CHAPTER 5. NUTRIENT BALANCES

The balances are calculated as soil surface nutrient balances according to the method prepared in Poland in the context of the MAINTAINE research project supported by EC (Fotyma et al. 2001). This method is in general agreement with the official OECD methodology (OECD 1998). The original method or model called MACROBIL was aimed at balancing nutrients at the farm level. It has been adapted slightly for the purposes of this report where regions and the whole of Poland are regarded as the equivalents of the big farms. The model was fed with the following data: area and yield of the crops, number of animals of different kinds and ages (calves, cattle, sows, fattening pigs and so on), consumption of mineral fertilizers and balance coefficients for phosphorus and potassium.

In the nitrogen balance, on the input side the nutrients in fertilizers, manure, and crop by-products incorporated into the soil, biological N fixation and N atmospheric deposit are identified. The same items, apart from biological fixation and atmospheric deposition, are given for phosphorus and potassium. The right side of the balance consists of the nutrients removed with the total yield of the crop. For the sake of interpretation of the final balance, the input of nitrogen is expressed in units of so-called acting N as well as total N. Acting nitrogen is the amount of N from the given source (e.g. manure, crop residues, biological fixation) showing the same effect as the corresponding amount of nitrogen in mineral fertilizers. The amount of acting nitrogen is calculated according to the formula:

acting nitrogen = total nitrogen * nitrogen equivalent

The nitrogen equivalents are derived from the result of field experiments and relate the efficiency of N from a given source to the efficiency of N in mineral fertilizers taken for 1. Nitrogen equivalents are less than one for some sources, such as cereal and rape straw, and may even be negative (nitrogen immobilization). Nitrogen equivalent for fertilizers equals one (by definition).

The amounts of removed phosphorus and potassium also are corrected by balance equivalents. The balance equivalent is the ratio between the phosphorus and potassium uptake at the actual (or planned) yield and the optimal rate of these elements. These equivalents were calculated on the basis of long term field experiments with increasing rates of phosphorus and potassium fertilizers. For calculations at the regional level these equivalents were modified and correspond to the soil fertility status.

Tables 17, 18, 19 show nutrient balances for regions in Poland as averages for 1999 to 2001.

Table 17. Nitrogen balance, kg N/ha of agricultural land

* The amounts of nutrients removed with the crop products.

The input of total nitrogen in the whole of Poland and in all individual regions is higher than nitrogen removal, giving a positive nitrogen balance or N surplus. Nevertheless only in a few regions, (Kuj, Lod, Pom, Wlp and Zpm), are the surpluses of nitrogen slightly in excess of 40 kg N per ha of agricultural land, which is considered to be acceptable in most countries of the European Union. The balance of acting nitrogen is in the whole country still positive but in several regions (Dln, Mal, Opl, Pdk) deficiencies of this element are recorded.

The input of phosphorus in Poland and in most regions except Dln and Sls is higher than the P removed by the crop and there is an apparent P surplus. Nevertheless the output of phosphorus should be slightly higher (the balance coefficient for Poland is 1.12) than removal and hence the phosphorus input and output are practically in balance. In six regions (Dln, Lod, Pdk, Sls, Wrm and Zpm) the phosphorus balance, taking account of the soil fertility, is negative.

Table 18. Phosphorus balance, kg P₂O₅/ha of agricultural land

* The amounts of nutrients removed with the crop products.
** Output = removal multiplied by the balance equivalent.

Potassium input in the country as a whole and in practically all regions except Dln exceeds removal but due to the poor potassium status of most soils, the output of this element should in any case be higher than the mere removal (the balance coefficient for Poland is 1.30). In consequence the potassium balance calculated as the difference of input and output for the country as a whole is negative and in some regions potassium deficiencies exceed 10 kg K₂O per ha of agricultural land.

It may be surprising that, with the low consumption of nutrients in fertilizers and manure, the nutrient balances show surpluses or comparatively low deficiencies. This can be explained by the low or even very low crop yields recorded in the country and consequently the low nutrient removal with the harvested crop products. The productivity of agriculture differs considerably among the regions in Poland. In this context, it is interesting that the nutrients removed per unit of yield are practically the same in all regions, which proves that there is a very close relation between the crop yields and the amount of nutrients at the disposal of the crop. Reversing this relationship, one can conclude that further growth of crop production depends considerably on the increased consumption of fertilizers and manure. The crop yields converted into cereal units and the uptake of nutrients per cereal unit are presented in Tables 20 and 21.

Table 19. Potash balance, kg K₂O/ha of agricultural land

* The amounts of nutrients removed with the crop products.
** Output = removal multiplied by the balance equivalent.

Table 20. Crop yields and nutrients uptake per yield unit in regions of Poland (averages 1999-2001)

* Cereal units as given in table 21 below.

Table 21. Equivalents of crop yields in cereal units