Widespread lack of nitrogen

Press, 30 September 1977, Page 17

Widespread lack of nitrogen

On the basis of tests done on soil samples taken from wheat paddocks from close to the Rangitata River to Cheviot between August 15 and September 16 the indications were that some 78 per cent of the paddocks were lacking in nitrogen for maximum wheat production.

The tests were done under a service provided by Kempthome Prosser and Company, Ltd, to help farmers determine whether nitrogenous fertilisers should be applied to their crops, and if so what sort of rates should be used.

The soil samples were analysed for nitrate nitrogen, as the nitrate nitrogen level at that time of the year gives a good indication of whether there will be a grain yield response to the application of nitro-

genous fertiliser. In giving details of the testing, which showed that some 78 per cent of 359 paddock samples contained less than 12 parts of nitrate nitrogen per million, Mr M. S. McGeorge, marketing officer for the company, said that this indicated that a lack of nitrogen was likely to limit yield in these crops provided that a lack of moisture later did not become the overriding factor in this respect.

While the developing wheat plants needed moisture now — and they had been getting this — he said that the crops also needed adequate moisture at the time that grain was forming. This was in about November and December in the case of autumnsown crops and a little later with spring-sown crops or crops growing well inland in foothill districts.

An objective of the service, he said, was to avoid the situation where due to moisture stress at that stage a lot of shrivelled and pinched grains were produced following application of nitrogenous fertiliser, which meant that, there was not an economic return from its use.

For this reason the comEany sought from fanners aving their soil tested information as to whether, in fact, their districts were subject to a soil moisture shortage in the early-mid summer period that could limit wheat yields and therefore make fertiliser nitrogen applications uneconomic.

In the light of this information and test results Mr McGeorge said that quite varying recommendations had been made to farmers, and in most cases he said that these had been followed, although in some cases he suspected farmers had been applying more

and less than the recommended rates as a check on them.

Mr McGeorge said that most farmers had been asked to leave an untreated strip when they were applying the fertiliser, or if the fertiliser was applied from the air to cover a piece of the area with a tarpaulin so that an assessment could be made later of the response that had been obtained.

Where on light land wheat was being grown after a good pasture and nitrogen levels in the soil were already naturally high, Mr McGeorge noted that in the event of a dry turn in the weather the same phenomenon of pinched and shrivelled grain being produced could result even if fertiliser nitrogen was not used.

A number of interesting points emerged from the tests this season.

In some cases, he said that they had retested areas after an interval of about three weeks to see if there was a further decline in nitrate nitrogen levels following further rainfall. This was done in areas that had not been sown to wheat. Mr McGeorge said he was a little surprised that the difference was not larger, but in one case there had been a decline in the level of 3 parts per million and in other cases of one or two parts. An indication that it is not possible to say without fear that where a crop of wheat follows a previous cereal crop the soil will be in need of additional nitrogen came from the testing. In the case of some 125 paddocks wheat was following a previous cereal crop but 21 of these paddocks were clearly not nitrogen deficient. All of the others were to some degree. Nor is it possible to say that because wheat follows a clover seed crop or lupins that the soil will always be high in nitrogen. There were 56 paddocks in this situation and here only 22 had very high levels of nitrogen. Peas also did not show up as being such a good

enricher of the soil, for where 71 crops followed peas only 17 crops, or 24 per cent, showed high soil nitrogen levels.

But here Mr McGeorge said that the length of the period in fallow following the crop and before sowing to wheat could have a major influence on the nitrogen level. In fact 21 per cent of the paddocks had very low nitrate nitrogen levels.

There was a similar story, too, following a brassica feed crop, with only one paddock out of 13 showing a high nitrogen level, and the majority being nitrogen deficient for wheat produtcion, and following potatoes there was also a fairly marked degree of deficiency with nearly 80 per cent tailing below 8 parts per million and only one again being in the top category for nitrate nitrogen.

Where wheat followed good pasture that had been down for four years or more, however, almost 40 per cent of the paddocks made the top nitrogen level and 30 per cent were only marginally deficient, but another 30 per cent tended to be fairly deficient in nitrogen too. One of the lessons that would seem to come from the testing would be that no matter the previous crop, it would be unwise to make any positive assumptions on the nitrogen status of the soil.

But Mr McGeorge said that where properties were well farmed with nondepletive rotations being used and also adequate fertiliser, it seemed that there was less likelihood of the nitrate nitrogen level being very low.

Even at the increased prices now ruling for nitrogenous fertiliser, Mr McGeorge said that where it was recommended its use was still economic. To cover the cost of 125 kg per ha or 1 cwt per acre of ammonium sulphate or 63 kg per ha or | cwt per acre of urea, or 112 litres per ha or 10 gallons per acre of liquid nitrogen, only about an extra 100 kg of grain per ha or 1.5 bushels per acre was required.