international fertilizer correspondent
No 1

K rules, OK?

Photo: K+S

Rising affluence and increasing urbanization are going to have an increasingly marked effect on the type of food that is in demand. As diet and food needs change, agricultural production will also have to change and, if demands are to be met, the input of plant nutrients will have to keep pace.

With increasing availability of disposable income, people change their eating habits. Initially they may simply substitute some of their consumption of cheap root crops by grains and rice is often partially replaced by wheat. Given free choice, total calorie intake from roots and cereals decreases to the advantage of vegetables, fruits and meat. Urban populations spend more on food than rural populations and urban populations are rising dramatically, a trend which is unlikely to be reversed. In the urban areas of Indonesia, for example, per capita expenditure on food is nearly one and a half times that in rural areas where cereals form a far greater proportion of the food consumed. Paradoxically, a diet in which meat is favoured over cereals exacerbates the demand for agricultural production of cereals. Production of concentrate feeds now account for much of the 21% of world arable land now devoted to livestock production

The only way to meet demand for more cereals is by intensifying production. Global cereal yields increased from 1.1 t/ha in 1950 to 2.7 t/ha at present and only 6% of this increase was achieved by expanding the area under production. Assuming constant per capita demand, yields would have to be increased to 3.9 t/ha by 2020 simply to match population increase. If the increasing conversion of cereals into meat (as livestock feed) is also taken into account, even higher future cereal yields will be necessary, of almost 5 t/ha as a global average. It will be a major challenge to satisfy the aspirations of billions of people for high value food in terms of quantity, quality and choice.

If that challenge is to be even partly met, the present downward trend in soil fertility in much of the world will have to be reversed. Higher yields inevitably mean higher nutrient turnover or, if unreplaced, higher nutrient loss.

Farmers are conscious of the need to replace nitrogen and phosphate but potassium is often forgotten even though cereals remove from the field as much potassium as they do nitrogen (approximately 30 kg K2O per tonne in grains and straw produced). For cauliflowers and tomatoes, the ratio of nitrogen:potassium uptake is 1:1.4, for potatoes and apples it is 1:1.8, and for bananas it is 1:4. When mineral nitrogen fertilizer is applied, but potassium is not, the imbalance is reflected in disappointing yields and poor quality of product. Farmers waste their input of nitrogen because, without adequate potassium, the plant is unable to make use of it. Excess nitrogen may be leached or volatilized, creating the further problem of pollution. Furthermore, when plants are under stress from unbalanced fertilization, they are more likely to succumb to pests and diseases.

If the potential of high yielding crops is to be realized, K replacement must be given higher priority. Continuous cropping with no K replacement leads to depletion of K reserves. If K is then applied to severely depleted soils, much of that application may be fixed in the soil, making it unavailable to the crop. Steady application of K at a rate that suits the soil and climatic conditions, the farming system and the crop is an essential first step towards meeting the needs of farmers and consumers.