Balanced Fertilizer use for crops

Balanced fertilizer use has always been the concern in India and there have been group discussions and campaigns to implement it. The optimum desirable NPK ratio given is 4:2:1. It is difficult to find out the source or the reasoning for this magic formula.

There are five factors that determine the fertilizer needs of the crop plants.

1. Crop, its variety and yield potential. For example HYVs need higher NPK application. Similarly rapeseed-mustard needs more sulphur.
2. Rooting system of a crop: It is another important factor in determining its capacity to feed the soil volume. Rice roots for example are clustered in the surface 0-20 cm depth and can feed from this soil layer only. Even if a nutrient such as P and K is present in deeper layers, rice roots will not be able to tap it. On the other hand, grain legumes such as pigeonpea and chickpea and cotton, etc. have deeper root system and can feed on P and K reserves from deeper soil layers. Incidently, P and K are fairly well distributed in deeper soil layers and as a contrast N is mostly present in surface 0-20 cm layer. Thus, despite a great deal of effort on studying P fertilization of legumes, responses and therefore applications have not gone beyond 30-40 kg P₂O₅ ha. This is rightly so because only in the initial stages when the roots are young and shallow they feed upon applied fertilizer P, but as they grow and move deeper they feed upon a larger soil volume and the P,K or other nutrients stored there in.
3. Soil fertility status: Soils differ in their nutrient supplying capacity; what has been recently termed as INS ( indigenous nutrient supply). For example, alluvial soils in northern India can supply much more potassium than the Ultisols in peninsular India. Required N:K and P:K ratios in a fertilizer material or recommendation cannot be the same for the two kinds of soils. A perusal of the nutrient status of soils of India for eight nutrients (N,P,K,S,Mg,Zn,Fe,B) as shown in the below given Table, shows widespread deficiencies of N,P,S and Zn.

Table : Nutrient status of soils of India

Sr.No.	Nutrient	Nutrient status category ( no.of development blocks)
		Low	Medium	High
1	Nitrogen	228	118	8
2	Phosphorus	170	184	17
3	Potassium	47	194	122
4	Sulphur	Deficiency scattered over 130 districts
5	Magnesium	Kerala,other southern states, very acid soils
6	Zinc	50% of 200,000 soil samples analyzed found deficient
7	Iron	Widespread in upland calcareous soils
8	Boron	Parts of West Bengal, Bihar, and Karnataka




4. Nutrient uptake pattern: This is the basis used for classifying nutrients as macro (primary and secondary) and micronutrients. This is the main basis which should be considered along with above factors in determining a balanced ratio for that crop and for that region.
5. Nutrient use efficiency: It is another important factor in determining fertilizer need. For example N use efficiency is only 30-50%, Thus about 2-3 times of fertilizer N has to be applied in relation to its uptake by a crop.

There is an urgent need for determining NPKS balanced formulae for each crop in a specific region. Agronomists and soil scientists need to do more home work on this issue during the first decade of next millenium, so that the farmers can make application of needed amounts of plant nutrients. This will also help the fertilizer manufacturers to make the desired grades for different regions.

Integrated Plant Nutrient Management or Supply System (IPNM or IPNS):

An efficient plant nutrient management on a farm should evaluate the reserve stock available and should carefully examine their inflow and outflow. Ange has likened IPNS to a capital system where the fixed capital refers to the soil reserves of plant nutrients and working capital is made up of residues returned to the farm and annual investment in fertilizers. It may be pointed out that agricultural systems differ from the natural forest systems, where the nutrients are completely recycled. In agricultural systems the nutrients in the form of grain,straw and other produce are taken away from the farm. If the nutrients continuously removed are not replenished as in Indian Agriculture, the reserve stock (the capital) is also continuously depleted. That is why the question of sustainability of present day high yielding multiple cropping systems such as rice-wheat has been raised in India. The observation generally made is "we have no right to mine the capital of future generations". IPNM aims at minimising the mining of the natural reserves of the plant nutrients and thus leads to sustainable agricultural systems. Truly speaking, mineral sources of plant nutrients applied as fertilizer and organic sources available naturally or applied as organic residues coexist and constantly interact in soil.

It is generally suggested that 25% nutrient needs of the Indian agriculture can be met by utilising various organic sources such as FYM, crop residues, urban and rural wastes and green manuring. Estimates of these tapale (80% human excreta, 30% livestock dung and 33% crop residues) by 2010 are:1.8 mt (N₂P₂O₅+K₂O) from human excreta, 2.10 mt from livestock dung and 2.34 mt from crop residues (totaling to 6.24 mt of N₂P₂O₅+K₂O). But to mobilise these sources will take much more than just the wishful thinking. The fact generally overlooked is that on per kg nutrient basis organic manures and residues are more expensive. Also, with the gradual decline in draught animals, free home transport is no longer available and the transport costs by tractor trolleys are too high. Also hired labour is too expensive and difficult to get and very few are willing to take up such menial work in the villages. Since most rural areas do not have home toilets some night soil is recycled but in a very unhygienic way. A more hygienic way is needed to use this source of plant nutrient. Human as well as animal urine is very rich in urea.

Most of the livestock dung is used as a cooking fuel in India, but even if ashes could be returned most K, Ca, Mg, Fe, Mn, Zn and some P, B and Mo can be returned back to soil. Although returning of kitchen ash to farm is practised in some parts, this needs to be given more attention by extension workers. This way mining of these nutrients from natural soil reserves can be reduced.

Green manuring, which is used to be practised quite widely in the past is now loosing ground despite substantive experimental data supporting it. It declined from 35.8 million ha in 1994-95 to 22.5 million ha in 1996-97. Probably a better alternative could be growing a short duration dual purpose legume such as mungbean, harvesting the grain and incorporating the green residue. In rice-wheat cropping system a summer mungbean residue can add 60-80 kg N ha.

Of the biofertilisers Rhizobium culture has found most favour with the farmers, particularly in soybean belt. It does good job in other legumes too but these Rhizobia are already present in the soil if the crop has been once grown there. Nevertheless advantages of Rhizobium culture are well accepted by the farmers. Despite a good amount of scientific literature, production and marketing of other biofertilisers have still not made a dent in agriculture in India or elsewhere. The biological nature of biofertilisers and their susceptibility to abiotic factors is responsible for their highly inconsistant performance.