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Reassessing your fertilisation programme after a bumper crop

June 2020

Written by a retired farmer

The 2019 to 2020 cash-cropping season is distinctive in many production areas, for drought conditions that delayed planting of crops in the ideal planting window. Other areas had more normal early season rains and so farmers could plant all their crops on time for a maximum possible yield.

The continuous follow up rains during the vegetative crop phase will result in above average and on some farms record crops of maize, soybeans, sunflowers and other crops. Permanent pastures and annual teff pastures will deliver above normal grass and seed yields. 

Some areas could only plant in the latter part of November and early December. Although these crops looked to be of very high potential during February and March 2020 the final yields realised will be affected by a shortfall of the maximum heat units required. The yields will also be further negatively influenced by the long periods of overcast weather conditions experienced in some areas which occurred during the critical grain filling stages of the crop.

After the very variable rainfall patterns and drought conditions experienced over the last five years farmers in many cases cut back on the fully recommended fertiliser programme for maximum yields. This lowers the financial risk for crop production but in a good rainfall year could result in a shortfall of nutrients to produce a maximum yield. The shortage of nitrogen at planting and subsequent further leaching of nutrients in the fertiliser applied at planting could be observed in maize lands during February and March 2020. A sulphur shortage can be masked together with a nitrogen shortage when yellowing leaves are observed prior to final physiological maturity.

The fertility programme for each land and crop planted should be carefully reassessed after the 2020 crop has been harvested.

If you harvest an unusually large crop this year experience has shown that in some soils, you will experience lower fertility levels due to the unusually high extraction of nutrients this coming season. Some medium potential maize producing lands could produce two or more tons over the best yields ever realised.

Good farmers will make detailed observations of the growth progress from planting to harvesting.

Some of the main deficiency symptoms that you might have observed in your maize crop for some main elements are summarised below.

Abnormal dark green or purplish colour of the leaves in young plants resulting in short internodes and thin stems and reduced cob formation.

Yellowing of leaves and plants not growing to a normal size, or stunting, showing first in the older leaves with an inverted V pattern along the midrib of some or more older leaves. It was quite evident in some of the maize lands observed this season. Plants with five cobs also weaned off some of these to end up with two or three cobs of various sizes. Ask yourself the reason!

Causes pale green leaves or yellowed plants and reduced growth. Although easily confused with nitrogen deficiency sulphur deficiencies show first on the younger leaves compared to nitrogen in the older leaves.

Shows up in older leaves with yellowing and dying of the leaf margins. 

It is suggested that you look at the previous soil samples for your lands and compare them to the recommended fertiliser applications for the target yield to the actual fertiliser applied at planting or top dressing and the actual yield realised.

It is highly recommended that you test for sulphur levels and use a fertiliser regime with a nitrogen and sulphur combination in your next crop. 

What is interesting this year is that in many cases the ideal combination of crop management, fertility, fertilisation, soil potential, soil condition, previous crop rotations, cultivars used and other positive factors will combine to show you what could be a benchmark high maize crop yield for certain lands.

In an ideal situation the pH should have been corrected with the ideal calcium to magnesium ratios and your phosphate status to at least above 25 parts per million (ppm). The previous soil tests should be compared with the one that will be done in August this year and compared with the nutrients extracted in the actual yield realized These can be seen in the ‘Fertilizer Handbook’ (ISBN 0-909071-86-1), mentioned in other articles.

If you have had a good year financially it might be an advantage for the future to lime some lands if needed and correct the phosphate status if too low.

Use your records and crop husbandry observations as to nutrient deficiencies and soil fertility to improve future fertilisation levels for optimum yields.

Publication: June 2020

Section: Pula/Imvula