Cassava is usually grown by poor farmers in Nigeria with minimum inputs. Under continuous cultivation, this will lead to soil nutrient depletion. Cassava takes up substantial amounts of nutrients (mainly potassium (K) and requires large amounts of nitrogen (N). The quantities of N and K that need to be taken up by the plant to attain a fresh root yield from 18 to 45 mt/ha are presented here. The harvested roots in particular contain large amounts of K – the NPK ratio in the roots being 5:1:10. This clearly shows that N & K are the big elephant you need to get right in ur cassava farming operations.
Cassava is well adapted to low levels of available P, it requires fairly high quantities of K, especially when grown continuously for many years on the same plot. Long-term fertility trials have clearly indicated that sooner or later K deficiency becomes the most limiting nutritional constraint if cassava is grown continuously without adequate K fertilization.
In the past, it has been presumed that as the cassava crop gives reasonable yields under conditions of poor soil fertility, its nutrient requirements are low. This theory came about because, in primitive cultivation systems which still prevail in large parts of Nigeria today, cassava is often grown on marginal fields at the end of the crop rotation. Extensive research conducted on the nutrition of the crop, however, has shown that in order to realize its full yield potential, nutrient uptake by cassava has to be very high, which in turn requires a well balanced and high rate of fertilizer application.
Cassava yielding 30 mt/ha of fresh tuber removes 180-200 kg N, 15-22 kg P2O5 and 140-160 kg K2O per ha from the soil. On average, cassava extracts about 4.91, 1.08, and 5.83 kg of N, P, and K respectively per ton of harvested tuber.
Role of potassium
Since cassava is a high carbohydrate producer, it requires a large amount of K which has a special role in carbohydrate synthesis and translocation. Abundant K supply favors the primary processes of photosynthesis. It also regulates the balance between assimilation and respiration in a way that improves net assimilation. This is a prerequisite for vigorous growth and the formation of reserve assimilates.
Nitrogen and potassium interactions
Various field test have reported that without adequate K supply, cassava does not respond to N fertilizers or it only responds poorly to N at very low rates of K application. These two major mineral nutrients interact with each other in cassava production, with N playing a dominant role in vegetative growth, including leaf development, and K being important in tuber initiation and bulking as described above. Optimal rates of N and P application were studied in a systematic trial in Colombia, where maximum yield was obtained by using 130 kg/ha N and 160 kg/ha K, indicating that relatively high rates of both these nutrients are essential to achieve high root yield for cassava.
In South India, cassava tuber yield was increased by N and K additions, the highest yield being obtained with 50 and 300 kg/ha N and K2O respectively . From this experiment, it was also observed that the total starch yield was markedly increased by the application of K.
Experiments in Nigeria, in which N and K were supplied, found that although N did not affect cassava tuberization, it significantly increased the numbers of tuberous roots. However, root diameter and weight, storage cell size and number, and dry matter allocation to roots were all significantly higher in plants supplied with either K alone or in combination with N. These were significantly reduced when only N was applied .
Potassium fertilizer practice
Potassium deficiency in cassava can be corrected by the application of 50-100 kg/ha of K as muriate of potash (potassium chloride), with rates being dependent on soil fertility status and expected yield levels. KCl should be applied in bands near the stake within the first two months after planting. In light textured soils, KCl should be applied in two split doses to prevent losses by leaching.
Compound fertilizers are most convenient, if available, but they should be either high in N and K2O or supplemented by top dressing with urea and KCl. In soils adequately supplied with P, compound fertilizers with an N-P2O5-K2O ratio of about 2:1:3 or 2:1:4 are recommended in order to supply enough K to prevent K exhaustion of the soil through cassava cultivation.
This clearly indicates the need to restore and maintain soil nutrient status during cultivation through the use of sound nutrient management practices.