Soil fertility in the Great Konya Basin, Turkey

Soil fertility was studied in the Great Konya Basin, as part of the study carried out by the Department of Tropical Soil Science of the Agricultural University at Wageningen.

The purpose was to find the agricultural value of the soils, to learn about the main factors governing soil fertility, and to work out regional fertilizer recommendations for winter wheat, the main crop in dry farming.

The study was in the field, greenhouse and laboratory. Because of results already available from Turkish scientists, only nitrogen and phosphorus were examined.

In 1966-7 and 1967-8 a total of about thirty trial fields were laid out on the most important soils suitable for crops, mainly Terrace, Bajada, and Marl soils. The trials were carried out on farmers' fields in the common wheat-fallow rotation. As rainfall is low (about 300 mm per year), the land is fallowed each alternate year to conserve moisture for the next crop. To study the significance of this fallow year, the course of soil moisture content was observed during the trial years.

In the greenhouse, short-term trials (3 weeks) were used. A technique was developed, by which young wheat plants could take up nutrients simultaneously from the studied soil and from a nutrient solution (Fig. 11). If a nutrient is omitted in the solution, plants can take up that nutrient from the soil only. The availability of that nutrient in the soil is indicated by the 'sufficiency quotient', the ratio between the relative growth rates of plants on deficient and complete solutions (S Q_N and S Q_P for nitrogen and phosphorus, respectively).

In the laboratory, several physical and chemical soil characteristics were determined; also grain and straw samples from the field trials were chemically analysed.

Wheat yields of the field trials ranged from 300 to 3000 kg grain per ha. The response to fertilizers varied with precipitation and soil unit. There was a reasonable relation between crop data in May (height and growth stage) and final dry-matter production in July.

The yield factors moisture, nitrogen and phosphorus were studied in more detail.

The amount of water stored in soil at sowing proved to depend mainly on precipitation in the preceeding fallow period and slightly on soil unit. There Was a clear connexion between the amount of stored water and maximum dry-matter production. Because of differences in spring rainfall, the relation was not the same for 1967 as for 1968. Between transpiration and maximum dry-matter production, a linear relation was found; it was used to calculate transpiration from each field, so quantifying moisture supply.

The different characteristics for nitrogen status in soil, namely organic nitrogen, nitrate nitrogen and SQ_N , corresponded well with each other. Yield increase with nitrogen fertilizer, nitrogen content of grain and nitrogen withdrawal from soil depended on soil nitrogen and on moisture supply. If grain nitrogen exceeded 2.0-2.2%, there was no yield increase with nitrogen fertilizer. Nitrogen recovery, the percentage of applied nitrogen absorbed by the crop, varied from 0 to 30%.

The results of soil phosphorus determinations by P-Olsen did not correspond with SQ_P . Both parameters, as well as phosphorus content in the crop, indicated a very poor phosphorus status of all soils. The interrelations between soil, crop, and fertilizer phosphorus were complex and were governed by moisture conditions. Phosphorus withdrawal from soil was low, did not depend on soil properties and was determined almost entirely by moisture supply. Phosphorus recovery, low for all soils, was lowest on Marl soils, probably because of the fine texture and the high content of carbonate.

On some fields, soil slaking, profile depth and slope were factors influencing yield.

Yield increments from the factors moisture, nitrogen and phosphorus were effected mainly by increased tillering. The greater number of tillers could cause moisture shortage later in the season to become more severe, and could sometimes cause decrease of seed set and of 1000-grain weight. Phosphorus did not much change the grain/straw relation; longer culms were associated with more grains per ear.

Differences in productivity and in response to fertilizers between the soil units could be ascribed to differences in content of organic matter and in moisture supply. Since those factors have been included partly directly, partly indirectly in many units of de Meester's soil map, a fertilizer recommendation map could be drawn. For recommendations, the profit- maximizing combinations of nitrogen and phosphorus were determined by an algebraic and a graphical method. For the algebraic method, regression equations were used that had been calculated for the statistical analysis of field results. The graphical method was based on the construction of 'maps' with iso-profit lines. On these 'maps' the optimum combination of fertilizers as well as financial consequences of non-optimum rates, can easily be found.

Appendix I gives details of the developed technique of greenhouse trials, discusses the course of relative growth rate and its consequences for the determination of S Q_N and SQ_P and shows that reference soils are needed.