Intermitterende voeding bij tarwe

The influence of intermittent nutrition on dry-matter production, on contents (meq/kg) and amounts (meq per plant) of inorganic nutrients was studied in water and soil cultures with wheat as a test crop.From the results of this study notably from chapters 9 and 12 it is clear that differences between the techniques of water (2.1.1) and soil cultures (2.2.1) do not lead to essential differences in growth and development of wheat plants. Supplying nutrients at a constant level (water culture) brings about a different way of ripening (4.3) and no losses of dry matter from plant parts (fig. 8). But the character of a normal seed-bearing annual is the same as in soil culture where the store of nutrients decreases during growth. This holds for main shoots and tillers (with and without ears); the number of the latter increases in water culture as long as nutrients, especially nitrogen, are supplied and conditions such as light and temperature are favourable for growth (Aspinall 1961, 1963; section 4.2).According to Dilz (1964) data of dry matter and N uptake from pot and field trials are only comparable if based on dry matter of plants grown with these techniques and not on number of plants, weight or soil surface (per pot or per field plot). In this pot trial with soil the average of 2.5 culms per plant is in agreement with the number normally (Brouwer, 1970) found for spring wheat in the field. When inner plants of the pot bear less tillers than outer plants (light side) this is called the 'side effect' (Dilz, 1964). Water culture plants with very pronounced tillering show this effect but in soil culture with 10 plants per pot of which 6 to 7 are outer plants the effect is of none importance. New tillers with or without few and small grains only develop with low (105 mg N per pot) dressings before sowing and suppletion of nitrogen at a very late stage of development (10.2.2.1 treatments 112 and 113). This situation is normally not found in the field because of light shortage.Values of main shoots from water and soil cultures as well as from field trials are similar. Therefore these main shoots must be to some degree independent from the tillers. Young tillers need the leaves and root system of their parents to get carbohydrates, water and nutrients. But in wheat plants at the stage of shooting Quinlan & Sagar (1962) found no transport of 14C from main shoot to tillers or in the opposite direction. This was verified by Williams (1964) with timothy. From this stage on the conclusion of independence of main shoots and tillers, seems reasonable. By comparing only main shoots the influence of treatments 0N, 0K, split application of nitrogen and potassium on numbers of tillers (Chapter 5, 10.2.2. 1, Fig. 33, Table 24) is excluded.With the technique of water culture used in these trials it is not possible to distinguish between roots of main shoots and tillers. In soil culture a part of the older roots is lost by cleaning them (Fig. 29). Therefore the balance of uptake, assimilation and excretion of nutrients will be incomplete although the amounts of nutrients in the roots of older plants are far less than in the aerial parts.The influence of intermittent nutrition in water cultures and split application of N, K and N + K in soil culture on chemical composition of main shoots of young wheat plants is summarized in Fig. 51 by the relationship N-org. vs. C-A (= organic anions). According to Dijkshoorn et al. (1969) with rye grass, uptake and reduction of NO, in aerial parts (leaves) results in production of organic nitrogen and an equivalent amount of organic anions (carboxylates) from which a part is translocated to the roots for maintaining the process of cation and anion uptake by decarboxylation and exchange of H +and HCO3-. The main shoots of wheat (and according to Fig. 11 the same holds for the tillers) grown under optimum conditions (as supposed for water culture) retain, in the period of 35 to 49 days after emergence (Fig. 10) nearly 1000 meq of carboxylates per kg of dry matter. This content decreases gradually in older plants (Fig. 10).Usually for cereals excess uptake of anions, especially nitrate, results in an alkaline effect in the growth medium (in the water culture this was corrected by refreshing the solutions (Table 2) twice a week) and in a store of nitrate (and sulphate) in the plant. The difference between the N-org. and C-A relationship for water culture (Figs. 51 and 26, treatment NPK) and soil culture (Figs. 51 and 49, treatment 130, i.e. 840 mg N and 400 mg K 2 O per pot before sowing) can be explained by the difference in N source. In water culture all nitrogen was given as NO 3 , in the soil NH 4 NO 3 was used and from an incubation trial it was evident that NH 4 was available for uptake at least untill day 60 after emergence. Uptake of NH 4 ions suppresses the uptake of metallic cations and positive charge of NH4+is eliminated without production of organic anions (C-A). This results in, relative to the water culture, a high N-org. content and a low C-A content as expressed in Fig. 51. Withdrawal of nitrogen (waterculture) or low dressings of nitrogen (soil culture) lead to a relative decrease of N-org. against C-A production; the latter, according to Dijkshoorn et al. (1969), proceeds slowly in the roots by using HCO3-. In soil culture (Fig. 49) there is the same process but because of mineralisation of nitrogen the transition is smoother than in water culture (Fig. 26) from which nitrate is withdrawn and replaced by chloride (treatment 0 and 0N).Uptake, distribution and redistribution in older (from flowering on) main shoots of wheat are also in good agreement for water and soil culture. The order of mobility K>N = P>Mg>Ca>Cl is the same for both. This mobility is, as regards translocation (redistribution), connected with uptake and dry-matter production for young plants as represented in Fig. 27 (Chapter 9) for water culture and in Fig. 50 (Chapter 12) for soil culture. Potassium is supposed to be the most mobile nutrient in this cereal because of the fast uptake of this element relative to dry-matter production in young shoots (Figs. 27 and 50) and because from the older plant part is retranslocated to the root medium. (Fig. 45). Nitrogen and phosphorus are only redistributed within the shoots; P from stalks only, N from leaves and stalks. In water culture (Fig. 13) 65 % of both nutrients are found in the ears and 15 to 20% in leaves and stalks of main wheat shoots. In soil culture (Fig. 40) these values for nitrogen were 85, 10 and 5 % for ears, leaves and stalks respectively. Dilz (1964) found in field trials with wheat and oats, 75 % of total nitrogen in the grains (for chaff (= ear minus grain) 10 % of total nitrogen can be used) The amounts of Mg, Ca and Cl increase in all parts of the plant until the end of the growth period, for these elements no net losses are found; more Mg than Ca is found in the ear (grains) so that Mg is probably more mobile. By decomposition of proteins in older plants some sulphur is stored as SO 4 in leaves. The amounts of Na in aerial parts of wheat (also in water culture with 1 meq Na per liter) are very low, especially if compared with K; in roots the contents are higher but still to low to influence (C-A) contents.Uptake and distribution of nutrients are generally determined by the qualitative and quantitative supply in the growth medium, the selection of the uptake mechanism and the translocation from one plant part to another. An example of different N source (NH 4 or NO 3 ) was already given. But also in a system with SO 4 or Cl as anions (Marschner & Ossenberg-Neuhaus, 1970) differences in amounts taken up and mobility influence (C-A) content and the distribution in the different plant parts.As was seen in Table 9 Na is taken up by roots in large amounts. However the small amounts found in aerial parts of wheat, points to a selectivity in the transport system for Na. As opposed to Na, K is taken up by cereals in large quantities and is transported easily.With water culture the continuous uptake of nutrients leads to stores so that changes in supply of nutrients (e.g. K) during short intervals do not influence development and growth of plants and only have a small effect on distribution. If, as in older plants, transpiration is important changes in uptake and distribution can be found. With split application of K (Section 11.3.2, Fig. 45) in soil culture, Ca in leaves and ears (transpiring parts) was higher than in stalks. In water culture replacing K by Ca (treatment 0K, section 6.3.2, Table 16) the same is found for potassium. These results can be explained by exchange of ions in the transport system (Isermann, 1969, 1970) and translocation of the exchanged ions to transpiring parts. The amount of Ca does not seem to influence the quality of grain of cereals.By split application of nitrogen (in this pot trial (Fig. 6) suppletion of N on day 34, 62 and 90 after emergence i.e. at tillering, earing and flowering), the effect on yielddetermining factors (Fig. 38, 10.2.2.1 till 10.2.2.4) is the same as under field conditions (Coic, 1956; de Jong, 1969). Nitrogen applied during tillering and shooting has the most important influence on the number of culms whereas nitrogen dressed at earing and their flowering gives more grains per ear and a higher (1000) grain weight. Yet none of the treatments with split application of nitrogen outyielded (10.2.2.4) the dressing of 840 mg N per pot before sowing (treatment 130). Related to this, soils with low level of available soil nitrogen give a remarkable effect on grain yield (Dilz, 1964). In field trials (de Jong, 1969) no close correlation was found between yields of the untreated plots (controls) and the effect of nitrogen (dressings in one or split application and expressed as yields of grains resp. straw) was found. To be able to forecast the effect of split application of N on different soil types as was tried by de Jong (1969), it would be better to use the uptake of nitrogen (kg/ha) either by grain or straw or both instead of dry-matter production (kg/ha of grains and straw) because in humid areas nitrogen is the most important factor for determining yield.