cost, with fertiliser prices still relatively high and 50-80 kg/ha of nitrogen costing around 8-15,000 Ksh/ha from urea and even more from CAN.

Timing is critical. Maize takes up nitrogen slowly until around the V6 stage, when the crop’s requirement jumps substantially, from about 20kg/ha of N to about 80kg by V10. Miss this and the crop will never meet its potential.

By grain fill there is typically 140kg/ha of nitrogen in the crop, with a peak of around 200-250 at the milky grain stage.

Most growers will plant the crop with a fertiliser containing some Nitrogen such as DAP, MAP or a blend based around these, which might supply enough N for the first few weeks of the crop’s life.

On light soil however I often see crops run short of Nitrogen by V5, even where 40-50kg/ha of N was placed in the seedbed. In order to avoid this, the crop needed to be topdressed by the three leaf stage so that the Nitrogen made it in the crop in time. It frightens me how often I used to see this in Trans Nzoia and Nakuru especially.

Is it possible to topdress too early? The downside is the risk of leaching in heavy rain before the plant can take the nitrogen up, or excessive nitrogen causing suckers or side tillers to form. A little-and-often application is the best way to reduce the risk, and crops with deep roots always manage to find the nitrogen… put that disc plough in the scrap yard.

I generally advocate a split topdressing with the second dose at the 7-8 leaf stage, but there is nothing to stop you doing three splits through the crop’s life at V3, V6 and V8-9. Small and frequent doses massively increase the efficiency with which nitrogen is used by the crop.

Per kilo of Nitrogen, Urea is always significantly cheaper than CAN (yes, even with a volatilisation loss factored in!), but the best thing about urea is how gently it becomes available to the crop. I have never visited a country where there is as much scaremongering around Urea as there is in Kenya: our soils are acidic, we have good rain, modest temperatures and are applying it where the standing crop reduces wind speed at the soil surface. In short, we have all the right conditions to minimise the volatilisation risk.

Urea takes time to be converted to ammonium in the soil so you need to apply it a week ahead of what you would with CAN. As an admission, I frequently use a urea fertiliser treated with a urease inhibitor; this is because the product is well priced and has a prill size and hardness that allows spreading to 27 metres. Not because there is any direct agronomic benefit from the inhibitor.

Topdressing should also consider the crop’s requirement for Sulphur. For small shambas where you can topdress by hand, ammonium sulphate will always be the cheapest way of applying this and 50-75 kg/ha will apply what the crop needs, but it is very hard to evenly apply the product with machinery unless you have a side dressing machine. There are plenty of good urea and CAN blends with ammonium sulphate available however that will spread to 30m. Using a blend usually has a higher price tag per kilo of sulphur, but it does give a chance to split the application.

As always, experiment on your farm, use leaf testing, soil testing and above all local knowledge to improve your techniques.

Potatoes FAQs

What is the best variety for…..?

There is no single answer to this and it depends on the region you are in and your intended approach to the crop. If you don’t intend to apply a fungicide every 7-10 days and want minimal dormancy to allow immediate replanting, then Shangi and Unica are probably the ones for you! If you have cold storage and believe that you can make the grade for crisping or fries, European varieties like Destiny, Markies or Challenger (for fries) are the only real option. If you have a history of Potato Cyst Nematode, then this overrides everything as you will need a variety with tolerance to either species.

What herbicides should I use?

Always start with a pre-emergence. This avoids crop competition from early germinating weeds and ensures that you do not have to hit the crop with a post emergence such as bentazone. A lot of weeds such as Amaranthus, Gallant Soldier and Nightshade are not easily controlled once established anyway.

Metribuzin is a good option depending on varietal safety, and several are approved for use on the crop in many countries. Adding pendimethalin to this will help Cleaver and Blackjack control.

Where brassica weeds are an issue or indeed if metribuzin is to be avoided on a particular variety, Linuron has wide approval in the crop and is very good on Amaranthus, Wild Radish and volunteer canola. In many countries clomazone, S-metolachlor and flufenacet are added to broaden the spectrum of control, with flufenacet especially good on Nightshade (‘managu’) and Cleavers, S-metolachlor for Fat Hen (chenopdium) and clomazone excellent on Cleavers, especially in dry conditions where its high solubility allows it to reactive after rain.

I always try to avoid re-ridging; the ridge become unsettled leading to more weed germination and erosion, the weeds on the top of the ridge are not controlled anyway, a lot of soil compaction is caused, and even if you use jembes, the number one reason to avoid is that re-ridging causes a lot more green tubers and growth cracks on the sides of the ridge.

Remember that there are plenty of grassweed control options with quizalofop and clethodim for example.

How much seedbed fertiliser do I really need?

As a general rule potatoes require a lot of phosphate. If you look at the requirements for a high yielding crop at 50-60 t/ha it removes – and requires – over 200kg/ha of phosphate on a low P soil. That is over 400kg/ha of DAP or triple super phosphate. The reality is most farmers do not achieve yields anywhere close to that and more typically 100-150kg/ha of DAP is common practice at planting.

For most potato fields I see, soil structure and Cyst Nematodes are the most limiting factors, followed by seed quality. But it is a catch 22. If you don’t occasionally test higher rates of phosphate once you have addressed the bigger limitation, you will never know if P is limiting. Soil testing and knowledge of the field is always your best guide to getting P right.

Remember that P is never just about the quantity of fertiliser put down however, it is also about the roots being able to access it. Cyst Nematodes, Free-Living Nematodes, Rhizoctonia and Verticillium all prune the roots and reduce their ability to use what is in the soil, so addressing these is part of improving your crop’s nutrition.

Potatoes are largely unresponsive to sulphur but it is always sensible to apply around 20kg/ha of sulphate to replace what is leached or removed by the crops.

Potassium is increasingly being touted by input suppliers but seldom do I ever see any responses to this. If your crops do need potassium you should be applying it by the cheapest and most effective way possible – muriate of potash at planting in the seedbed.

Minor nutrients such as magnesium can be important on lighter soils where the soil test is below 200ppm, while leaf tissue testing and visually walking crops remains the best way to identify and address deficiencies of Copper, Zinc and Boron. I am yet to see a benefit from Calcium either from granular lime, TSP (a useful amount of calcium) or foliars.

Till next time,

David Jones,

Independent Agronomist

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David is an independent agronomist in Kenya and a member of the Association of Independent Crop Consultants. David gives independent advice based on scientific trials and experience. Currently works with the Centre of Excellence for Crop Rotation.

Think Agronomy is brought to you by Cropnuts and the Centre of Excellence for Crop Rotation. We share the same vision for sustainable, dryland farming across Africa, and Think Agronomy is our independent voice to promote profitable, climate-resilient farming through better management of soil health, systems-based agronomy, crop diversification, and farm mechanization.