Nutrition and Water-Air Balance in Crop Production

LAST week a farmer sent me pictures of her tomato plants which were showing a characteristic yellowing of leaves. Her narration was that she had followed the fertiliser recommendations as prescribed by the company that supplied her the seedlings. This was on a one-hectare field and she was elated to grow the crop because of the ever improving tomato fruits. A box of tomato has now hit a K200 mark when less than two weeks ago, the box was fetching a K100 and less; these are the dynamics of commodity prices that no one can predict with ease.

The farmer in question was suspecting that the crop could have been affected by some blight; a fungal disease in tomato. After close inspection of the pictures, I decided to pay the farmer a visit to have a close inspection and properly investigate what the problem was. I should make mention that this farmer has been loyal, but this was her first time to grow tomatoes. She had planted the seedlings in potholes and they were irrigating using buckets or watering cans which system I did not like. Just at the sight of the field, I could tell with ease that they might have been applying too much water, and her justification was that since it was too dry and hot, she opted to be irrigating twice a day. The potholes were slightly deeper as she had put some mulch around the plants as well and water used to stagnate in around the plant holes. After thorough checking of the field, the yellowing was not caused by what the farmer suspected. To make matters worse, she had even applied the fungicides to control the suspected ‘blight’ but the crop was not responding. The trouble with the plants wasn’t a case of disease but it was lacking nutrition. Initially, the farmer refused when I diagnosed the crop to have been ‘malnourished’. She retorted to say that she had been applying fertilisers on a weekly basis but after explaining to her what could have been happening in the soil, she calmed down and decided to try my ‘prescription’. I was so elated and happy when she called me early this week to indicate that the crop had recovered and that it was actively growing and ‘smiling’ at her again notwithstanding that she has already compromised on the yield.

This experience is not only peculiar to vegetable growers alone, but all crops can be susceptible to such conditions. What happens in the soil is that every healthy soil has a balance of nutrients adsorbed to the soil particles and organic matter which acts as reservoirs for nutrients. Between the soil particles are found air spaces and these contain air which is important for microorganisms, and around the soil particles are found films of water and sometimes this occupies part of the airspaces. If all the air spaces are occupied by water, the soil becomes waterlogged and this is not healthy for the plant roots as they will not be ‘breathing’ easily. For the roots to take up the nutrients, they take out what are called exudates on the roots and this is to maintain the plant osmotic potential but if the soils are waterlogged, the roots will not easily breathe and they won’t have enough energy to take up the nutrients even if the nutrients may be available in the soil. This is like blocking your nose when you are about to swallow some food; you won’t be able to swallow easily or at all. Those that could have watched some documentaries of how the Americans made the suspected terrorists that were detained in Guantanamo detention centre in Cuba where using the same tactic; they would blindfold the victim, cover his mouth and nose with a cloth and start slowly pouring water to prevent the victim from breathing easily (though that was not humane). It is therefore important to determine the right water-air ratio in the soil when growing crops. For this lady, it was too much to apply such amount of water especially that the soils were heavy (clay). We know that clay soil tend to hold water longer than sandy soil because of the finer air spaces as opposed to sandy soils that have larger air spaces that easily drains. Additionally, applying too much water than is necessary can also lead to leaching of nutrients out of the root zone. Many of the farmers think that just because they have applied the fertiliser, the only activity required thereafter will be giving it water. Nurturing a crop is a skill that is equivalent to looking after a child that is very young. For the crop to effectively extract the nutrients from the soil, there needs to be a good balance of air-water ration in the soil. It is important to understand your soils very well and know how much water it does require for particular soil to reach field capacity as well as how long will it take without watering for the field to reach wilting capacity of a particular crop. When these two parameters are known, you can easily deduce the water available to the plant.

Lastly, we ask farmers to regularly take their soils for pH analysis not because we want to increase their production costs. pH is a very important parameter that can help us deduce what could be available in the soil for plant uptake and what may be missing. It is like taking the body temperature of the patient at the hospital. When we know the soil pH, we can know whether we need to lime or add acidifiers depending on the crop that we want to grow. Soil acidity amongst the smallholder farmers will be one factor which will lead to soil degradation and abandonment of many fields because of low productivity. As Agronomists, we need to start advising our smallholder farmers to be taking their soil for pH analysis at least every other season if production has to be sustainable. If there will be no ‘burning’ questions from you the readers, we will try and discuss salinity in crop production. I have been elated by the number of emails I have received from farmers asking about information on watermelons, soybeans and groundnuts based on the articles done some few years ago; five years to be specific. As a result most seed companies have run out of soybean seed but please don’t plant your soybeans without inoculants, they help. Histick is one of the friendliest product around for soybean inoculation.

This author is an agribusiness management consultant. ftembo2001@gmail.com

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