The importance of balanced fertilization

The discovery of the possibility to convert inert atmospheric nitrogen into ammonia in 1913 led to a considerable increase in nitrogen fertilizer production. 

The rapidly growing food demand, which is linked to an exploding global population, explains the development of nitrogen fertilizer consumption in agriculture. Apart from environmental issues, this massive use of nitrogen caused an imbalance with the other major and secondary elements, which in turn did not follow the same rate of development. 

The consequence of this has been a change in the nutrient ratio, especially the NK ratio. The NK ratio of fertilizer uses in developed countries decreased continuously from a fairly balanced ratio of 1:0.8 in the 1960s and 1970s to a current N:K ratio of 1:0.36 today. The NK ratio for fertilizer use in developing countries has changed little and it remains very wide at 1:0.23, ranging from 1:0.10 in the WANA region, and 1:0.13 in South Asia, to 1:0.22 in East Asia. An exception is South America, which has an N:K ratio of 1:0.96, due to the large area of soybean, which is very responsive to potash. 

The very wide NK ratio for fertilizer use, for instance in Asia, contrasts sharply with the NK ratio in plants. Cereals take up N and K in almost equal amounts, while root and tuber crops, leguminous crops and vegetables take up even more K than N. One consequence of the wide variation in the NK ratio in fertilizer use is that the ratio of K input to K output has become greatly unbalanced. The highly negative K balance indicates considerable mining of soil K reserves. In developed countries, up to the late 1980s there was an increasing N balance, which was followed by a rather sharp decrease to currently approximately + 40 kg/ha N. Nevertheless, the N balance is still positive. 

On the other hand, the K balance for Western Europe declined steadily from a rather comfortable surplus of almost 40 kg/ha K2O to currently less than 10 kg/ha. Livestock farms still have a positive K balance due to the import of K in feed concentrates, whereas arable farms already have a negative K balance. 

The input/output balance of P is intermediate between that of N and K. The consequence of the N/K imbalance can be summarized as follows: 

• K is a highly versatile and mobile nutrient in plants. 

• K is involved in all major physiological processes - from the assimilation and the transport of assimilate to its conversion into storage products such as sugar, starch, protein and oil/fats. 

• K also plays a prominent role in the N metabolism. 

• As a cation, K accompanies the nitrate anion as it is transported from the roots to the shoots where the nitrate is reduced to NH3 to be incorporated into amino acids, which are the precursors of protein. K deficient plants have a repressed activity of the enzyme nitrate reductase. 

• K accompanied by malate is then re-translocated from the shoots to the roots, where the K-malate is oxidized, yielding KHCO3, which is exchanged for KNO3, and the cycle continues. 

• Plants that are inadequately supplied with K fail to transport nitrate efficiently into the shoots. 

• This leads to nitrate reduction and the accumulation of amino acids in the roots which signals, via a feedback effect, to the roots to shut down further nitrate uptake, despite the fact that nitrate might be present in the rooting zone (Marschner et al., 1996). 

• Any surplus nitrate in the rooting zone of plants that are inadequately supplied with K is likely to be leached into the groundwater or lost to the atmosphere as NOx gases. 

• The accumulation of nitrate in K deficient plants leads to a reduced protein content. 

• Plants supplied with excessive N and/or inadequate K are more susceptible to pests and diseases and less resistant to soil-borne and climatic stress than plants with balanced nutrition. This also lowers the yield and therefore affects the fertilizer use efficiency. 

There are numerous results from field trials that demonstrate the beneficial effect of balanced fertilization on the efficiency of mineral fertilizers use as shown by the following examples. 

The effect of the K status of the soil on the efficiency of N fertilization: in a long-term field trial with spring barley, Johnston et al. (2001) demonstrated that the grain yield increased by more than 50% with the same amount of N fertilizer only when the plants were grown on a soil that was well supplied with K (340 ppm Kex). 

The plants grown on the soil with inadequate exchangeable K (Kex) (60 ppm) were unable to accumulate enough K to meet the physiological demand of the plant. Similarly, barley cultivated on a soil poor in P (2 mg/kg Olsen-P) yielded only one-half of the crop as compared to barley that was grown on a soil with 6 ppm P although receiving the same amount of N fertilizer. ​