The Fertilization of Natural Grassland

                                           Introduction

     Research on the fertilization of natural grassland has been undertaken in South Africa from time to time under fairly different environmental conditions. The results of the work done  have in almost every instance shown that yields of herbage have increased and that the herbage  will have higher protein content. However the practice cannot, at this stage be widely recommended because in nearly every case where the applications of nitrogen have been fairly heavy or have been continued for a number of years, there has been a change in the botanical composition of the sward. Usually the reported changes have involved the disappearance of the climax or near climax species and their replacement by pioneer grass species. Such a change in any  natural grassland, under normal circumstances, is generally considered retrogressive and the invasion of pioneer grasses undesirable. If, however, the herbage and protein yields are substantially increased and the ground cover not decreased then such a change should be welcomed. This is however not altogether evident on the fertilization trial at Ukulinga.

    The investigation reported in the thesis is concerned with the botanical change that has occurred since the initiation of the experiment  in 1951. Booysen  (1954) reported on different aspects of the effects of the various fertilizers applied, viz; the effects of the  fertilizers on yield, crude protein percentage, the economics of hay production and the botanical composition. From results recorded by the officers of the Department of Agricultural Technical Services it would appear that there has been little of real significance that can be added to the work done by Booysen (1954), apart from the considerable change in the botanical composition  since 1954. BACK

                                                   Chapter II.

                                         Review of Literature.

   While there is a large amount of literature on the effects of fertilizers on the sward of grasslands in general, little has been written on the changes that occur in the botanical composition in natural grassland in the climax or sub-climax stage. Much of the literature deals with the grasslands which have been disturbed in one way or another. This review then, is confined to the effects of fertilizers on natural grasslands or veld as opposed to leys, cultivated pastures and permanent but not natural climax grasslands.

    When nitrogenous fertilizers are applied to natural grasslands there tends to be a change in the botanical composition.

In the very first experiments in veld fertilization in South Africa, Staples(1931) made it an object of his experiment to determine the effect on the botanical composition, although he was most interested in increasing the number of legumes by top dressing with photsphatic fertilizers.

   Hall(1931) also paid careful attention to this aspect in experiments carried out on private farms in various parts of South Africa. However, these experiments included only small applications of nitrogenous fertilizers, which in the light of more recent work, have been shown to bring about change very slowly. Further work by Meredith(1948) and Hall, Meredith and Altona(1950) has shown that very heavy applications bring about a rapid change. Roux(1954) cites Rose in personal communications as stating that at comparatively low rates of fertilization the change may take a number of years, but if larger dressings are applied the climax grasses may disappear within two seasons.

   At Frankenwald, the Botanical Research Station of the University of the Witwatersrand, the original grasses consisted of Tristachya hispida, Trachypogon spicatus, Elyonurus argentous, Eragrostis chalcantha, Digitaria solenoids and Heteropogonm contortus, but with very heavy dressings of nitrogenous fertilizers, these grass species had been almost entirely replaced by Eragrostis curvula and Cynodon dactylon (Hall Meredith and Altona,(1955)

At Ukulinga the climax grasses, Themeda triandra and Tristachya hispida tended to give way to Setaria nigrirostris and Eragrostis curvula after a period of five years (Booysen,1954) Hall et al (1955) state that in South Africa a change can be expected, and that usually thiss will mean an increase in Eragrostis Sp. of the E. curvula and E chloromelas types, and also an increase in Cynodon dactylon.

In Kenya Hall and Allen (1938) laid down a series of experiments on Pennisetum schimperi and Andropogon chrysostachyus dominant veld which had isolated patches of kikuyu (P.clandestinum) and clover (Trifolium jonsonii) in a stunted condition. It was noticed that after a year that Andropogon chrysosthchyus had increased on the nitrogen paddocks and that it was a precursor to kikuyu wherever the fertility was increased.

   In the United States changes varied considerably depending on the area concerned. Harlan (1960) working in Oklahoma states that the change in the botanical composition as a result of soil amendments has been one of the chief hazards in attempting to increase the yeilds by this means. Wayne and Eder (1960) state that fertilizers only increased weed production. Attempts to improve the native grass areas have rarely been successful in Oklahoma.

Rogler and Lorenz (1957) reported a yield improvement due to an increase in western wheat grass over Gracelis bouteloa which was dominant prior to fertilizing. Patterson and Youngman (1960)state that ranges in central Washingtonhave become infested withan introduced grass,Bromus tectorum(cheat grass),that has now essentially become naturalized.Increased rates of nitrogen aided cheat grass at the expense of native grasses. Festuca idahoensis and Agropyron inerme were severly decreased by the cheat grass competition,while Poa secunda was not greatly affected. They comment that this may seem strange at first as the two species depressed the most are much taller growing than Poa secunda, but the latter commences growth much sooner and can complete its growth cycle much ahead of the other two species which might explain its success with cheat grass. The results show that cheat grass becomes completely dominant at high nitrogen rates. The control had 13% cheat grass, while rates of 20 pounds of nitrogen per acre had 47%, 40 pounds nitrogen- 56%  60 pounds nitrogen- 78% and 80 pounds nitrogen-82% cheat grass.


In South Australia,Tivers and Crocher(1951) gave an ecological description of the changes under a 20-24in. Rainfall condition.The natural Eucalyptus camaldulensis dominated savannah,when cleared gives rise to a Danthonia spp. Stipa sp. And Themeda sp. Associes. Small dressings of superphosphate (90 p0unds per acre) results in a stimulation of voluntary introduced annual clovers, including Trifolium dubium, T, cernuum while annual grasses, Vulpia sp. Hordeum murium and Bromus spp.also invade.

The improved nitrogen status of the soil resulting from the increase in legumes, stimulates the annual grasses at the expense of Danthonia sp. which begins to die out. With larger dressings of super phosphate,the more valuable Trifolium subterraneum takes control,and animal droppings stimulate the growth of annual grasses; Hordeum murinum/Trifolium subterraneum sword is formed. Under higher rainfall conditions Holcus lanatus makes its appearance at this stage.

At Gwebe in Southern Rhodesia,the most striking response was the increase in Setaria sphacelata. S. pyramidalis and Heteropogon contortus also increased with applications of sulphate of ammonia per acre per annum, and then declined with higher applications. The climax dominants Hyparrhenia hirta and Andropogon schinzii also declined. Digitaria melanin, a palatable creeping grass showed a tendency to increase. (Barnes,1956).
 Roux (1954) states that the discovery that the addition of nitrogenous fertilizers actually sent the succession backwards, made it necessary to revise the opinion that plants coming higher in the succession were avle to take advantage of the fertility resulting from the gradual accumulation of organic matter in the soil, whereas plants  coming early in the succession were able to with stand lower soil fertility. Here of course it is necessary to distinguish between actual fertility and potential fertility. (Theron and Haylett,1953). With this knowledge Roux(1954) and Jong and Roux (1955) carried out studies into the sensitivity of different veld grass species to nitrogen. They found that the degree of tolerance of these grass seedlings to ammonium sulphate diminished as their place in the succession advanced. Thus they found in decreasing order of tolerance:
         1.     Elusine indica, representing the ruderal stage which are plants first to colonise disturbed lands .
         2.      Eragrostis curvula representing  stage i
         3.      Hyparrhenia hirta  representing stage  ii
         4.      Elyonurus argentues and Trachypogon spicatus  representing stage  iii .

                 
Further studies were carried out on the annual dicotyledonous weed Tagetes minuta, Conyza ambigua and Bidens pilosa all of which reacted in the same way as the climax grass Trachypogon spicatus.  Roux(1955) cites    observations by Davidson(personal communications) who states with the higher levels of nitrogen fertilization annual grasses occur in the place of ruderal cotyledons.          

                                                                        Discussion

One observation is that pioneer grasses and weeds are ubiquitous and are opportunistic invaders when the conditions are suitable to them. In most cases then they are unable to invade undisturbed climax communities. In forest areas pioneers are always found on the fringes and move in after logging occurs but are unable to do so under normal circumstances. Gum trees invade grasslands in Natal but do so from the edges where they shade out the grass dropping the seeds and gradually move in. However this is a different situation.

There is another possibility. Some plants give off allelopathic chemicals that are toxic to others and this may prevent the invasion of climax communities but when the climax grasses die out with the heavy Nitrogen fertilization bare patches are left ready to be colonised. In this experiment these seeds would have been present on the whole area but were unable to germinate and or develop into viable plants on the plots that did not receive nitrogen. ‘ Naturally occurring allelopathic compounds play an important role in regulating plant biodiversity, dominance and succession and climax of natural vegetation‘.(Chang Hung Chou, Vol 5 Biochemical Interaction among Plants.)Allelopathy has also been implicated in plant invasion. The Novel Weapons Hypothesis proposes that ‘invaders bring unique, species-specific biochemical impacts to naive native plant and soil microbial communities’ (Callaway, Ridenour et al. 2005). The invasion of pioneer grasses and weeds may have in fact started the demise of the climax grasses once they started to get a foothold.


There was absolutely nothing consistent about the different treatments. The result seemed to be random depending on the seeds that landed on the plots. As an example the three plots 41, 4, and 50 all had the same treatment of Lime,Phosphate 900lbs per acre Ammonium Nitrate per annum yet n3PL Plot4 had 7% cover predominantly weeds(see photo) Plot41 Predominantly Paspalum
dilitatum (see Photo) and Plot 50 predominantly Eragrostis curvula similar to plot 49. While N3PL Plot 52 was predominantly Eragrostis curvula sedges and weeds, Plot 91 predominantly weeds,with Ryhncelytrum setifolium, Hyparrhenia hirta and Aristida barbicollis and Plot 74 cover very poor many bare patches E. curvula dominant with Panicum maximum, R. repens, H. hirta and sedges.

The results show the necessity of providing seed of grasses that are most suitable for hay production and not rely on the random opportunistic grasses that may or may not be present and in so doing also avoid the bare patches and production of useless weeds.

The experiment has been in progress for over 60 year and all possible information must have been garnered from it. However it is there for the edification of students of pasture science and for some to gain experience in pasture research.

However what remains unanswered is the question as to why climax grasses actually die out. It is appreciated that high nitrogen does not suit or is detrimental to these grasses but it was not known why they should die. It could have been physiological or something to do with lack of moisture in the soil caused by the increased leaf growth. Attempts were made to analyse the ammonia content of the grasses as it was known that even minute quantities of ammonia were toxic to plants but the apparatus available was not able to detect such small amounts at that time. This could be easily done now.